Loss of power on track

Recently I took my 15 GTPP to Auto Club Speedway for a weekend of tracking with Speedventures. It was a blast! However, I did notice a sudden loss of power on the second day of racing, and I'm not sure why.

On my second day, I was in the middle of my second session and had just set my best lap time. All of a sudden, the next lap the car felt a little slower and I started getting passed. I wondered if I had suddenly done something with my line or had started driving worse, but once I looked at the data on my phone, I noticed that the car was running about 10 mph slower in most areas of the track. I was reaching 145 on the long straight on every session previously, but now could barely reach 130. My lap times went up by 2-4 seconds for the remainder of the day.

Temperature was similar to the day before, oil and coolant levels were within normal, tires were at the same PSI and wearing fine, and nothing was showing that it was overheated. I really could not figure this one out...

My lap times had improved every session up until the middle of the second session of the second day, getting drastically slower after I set my best lap time. Any theories to why this could have happened, other than I suddenly started sucking at driving?

No way to know without logs. Car could've been pulling timing if you had to refill your tank for the second day. Maybe lower quality fuel?

Barrel said:

No way to know without logs. Car could've been pulling timing if you had to refill your tank for the second day. Maybe lower quality fuel?

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This, sounds like octane adjustment. Shitty fuel, but the car can make up for it and still run okay, just less ignition timing and it might fuck with the cam timing too, need to datalog it.

aleccolin said:

This, sounds like octane adjustment. Shitty fuel, but the car can make up for it and still run okay, just less ignition timing and it might fuck with the cam timing too, need to datalog it.

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Thanks for the insight everyone. Fuel was from a Chevron both days, so that's pretty bad advertisement if their fuel was somehow bad quality.

How do I go about datalogging?

Since you (like most people) weren't logging data maybe some basic questions might turn up the cause if it is simple.

Did the car cool off between sessions and if so was it slow immediately in the next session vs needing to get "hot" first?

Are there any check engine codes or have you run something like Torque to check codes?

Any modifications to the car from stock?

Any chance the air intake ingested something that blocked the air filter?

How is the car running on the street compared to previously? Will it pull strongly to the rev limiter? How is the idle? Still smooth or is it "different" now.

Did you notice anything such as change in air fuel ratio either on track if you had a chance look or later on the street?

Any chance you somehow vibrated or otherwise disconnected a hose or wire because it was loose?

And really stretching for ideas does the throttle pedal still go all the way to the floor?

Note that if you use something like Torque OBD throttle position may not read 100% even at full throttle . . . I'll let somebody who knows more about this comment with actual facts since I'm going by memory of what I think I've seen in the past and its "been awhile".

Edit to update: I checked my Race Technology DL1 OBD data for throttle position. Direct OBD throttle position % was about 89% at full throttle in my 15 and and is about 89% in my 18. Idle (zero throttle) reads about 10 - 20% or so after warmup.