1/4 MPH vs. Air Temp

GT_Rich · Jul 19, 2008

First off, I don't make it to the track very often. It's been once a year for the past three years. Last year, after my engine was built and dynoed (see sig), I was trapping 106.5 MPH in the spring (65F outside). My 60' times were horrid b/c I had a stock suspension. I was also breaking loose on every shift (even 3-4). The best time I made was 13.46 @ 106.5 w/ a 2.3 60'.

Last night I went again (90F outside, probably dropped to 80F during the night, high humidity, low atmos. press). 60' times were better (not great) and it was really hooked up going down the track (no spinning other than just out of the box). Best run was 13.41 @ 101.4 w/ a 1.89 60'.

So, my question is, can the hot weather really take 5 MPH out of the car? Seemed like a lot to me. I had hopes of high 12's last night, not 13's

Bullitt347 · Jul 19, 2008

The hot temp will take some of the mph out of the car, sometimes it is significant. The faster 60 ft will also take some mph out of the car as the car from a time standpoint, does not have as much time to accelerate. An example is my friends car on a cool evening went 10.85 @135mph but only had a 1.68 60 ft. A week later on a hotter day (by 30 deg) the car went 10.62 but only 129mph but had a 1.46 60 foot. The car was quicker (et) but slower (mph) and the 60 ft was a lot better. So some of the mph loss was because the car had .26 sec. less time to accelerate and the air temp was 30 deg hotter. .26 sec. does not sound like much time, but it is all built on momentum, and it does add up. Also if your car is a stick shift, time loss from shifts can play a big roll. Maybe you nailed the shifts on the faster (mph) run and thus lost less momentum, compared to your slower (mph) run? Just a thought.

Vipersix · Jul 19, 2008

25 degrees is significant and 15 degrees only slightly less so. However, I would bet most of your variances can be attributed to inconsistency rather than atmospheric conditions. If you ran identical runs on a 65 degree day versus a 90 degree day you will see a larger gap than the 0.05 second improvement you experienced. Regardless, your times are right on par for a 2900 lb car at your hp level.

thumper460 · Jul 19, 2008

you bet the temp is important!! the cooler the air the denser the fuel... ever notice the car seems to have more pep at night, or after a rain?? thus the "cool cans" and icing the intakes..

Just me.........................

Thumper

Vipersix · Jul 19, 2008

Fuel experiences negligible density changes in average ambient temperatures. The key component is air density, or more accurately in this case, oxygen content. As the temperature rises the density of air is reduced. An increase in 2 degrees of temperature is equivalent to 1,000 feet of altitude. So in the example above, driving at a track at sea level in 85 degree heat is equivalent to driving at a track at 10,000 feet in 65 degree weather. Pretty significant.

So the cold air intakes and icing and all of the other tricks of the trade are intended to cool the air coming into the intake. The air coming in will be more dense and therefore will have a higher oxygen content. Oxygen + Fuel = Power!

Mustang5L5 · Jul 19, 2008

Humidity kills you too.

All those water droplets in the air do not burn and take the place of combustable air. So you are actually getting less air into the engine on a humid night.

There is a reason why cars run like raped apes on cool dry fall evenings. More air + denser air = more pwoer

57fairlane · Jul 19, 2008

last night sucked . . . I went with 5 people to the track and everyone was down about 3-4mph.
in my neck of the woods it was 85-88*F till 10pm and ~70% humidity

GT_Rich · Jul 19, 2008

Thanks for the replies everyone. I feel better about last night now.

25thmustang · Jul 20, 2008

Vipersix said:
25 degrees is significant and 15 degrees only slightly less so. However, I would bet most of your variances can be attributed to inconsistency rather than atmospheric conditions. If you ran identical runs on a 65 degree day versus a 90 degree day you will see a larger gap than the 0.05 second improvement you experienced. Regardless, your times are right on par for a 2900 lb car at your hp level.

A 2900 lb car making his power should NOT be a 101 mph even in hot air (the 106 is much closer to what it should trap). Our 3000 lb 290 rwhp (okay 288) traps 108-110 in 90 degree humid weather.

You will see some mph loss, maybe a few, but that seems to be quite a bit. Have you done new plugs, wires etc since? Did the tune on the car change? Does your track show you what the DA is on the timeslip?

Not something to worry about really, as the only way to know is go out in cool weather. But with that large a drop on the 60' you should have been touching the 12s I think.

Vipersix · Jul 20, 2008

A 3000 lb car driven by, say, Mike Murillo will turn a much quicker trap time than our friend GT_Rich. Of course, this is what makes racing so much fun!

GT_Rich · Jul 20, 2008

The ONLY thing on the car that changed from last year is rear suspension and some drag radials. The tune, plugs, wires, timing, fuel pressure are all the same. I should note this car is only driven 3-4k miles a year, so I see no need for a tune-up at this point. I could go back and check fuel pressure and timing (I periodically check them, but they haven't moved since the car was put together). My tune was done w/ the timing at 10 deg, spout out, so that's where I leave it.

I agree that driving ability is very important. However, my WORST run last year trapped 104 MPH. My BEST pass from Friday night felt 100x better than any pass I made last year, as far as speed of shifts and traction, but only trapped 101.4.

Then again, if 57 fairlane was seeing -4 MPH Friday night, that could be a good indication I should stop worrying.

25thmustang · Jul 20, 2008

The only way to know is when the good weather hits. We just rarely see a huge difference in mph even on hot days. My car only loses a couple mph even in nasty conditions.

Also to note, Mike Murillo doesn't drive our cars, all of my family members do, and each one of us has been 108-110 in the car. My 258 rwhp, 3200lb car has been 107, and even on 100 degree days was running 105 mph?

Vipersix · Jul 21, 2008

I really can't figure out why you guys are getting wrapped around the axle on the trap speed. Your opponent could have a figure 20mph faster than you yet you can still beat him to the finish line. Drag racing is not based off your speed, it is based off your time. I know you realize that because the times in your signature prove this, but this conversation is straying into weird arenas.

There are basically three factors which directly determine how well a car will run: weight, power, and traction. All else remaining the same, environmental conditions WILL affect power output and you will see variations in the outcome.

25thmustang · Jul 21, 2008

Noone seems to be doubting that, I just am saying there could be more to it than environmental issues. The car lost a solid 5-6 mph since his last outing. Now weather does affect a car, mph, times, etc... but how do we know the car isn't having other issues? I'm merely going off my own personal experience, but we have a great track (Englishtown) and a not so great track (Lebanon Valley) that I frequent a lot. I have been 107 mph at Eown in cool air, yet even on the hottest days at LVD (Im talking 100 degrees at a track thats well over 1000' DA even on a cool afternoon) the car still traps 105 mph. This is the only reason I question things like plugs, wires and tune. Again thats just a personal experience of mine, with each and every one of the cars we race.

Not arguing, just bringing up things that might also account for lost mph. After all it doesn't hurt to check simple things and make sure the car truly is running in top shape. Low mph just might be the indicator that things are changing on it.

Vipersix · Jul 21, 2008

Ooohh.... Yeah I see what you're saying now. Good point and even more noticeable when you run the car consistently throughout the year.

SVT32VDOHC · Jul 21, 2008

Let the car sit a long time on hot days at the track. My best times all day were 14.50's and 14.40's at 97-98MPH. I let the car cool, with the hood up, and we had a hell of a wind that day too, so I pointed the nose in the wind. Next two runs were 14.25's at 99.something....JUST under 100MPH. At least this helped me a little.

25thmustang · Jul 21, 2008

Hot lapping will hurt times and mph too. Heat never helps a car.

57fairlane · Jul 21, 2008

I have seen huge changes with temperature . . .

PV/T people . . . every single one of our motors is based on laws.

We had a car that ran extremely consistent 6.10s-6.12s @ 111-112 in february/march and last month it ran mid 6.20s @ 108-109 and that is dropping a consistent 2mph in the 1/8 . . .

We also had a car on the dyno (502bbf, ported super cobra jets, solid roller) that picked up 30rwhp (with a glide!) by building a air plate that made the carb only draw air from out under the cowl area over letting it draw air from anywhere.

Ambient temp was in the 88-90*F range . . .

1/4 MPH vs. Air Temp

GT_Rich

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