Does wheel size affect BHP on a rolling road?

[BILLY-R5GTT]

After my car is built back up i'll be taking it over to Scoff for him to fit my ECU, wiring and set it up to run in and then I'll be going back for a power run at a later date to get the car set up properly and rolling roaded.

I am currently running 18's with 215/35/18 tyres will this give a lower bhp and torque result on the rolling road compared to a set of 15's? And if so what sort of difference will I be expecting?

Just curious...

Cheers

[SCHWARTZ]

I have all ways wondered this would have thought it would affect the torque but surely there must be something that calculates it all.

[Tony Walker]

i think the gear ratios are calculated by the speed of the rollers and the engine rpm at different speeds?

[BILLY-R5GTT]

I have no idea how it woks... I assume some of it is done by how fast the rollers turn?

If an engine was at 200bhp on 15's and you put a set of tractor wheels on the front surely you would see a big difference on the graph

I know when my f7p was standard on 15's I could get my 19 16v to 140mph on the speedo quite often but when I put my 18's on it struggled at 125mph on the speedo.

[Tony Walker]

i think thats why reference points are taken at set rpm to measure the speed of the rollers and calculate the ratio from those measurements.

[Scoff]

It doesn't really effect it billy. I can run the same car in 4th or 5th gear and get the same result. The dyno is calibrated to engine RPM before we do any tests which removes wheel size as a factor. It's like saying different gear ratio's affect dyno results, which they don't.

On some level you could argue the bigger, heavier wheels have more inertia than smaller ones and that it takes longer to accelerate the heavier wheels but since the dyno is a braked type any small changes in inertia have almost no effect on the result. Certainly above 200hp its neither here nor there. The brake is absorbing most of the load and so inertia plays a smaller part in the calculation.

[BILLY-R5GTT]

Excellent news I don't have to drag a pair of 15's over with me then

Cheers