[Fot] 89MM piston question

[Larry Young]

Marcel,
I suspect if you brought this question up with the dyno operator he 
would look at you like you were from Mars. I think it is a safe 
assumption that the inertia of the engine/drivetrain is small relative 
to the inertia of those large drums. I've often wondered about the 
differences in rolling resistance, since it depends on road surface and 
weight (including strap down force). I suppose this can all be lumped 
into drive train losses.
  - Larry

On 9/5/2018 3:33 AM, van.mulders.marcel--- via Fot wrote:
> It seems I've a chance someone here knows what the inertia is of the 
> rotating parts of a (standard) TR4 and TR6 engine?
>
> PS : most chassis dyno's are measuring horsepower out of the 
> acceleration of the very heavy cylinders (great inertia). The  wheels 
> of the car are resting on these cylinders and accelerate them during 
> the runs. But the engine has also to accelerate the rotating parts of 
> the car(crank, transmission, wheels..). I suppose that the dyno 
> operator has to make a guess of the  inertia value of these parts and 
> add it to the inertia of the cylinders. Here is a comfortable 
> opportunity for the operator to make the customer happy : if he is 
> adding an exaggerated  number for the inertia of the rotating parts of 
> the car, then the horsepower is exaggerated. As far as I know , 
> chassis dyno operators never mention or ask about the car inertia. It 
> would be very interesting though to know the value he is using, 
> because then you can always use this same value for runs on different 
> occasions and compare the results a bit better. A chassis dyno is 
> really only usefull to compare the results of changing things on the 
> engine during one and the same session.Don't be fooled by the absolute 
> numbers : Once I went to a chassis dyno with my TR3 :  my car must 
> have looked pitiful against the modern cars the operator was normally 
> testing, because the result was 210 bhp at the wheels and I knew it 
> surely had no more than 180 bhp at the flywheel! I don't know if some 
> chassis dyno's have a brake to hold the engine on a constant rpm at 
> WOT : then the inertia of dyno and car doesn't matter and you know the 
> right horsepower at the wheels. About right at least : you can cheat 
> yourself a bit by overinflating the tyres and the dyno has still to be 
> calibrated rightly.
> Marcel
>
> ------------------------------------------------------------------------
>

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