I had my 2002 C5 at the Magnaflow dyno a couple of weeks ago. We did two pulls. The horsepower on the 2nd pull was greater than on the first pull. Why?

If I did a 3rd pull would the horsepower have been even higher?

 

How much greater? +- 1-2HP is normal between runs. Had you reset the PCM lately and not gone WOT yet? The PCM simply could have learned a little better...

 

First pull was 284 hp.

Second pull was 292 hp.

 

I'd hazard the opinion that you seldom do wide open throttle runs with your car.

I suspect that if you were to do ten 20-120 mph runs at WOT and then re-dynoed shortly thereafter, your car would make around 300 rwhp.

Saw this jump in two Z06s that were initially driven very conservatively and then run fairly hard subsequently.

Ranger

 

Ranger

I don't do a lot of wot runs.....too much traffic in Orange County, Ca. Also, car has around 3000 miles on it.

Do you know what causes this change? I have heard that the computer adjusts based on your driving style. Is it changing things like advance and fuel mixture?

 

In addition to the Engine controller learing that you driver harder--your engine oil could be thinner due to the heat load of the first pull and thin oil is easier to pump than thick oil. BUT also

Consider that the very best dynos are only accurate to 1% (~= 3HP)
and are repeatable only to 1% (another 3 HP), so anything within a 5 HP window accurately reflects the ability of you engine to perform work. Anyone who thinks they know the HP of their engine to more than 2 significant digits is fooling themselves.

Also note that the same engine, under the exact same conditions will produce less HP in the lower gears than in the higher gears because the mass of the rotating and recipricating components is a higher percentage of the drive
train inertia. What does this mean. In short it means that a the equation relating TQ to HP (HP = TQ*RPM/5252) is only accurate when the engine is pulling a steady load, NOT accelerating. It also means that an engine built with ligher internal components (like titanium/aluminum rods, aluminum flywheels) will accelerate a car faster than an engine built with heavier stuff, even if they dyno exactly the same on a steady-state brake dyno.

Finally consider when comparing HP numbers from around the world that different regions use different standards in converting TQ to HP {SAE, DIN, PS,...}. DIN tends to be about 3% higher than SAE. This means that all dynos only measure TQ (and rolling dynos measure rear wheel thrust). TQ is then used to compute HP. TQ is actually doing the work. HP is getting the credit.

 

I think mitch touched on the cause of your variation.

Oil temperature plans a big role. The LS1 will make the most power with the oil around 200 degrees F. On your first run the car had sat for a while and the oil cooled. The first run heated the oil so the second run produced more power. I saw this very thing on the three pulls I did at one of the R&D events. I saw a big jump in oil temp and hp from the first to second run. I had done several WOT runs prior to the dyno day. My third run was with only slightly higher oil temp and the hp was just slightly higher. I would have to pull the sheet out again to give you the exact numbers but hp was directly proportional to oil temp. This, of course, will only be true up to a point where the engine gets too hot and starts making less power due to other factors. As I said optimum is around 200 degrees.

 

SoCalC5/Mitch

Thanks for the input. The car sat for at least 2 hours before the dyno. So that makes sense.

Thanks guys....I love this forum!


[Modified by RWD, 3:52 PM 9/28/2001]