I just completed a track weekend at Carolina Motorsports Park. I've run there about 9 times prior to this. I ran there last February in cold but not frigid temps and set my best lap times ever. Stock except for Carbotech pads. Street tires, 265/40 square.

This weekend it was near 88 degrees and humid and I was a good 2-3 seconds slower per lap on my best laps and I noticed my top speed in the straights was anywhere from 5-10 mph slower than in February. Same exact tires, brakes, etc.

So, maybe I just wasn't driving as well. Maybe the tires were getting a little greasy. But it just didn't seem to have the same power in the straightaways as last time.

I'm not looking for excuses but does the ambient temp really make that much difference in horsepower?

 

Yup. Hotter air is less dense, less oxygen molecules to combine with gasoline equals less torque and power.

 

I had always heard that but just did not know if it would actually be noticeable to me. At NASA events in the winter I remember them warning in the driver's meeting that in the cold you'd have more HP and less traction. Had no idea I be able to feel it in a stock Z.

 

10% of loss or even more, depending of the ambient temperature should be noticeable on lap times.

 

Most likely you had less grip with the tires as they heated up, your driving was not as good, or the track surface had less grip. 88 degrees is not hot enough to cause a measurable power loss if you were running good fuel.

 

It is hard to figure. All of those are possible, of course. Not driving as well is probably the most likely answer. I was running in a higher class than I have run before (DE 3 level) and I was not getting passed much and was passing a lot of other cars. I know, weak data point. But, I felt like I was driving as well or better than February. And, a quick look at lap data compared to February shows that my corner speeds were similar and my speed in the straights was lower, which would seem like a HP issue.

Was running 93 pump gas, no ethanol.

The guy I went with was running his 67 Mustang (500+ HP, track tires) and his lap times were down too. No speedometer or data acquisition in his car to tell about top speed or average speed.

Anyway, I heard one instructor telling someone that all the rains we had this spring had wash the track and it was not as grippy as usual. Not sure if that was true or not.

It just has to be something other than bad driving. At least I was consistently bad................

 

Mostly true especially if your part of the country gets a lot of rain.

 

CMP is one of those places that really fluctuates through out the year. It's a rough track so it collects rubber well, but gets a lot of rain that resets it. Then you have all of that sand right off track. In truth all that plus the weather, and you all effect the lap times. Sometimes it's best to look at the relative lap time vs the raw data. Compare the times to others around you, especially those you run with regularly.

 

Good points. It was frustrating to me because over the last 2-3 events at CMP I was steadily improving in terms of lap times and then this time 2-3 seconds off my average times and 5 seconds off my best times. That seems like a big step backwards to me.

Looking at some video I was consistently turning in too early on some key turns so I'm sure I was not driving as well.

In this clip I turned in way too early at the kink carrying more speed than I usually do:

 

As you get faster you will find yourself turning in earlier. It's a natural instinct and one you have to fight, or learn the art of scrubbing off the speed with slip angle.