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Loss of 10% whp between dyno runs ?

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Hey guys.
I dyno tested my car back in April, and it was cool, and dry. My car (02) put down 258hp and 202 torque.

Fast forward to today, it was hot, and humid. After the first dyno run the car put down 224hp and 181torque. Nearly 35hp drop, and 21 lbs of torque at the wheels difference. A second run showed the exact same power and torque numbers. We did notice a big drop in air fuel ratio. Ie, when the car was running at 258hp, the a/f averaged almost perfectly 13:1. When the car had 224 hp, it averaged between 12:1 to 11:1.

Now, I know that running the car on a hot day like today (95degrees, 50% humidity) would affect the cars power output, but figured it to only be 5-10 hp at most at the wheels. Also, temperature changes like between the two dynos shouldnt cause a difference in almost two full points for air fuel should it ?

Has anyone else run into situations like this ? Ive changed oil recently with same type as last time, and theres no check engine light whatsoever. Everything seems fairly fine about car except for dyno numbers. (The car does feel a little bit slower lately though, backing up the dyno numbers we are seeing)
 
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Ive refilled my gas tank tonight, and the car does seem a little bit more peppy, but I cant imagine there being a difference of 35+ hp from just adding gas. Would a batch of low grade octane gas from a gas station not correctly labeling their gas grades make this much of a difference ?

Any other ideas guys ?
 
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My car is runnign a LOT slower with 90+ heat and 80+ humidity. You are not alone.
 
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MCM said:
No on the gas.
Was the car run on the same exact dyno on both pulls?

Yes it was.
Only difference was dyno was run nearly 3 months apart, different batch of gas of course, fresh oil change. Nothing else changed.
 
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NetViper said:
My car is runnign a LOT slower with 90+ heat and 80+ humidity. You are not alone.

Ok, well you make me feel a lot better telling me that.
IIts still hard to believe the difference would be 35hp at the wheels due to temperature and humidity though.
 
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Modernceo said:
Ok, well you make me feel a lot better telling me that.
IIts still hard to believe the difference would be 35hp at the wheels due to temperature and humidity though.

An engine should definately be less efficient when it's hot and humid (and at high altitude). Does their dyno software have adjustments for heat and humidity? I thought they usually did, but have never looked close at the software. It would be interesting to change the adjustments and see how much the adjusted numbers change.
 
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Modernceo said:
Yes it was.
Only difference was dyno was run nearly 3 months apart, different batch of gas of course, fresh oil change. Nothing else changed.

Same oil weight and quantity? 10-30 vs 20-50 maybe? 5 qts vs 6 qts?
A pony or two for each delta, plus the delta from run to run, plus the delta for engine (and intake manifold) temps, plus the HUGE delta for ambient temp, you are probabally within a reasonable power delta.

JMHO,
MB
 

MJK

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Did you leave the A/C on?
 
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