[ggggbmw]

I recently did an HPDE at Road America, and I was a bit disappointed that my times were not matching what I did last fall there. Even though I felt I was driving better (had some great instruction, both in-car and in-class), my best times were off nearly 5 seconds.

Does anyone have a formula that can quantify the impact of IAT temperatures on performance?

I'm thinking that the warmer temps were having a major impact on my acceleration. Here are some speed differentials from turn 14 to the finish line.
Fall - ambient temp ~50F , IAT peaking at 99F
54 to 128
54 to 127
Summer - ambient temp ~80F , IAT peaking at 130+
57 to 119
61 to 121

Even though I'm taking the turn at 3-6 mph faster, and a better exit speed, my speed at the finish line is 8-9 mph slower.

Yes, I know it's not a race, and if I dig into my individual sector and corner times, I am smoother/faster this past event. But it's still a bit disappointing to have a lap time that much slower.

Another factor is that this time I had an instructor in the car. Small skinny guy, but still at least another 160 lbs in the car. Could that much weight have had this much impact? Hmm, he would add about 4.5% to the weight of the car, and my times were ~3% slower.

So, which is the bigger impact? The higher temps? Or the additional weight?

[aerobod]

A basic calculation would indicate your air density at 99F (310K) is 5.8% higher than at 130F (328K), all other things being equal, you power loss would be of the same order, but it may be even more if timing is being pulled due to too high combustion chamber temperatures causing pre-ignition. As you mentioned weight difference would be about 4.5% due to the passenger, effectively reducing the power to weight ratio by that amount.

Nominally your power to weight ratio is going to be about 10% different between the two scenarios.