"The Truth About Chassis Dynos"-- Hot Rod article by Marlan Davis

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.

Originally posted by OldSStroker
FWIW, I couldn't find any ads in Hot Rod for the dynos discussed.

This is a good thing.

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

Originally posted by OldSStroker
..... The cover has a white Chevy II on it, not Valarie Baber from the April issue.

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.

Originally posted by rskrause
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


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

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

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
============================================


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

Max Race- I can totall understand that story. I LIVE AND BREATHE street engines. The number of times I have found simple stuff holding an enigne back AS INSTALLED IN THE CAR is a lot. Fuel supply and voltage/ignition problems are almost always the culprit. At high speeds I can understand hood scoop/air inlet problems also causing hair-pulling situations, although my motors don't often get connected up with true forward-facing scoops.

The difference between and engine dyno and how it runs in the car under actual racing conditions can be HUGE depending on stuff just like that. That's absolutely the truth.

My 78 Malibu street cruiser with the blower motor puts out 390 to the wheels. Put on the air cleaner and I bet I whack 10-20 HP off that number. Why? Becuase of hood clearance I have to run a too-short air cleaner assembly that I just KNOW and can FEEL is restrictive. I have to do this just to fit it all under the hood. In my case it's a trade-off that I knew about going in and I accept it. In a race car stuff like that can drive you bonkers.

Anybody not running an EASILY VISIBLE fuel pressure gague and voltage gague on a race car is just ASKING for frustration. I do testing on my street cars with them hooked up temporarily (often duck taped to the windshield) to make sure all is well, then I take them off when I'm satisfied that everything is working correctly and running to the best of it's ability.

I help out at my local Dynojet dyno shop a couple of times a week.

It amazes me how people who swear up and down that the new part they just put on makes the car so much stronger...then you dyno it and see like 1 rwhp difference....

I enjoy seeing cars dyno because that is where the bs stops..just like at the track...

But from what I have found...some cars that are boosted are actually putting down lower #'s...like my Lightning, dynoed last night...put down 358/430 SAE..only 4 more rwhp than last time...but my truck is stronger on the h-way...under a load I make another lb. of boost and my modified airbox works when my truck is moving...something a dyno does not do.

And I have had lately some problems coming up with an equation with my truck. I G-teched it last night...0-60 in 5.12, 13.56@108.4...you plug my rwhp vs weight of 4900 lbs..and crap aerodynamics...I know G-techs are high...but let's say it runs 104 in the 1/4...the numbers are not working out.

But as a tuning tool...a dyno is hard to beat unless you have the ability to simulate the conditions every time. Same engine temp, air temp, start rpm, everything the same...try that at a track when there is a 20 deg drop between a few runs.

I can't remember how SAE hp is derrived; but It seems like it's lower. In general wasn't it an attempt by the big three to avoid insurance problems with customer's cars?

I assume your using the pro-stock et formula. Don't give up on them; they are based on a nearly perfectly tuned (all components match) naturally aspirated prostock car, not a supercharged truck.

Based on MPH if you can acheive the et they did, you're doing an excellent job with your combination (matching up the entire drivetrain). Another excellent class to compare to is nostalga super stock, those men know how to lay it down.

I just got the G-tech for christmas, finally convinced the wife street tuning is safe, so I haven't correlated the numbers to the dragstrip results. I will. Excellent tuning tool, just like a dyno. But the strip is still what seperates the men from the boys.

SAE on dyno's are to give a more fairer comparision against other dyno's numbers. STD is what the vehicle made in the current conditions...like my truck really made 365/440 STD...but I guess I am a guy that likes to know how it does against others...sorta like putting the weather, elevation, DA(if known), the day I race my car...keeps it fairer in my eyes...

I agree, without a baseline (track times or other racers) how does one know how he stacks up. Everything in life, as well as race cars, is all about coutinuous measureable improvement. Anything less is a waste of one's time.

SAE hp is an equalization factor to compare different dynos results (I looked it up). But that still says the dyno was accurate or calibrated to begin with. And, if you read between the lines, SAE hp includes: full exhaust, accessories, and etc (and the date 1972) it was an attempt to more accurately represent what the consumer could expect from the powertrain and an attempt to reduce high insurance preimums for consumers buying sports cars. One should also consider the ramifications of the actions of the CARB, they were begining to ramp up at this point in time. SAE hp numbers are serious business; cars are sold and the big three is sued over these numbers.

On your eariler point, you see people (probably) daily who spend big money on parts that don't add any power. I see it all the time at the track. Now, imagine you're spending $5-10k on a "dynoed" engine that doesn't produce. Wow! Buyer beware.

There still needs to be a better way.

What was the general outcome of the Hot Rod "Chassis Dyno" comparisons ???

Originally posted by CANTONRACER
SAE on dyno's are to give a more fairer comparision against other dyno's numbers. STD is what the vehicle made in the current conditions...like my truck really made 365/440 STD...but I guess I am a guy that likes to know how it does against others...sorta like putting the weather, elevation, DA(if known), the day I race my car...keeps it fairer in my eyes...

Actually both STD hp and SAE hp are correction factors established by SAE. Actual or measured HP uncorrected is usually listed as 'actual HP' such as seen here: http://silver2kws6.tripod.com/images/dynosheet.JPG (copy and paste). Here (http://www.personal.psu.edu/users/b/m/bmz107/dyno2/data351.JPG) is a sample of a STD hp data sheet and you can see it lists a correction factor of 1.04.

'SAE hp' refers to a modified version of the SAE J1349 standard of June 1990. Power is corrected to reference conditions of 29.23 InHg (99 kPa) of dry air and 77 F (25°C).

STP (also called STD) standard is another power correction standard determined by the SAE. This standard has been stable for a long time and is widely used in the performance industry. Power is corrected to reference conditions of 29.92 InHg (103.3 kPa) of dry air and 60 F (15.5°C). Because the reference conditions include higher pressure and cooler air than the SAE standard, these corrected power numbers will always be about 4 % higher than the SAE power numbers.

http://www.superflow.com/support/cycledyn-theory.html#CWP That page also shows you the actual formulas.

-brent

I've been saying it for a long time. The dyno was never meant to be the track, it's a tuning tool. You can't really compare numbers with other people unless you both use the same testing methods.

If you are one of the people who ice down everything, use nitrous to supercool parts, and change setups you normally wouldn't just to get nice peak numbers then you are wasting a very good tool. Things like this are the reason you should be careful when reading claims, as you don't know what the testing methods were.

With that said, each dyno session should be done exactly the same as your others with only the changes in question being different.

This is one of the main reasons many Supras have a "Dyno Queen" stigma, as the little dyno tricks have a greater effect on FI cars.

Not to beat a dead horse but I scanned and posted it here.

http://briefcase.yahoo.com/bc/robss96dcm/lst?.dir=/My+Photos

Link doesn't work.

Link doesn't work.

Link is good, its just been hammered. over 200 hits since yesterday so yahoo shut off the tap. Just ck back when you get a chance and it should be good.

I help out at my local Dynojet dyno shop a couple of times a week.

It amazes me how people who swear up and down that the new part they just put on makes the car so much stronger...then you dyno it and see like 1 rwhp difference....

I enjoy seeing cars dyno because that is where the bs stops..just like at the track...

But from what I have found...some cars that are boosted are actually putting down lower #'s...like my Lightning, dynoed last night...put down 358/430 SAE..only 4 more rwhp than last time...but my truck is stronger on the h-way...under a load I make another lb. of boost and my modified airbox works when my truck is moving...something a dyno does not do.


But as a tuning tool...a dyno is hard to beat unless you have the ability to simulate the conditions every time. Same engine temp, air temp, start rpm, everything the same...try that at a track when there is a 20 deg drop between a few runs.


Something very important that most people miss is putting too much emphasis on peak numbers. Peak numbers are a good indicator, but also is distribution of power. In other words…….. the curve of the powerband.

My last combination made 445rwhp and the current is 472. less than a 30 hp gain if you just look at peak numbers. However, at 7000 rpm I’m up 100 rwhp and the car is .35 seconds quick accelerating on the dyno from 5200 rpm to 7000 rpm.

A buddy of mine has a F-stock F body that runs very low 11’s. I’ll not tell his secrets but the peak power was way below what you would think a low 11 NA car would be. But the powerband went forever and was amazingly flat.

“Average hp within intended rpm range”………. My hero, John Lingenfelter

John beat that dead horse over and over. I remember when I read his SBC book he pretty much brings it up again in every chapter even though he covered it thoroughly in Chapter 1.