#BHP vs MPG

Hello All,

I hve had a good read through the tuning guides for the 20VT and was wondering if there are certain mods you can do that effect MPG less than others or is it a case of the higher the BHP the lower the MPG???

?! i think something is missing... .

here comes Nigel.....

the basic ones like air filter and exhaust will not affect the mpg much

Depends what car mate, in my diesel runaround it has to be MPG.

But in my modded quick coupe mpg is not in my head, i'm just think BHP.

Coupe isn't really a MPG car, MPG is more of a diesel thing or petrol 1.1. Deffo not acoupe anyway, all tho a standard coupe is not that bad off boost and drove sensible.

Hello mate, werlcome to the forum.

I modified mine from standard to 325bhp and had no noticeable difference in MPG.

you should only really notice big mpg differences when you are accelerating hard, with normal driving it should be quite similar (better if you have headwork). But like J says I'll let Nigel have his moment

If you drive it economically as long as the modifications are well thought out there will be little or no effect on MPG. Tuning the ECU will actually improve like for like steady running as more ignition advance improves the combustion of fuel.

Anything that reduces the impedance of airflow into or out of the engine will generally improve economy.

If there was ever a topic that was made for Nigel, this is it. He's probably composing a 2000 word essay as I write this....

Yes, almost certainly quoting his best tank range efforts on that 1 historic (but dull) drive

i am trying to work out how long it would take to recop the cost of a 400bhp coversion if i got another 10mpg as nigel gets between 35 and 42mpg and i get between 25 and 30mpg.
i do 15000 miles a year so not to long lol :-)

my caluclations as a 10mpg difference i would make back about £790 a year.
how much would it cost to hit 400bhp?

Sounds like good news then.......mod as much as you want and you could even potentially improve MPG.

well the main thing is increasing the mpg on these or any car is having a decent port and polished head, decent balanced engine using low resistance parts and most of all a good map.

If you do the air filter and exhaust do you get much of a BHP increase?

If you are looking to modify a Coupe engine on the basis of saving money on fuel, well... That's just

Once you go above 350bhp you will require a complete engine rebuild. That will cost about £1200 in forged parts alone and that's before you start on the ancilaries like bearings, gaskets, etc, etc. Then you have labour on top of that, bigger turbo, FMIC, exhaust etc. It really is an expensive and time consuming business not to be undertaken lightly. if you are on a budjet, don't do it, as there is always something else to do.

The best bet is to take a look at TurboJ's 20VT tuning guide, and make a decision after reading that.

Also, don't forget that if you modify a car to give it improved performance, yes, it will improve MPG if you drive it cautiously, but who is going to drive a 300+bhp coupe like an OAP? You are bound to take it out for a good thrashing every now and again, that is what the car is for!

If you want to save money, buy an economical diesel run around.

007 it surprises me because when I modded my bravo hgt the fuel comsumption went rediculously high. I couldn't belive it. havn't noticed it yet with coupe but my project has a long way to go so i expect it will soon.

I was a little surprised to. My best run has been 368 miles, that did include a long run back from PF but I usually get around 320+ and never less than 300. I don't drive like a nut all the time but I definitely enjoy a good blast off a nice roundabout or the odd fast bend.

Buy a 60MPG+ car and spend the savings on tuning your coupe
Based on business miles that I get paid at the gov rate I realised that by running a 60MPG car instead of a 30MPG car for my 35000 business miles/year I could save a few grand a year in total car expenditure.

So now I use a Audi A2 TDI for work. £35 a year to road tax, £200 to insure and does 62MPG without much effort. Takes 25,000 miles to wear out the tyres and is made from lightweight aluminium so will never rust.
Helps pay the insurance and road tax on my other 4 cars that I get my fun from!!

yeah but i only use my car to drive for pleasure not work and i do 50/50 motorway and late night b roads so i normally cruise on the motorway which would give me great mpg and then have 400bhp tp have fun with on the way home :-)

i never said it was the best cost effective method but atleast it would be fun as i don't just want a car i drag out the garage on a sunny day and driving a boring car most the time as whats the point,

rule number one of engine tuning is the more fuel and air an engine can consume the more power it will produce

Obviously there are other factors that effect fuel consumption, these are effiencys like thermal effiency, mechanical effiency, we will ignor these as they are not going to be very different from one 20vt to another 20vt although things like wheel diamiter can effect these

Ontop of this and as john has said how it was mapped can effect it also


But you will find with turbo charged engine that have larger turbos fitted they will be laggy in comparison to the standard setup, this means they will be producing boost later in the rev range than the standard setup, in some cases this can be higher than your cruising speed on the motorway

So if you sit at 70mph and 3000rpm you may, depending on your setup be producing very little boost and as such you will not require so much fuel.

So if your clever with your setup you can make a very powerfull car but also in a certain situation very economical

Ricky

Tricky I'm sure you meant to say this but I'll clarify just in case. When cruising on the motorway at 3000rpm, it doesn't matter what turbo you have as long as the engine is under no load i.e. very light part throttle, then you will be running between 0 to -0.1 vacuum pressure. Effectively you should have the same fuel economy as an efficient 2 litre car.

Re the mapping side of things, I invariably give a lean burn as well as advanced ignition at common cruising speeds to improve fuel ecomony.

Thank you Flea, I did not make that very clear and that in itself makes clear that driving style makes a massive difference to fuel consumption

LOL - am I getting THAT predictable?

The best mods you can do for performance AND better mpg is headwork

The cylinder head is the most restrictive part of the engine and any gains here will always result in better flow (which equals better power and/or better mpg

I'm running around 400bhp and I have got a best of 43mpg. However, when I got the 43mpg, I doubt that I produced more than 40bhp all day.

I can get mid to high 30's mpg cruising at motorway speeds. Even my daily commute gets me 30mpg if I turn the boost down.

I conservatively reckon that when I'm producing peak power, I'm probably doing 6 or 7 mpg at best. I once got 7.9mpg while on track (and that was with 300bhp)

A lot depends on the mapping too - Flea currently has my car for some more tweaks and I'm hoping he'll keep it fairly lean off boost so I can get decent mpg on long motorway journeys.

So - spend £1000 on a mildly-worked cylinder head and get an extra 30 - 50 miles per tank - it won't be long before its paid for itself

Yea and the rest

£1000 for a modified head is a lot of money! how come its so expensive on these cars as any other head i have had port polished and larger valves and so on has worked out about £450-550 but then thats with me fitting it.

i recon that if you picked up very good hardly used secondand parts and built it up then 400bhp could be done for about £3000 if i done all the work except mapping.
so it would take about 2.5 years to pay off in better fuel consumption but more relisticly you would maybe lose 2k in extra mapping and parts failing but then that would be a fun 2k.

Do you get any flow results with the head? A properly modified head will easily cost in excess of £2000+, there is a lot of work to be done, especially when you are watching how the airflow goes through the head, matching the inlet / exhaust ratio to keep it around 75% or 85% (can't remember), lapping, grinding, modifying the seat, fitting race spec valve guides, cleaning everything

maybe i am used to old school engines as i used to know a guy who used to work for vulcan and he done all my head work for me. and most the old school guys i used charged around £450 or a 4 pot 8 vlave £550 for a 4 pot 16v and £600 or a straight six.
i am not saying it is easy work but the haed for my old 2.1 pinto that made 212bhp cost me £300 to get sorted lol.
might see what these guys can do for coop heads as they are very good at what they do.

all I can say to that is dream on!

I've spent easily over that on parts alone to get it from 360-370bhp to hopefully 400+! When I bought the car off Joe I worked out that he had spent close to £15k. Tuning isn't cheap!

really? i think i might give up on the coop then as pretty much any other turbo car i ahve had i could get 400 bhp for around £3000 to £3500 using secondhand parts.

it wouldn't cost nowhere near what has been spent on mine, but I think you are looking at least double what you've said to have a "reliable" 400bhp. And thats without suspension and brakes.

This has been done a thousand times, but let's do it once more with secondhand parts prices

Rods - £500
Pistons - £500
Flowed head - £500
Turbo - £750
Downpipe - £200
Boost control - £250
Intake (filter, SIP) £100
Intercooler - £300
Dumpvalve - £50
injectors - £200
Mapping - £500

Total = £3,850

This assumes you can actually FIND all the parts secondhand - you may end up with a very long wait.

It also assumes you fit everything yourself - it doesn't include all the extras (would you fit a flowed head without replacing the cambelt? - no, neither would I)

I reckon this is the absolute minimum you would need to get to 400bhp - however, IMHO, you would also need to spend another £1000 or so on making the car stop and handle as well.

To do it properly and with someone fitting bits for you, I still reckon you're looking at £7,500 minimum

peanut - the reason you can get to 400bhp on other turbo cars is because you're thinking of a Skyline, which starts off with a bigger displacement, higher starting power and more easily accessible (and cheaper) tuning bits

If you're thinking about a Rover Tomcat, you cannot possibly get one of those to 400bhp for £3k

Those bits that Nigel mentioned like cambelts soon add up! rebore, gasket set, spark plugs, bearings, the list goes on.

Nigel can I shop where you do for cylinder heads please

yeah you could get 400bhp using secondhand parts on a tomcat.
infact there was a full setup including new forge block bored to 2.1 and so on for 2k including turbo rated to 380bhp for 2k on rovercoupe forum.
so maybe 400bhp would be achivable on the tomcat.
well maybesay 4k if i done the work myself on the coop.
using secondhand parts a gtst would be about 3k and a rb26 gtr even less.
i have some of that list waiting to go one or already on so im getting there :-)
i agree with the handdling and brakes but thats another story. i know one guy with 610bhp on a certain car and he had standard carpy brakes. he was happy with it but then he is a nutter lol

i recon acttully 4.5k with the bits and you would be in there with a uprated clutch and so on.
to be fair though 400bhp coop should be worthn the money and be very quick.

also i recon that you would driven day to day achive 10mpg better than you would at 340bhp and the difference is price on parts is maore due to a rebuild but that coule be recouped.

Me too, coupe for bhp and fun

turbo diesel run around for mpg and genral running around in confort and economal movement.

well i just think that unless i get really rich then i want one car that does it all and a coop that could get 35-40mpg driving carfully but has the neck breaking power when putting my foot down would be ideal.

out of intrest if i went from 300bhp to 400bhp i doubt any parts really are interchangable so i was just going to save and go for the 400bhp from the off or do you think its worth hit 300bhp first?

most of the parts you are going to use getting 300bhp will still be used getting 400bhp. The only things that you won't are the turbo and the map. Just go for a downpipe which fits onto a GT series turbo and also a clutch which will support 400bhp (I know clutches are rated to torque )

300bhp

GT28R
FMIC
Full Exhaust
Walbro Fuel Pump with wiring mod
SIP with Filter
EBC
Dumpvalve
Some sort of map/chip

400bhp

GT****
Rods
Pistons
Flowed head
Larger injectors
Bigger MAF
Maybe Cams??
Remap

another idea would be to install a gt30r say for 400bhp and the fmic, down pipe, full exhaust,welbro fuel pump,sipand map it on a low boost setting and deal with slightly more lag and then build the engine up and add the forged bottom end, head, injectors, maf,cams and then get mapper later on?

maybe, but for the little money you are going to lose by the time you have sold the GT28R (presuming you bought it secondhand) I would run a smaller turbo. It won't be much fun having a big turbo with no headwork.

Just to set JIM16VT's mind at rest. When I asked the original question I wasnt trying to find out what I can do to improve MPG, I was just wondering if there are certain mods that can improve both BHP and MPG. IMO if you can get the horse power and keep the same MPG or improve it, that makes financial sense for those who cant afford to run 2 cars.

Ok mate, one thing I do know is keeping air filter clean helps. And also just genral looking afterr and maintaing car too.

As for the mods I have always thought that it makes mpg go up, who knows tho.

basicly headwork and a decent map will increace mpg and bhp together but head work isn't worth it unless rebulding the bottom end so really just a decent map and keep the car serviced well.

Supose everyone has there own view and story on how they want there coupe.

IMO, tho there is always going to be a issue with burning MPG in a 2L-turbo

To me it seems crazy talking about trying to save petrol with a engine like that.

i know mate i wasn't really being serious. that is just what you tell the missis why you have to get a remap and a bigger turbo lol
it will get better mpg but you wouldn't just doing it for that but more the fun factor.

I agree with JIM16vt. There is no point buying a 2litre turbo engined car and then worrying about fuel costs, I just want to make sure that when I get mine (Hopefully next month) and start doing some work on it, I dont end up going a bit too far and ending up with a car thats a massive amount of fun, but I can only use it on the weekends because I cant afford to put petrol in the tank.

Bull - why re-build the bottom end if it does not need it?

Why not just get a big V8 then? If you only want one car to do everything and a lot of miles then a turbo gives you the best of both worlds.

I'll sum this up. A flowed head a good map = efficient engine for cruising and more MPG.

However....

High boost and a big turbo mean when you put your foot down MPG will be in single figures, so depending on how you drive I reckon the average is probably about the same. The only exception is if you do a lot of motorway miles.

Why not get a v8?

For me tubos are much more fun to drive than Nateraly asperarated cars,Not matter how fast they are. Plus it is alot easyer and mcheaper to mod and increase power of a turbo than it is with non-turbo.

fine - but they are also better on fuel on a run - so you are onto a winner there!

Keeps you away from the MRS

Suba the thing is , a V8 will kill the fuel weather it has 5 liters or eight. Turbo or not engine that size with 24 valves and 6+ cylinder will kill fuel.

Think tho, knowbody who buyes cars with engines like that gives a crap about the mpg.