Larry, I believe that your comments are correct but for different reasons
then you mention. The higher apparent HP reading is an actual reading.
Everything in the car that rotates consumes power, engine internals,
transmission internals, belt driven accessories, driveshaft, axles, wheels
and tires. If any one component is lightened then the power required to
rotate that component is reduced, thus giving back horsepower that you were
already making but were using to turn car parts.
If you look at chassis dyno results, it is almost always measured in fourth
gear because it is 1:1 and is the most efficient gear in the transmission.
Occasionally you will see tests done in other gears and you will find large
losses in horsepower. Even though physical acceleration is fastest in first
gear, you are consuming much more power in the more rapid acceleration of
the reciprocating mass of the parts. Something on the order of a constant
times the square of acceleration. It is not unheard of to see 50% less
horsepower to the wheels between first and fourth gears.
This in one of the major differences between american and european
transmissions, americans use low gearing and a high overdrive to loaf at
speed, while european cars use tall gearing and a 1:1 final drive so that
acceleration at high speeds is improved via more available power.
There are two types of dynos one is an eddy current and I forget the name of
the other, one is designed to measure rate of change of rpm as you suggest,
because the dyno computer knows how much power it takes to accelerate the
rotating dyno parts. The other type varies it's resistance and will allow
you to hold unnatural throttle openings indefinitely. For instance, if you
wanted to tune at WOT, the dyno will allow you to vary the dyno resistance
so that you can hold full throttle without accelerating. They both have
their uses but the first one that I described is the more common type.
The behavior I described is the math behind why when someone changes to
larger rims and tires (with same overall diameter) acceleration will
typically get slower. The new wheel/tire combination is often heavier and
even though gearing is the same they will take more power to turn, thus
slowing acceleration, and showing lower numbers on the dyno. If you were to
put your engine on a dyno and then put the whole car on a chassis dyno, the
difference in power ratings is the amount of power being consumed by the
driveline in order to turn all of those parts. Lightening, and thereby
reducing polar moment, will not make power, but it will give back some of
what you already paid for when you built your engine.
James Nazarian
71 MGB Tourer
71 MGBGT V8
85 Dodge Ram
----- Original Message -----
From: "Larry Colen" <lrcar@red4est.com>
To: <mgs@autox.team.net>
Sent: Friday, October 17, 2003 9:49 PM
Subject: Chassis dyno theory question
> Since I've been asking about lightened flywheels, here's a related
> question:
>
> Somewhere, I picked up the information that chasis dynos measure power
> by measuring the rate of change of RPM. The faster that your car can
> accelerate the rollers, the more power it makes. If this is so, then I
> believe that reducing the rotating mass of the driveline will result
> in an apparant gain in horsepower on the dyno. Lighter flywheel ->
> more apparant power. Is this true?
>
> I suspect that this may only be true on some dynos as the Clayton smog
> dyno I've used, can tailor resistance to either a specific level, or
> to represent "real life at that speed".
>
> I do suspect that unless you are measuring RPM at a steady speed, that
> they would need to take the rotating mass into account, and that
> changing the polar moment of the flywheel would have a small but
> measurable effect on the readings.
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