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Re: A couple mistakes (archive)

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Posted by JS on June 12, 1999 at 01:49:37:

In Reply to: More bad news for the Boxster? posted by Tim Milligan on June 11, 1999 at 11:12:31:

The author makes mistakes that indicate a real lack of understanding os how dynojets work.

However, it is extremely difficult to find a
reliable constant power source of say 100 horsepower as a reference for your
calibration.

That's one way to do the calibration, but certainly not the best way. To make a measurement with a dynojet you need to know the moment of inertia of the drums, the diameter of the drums and the speed of the drums as a function of time. The moment of inertia can be calculated from the dimensions (the blueprints), this would give much better than 5% accuracy. It can also be measured invarious ways that don't require a "100 horsepower reference". For instance, spin the drums up, then see how quickly they decelerate while driving a generator that you measure the output of. The measurement of the motion of the drums is done electronically and should be good to a small fraction of a percent error.

the
chassis dyno, which measures horsepower at the rear wheels (which is flywheel
hp minus transmission losses).

When you make a measurement on a dynojet, the measurement differs from flywheel horsepower due to frictional losses in the drivetrain (including tires, tranny and what not) and the work that must be done overcome the inertia of the engine and drivetrain - which is a big effect, and is why you will get different numbers from the dynojet if you make runs in different gears.

a
dyno is used to measure the torque output of a vehicle and not the horsepower >/i>

True for a brake dynometer, but not a dynojet. The measurement made on a dynojet (change in velocity of the drums (work done on them) over a given time) is much more closely related to horsepower than torque.


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