May 2004 Hot Rod magazine has an excellent 6- page article by Marlan Davis about all types of chassis dynos, how they work, how they compare, how they measure and calculate, as well as how to "cheat" or fudge the results.

Covered types are inertia only (Dynojet), hydraulic/water brake(Dynapack axle mounted), electric or eddy-current (Mustang and Superflow).

They also took a new 4.6L SOHC manual trans Mustang to each type of dyno and compared the results.

IMO, it's the best summary I've ever seen. Marlan Davis has consistently written factual articles about technical automotive subjects. I think a lot of his knowledge, and his No-BS presentation. FWIW, I couldn't find any ads in Hot Rod for the dynos discussed.

I'm not sure of the legality of scanning the article onto this website, so for now you'll have to read it at the supermarket. The cover has a white Chevy II on it, not Valarie Baber from the April issue.

 

Very good and thought provoking read, makes you wonder.

 

This is a good thing.

 

whats on the cover of this issue? i dunno if i got it yet

 

 

Could you summarize? I don't buy/read car magazines from one month to the next these days.

My very limited expereince with dynos tells me that I'm better off just running my car at the track and comparing ET, MPH and vehicle weight (as I have done for years). The number of times I've gone qucker/faster than some car that allegedly dyno'ed a mind-bending HP number is a lot.

Most Dynojets seem to give reasonably sane numbers, near as I can tell. Mustang dynos can be totally "out to lunch." My brother's 87 GTA laid down only 240 HP on a Mustang dyno but regularly ran 110-112+ MPH through the traps at the strip. At a race weight of 3900 lbs. A little more than 240HP in my book.

 

got the mag today in the mail good article ill post info about it once i get home for ya.
also the 77 street drivin 11 sec cars were pretty cool IMO..

but that one tubbed camaro was WAY over kill cool none the less

 

Yea, of those cars there were a few that impressed me , they seemed well thought out and overacheiving, but there were a few that were plum overkill.

 

I agree with you damon, between engine dynos and chasis dynos the only true measure of real hp is weight and mph in the quarter mile. Sounds like I'm not the only one using the pro stock et formulas.

Now you pepper the weight and speed with estimated et and you really get to see who the real men are at the dragstrip. After all these formula are empirical, based on very tough pro stock competition.

I always shoot for the highest mph I can get knowing full well that I can "tune" in the et later with the appropriate changes (gear, converter, tire, trans, suspension, etc).

By the way have you done any evaluations with with supercharged or nitrous cars. They violate the pro stock et formulas for efficiency. The formulas were based on NA motors and their particular power curve. I regularly exceed the et formula by 5%-10% with nitrous. Just wondering what your experience shows. I'm afraid I'm on the low side. Anyone?

 

I haven't read the article yet. But FWIW, I don't care if the chassis dyno gives the "correct" hp. For me, it's a tuning tool. I see a very close correlation between improved numbers from the Dynojet and improvements at the track, for a given car. Can't use to closely compare different combos though. That's why we race at the track and don't race dynos!

Rich Krause

 

Andy- I haven't done anything that in-depth. I regularly run nitrous, limited experience with blowers. Main thing I see that could throw it off is the massively disproprtionate low-mid RPM torque you get with a power-adder. MPH through the traps is really dependent on AVERAGE HP in the RPM range the motor sees through the run, not so much the absolute peak HP. Since a blower or nitrous setup often raises the average HP so considerably, and the torque curve is often less "peaky" than a N/A motor, I imagine it could easily blow the lid off the usual N/A calculations.

 

I think you hit the nail on the head, massive low end torque. Our nitrous cars regularly run 105%-110% faster than the pro stock formulas allow. I actually turn the et's back into hp then base an effiiciency off of that. So I sorta misled you by saying 105-110% of the et. It is hp % change based on et (sorry, I checked my calculations just now). Anyway, the cars violate the formula. And, as you point out the formula should represent average hp which is hard to influence.

It seems the more an engine is taylored to low end torque the greater this phenomena. For instance my friends 85 Vette ran 11.91 @ 110 mph. No matter what we did, it would never run over 110, yet the ets would vary directly with 60' times. With my calculations it should be a 12.30 car at best. It's a TPI car with a 3.08 rear end.

My LT1 went 11.52 @ 115.98 with a 1.55 60'.

Of course we spray off the line.

I have actually made a spreadsheet that one can punch in the numbers on motor and on spray to see how well the car responds and how much power is put down. It's pretty cool and it regularly has been able to predict ets to within a tenth once a baseline run has been established. I also use desktop dyno and quarter jr to predict ets when engine or chasis changes are made.

 

I'll go even further...
I don't care if the dyno is wildly inaccurate, provided it's consistantly inaccurate.

 

Thanks for saying what I was thinking better than I said it

Rich

 

Originally posted by LameRandomName
I'll go even further...
I don't care if the dyno is wildly inaccurate, provided it's consistantly inaccurate.
--------------------------------------------------------------------------------

Thanks for saying what I was thinking better than I said it

Rich
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i also agree, but most times its very helpful if Engine Dyno is
calibrated "Honestly and Accurately"

from Dyno data and car data, you can predict how fast RaceCar can run

Example=> i had a Racer bring his new Reher-Morrison 850 HP BBC engine to Dyno test on my dyno.
He was unsatisfied with the RM engine,
after trying all sorts of changes to different parts on car,
the car was about a second slower than he thought it should run

1st Dyno test on my Dyno was 832 HP at 600 Rpm/Sec accel rate
very close to Reher-Morrison's numbers

from dyno information and ET/MPH progressive incremental times
i plugged in info in my computer program and saw he was putting
830+ HP to 60 FT point, but after that ET times were progressively off .

i told him theres nothing wrong with your engine ..the problem is in your RaceCar ...and most likely fuel system problem , maybe also hoodscoop problem

The next day after dyno test, he calls me up saying he found the problem => when he put in new RM BBC engine , he also put in new Harwood fuel cell, and forgot to unplug the vent

he was leaving starting line with 830+ hp, but as RaceCar went down DragStrip, the unvented fuel cell restricted fuel flow to engine