[minime s]

Does wheel size make a difference on the dyno ?

I know JLM,Andy and Cheese should have some good input.

I know my car is alot faster on the street and track with the lightweight wheels(15") vs. the 18" superheavyweight Kahn's

[RUKnight]

It's just like real life, the smaller and lighter the wheel the easier it is for the car to spin and therefore the more power you'll make on the dyno.
Mike

[helix13]

Yup. Or more specifically the power curves will be higher because of less drivetrain loss, but you must adjust your drivetrain loss coefficient when calculating crank horsepower. That's why it's better to compare wheel horsepower than crank horsepower when bench racing.

[minime s]

I have to bring my car back to you for a dyno pull with the smaller wheels and a nitous pull also.........

[TonyB]

A smaller diameter wheel/tire set-up (rolling diameter) will be more torquey. Since we lack in that area, that is a strong reason why I'm going the way I am...

To better visualize this, think extreme - like a monster truck MINI. Much slower off the line. However, once rolling, top end will be better, as well as improved fuel efficiency, in that range...

[jlm]

I find it hard to believe that the amount of whp is wheel size dependent.

[Andy@Ross-Tech.com]

It depends on how fast the dyno is spinning. It takes about 16 seconds for a MCS to go from 2k to 7k on Helix' Mustang dyno. That's quite a bit of time, as far as rotational inertia is concerned. Heavy wheels hurt your acceleration most in first gear, then a bit less in second, etc. In 6th gear, I doubt there is measurable difference in acceleration.

[Mugami]

Looks like Minime s read something i put in another post =)

[macncheese]

From a physics point of view, wheels only give about 1/3rd of the weight reduction benefits in second gear compared to first but i've seen plenty of articles talking about how a wheel/tire change can result in drastically different dyno results.

--
Cheese

[friedduck]

Inertia dynos will record a higher reading, as you can spin up the drum faster (less inertia as you've all pointed out.) I don't think brake dynos (Eddie Current) are affected.

I have no idea as to magnitude. (I've read about large differences, but it was bikes, not cars.)

Jeff

[minhi]

http://www.nissanperformancemag.com/may01/dyno.shtml

here's an old article, but they do back to back tests of light wheels vs heavy wheels.

unfortunately they don't show the graphs, i think that would show some advantages for the light wheels too.

[Andy@Ross-Tech.com]

That article shows graphs.

Less than 1 hp difference between the light and heavy wheels at most rpm: