By
ever_green, July 7, 2011
ever_green
well i coded the scanggauge 2 to display brake horsepower. i did this couple months ago when the weather was cool, below 15 degrees Celsius. during highway merging when i floored the pedal, it would go as high as 132-137 horsepower. however in light of recent warm weather and high humidity the highest number i have ever seen has been 109. the car feels sluggish as well and seems to have less mid-high end power. is this normal? fuel economy average has been good however.
Sounds right - car's tend to loose power when the ambient temps are high. The Scangauge is making it look a bit worse than it actually is, but you could see 3-5HP loss with a 15-20 degree Celsius temp swing. Probably seeing more of the timing being pulled at higher temps, to protect the engine. Rough order of magnitude - for every 10 degree temp change, you could gain/loose 1% of your HP. Most of the HP ratings posted for cars are at "standard" air temp of 77F (25C), 29.235 Inches Hg altitude-corrected barometric pressure to sealevel, and 0% relative humidity.
Dan_H
I consistently see decreased power and mileage (~2-3 mpg) due to the ECU backing off ignition timing quite a bit in the summer. I'm in Arizona, where official temps reach as high as 50C 
. Higher air temps (lower air density) contribute to the power loss, but I can tell that most of it is timing in my case. I'll hear slight knocking under acceleration, followed by a noticeable power loss once the pinging stops.
Mid grade or premium fuel helps if your car is backing off the timing due to pinging. I run higher octane fuel mid spring through early fall, and the power/mileage drop isn't as bad. For me, 91 octane is the minimum if I tow anything in the summer.
No doubt the heat is the culprit. I went ahead and switched brands of gasoline this morning, just to see if I could tell a difference in a day or two. The heat index is supposed to be 105° F today, so I won't blame my '09 Sport for being a little slower!!!
