Ask them if they think what gear the car is in makes a difference on to the overal power results of a dyno.

Then ask if they think the diff ratio makes a difference to rwhp.

Then ask them to show you the mathematical formula that demonstrates these relationship

Power = torque * rpm is all there is to it.

Where is tyre size in this equation, where is gear ratio in this equation.

Get them to give you a different mathematical equation for power - and when they do, quickly take that equation down to the patent office so that later you can collect your nobel prize.

It's as simple as the mathematics - if they can refute your formula P=T*rpm/5252 then they have an argument - otherwise direct them to their arsehole and their elbow and see if they can tell the difference!

I told them not to bring it up aging unless the can find a form of proof,

and now they are trying to change the argement to save face.

they also also as a side not believe dyno and real road rear wheel horspower differs.

Sorry to come in a little late. But I am interested to know if one was to increase the size of the rim/tyre this may increase the weight of the rim/tyre. In doing this would it not take a little more HP to turn the increased mass of the rim/tyre combination. You may loose a little HP initially but it should not matter up in the RPM. Personally I reccon leave the DYNO for the number chasers and and let your quarter mile times do the talking.

Sorry to end the argument but no difference at all - been there, done that with the A14. Used 13" wheels against 15" wheels and the diff was 0.2 rwhp on 2 different occassions. As previous replies have stated, the curve just moves in relation to rpm. Same curve, same output, just located at higher/lower rpm. Yes tractive effort changes slightly due to mechanical advantage but HP is "you got what you got"
Cheers

it's a gearing change so to speak.

A dyno usually doesn't show any power change, if you take into account the error margin on the readings.

Dyno's are a good general tuning aid. The 'look at my power graph' manner that most people go and get the tuned based on, overlooks the best a dyno really has to offer.

For instance you can 'time' the accelleration across the graph to determine intertia or gearing benifits in comparison. It's a very handy thing to do, it tells you more about how fast your car is than the power curve.Although it still doesn't account for everything (like aerodynamics /weight etc.) Most people have no clue about this fact including 99% of workshops who own a dyno (perhaps I'm being harsh). Somewhere in the whole dyno fad people forgot what they wanted more power 'for'.

Next time you go to tune your car on a dyno get a time axis on some of those graphs. You can see what all that lovely extra power is really doing or check out the gains from the new light weight flywheel or carbon fibre tail shaft. You can even see the reason why fattening up the power curve at the expense of a little top end is a good move, especially when you have poor ratio spacing in your gear box.