14 replies to this topic

[racingbob]

my mini seems to be losing about 19% 5.5 wheels 12". what would my mates mpi be losing with the 7x13

[cal844]

More due to drag, I'd guess 25/30% total possibly more

Edited by cal844, 21 May 2018 - 01:23 PM.

[Dusky]

An 11% increase by just fitting wider wheels? A mini on 10x3.5 would gain power then lmao.

[cal844]

An 11% increase by just fitting wider wheels? A mini on 10x3.5 would gain power then lmao.

Quite possibly due to rolling resistance and inertia

[Steve220]

Be careful on using nova maths for transmission losses. Remember it's AT the wheels, how it puts the power down is a different kettle of fish.

[Northernpower]

At the wheels is very subjective. It depends on the grip of the rollers and the grip of the tyres. It also depends on the climatic condition at the time of the run. Rolling roads are great for setting up the engine across the entire rev range in a controlled environment but you can't compare like for like unless it's on the same rollers with the same amount of downforce on the tyres on the same day with the same atmospheric conditions. I know what mine gives at the wheels but I have no idea what it gives at the flywheel. You can only guess because new CV joints, new pot joints, new wheel bearing and a rebuilt gearbox all have an impact on the wheel horsepower.

[Cooperman]

It would be unlikely that the 'at the wheels bhp' would be much diffferent with 10" or 13" tyres, although as stated above it will be slightly reduced.

 

What is true though, is that the maximum speed and the acceleration in the higher speed range will be reduced on a car with 13" wheels/tyres due to the much increased drag.bThe fuel consumption will also be higher.

 

If the frontal area of the huge wing mirrors, the very wide wheel arch extensions and the tyre width is calculated it is clear that the additional drag will make quite a difference when compared to a car with no arch extensions, 3.5" or 4.5" wheels with 145 tyres and no wing mirrors (legal on  Mk.1 & Mk.2 cars and on some Mk.3's). One can only wonder how many bhp is used up by these parts on a late car at, say, 75 mph.

[Dusky]

 

An 11% increase by just fitting wider wheels? A mini on 10x3.5 would gain power then lmao.

Quite possibly due to rolling resistance and inertia

 

You have a very special dyno, for it to measure inertia.

13" wheels, stock engine I had 22% loss
On the 1330 clubman ( 13" avon slicks, sccr box) we had 15% loss.

[racingbob]

has anyone with 13 x 7" got figures on fly and wheels

[Dusky]

has anyone with 13 x 7" got figures on fly and wheels

140 @ the fly
118 @ the wheels

[Steve220]

has anyone with 13 x 7" got figures on fly and wheels

140 @ the fly
118 @ the wheels

16% loss, that's not bad!

[racingbob]

worked it out seems like 19% as mine

[ings]

Dont waste your time compairing some numbers.

maybe you loose some acceleration but you gain more grip in corners or better braking performance.

[Retroman]

or less in the wet or snow

[MIGLIACARS]

On the dyno i use its measured at the wheels. Make no difference as to what tyre you run as its dyno packs so no tyre is in contact with the ground it bolts to the front hubs.   True power figures at the wheels.