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Marcel,<br>
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.<br>
- Larry<br>
<br>
<div class="moz-cite-prefix">On 9/5/2018 3:33 AM,
van.mulders.marcel--- via Fot wrote:<br>
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<div>It seems I've a chance someone here knows what the inertia
is of the rotating parts of a (standard) TR4 and TR6 engine?</div>
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<div>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.</div>
<div>Marcel</div>
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