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Discussion Starter · #1 ·
Why is there up to 20 rwhp differences in stock dyno results? One dyno result for a 2002 Z is 337 rwhp and mine ran 357.1 before any mods. Shouldn't two stock Z's run about the same?

I'm new to dynos and how they really work.

Thanks!
 

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Two things going on here. 1) Production tolerances - some cars are just stronger than others. 2) Dyno variance. There are all sorts of things that can effect dyno results.

Filling your tires up to 40psi should give you even more horsepower readings on the chassis dyno, lower pressures less power. Also, was your reading SAE corrected? This is a formula that takes into account ambiant air pressure, temperature, humidity, and probably some other things I forgot, and gives you a number that is "corrected", meaning it should be repeatable.
Kind of like drag racing where you'll see "results corrected for altitude."

Also, some dynos (cough mustang) seem to give much more umm "optimistic" results. We're talking 10-20hp here, that could account for things. Its pretty much accepted that you want to use a Dynojet 248 dyno.

Somewhere (not sure of the thread, maybe someone can help) Jim Hall wrote a bunch of stuff on the dyno, and how results can be skewed, and the processes they use over at Halltech to make sure the results are repeatable.
 

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Discussion Starter · #3 ·
Yes my numbers were corrected--the tech that did the dyno for me gave me both sets of results on a 3.5 floppy but warned me--"Dude, if you post these uncorrected results on Covette Forum or somewhere, you are going to hear about it!.....don't do it."

The uncorrected reads 361.8 rwhp and 353.1 torque.

Thanks a lot for your info---
 

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Don't forget about gas octane/quality, intake air temps, heat soak and knock reduction.

A lot of the dyno runs with lower numbers seem to show Air-fuel ratio problems.

I don't see O2 content of the ambient air mentioned much. This is a bigger problem in small dyno cells. You will lose power if the ventilation system isn't bringing fresh air to the engine. Hopefully this isn't generally a problem.

Some folks insist that SAE correction isn't 'correct' in all cases. Trying to compare a 50 degree run to a 95 degree run, even with correction, is probably more likely to introduce variability.
 

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Another thing is the smoothing used on a dynojet run.

If you play with the smoothing (goes from 1 to 5) you can get 4-6 hp and ft-lbs of torque difference because of what value is used.

Smoothing helps to get rid of the choppiness in the graph. Which is actually real, thats a hard question to answer depending on what is causing the "unevenness" of the graph to begin with.

Alex
 

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BrianK said:

I don't see O2 content of the ambient air mentioned much. This is a bigger problem in small dyno cells. You will lose power if the ventilation system isn't bringing fresh air to the engine. Hopefully this isn't generally a problem.

Some folks insist that SAE correction isn't 'correct' in all cases. Trying to compare a 50 degree run to a 95 degree run, even with correction, is probably more likely to introduce variability.
Very true, and even moreso when comparing a forced-induction motor to a N/A one. For instance, you can really toss-out any correction factor when using nitrous oxide. You want to run at an effective altitude below sea level...? Use nitrous. Or even an efficient supercharger. It packs in a lot more O2 than any N/A engine could do given the same conditions...which is why turbos/blowers work so well at high altitudes.

I will _assume_ that jnation got his car dyno'd at SpeedWorks which is a shop close to him (and myself) and can verify the accuracy of this Dynojet. I've done a LOT of testing there myself and have also tested my vehicles at other local dynos as well.
 
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