chassis dyno

[Ed Wright]

Originally Posted by bill dedman

My car, running a normally-aspirated VERY MILD 360 Magnum Mopar engine made 260 RWHP on a Dyno Jet dyno, with a 2.76 rear axle ratio.

If I had swapped a 4.56 gear into my car on the spot and had immediately, re-run the test, I am SURE I could have accelerated that inertia roller a lot more quickly.

Would that have altered the "reported" HP output, since it did the same amount of work more quickly?

It should have, or maybe I don't understand physics.... a definite posibility.

If NOT, then how is the Dyno Jet type of computer able to deal with this variance, and give us a legitimate HP number with both rear axle ratios?


Just askin'....

A chassis dyno is only a tool for measuring changes on that car, on that day. You can move it off the rollers, then put it back on and tie it down again and often get a couple hp difference, due to tire's alignment with the rollers. Lower gears on a Dynojet (or any inertia dyno) will often show the added loss through the gear set. It calculates the actual gear ratio, so there is more to it than simply how fast the rollers spin up. Lighter tires, wheels, drive shafts, flywheels, etc, show more power because the rollers will spin up faster. Comparing rwhp between cars with different drive train components is a joke. They are not as accurate as an engine dyno, day to day, for many reasons. It is easier than pulling the engine to use an engine dyno. It is simply a tool, with it's limitations. All tools have their limitations. My car, with it's 6000 RPM converter, won't repeat within a couple hp because tranny fluid temps vary so much, effecting the converter's slippage. Two pulls within 5 rwhp is close. It's quicker and easier to check your air/fuel ratio and seat the rings. My car has not been on my own dyno for over a year now.