Assuming it is practical, what sort of cost would be involved in raising ratio on 5th or 6th or overall gearing?

Presumably raising the final drive on cars with over 300 bhp would help avoid wheelspin in lower gears whilst improving economy by dropping below 2500 at hi cruising speeds - the usable in gear acceleration should be broadly unaffected as you simply use a lower ratio if need be

using a longer final drive to increase the gear ratios will massively effect the acceleration of the car, the coupe's standard gear ratio's are very long already with 3rd being good for about 106mph and 4th being good for about 139mph, you do not want to raise them

also while the idea of dropping off boost by having a longer 6th gear sounds great the real outcome will be that the car simply cannot pull a gear ratio longer than the 6th which is already in the car

in 6th gear the car does about 2500rpm at 70mph, imagine if you increase your gearing to say 30mph per 1000rpm in 6th you would only be doing about 2300rpm, any sort of incline and you would be having to change down a gear, the problem would be exacerbated in a coupe with a bigger turbo as you would have even less torque lower down the rev range where you need it for cruising

to change the overall gearing by replacing the final drive shouldn't be that expensive as assuming you are using a 6 speed box there are a number of alfa's which have final drive's which are interchangeable with the coupe's although as far as i know they all make the gearing shorter

With my increased rev limit, I'm currently geared for around 200mph at the rev limiter in 6th - with my power, I doubt it'll do much more than 175-180, so there's simply no point in raising the final drive ratio

I'm already at the point where motorway-legal cruising is well below the spoolup point, so dropping the revs wouldn't make much difference

it's already at the point where if I'm cruising at sub-60mph, I tend not to use 6th, as it labours the engine too much. The real sweet spot is about 2,500rpm, which equates to about 65mph in 6th. Anything slower than that and I change down

I was expecting the poster to ask how to lower the final drive.

If anything, the best option would be to re-space the ratios on the six-speed so that the steps are more equal (6th is only a small step from 5th)

ISTR that somebody has already done this, using Alfa internals

yep, its me lol

http://www.fiatcoupeclub.org/forum/ubbthreads.php?ubb=showflat&Main=86943&Number=1043810#Post1043810

and me, just swapping the Alfa 156 2.5 V6 internals straight into the Coupe 6 speed casing. Think I'm geared for 167 mph @ 8000 in 6th, unless I take the kappa to Bruntingthorpe or round the Millbrook bowl, that will never be an issue

Changing gear ratios will make no difference to the acceleration of a car. What it might do is reduce or increase times through speed increments. It will not affect speed over distance.

Hi Trap
err isnt acceleration the rate of change in speed related to time? - Distance is not a factor?

Lovely spec to your car by the way

Trappy not sure what you have written but it's wrong, changing the ratios definitely alters the acceleration of the car.

I'm done with the days of forum arguing so instead I'll just post up some number from my calculator and let you look over the numbers

Fiat Coupé 20vT (Standard final drive 3.110)
Max speeds
1st: 41
2nd: 70
3rd: 103
4th: 135
5th: 171

0-30mph: 2.53
0-40mph: 3.41
0-50mph: 4.78
0-60mph: 5.98
0-70mph: 7.85
0-80mph: 9.71
0-90mph: 11.83
0-100mph: 14.87
0-110mph: 18.11
0-120mph: 22.26
0-130mph: 28.63
0-140mph: 37.4
0-150mph: 59.31

60-100mph 8.89
60ft Time: 2.58
60ft Terminal: 30.5
330ft Time: 6.41
330ft Terminal: 63
1/8 Mile Time: 9.59
1/8 Mile Terminal: 79.4
1000ft Time: 12.21
1000ft Terminal: 91.6
1/4 Mile Time: 14.56
1/4 Mile Terminal: 99
0-400m Time: 14.51
0-400m Terminal: 98.8
1km Time: 26.19
1km Terminal: 126.4
3/4 Mile Time: 29.78
3/4 Mile Terminal: 131.6
1 Mile Time: 36.41
1 Mile Terminal: 139.1
2 Mile Time: 61
2 Mile Terminal: 150.3
0.75 Mile Time: 29.78
0.75 Terminal: 131.6
Top Speed (mph): 152.5

Fiat Coupé 20vT (with 4.000 final drive)
Max speeds
1st: 32
2nd: 54
3rd: 80
4th: 105
5th: 133

0-30mph: 2.53
0-40mph: 3.7
0-50mph: 4.67
0-60mph: 6.26
0-70mph: 7.72
0-80mph: 9.88
0-90mph: 12
0-100mph: 14.98
0-110mph: 18.19
0-120mph: 22.41
0-130mph: 29.4

60-100mph 8.72
60ft Time: 2.58
60ft Terminal: 30.5
330ft Time: 6.48
330ft Terminal: 61.5
1/8 Mile Time: 9.66
1/8 Mile Terminal: 78.9
1000ft Time: 12.29
1000ft Terminal: 91.2
1/4 Mile Time: 14.65
1/4 Mile Terminal: 98.9
0-400m Time: 14.59
0-400m Terminal: 98.7
1km Time: 26.28
1km Terminal: 126.4
3/4 Mile Time: 29.88
3/4 Mile Terminal: 130.5
1 Mile Time: 36.67
1 Mile Terminal: 133.1
2 Mile Time: 63.72
2 Mile Terminal: 133.1
0.75 Mile Time: 29.88
0.75 Terminal: 130.5
Top Speed (mph): 133.1

Fiat Coupé 20vT (with 2.500 final drive)
Max speeds
1st: 51
2nd: 87
3rd: 128
4th: 168
5th: 213

0-30mph: 2.53
0-40mph: 3.4
0-50mph: 4.4
0-60mph: 6.05
0-70mph: 7.55
0-80mph: 9.36
0-90mph: 12.02
0-100mph: 14.71
0-110mph: 17.89
0-120mph: 22.6
0-130mph: 28.62
0-140mph: 37.27
0-150mph: 61.16

60-100mph 8.66
60ft Time: 2.58
60ft Terminal: 30.5
330ft Time: 6.35
330ft Terminal: 62
1/8 Mile Time: 9.49
1/8 Mile Terminal: 80.6
1000ft Time: 12.12
1000ft Terminal: 90.4
1/4 Mile Time: 14.49
1/4 Mile Terminal: 99.3
0-400m Time: 14.44
0-400m Terminal: 99.1
1km Time: 26.12
1km Terminal: 126.2
3/4 Mile Time: 29.71
3/4 Mile Terminal: 131.6
1 Mile Time: 36.34
1 Mile Terminal: 139.2
2 Mile Time: 60.95
2 Mile Terminal: 150
0.75 Mile Time: 29.71
0.75 Terminal: 131.6
Top Speed (mph): 152

In all honesty, I don't know WHY it makes no difference but, the way I see it, you have a fixed power output and a fixed amount of effort required to do a fixed amount of work. No amount of leverage or gearing can change this (I'm sure one of the forum physicists could explain this )

Short ratios gear boxes feel more sporty and enable the driver to help keep the engine 'on the boil' but they won't make it faster in a straight line.

more torque delivery, the gearbox multiplies the engine's torque by a factor equal to the (gear or) final drive ratio.

shorter gearing (higher ratio) provides better acceleration but at the sacrifice of top end

Yes Rudi is spot on, the lower (or shorter as people seem to call in nowadays) the gearing, the faster the acceleration.

The answer is that you don't have a fixed power output; the difference between the 3.11 final drive and the 4.0 final drive is that the engine is spinning faster and you have 1.286 more power strokes per unit of time in any given gear. Assuming a flat torque curve (for simplicity) you will have 1.286 x the BHP of the engine running at a slower speed.

As mentioned above by others, what produces the acceleration is the amount of force delivered by the tyre to the road, and a 4:1 final drive muliplies the engine torque 1.286 more than the 3.11:1 - so more force to accelerate the car.

I think your calculator is making the wrong calculation with regard to engine torque/engine speed and BHP.

Not wishing to start/enter an argument; this is what my excel calculator outputs for different final drives:


.........................4.......3.11....2.5
0-60mph..........5.47....5.9.....7.07
0-100mph.........14.27....14.57...15.7
60-100mph.......8.8.....8.67....8.63
1/4 mile...........14.83...14.92...15.89
1km.................26.34...26.79...27.5

The forum doesn't like copied excel cells; hence all the decimal points

ooops, didnt mean to start a theoreticians debate in applied mathematics...

Not yet owning a Coop, I hadnt appreciated just how highly geared it is, quote:

Fiat Coupé 20vT (Standard final drive 3.110)
Max speeds
1st: 41
2nd: 70
3rd: 103
4th: 135
5th: 171

What changes to above are made with six speed box, simply adds a taller gear or are the main five re balanced? - Will it really pull 135 mph with two gears to go?!

I would have thought with the gearing being so tall, wheelspin can easily be suppressed if boost transition and pressure isnt too abrupt in 1st and 2nd (even with target 330 bhp) - what options have been used for running limited boost / softer transition in first and second / low speed - can std ECU retard boost when fed an external variable such as road speed / gear selection? - Presumably yes, if std car runs similar feature to control wheelspin - what methods are employed other than changing ECU - and keeping subject separate, what are these Boost controllers (Geddes?)- they sound expensive, can they be worked around?

Trappy sorry mate but what ever that calculation is its wrong, plain and simple, lowing the gearing for faster acceleration is absolutely certain. The only time it wouldn't make it faster would be some ridiculous low gearing such that a 60mph dash might take 5 gear changes with the advantaged wiped out in gear shift time.

griffster on the standard 5 speed box the gear ratio in 5th is 25.18mph per 1000rpm, on the 6 speed box the 5th gear is something like 24.5mph per 1000rpm and 6th is 27.5mph per 1000rpm so as nigel said earlier its about half a gear longer, there is virtually no difference in the intermediate gears

something like a apexi boost controller can be set to give different boost levels in different gears

i have a stage 3 ball bearing hybrid which must be close to 300bhp and i get full traction in 3rd with my shorter gear ratios, in the bone dry i get full traction in 2nd but if its wet and i apply full boost in 2nd the wheels will spin even with the continental tyres i have so traction shouldn't be an issue for you

That is really astonishing Doug that an old, presumably fairly simple design FWD copes so well with over 300bhp (it is a given that 1st is not provoked abruptly)
For it to take a full throttle full boost transition in second with barely a wiggle of the wheel and full traction certainly dismisses the old adage that over 200 bhp and FWD are not good friends

Presumably, once in 3rd, because of very tall std gearing there would be no fear of transitional wheelspin in normal wet conditions

What would be minimum nice riding sized tyres that would cope as described - 205 50s? - Would prefer avoiding going lower than 50s to maintain a good ride and avoid camber hunting - also prefer progressive breakaway to optimal grip so wouldnt fit widest possible

loss of traction in a 20vt is more often down to HOW the power is delivered, rather than how much of it there is.

I can happily put down well over 400bhp in the wet, even in 3rd and occasionally in second (surface-dependant)

As for tyres - yes, 205/50 are (in my opinion) the most suitable tyre - just make sure you get a good make, as cheapie brands will not cope

wouldn't say there was no wiggle of the wheel in 2nd when giving it full boost lol, but if you keep a grip of the wheel then it's not a problem and that running standard sized 205/50/16 tyres

as Nigel says make sure you put decent tyres on though, i am running continentals and before that had eagle f1's, would never run a budget tyre as don't think they would cope nearly so well

I knew my statement would open a bit of a can of worms but, to tell you the truth, I've only just managed to get through to people in the office about contact patch/grip etc, so don't want to go through it again for hours here!

Instead, consider this to see just how ridiculous the notion that shorter ratios improve acceleration is...

Car A has ratios that yield the following speeds;
1st: 41
2nd: 70
3rd: 103
4th: 135
5th: 171

Car B has ratios that yield the following speeds;
1st: 28
2nd: 41
3rd: 70
4th: 103
5th: 135
6th: 171

The ratios are indeed shorter on Car B... infact they're short enough that they're the same as those on Car A, albeit a gear higher. Do you really think Car B would be faster than Car A?

The simple fact is that you can't improve a car's acceleration by shortening it's gear ratios. You can optimise the power at the wheels by tailoring ratios to suit the powerband, and you can set it up to help on a drag strip in terms of number of changes and not changing up just before the finish, but it will always reach the same distance in roughly the same time and at roughly the same speed until it redlines . ALL that matters, is which part of the power band is used

have to disagree here trappy, if you don't think lowering gear ratios effects acceleration then try accelerating in 4th gear over 50 to 100mph and then try the same acceleration in 3rd gear, which gets you between the same points of speed quicker, 3rd gear does of course because it is lower geared

in a drag strip situation the car with the shorter gear ratios, all other things being equal, will get to the end of the track quicker, why, because it can reach all points in the speed range quicker and will therefore travel further as it is going at a higher speed at all times

I think I'll stick with Newton's second law in preference to trappy's first law

Oh give me some credit!! I'm well aware of this, it's the whole range of ratios working together that I'm referring to.



You do realise that some of the gears have to be longer to enable the car to reach higher speeds? You can't have 5 ratios and expect them all to be short... Please see my previous post for perspective.



Ironically, Newton's second law makes my point quite nicely...

to put it in perspective, given that the car is a road car, i would much rather have a car that was faster between 0 and 100mph and had a limit of say 130mph than a car that was slower between 0 and 100mph but could go on to 150mph, assuming that the car is to be used on the road surely the faster acceleration is far more useful than a higher top speed

granted you will be running at a higher rpm in the lower geared car although by using the final gear from a lancia you can run the same mph per 1000rpm as a standard box in 6th and have shorter ratios from 1 to 5 thereby having the best of both worlds

In what way? What are you considering as the accelerating force?

You're assuming it would be faster; it wouldn't.

Would a 'non-believer' please answer this question?



It really isn't difficult to understand... Is it?

Car B would get to 28 MPH in less time than car A, therefore it would get to all higher speeds in less time too (even if by only the time saved in getting to 28 MPH)

errr, yep otherwise you wouldnt be standing alone at the mo!

At this point, I would concede to being intrigued by your argument..which was nicely put above!

So what happens from a 40mph rolling start? Car B has shorter ratios than Car A so it must be faster, right!?

to simplify things,
1) what if we assume the no of gear changes is removed from argument by fitting only one gear to each car, one being twice as high as the other - ignoring clutch slippage delay - which will hit a speed within the range of the lower geared car first...? -- hmm, that brings powerbands into the equation...it was always at the route of the mystery anyway.

2)Now take a constant speed transmission where the engine will always run at optimum power at full throttle, no gear changes and no driver finess in pulling away and the gearing is infinitely changing, but with a twist - car twos same variable transmission goes through a final drive twice as high.............- in practice the variable transmission will detect hinderance of higher final drive in accelerating and remain longer at the shorter end of the infinite gearing for longer ----from this, what can be deduced...?

Do I hear the sound of goalposts moving?

Nah, I'm sure it was a penny dropping

Lanci, if you mean by my apparent siding with shorter gearing having first asked about taller....yes....err, and no!

I too have always believed that 5 close ratio gears, with fifth being direct and an interglactic 6th like the Holden Manaro / Chevy would be the way to go to combine relaxed economical off boost cruising with a sprint box set up - after all, dont all competition cars alter their gearing when a hi top speed is not required - why bother if there isnt a trade off in accceleration

Because it allows the driver to use the best part of the powerband more often.

This particular penny dropped for me back in 'O' level physics....

Physics dictates that the higher acceleration would create higher g loading (Newtons third in action?) - the harder the car is forced forward the harder the occupant is pushed back - floor a Scooby in first compared to fourth to experience difference in acceleration - first would put you through the back window if it wasnt for the seat resisting the counterforce and fourth would barely raise a smile - difference? - Gearing?

of course it would be faster, if a car accelerates at a faster rate then it is going to reach the desired speed in a shorter time therefore making it faster, am i missing something here, that is surely very simple

unless you are saying that the desired speed is something like 150mph then yes the shorter gearing wont reach that speed but in the example of our cars then i'm sorry but making the gears shorter definately makes is more more accelerative, i know this for a fact being one of only a handful of people to have actually done this on the coupe

i made no other changes to the car other than the gearing and when the new box was fitted the car accelerated faster because the intermediate gear ratios were shorter, sorry but that's just fact

so how does that perfectly acceptable explanation of yours side with the constant engine speed transmission comparison sited above - the gearbox seeks out lower gearing when held back by the higher final drive, thus higher gearing was impeding progress

I have to say, I am warming to the powerband reasoning, but not certain either way..

....floor a Scooby in first compared to fourth to experience difference in acceleration - first would put you through the back window if it wasnt for the seat resisting the counterforce and fourth would barely raise a smile - difference? - Gearing?....

Do you have any proof? Or does it just 'definitely feel faster'?

Getting back to Isaac Newton, in a=F/m, what are you using for F?

..is this keyboard worki..........

no i don't have proof as i didn't go out and strap timing gear to my car or any such other thing but it definately accelerates faster

i would refer you to my earlier comment when i suggested that you accelerate faster in 3rd than you do it 4th between 2 speed points, which you agreed with, lowering the gear ratos merely make the 4th gear closer to the 3rd gear therefore making the car more accelerative, so can you tell me why you still insist given this is a fact that altering gear ratios makes no difference to acceleration

car manufactures pay huge attention to gear ratios, do you think they would do this if it made no difference to acceleration

Interesting discussion....

If two cars of the same power but different gear ratios accelerated ONLY in one gear, the car with the lower gear ratio would accelerate faster than the car with the higher gear ratio and the car with the higher gear ratio would end up travelling faster by the end of the exercise.

However, if ALL the gears are taken into account, the end result will far less different (although probably not quite identical)

The problem with shorter gear rations is that they run out quicker and you have t change up (to a higher ratio)

Let's simplify this a bit....

You have two bicycles. One has a six-speed rear deraillieur with a HUGE first gear and the other one has a five-speed rear cog, with the same ratios as 2nd, 3rd, 4th 5th and 6th on the 6-speeder.

If the test was how quick you can accelerate in 1st gear, the 6-speeder will win, as it has the lowest ratio.

However, if the test is how quickly you can reach top speed (which of course will be indentical, because the top gear ratios are the same), then the difference between the two bikes will be negligible

The reason for gear ratios is to make best use of the range of engine revs (whether it's for efficiency or power, depending on application)

Some good posts here, carry on. . .

In trappy's two cars above, if it was standard VT power and weight and assuming a 'zero-shift' box, car B would be about 3.9 metres ahead at 28 MPH - and stay there all the way to top speed (because of the identical gearing thereafter).

If you switched from a 3.11 final drive to a 4.0 final drive, thus changing all the gears in proportion, the advantage in all gears would give the car with the 4.0 final drive better acceleration in all gears, upto it's maximum speed of course, which would only be 0.7775 of the speed attainable with the 3.11 final drive (given that the standard car actually has enough power to pull 5750 RPM in top gear, which I doubt it has)

I started thread but it seems to have been hijacked and offerings ignored... so off for a sulk..

Right, let's have another stab at this.



I wasn't sure what you were getting at in your earlier post, but it's fairly obvious that a car with a gear ratio that's twice as long as another will not accelerate as quick...



Yes, gearing, and also grip (let's not go there though).



Now I'm home, I'm looking at the formula on my spreadsheet(I made it so long agao now I can't remember how I measured force).

I've got the formula 375*(bhp/speed) to measure the force
AND
the forumla 9.80665*(Force/weight) to measure acceleration.

The rest of the spreadsheet applies these to the current speed and works out how far the car travels over time. It's a big old beast and not easy to read if I'm honest. It kind of grew as a bit of a hobby so didn't have a clear direction that would be easy to understand later...



OK, so in your argument, what do you think happens when you switch into 5th gear in the 'low ratio car'? You're now at the same speed as the other car but with a higher as he's still in 4th... You're gonna have that situation where each car pulls a length between changes... Ever race a SEAT Leon Cupra R in a standard 20vT??



What you fail to mention, is that the car with the shorter ratios will only have the advantage while both cars are at the same speed in the same gear. I'll try to explain with a demonstration;

Car 1 3.11
1st: 41mph
2nd: 70mph
3rd: 103mph
4th: 135mph
5th: 171mph

Car 2 4.0
1st: 32mph
2nd: 54mph
3rd: 80mph
4th: 105mph
5th: 133mph

At certain speeds, both cars will have more 'leverage' due to gearing.
0-28mph Car 2
28-41mph Car 1
41-54mph Car 2
54-70mph Car 1
70-80mph Car 2
80-103mph Car 1
103-105mph Car 2
105-133mph Car 2
133mph- Car 1

Here we have a whole 'swings and roundabouts' thing. While this is happening of course, you also have to consider the powerband, which of course is the same for both cars in this example. Does this make sense?

Just to add, I used this website a LOT when putting my spreadsheet together.

Well, that's where you are going wrong...

I suggest you start again from fundamentals.

p.s. apologies to griffster - I am just trying to make sure what you read here is not misleading.

I have proof, that changing overall final gear ratio (in my case by changing wheel diameter) affects acceleration a lot..

On my tipo 2l 8V I used standart 185/60/14 (182cm circumference) when I swapped to smaller ones(195/45/14 and 164cm circumference) the acceleration through the gears improwed a lot.

On standart wheels it was impossible to catch standart 16v tipo in good condition.On smaller wheels, there was no difference in acceleration between my 8v and mates 16v.., and after few tweaks done to engine (intake, exhaust and fuel delivery/ingnition mods) the acceleration was impressive (15,5s for 1/4mile...)

On my heavily modified coupe, the time for 1/4mile with ordinary tires 195/50/15 (181cm cirumference) was 14,8s and top speed 153km/h.

because of the (in some term) high top speed indicates heavy traction problems I borrowed slick tires from mates 20VT - In the same day.
The slicks were 200/605/16 -these had circmference 190cm... after this swap I was unable to get under 15,2s and best top speed in finish was 148km/h.. so swapping to "longer" gear ratio meant in my case notable 0,4s on 1/4 track.... On smaller slicks wich I have currently (200/580/15) the time went under 14,5 territory with top speed unchanged (152-153km/h)

this change of tire circumference is comparable as change from standart 16v final gear 3,733 to 3,56 (wich is for example in tipo..)

Trappy you issue here is you confused as to what a ratio is. The ratios in the gears you mention are the same except for 1st on the 6 speed example thus there is no difference they are not lower they are just assigned to a different number on the cluster, hence starting in the same ratio gear from 40 mph to say 100 there won't be a difference. But this thread was about changing a final drive which if you don't know will lower the gearing on all gears such that the max achievable speed in each gear will be lower. The car with lower FD will accelerate faster to its maximum speed (which will be lower than before). We are totally ignoring the power curve and % torque drop in each gear; thus for example 2 std 20vt's one with say 3.5:1 FD and the other 4.2:1, the 4.2 will out accelerate the 3.5 everywhere because it develops more axle torque to accelerate the vehicle (ignoring the factors I mentioned).

This chap actually write it up well http://www.rubydist.com/Family/Power.html

I am well of aware of what the final drive is... in fact changing it makes no more difference than changing the wheel/tyre circumference. You can change ANY of the transmission variables, ratios, the final drive, the tyres, the wheels etc, etc, it simply won't make any difference to the time taken to reach a certain point if the car's driven in a straight line.

For me, the frustrating thing is that I didn't document or set up the spreadsheet so I could later see what I did... For information, there are 183,464 cells containing formulae that work the stats out... Here's a 'biggy' from one of the cells...

=IF(AND(V16=1,W16<VLOOKUP(MAX($B$27:$B$389),$B$34:$C$389,2,0)),375*((MAX($B$27:$B$389)-U16)/T16),375*(X17/T17))

From what I can tell, it's seeing if the max force is greater than the max tyre grip and limiting it if so but, if not, it's working out the force available for acceleration (If the car's in first gear, it allows it to use maximum power up until it reaches max bhp rpm to represent slipping the clutch). A different column has already reduced the power to allow for wind resistance. More columns then work out how much the car accelerates for that tenth of a second, how far the car travels in that tenth, and then tots up the accumulated distance and speed. Simples.

All I can remember is that the 375, hp and mph formula was the key to making the whole thing work by somehow circumventing the neccessity of sodding around with torque calculations... and it does work. I add the following values along with wbhp across the rev band and it works out the performance to within fractions of actual road test data for ANY car.
Vehicle Name:
Driven Wheels:
Weight (kgs):
Weight Distribution (% on front):
Wheelbase (inches):
Gear Ratios
Final Drive:
1:
2:
3:
4:
5:
6:
7:
Rev Limit (rpm):
Tyre Width (mm):
Tyre Profile (%):
Tyre Size (inches):
CD (Co-efficient of drag):
Frontal Area (feet):
RPM and Power (BHP @ wheels)
2000:
3000:
4000:
5000:
6000:
7000:
8000:
9000:


I've been sitting on it for years now with the hopes of one day selling it but I can't see that ever happening now. If anyone wants a look, send me your email and I might send it over... It really is pretty handy and might prove that I'm not talking shite!

I assure data quality for a living, so please trust me when I say I wouldn't put my name to it if I thought for a second it was wrong! For what it's worth, Engineers (especially in the rail industry) rarely know their arse from their elbow in my experience, so a few words at the bottom of a paper don't hold much weight for me...

Clearly it doesn't!

Add just 64 cells and you can arrive at a set of accurate figures for F. Then a=F/m will work fine and all will be well.

To be honest some of the stuff on here is far beyond my understanding. THis helped a lot http://craig.backfire.ca/pages/autos/horsepower

I am pretty certain that changing the final drive ratio DOES affect acceleration times and maximum speed.

But to be fair, I am not clever enough to prove it, so I am enjoying and learning from this thread. 8)

I must say this is quite facinating but I dont think you have understood SediciRich's point which is the key to this discussion. You have not provided a decent arguement against it either. You cannot refer to your spreadsheet as proof in doing so. We dont know how it works or if it is even correct.

looking at the whole speed range of the coupe and comparing accelerative ability of 2 identical coupe's, one with shorter gearing (15% shorter)and one standard

standard vs shorter gearing (per 1000rpm)
1st 6 vs 5.1
2nd 10 vs 8.5
3rd 15 vs 12.75
4th 20 vs 17
5th 25 vs 21.25

looking at the speeds from 1000rpm to 7000rpm in each gear
with the shortened ratios the coupe can accelerate to 35.7mph in 1st before it needs a change to second so from 0mph to 35.7mph it will accelerate faster than the standard coupe

from 35.8mph to 42mph the standard coupe will accelerate quicker as it will stay in 1st

so in first the shorter geared coupe can accelerate quicker for the first 36mph then the standard coupe is quicker for 6mph

from 42mph to 59.5 the shorter geared coupe will accelerate faster, an increment of 17.5mph

after which point it will require a change to 3rd so the standard coupe will be quicker from 59.6mph to 70mph as it can stay in 2nd, an increment of 10mph

from 70mph to 89.25mph the shorter geared coupe will be faster again over a 19mph increment

i don't know about anyone else but i spend 99% of my driving below 90mph and from 0mph to 90mph the shorter geared coupe will be faster over 72mph of that speed range hence making it usefully faster in everyday situations

if you want to continue to expand the comparison to almost max speed then from 89.26mph to 105mph the standard geared coupe will be faster by virtue of the shorter geared coupe having to change in to 4th, an increment of 16mph

the short geared coupe is then faster over the increment of 105mph to 119mph, an increment of 14mph before it has to change in to 5th allowing the standard geared coupe to be faster from 119mph to 140mph an increment of 21mph

therefore from a total of 0 to 140mph the shorted geared coupe can accelerate faster between speed ranges 0 to 36mph (36), 42 to 59mph (19), 71 to 89mph (18) and 106mph to 119mph (13) giving a total of 86mph over which it will accelerate quicker

with the standard gear coupe able to accelerate quicker from 36 to 42mph (7), 59 to 70mph (11), 89 to 105mph (16) and 120 to 140mph(20) giving a total of 54mph over which it can accelerate quicker,

20mph of that increment being over 120mph which really is of no consequence in the real world

so basically the higher the speed the less impact the shorter gearing has, in fact over 120mph due to the standard coupe's gearing it will be quicker after this speed, so if you live in Germany and regularly use the autoban then stick with the standard gearing as it will likely be quicker, if however you live in the uk and drive normally the shorter geared coupe will be quicker in everyday driving

Trappy it doesn't appear you know what this sheet is doing I do not understand why you give it so much credence in that case. The sheet has far more vaibles then your A vs B comparison which displays a fundemental error in you understanding irrespective of other variables.

Now if we put the variable of a torque curve behind the wheel torque forumla Road speed= (rpm * tyre circ)/(overall gear ratio *1050) with a set of gears (overall ratio * engine torque = axle torque) we can end up with a table which can show the drop in axle torque, if this drop is overly large then the car's acceleration will be diminished, at we go up the gears the torque drop will be less but wind resistance will increase meaning less of a torque drop is acceptable. If the gear change puts the engine at and RPM where the torque is greatly reduced then the situation is really compounded. Race gear sets reduce this torque drop compared to road boxes, and high level MS will use different ratios for different tracks.

Now this XL sheet has variables for traction and that is important too, if the tyres cannot provide the traction to tolerate the wheel torque then we cannot utilise that wheel torque and lower gearing here which increases that wheel toque will be pointless - that is clear but your fundemental dismissal of lowering the FD to improve acceleration is incorrect when we assume (quite reasonalbly from a rolling start) that the wheel torque will not overcome the tyre's traction.

Errr, so (Quote): ...'what sort of cost would be involved in raising ratio on 5th or 6th or overall gearing?!'

Thread author

If you can get the parts second hand then labour in the box build will be the main thing. But you would need either ratios which are higher and a matched pair to replace the current gears or a final drive and crown wheel with a lower numerical ratio both of which I'm not sure you'll get, But try finding out the ratios & FD for the alfa 3.2gta, 2.4jtd boxes as a start.

Oh about 8 hours labour to build.

i am using the final drive from the 3.0gtv and that is pretty much a straight swap, that will lower all the gear ratios and as sedicirich says the parts for that second hand aren't that expensive, labour will be the big cost as you will have the labour for the build of the box, the labour to take the box in and out and you might as well put a new clutch in while you are at it as well so unless you are very handy with a spanner you will be looking at least 12 to 14 hours labour charges

to shorten the intermediate ratios and have a higher 6th gear which i chose to do is a different ball game as the only way i could find to do that was to use the whole main shaft from a lancia thesis 20vt and getting one of those second hand proved to be impossible to I bought a new one along with the final drive from the 3.0gtv and a host of new parts and the cost for all that is not something i am going to share on an open forum, if you want further details about that then i would be happy to discuss via pm with you

Right, I've done some digging and found the formula explained (very nicely indeed ) on this website
Horsepower and Torque

Please have a read through and, in particular, look for the power method and the formula I have used in my calculator.



Force= 375*(HP/Speed)

I have the powerband of the car at the wheels, the weight of the car, the gearing of the car and the aerodynamic drag of the car. Throw in some formulas to show clutch slip off the line and maximum grip to represent tyre adhesion and you can see why my spreadsheet works.

Oi Trappy that's the link I posted earlier on in this discussion!

Works for me - must be your spreadsheet. See my next post:

+ speedo recalibration if the feed is from the box

Damnit, the work filter blocked it at the time! I wish I'd seen it then now...

RPM = 5750
BHP = 216

Speed 1st gear, 3.11 final drive: 40.52 mph (my tyre spec)

Force = 375 * (216/40.52) = 1998.773

Acceleration = 6.42 m/s^2

Speed 1st gear, 4.0 final drive: 31.50 mph

Force = 375 * (216/31.5) = 2570.769

Acceleration = 8.25 m/s^2


QED

Are you answering a question Griffster previously asked or has a point been made there?

Read my post above - #1316861

Your spreadsheet is obviously wrong. You can see from my latest post that, using your formula, acceleration is greater with the 4.00:1 final drive than the 3.11:1 final drive.

The same would be true if:

a) the tyres were smaller in diameter
b) the gear ratios were changed rather than the final drive

If you can't see that, then I don't know what more to say.

You're right acceleration is greater in your example... because you used a lower speed!

To make any comparison fair, you NEED to use the same speed. To work out which car is faster, you must find out what the power output is at the wheels at that speed. The fun bit is using the gear ratios, tyre size etc, to work out what power the car is making at any given point.

For the sake of argument, I'll stick with the 3.11 and 4.00 final drives. Let's not use first gear though, as the FIAT Coupé 20vT can only put down a force of 1611 on road tyres which equates to 5.13m/s^2 at these speeds...

Let's do a few 'rolling starts' and what out what would happen if both cars were in the optimal gear and booted it from a few different speeds.

Here is the approximate power at the wheels data for a standard 20vT;
2000: 34.2
3000: 98.4
4000: 157.0
5000: 175.1
6000: 176.8
6800: 148.9

So, from a;
50mph roll
3.11
2nd gear @ 4,856.4rpm
166bhp@ wheels- wind resistance
Force=1241
M/S^2= 3.95

4.00
2nd gear @ 6246.2rpm
165bhp@wheels- wind resitance
Force= 1221
M/S^2= 3.89

60mph roll
3.11
2nd gear @ 5827.7rpm
169bhp@wheels- wind resistance
Force= 1055
M/S^2= 3.36

4.00
3rd gear @ 5097.6rpm
165bhp@wheels- wind resistance
Force= 1033
M/S^2= 3.29

80mph roll
3.11
3rd gear @ 5284.5rpm
154bhp@wheels- wind resistance
Force= 719
M/S^2= 2.29

4.00
4th gear @ 5169.1rpm
152bhp@wheels- wind resistance
Force= 710
M/S^2= 2.26

100mph roll
3.11
4th gear @ 2023.7rpm
126bhp@wheels- wind resistance
Force= 470
M/S^2= 1.5

4.00
5th gear @ 5108.8rpm
127bhp@wheels- wind resistance
Force= 475
M/S^2= 1.51

The simple fact is this. The current power developed determines the current rate of acceleration. Gearing can be used to keep an engine in the power band more of the time. It can't be used to increase power; so it makes no difference to acceleration. It makes it easier for the engine to turn, but the work done always remains the same.

As an aside, you'll note that the gear each car is in in my examples above will sometimes be one higher than the speed the car can reach in a lower gear. That's the short-shift function kicking in at the optimal change-up point

You can lead a horse to water......

true


true, kinda, but a bit of a moot point. cam profiles usually establish the power band and gearing is matched to the engine spec, not the other way round.



true in its strict sense, ie the engine will deliver certain power and this wont change
BUT
it is not true in relation to the torque applied at the tyre or box


wrong, it has a significant effect on acceleration - and top end



????

I dont think you get the maths bud, relying on a spreadsheet incorporating all the mitigating factors such as shift time, mechanical grip, wind resistance only confuses the matter of the extent to which gearing & 'torque' affects acceleration

Trappy

From a quick look, the constant in that formula makes the formula work for imperial units yet you give acceleration in metric?

Without checking your maths I believe in the science but not your analysis. Most people understand torque but they don’t really understand power. Since you are taking a power approach, this is quite interesting. You have really had me thinking over the last day.

In your examples, you match the RPM by selecting a different gear, this is a scientific error, as it result in an unknown ratio, maybe even a similar overall ratio. It is at least going to be different by more than the FD. If the ratio is similar, then sure, it will accelerate the same. However if you use the same gear, RPM and therefore power will be different, which is the point. Also, in your example it does not take into account the change in acceleration throughout the gear. If you have truly shorter gearing it will rev out faster so you can achieve more power sooner.

Shorter gears will allow you to operate the engine around its peak power output more of the time. You may drop 2500rpm per shift rather than 4000rpm. Hence if you plot power output as a function of time or distance, it's going to be higher integral (more area under the curve) for a shorter ratio box than a longer ratio box. I.e. you are extracting more power from the engine over the same distance.

in very simple terms, take a modern diesel car. 140mph+ top speeds are the norm, but they still take ages to get there because they are so high geared

Partly...

Modern diesels are high geared because their range of useful revs is so narrow, but the torque is high. This is why there are many instances of six-speed diesels and (7+ speed when an auto-box is used)

why you led this disscussion - the cheapiest way how to check if it is faster or not is to fit wheels with various circumferences (as I did it and found that shorter final gera means faster almost by 0,4s..)so set of some standart 205/50/16, some set of 17/18or even 19" wheels will be necessary (also 15" wheels fit, but very limited amount of rim type -ATS DTC for example..) a do some ride with some Gtec, racetech and simmilar clever boxes..

and as was already written - the final ratio cost few $.. I get my final gear from AR 1471.6 88kW for only 40euros... the swap is about a working day...so te cost depends on hour taxes of garage... I have my own record in swapping final gears and some sincro rings on my mates skoda felicia kit car in 3ours...

375*(bhp/speed)= force-lbs

9.80665*(Force-lbs/Weight-lbs) gives me the m/s^2

In order to understand how this works, you need to understand and accept the following statements;

-If the vehicle's speed and the power of its engine are known at a given instant, the force of acceleration can be calculated without knowing anything about the drivetrain gearing, tire diameter, or even the engine torque.

-Power is what gives acceleration, not torque.

-The only reaon cars have gears, is because the power is very low at low rpms. If a car had a completely flat power curve (note not torque curve) then it would accelerate at the same rate in ay gear according to Force= 375*(HP/Speed) (let's discount aerodynamic drag for now). This is fundamental. There, with a flat power curve from 0rpms, a car with a single gear capable of taking it to 150mph would accelerate at the same rate as another car with 5 gears to do the job if the powercurve was flat.



I can't understand your point here. In my examples I'm using a fixed speed, which I believe is the whole point of this discussion. We want to establish if a car is faster with a closer ratio'd gearbox at the same speed. With a different FD, both 'cars' will have a different power output at the same speed because they will be using a different part of the powerband. The gear is irrelevant.



The change in the rate of acceleration throughout the gear is determined by bhp, nothing else. This can easily be determind by using a simple vlookup function of rpm vs bhp.



True, but if you modify only the FD, then you will have EXACTLY the same power plot through each gear! That's why it makes no difference. In order to improve the acceleration using gearing, you would need to customize every ratio independently and even then, you must remember that ONLY the current bhp determines how fast it accelerates.

No no and no agian that exactly what we are NOT talking about.
You have fundemental gap in your knowledge between instantaneous acceleration and what the rest of us are talking about is average acceleration a=(v1-V0)/t.

To maintain a body at speed need an instantaneous acceleration as you say it doesn't mater what gear ratio you have at what speed as long as force can overcome the forces of resistance then the speed will be maintained, BUT that is NOT the main theme of recent discussion. The rest of us are talking about the time it takes to change the velocity of the body from one speed to another. The stuff you have typed at fixed speeds is true but that is not the acceleration that will be changed by FD ratio change while the time it takes to change one speed to aother will and thats what the rest of the world on here are talking about. Even the article you linked to explains this when looking at gearing, in fact I'm not sure you understood that article in its entirity.



oh and a vlookup means nothing to anyone without the value of the cell you are looking up and the table you are looking up against, obviously not some sort of empirical maths command.

I can only hope you start to see the differences in the 2 scenarios, otherwise I'm afraid you will not see what the rest of us are talking about. 2 identical cars one with a 3.5:1 will be out accelerated by the other with a 4:1 ratio where the test conditions are say idle velocity in second gear on the 3.5:1 to max rpm in second gear with max velocity being set by the red line speed of the 4:1 FD car. Here we set the v0 and v1 speeds to be the same, but t will be reduced increasing a.

What I find most astounding is that, although I'm the only person in this discussion that actually has a tool that accurately estimates the performance of a car with just a few attributes, you're all keen to dismiss anything I say.

The very fact that I do have this calculator proves that I know what needs to be known to make such a thing! And of course I have those formulas, they're what make it work.



I'm using a fixed speed because if we want to determine if one car is faster than another after the fitment of a shorter FD, then you would want to use the same speed to compare them at!? (I'm getting worried here fella ). This has been the whole point of this discussion from the beginning (except maybe the cost of it eh Griffster ).



I don't understand why you are suggesting I don't have a grasp of the time it takes to change the velocity of the body from one speed to another. The data below shows that at 60mph, the two cars would be in the gear listed, have the power available for acceleration and the rate of acceleration that would give. My spreadsheet records this at every 0.1mph increment.

60mph roll
3.11
2nd gear @ 5827.7rpm
169bhp@wheels- wind resistance
Force= 1055lbs
M/S^2= 3.36

4.00
3rd gear @ 5097.6rpm
165bhp@wheels- wind resistance
Force= 1033lbs
M/S^2= 3.29



This doesn't make sense. At a fixed speed I can use the gearing to calculate exactly how much power the car is developing at the wheels by running a vlookup over the rpm at that speed against the power at the wheels. This is reduced by drag at that speed, converted to a force and and then converted into acceleration. Subsequent columns within the spreadsheet then tally up how far the car travels during this speed increase increment (using the forumlas you posted) and give me the full picture. I can then interrorgate the columns to bring back any metric I want (distance traveled at x speed, speed at x distance, time to reach x speed and so on).


Let me ask 'the rest of the world on here' to provide one shred of evidence that shorter gearing actually provides faster acceleration.

Well the only evidence I can think of is to test acceleration from. 0-30mph

Try it using 1st gear
Then try it in 2nd gear
Now try it in 3rd gear

Now which one gave the quickest time? Or am I missing something?

Sigh... I'm gonna update my signature to this soon!

Accelerative Force(lbs)= 375*(bhp/speed)

If you were to plot power against speed in those three gears, what do you suppose you would see!??????

Here's a good website that explains what a gearbox is for http://craig.backfire.ca/pages/autos/transmissions

I thought you asked for evidence that:

"Let me ask 'the rest of the world on here' to provide one shred of evidence that shorter gearing actually provides faster acceleration."

You asked, I gave.

And isn't that the link that I provided at start, why do you keep referring to it...

You didn't, you stated something that clearly points out that you haven't been reading what I've previously posted.



It isn't, it's a different page from the same website talking about gear ratios and why they're selected.;

In short, it's to optimise the range of the powerband in play. This whole multiplying torque = more acceleration nonsense isn't mentioned.

Trappy,

Sorry mate I thought we were simply discussing the following:

"If I put shorter gearing on my car, my car will have a lower top speed but should accelerate faster"?

Now while all this talk of spreadsheets and formulae is going right over my head I can see the practical side and the only way I can think of to prove or disprove your theory is this:

Shorter gearing (eg first gear), will get me to thirty MPH faster than second gear (taller gearing). Obviously my maximum speed in first will be much less than it would be in second.

I thought that was the crux of the argument. If it isn't then I apologise. Perhaps you could explain what we are actually discussing then? (Please avoid any talk of Excel spreadsheets or mathematical formula or physics... though, just in layman's terms.)

In fact according to your own figures acceleration is altered. (Page 1 of this thread).
Fiat Coupé 20vT (Standard final drive 3.110
0-30mph: 2.53
0-40mph: 3.41
0-50mph: 4.78
0-60mph: 5.98
0-70mph: 7.85

Fiat Coupé 20vT (with 4.000 final drive)
0-30mph: 2.53
0-40mph: 3.7
0-50mph: 4.67
0-60mph: 6.26
0-70mph: 7.72

You will have the same power curve as a function of RPM but you will have a different power curve as a function of speed. i.e. To achieve a higher speed, you will have operated the engine over a different power range. If power range were to be the same then the gear ratio must also be the same.

OK, from that very link



Now 3 gears or 6 gears should make no dfference to your arguement. The only thing that has changed is the gearing.

The 3 speed could theoretically have another 3 higher gears again only they were not used as the first 3 covered the same speed range as the other 6 speed gearbox. This is basically the same as changing the FD by a huge ammount.

Oh I do *love* an argument where people are so fiercely agreeing with each other!

Here's the short and nasty: if you increase the final drive ratio (i.e. more teeth on the pinion or fewer teeth on the crown wheel), without making *any other change*, your vehicle will accelerate more quickly in all gears but will have a lower top speed (limited by engine maximum revs). If you reduce it, the opposite effect will occur. This is exactly the same effect as putting smaller or larger tyres on, respectively.

What determines the instantaneous acceleration is the instantaneous power available - and note that the peak power output from *any* engine is always above the peak torque output.

The function of the gearbox is to match available power to desired speed. Ideally, for the best acceleration, you want the maximum power transferred to the wheels at all times, so you match the ratios in the box such that having passed the power peak, changing up will reduce the speed of the engine to achieve the same power on the 'uphill' side of the power curve.

The closer the gearbox ratios are, the nearer the peak power you will be when you change and the more power you will deliver to the wheels, and therefore the greater the acceleration (ignoring such delights as the absolute number of gears in the box, the time it takes to change between them, the physical momentum of the engine parts, friction losses in the box etc).

The manufacturer will play with the gears generally not to give the maximum acceleration but to meet a number of other criteria - acceleration at the bottom end (because 0-60 is *important*, right?) or midrange; quietness or fuel-efficiency at nominated cruising speeds; even avoiding bodywork resonances at local speed limits. But within those constraints, they will generally attempt to arrange things in the box such that if you change down at optimum revs, you'll get much the same power on the other side of the curve.

Example: 16vt (for which I have the Fiat curves to hand) has a power of about 180BHP at 6000, and the same power at about 4500; between those two points the power increases. So you'd expect the gear ratios to be about 4500/6000 (0.75) between changes:



Looking at the NA, we see essentially the same


Looking at the NA curve, we see that the power at peak revs is about 135BHP at 6500; the same point on the up-side is around 5400 rpm. That would suggest a better performance could be achieved with ratios of 0.83 - but you'd need a seven speed box to do that, and you'd spend so little time in the lower gears it would probably not be worth it. (And that's probably why the ratios in first and second in both boxes are below the expected values; the acceleration has to stop while you change gear so you don't want to do that too often...)

One final thought: it's quite possible to get the seemingly contradictory position that decreasing the final drive ratio will *reduce* the top speed... maximum speed when maximum power is transferred to the wheels. You'd expect this to be the engine's peak power, but consider this scenario: at any speed on the road, what is holding you back is the friction with the road surface and the aerodynamic friction (form drag). Form drag is proportional to the square of the wind-speed, so ignoring tyre friction you need four times the power to go twice the speed. It's quite possible to gear the engine such that at a given speed, the power required to accelerate is more than the engine is capable of supplying at that instant, even though the engine is capable of delivering more power. This is why some cars with a long top gear are faster in the gear below...

Honza did just that.


so confirm to calculate the time it takes to go 1-30 you are using the value you have for acceleration available at 1mph and are using that rate as the average acceleration for the speed change of 29mph to derive a time.

Of course you could just mail me the sheet and I'll answer this myself if thats easier.

I've been wondering if this was the case for a bit now...





You see I believe these two statements are conflicting. By increasing the final drive, you will not only be decreasing the time to get to the peak power in thenext gear, you are also decreasing the time it takes to pass it.



It's also possible for this to happen in gears shorter than the top gear to a lesser degree. Sometimes a car will have passed the peak power and be on the tail end of the power curve when the next available gear can supply more power before reaching the redline. Here we have the need for a short shift though it's not as clear-cut as the instantaneous cross-over point; the time to change gear needs to be factored in to see if it's effective... that took some doing on the spreadsheet!



To calculate the time it takes to go from 0-30mph, I am calculating the time to go from every 0.1mph increment from 0.1 to 30mph.



I've left my portable hardrive at work (out on the lash Frday night and it bores people in the pub on a laptop) but will happily send it over in the week. I'm going to have to do a little work on tidying it up and using some headings/ descriptions to make it clearer because it's just a page of numbers at the moment and a lot of it isn't relevant to this discussion.

I'll also use it to see how it covers the 3-speed vs 6-speed argument posed in the website as all the data is there for it to calculate the comparison. PM me your email and I'll send it over with a covering email to make it clearer.

trappy given the above is a fact, can you explain how up to 120mph given that the shorted geared coupe can accelerate faster over 86mph of increments against the longer geared coupe's 34mph of increments, how it wouldn't be quicker

Because 'having shorter gear ratios' doesn't mean you accelerate faster. The only thing pushing your car forwards is the power at a given time.

Here's a fact;
*200wbhp at 60mph in 5th gear will give the same acceleration as 200wbhp at 60mph in 2nd gear*

To calculate the time it takes to go from 0-30mph, I am calculating the time to go from every 0.1mph increment from 0.1 to 30mph.



I've left my portable hardrive at work (out on the lash Frday night and it bores people in the pub on a laptop) but will happily send it over in the week. I'm going to have to do a little work on tidying it up and using some headings/ descriptions to make it clearer because it's just a page of numbers at the moment and a lot of it isn't relevant to this discussion.

I'll also use it to see how it covers the 3-speed vs 6-speed argument posed in the website as all the data is there for it to calculate the comparison. PM me your email and I'll send it over with a covering email to make it clearer. [/quote]
------------------------------------------------------
thanks when you are ready I'll PM a mail address

of course shorter gear ratios means you accelerate faster, you accelerate faster in 2nd than you do in 5th, why, because the gear ratio is shorter, the power from the engine hasn't changed, the only thing that has changed is the gearing and as the wbhp constantly changes you have to look at it over the entire rpm range

You accelerate faster through second gear for two reasons.

One, the force generated by the same amount of power differs because of this formula;

Accelerative Force(lbs)= 375*(wbhp/mph)

*The same amount of power at different speeds give a different amount of force*



Two, aerodynamic drag in 5th gear is a LOT more than it is in second gear when you're using the same part of the powerband (say at 5750rpm in both gears).

You can therefore state that accelerative force(lbs)=
375*((wbhp-drag)/mph)

Where are we with this discussion?

I have read the thread again and have come to the conclusion that the formula Trappy has used is sound. But I believe there maybe some false data lurking in his excel sheet. Maybe as a result of some false assumptions.

I say this due to the example Trappy gave on page 3. The example shows 2 gear boxes where 5 of the gears in each box actually have the same ratios. However Trappy seems to imply the ratios are different.

It's on on one page for me, but I presume you are referring to the following post?



My point was that Car B has (for all intents and purposes) a closer ratio'd gearbox because every equivilent gear is shorter or has a different ratio to the same gear on Car A.

What I was highlighting (in a slightly exaggerated way), was that this change to all ratios makes no difference what-so-ever to the performance.


Back to the thread, I was going to send the spreadsheet on but haven't found the time at work (after hours) and haven't been at home long enough to tidy the spreadsheet up. I'm working on it though; it's a lot more complicated than I recall and I'm, at times, struggling to work out what most of it actually does! It's only through deleting code that I can see what metric it works out when it returns an error...

i am not going to argue this point anymore as i am happy to agree to differ, all i would say is that i haven't relied on any spreadsheet or equations rather i have done it the old fashioned way and actually made the change to my car and after doing it the car is definately faster

Maybe this is just a poor example but,

The OVERALL gear ratios to the ground in 5 of the gears are not shorter in your example. Please understand that concept, they are exactly the same, only the position of the gear stick will be different when driving.

Here is an example where car B has shorter ratios than car A

Car A has ratios that yield the following speeds at redline;

1st: 41
2nd: 70
3rd: 103
4th: 135
5th: 171

Car B has ratios that yield the following speeds at redline;

1st: 37
2nd: 63
3rd: 93
4th: 121
5th: 154

All I have done is made Car B 10% shorter than Car A

Note, not only are the speeds lower in each gear but the key is the difference between each gear is also smaller, thats where the nomenclature "shorter" comes from.

Lets assume our engine has a flat torque curve and a linear power curve, maximum power is achieved at redline. (i.e. a good NA engine)

Accelerating from 0-154 will involve the same number of gear changes and if we discount any traction problems or wind resistance etc, car B will reach 37 at red line, it has already extracted a full power sweep, while car A will reach 37 at less than redline, i.e. not yet extracting as much power. By 63 car B has extracted 2 power sweeps, but car A has not, in fact engine speed will be even lower than it was in the compareson of 1st gear. The problem for car A is going to compounded, every gear RPM is falling behind car B to a higher extent. When they reach 154, car B has extracted more power from the engne than car A.

Rev drop, I have this calculator saved on my desktop I find it very good at showing speed vs ratio and rev drop at redline, well worth downloading for anyone interested.

You will need some kind of spreadsheet program (excel etc) for it to work.

Calculator