Yes it's the heat.
It increases your intake temps, and the intercooler can't work as efficiently because it can only reach ambient temperatures at best. Hotter intake temps are more likely to cause detonation, so your car may be trying to fight it off by pulling a bit of power.

If you're running any kind of intake it will make it worse, because the intake filter is stuffed inside the hot engine bay. The factory intake is still the best because it only pulles ram air from outside which makes it even MORE effective on hot days.

 

ya i drove it tonight cause it wasnt muggy or anything and its a beast again....

 

actually it's the opposite..
Colder air is more dense because the molecules aren't moving as much so they can squeeze together tighter Hotter air is less dense because the molecules are moving around too much. Air expands when it's heated.

 

You probably have Al's famous 8* of timing and may be knocking.

 

I just got dyno'd in VA with 90+ degrees ambient temp and high humidity. When I was first tuned I put down 340hp (75 degrees w/ very little humidity) with the heat I made a whopping 294hp. Heat kills power, plain and simple.

All the appropriate controls were doing there job so I am not upset but just giving you a reference point. The stock ECU pulled tons of timing and even though it was crazy hot and i had a couple of lean spots because of the heat there were zero (0) knock counts.

cliff's notes: You will loose power but the ECU is saving your engine (and about 8 grand)

 

I <3 stock ecu saving me money.