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!