chassis dyno

[Jeff Stout]

UTI had a invite and I was able to have 2 free pulls. Like stated you need to be in high and close to stall. First pull n/a with 406sbc was 525 hp. Car was pretty steady tied down. Next pull with N2O car made 770 hp and with loose front being tied by operator you would have thought the car would not move. The higher the rpm's went the higher the front end would rise. It was getting pretty scary and the kids started moving away.Anyway with those numbers it showed around 20 percent loss using et of car and BG calculator to calculate flywheel hp. 10.2 et n/a and 9.1 et on n2o.

[Chris "drooze" Wertman]

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'....

Uh, my understanding and Ive never run a car on an inertia dyno.

But Gear ratios are taken into account on a Dynapac.....

I wondered the same thing until we had to plug that data in.

[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.

[Chris "drooze" Wertman]

Originally Posted by Ed Wright

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..

Very good point, I guess you just take certain things as a "given" but others may not know this.

Years ago on small engines my brother and law, we used a well pump with a captive air tank, hook the engine up and see how long it could it took to make X pressure, if the next run was better we knew we had an improvment, worked good for baseline from a given. No way to rate horsepower but it gave a refrence point, until the tank went farther south that it already was and we scrapped the whole contraption, I have always wanted to rebuild the design with computer refrence to somewhat accuratley measure HP.

It was actually a wonderfull tool, it really helped up peak out the engine we did it for, better than "seat of the pants" which at 75 across a plowed field was a bit dicey on the cart (an Oddesy like buggy)

A "hub" setup for a car , a "home" dyno as it were would be slick.

We had lots of problems getting an accutate measurment with our converter as wel.