Well I went to the track this saturday and was dissappointed. Got a best of 13.20 @ 105.6. My previous best was a 12.98 @ 108.4 mph. I have since added the predator which added about 7 rwhp so I was hoping for at least a 12.8 When I ran the 12.98 it was 50s, dry and no wind. On Saturday it was 70s, 80 percent humidity and a 15 mph head wind. Could this make up for the 3 mph difference and 2 tenth et difference?

 

I was kinda wondering the same thing. A few of the people at the track were saying they lost a few tenths and few mph because of the head wind we had. One guy said he lost .6 tenths and 6mph. But he was also in a Bel Air (i.e School Bus).

I'd say you lost MPH more than anything, and probably like a tenth or two. One thing I noticed though was how in the past .1 in the 60' was like .15 to .2 in the 1/4. I was getting .1 to .1. Maybe the wind had something to do w/ it.

Later,
Jason

 

Well I hope that was the problem. There were some other cars there that were running worse than I thought they were. A 98 A4 SS with lid and exhaust could only pull off a 13.7. So hopefully it was just hte day

 

Heat and humidity are killers on HP. The hotter the air is, the more it expands. The more it expands, the less air molecules are in a volume of air.

Lets say a car has a 750 cfm carb. 750 cubic feet per minute of air is rushing through the carb at WOT. The amount of air flowing through can never get increase more than that. Hot thin air will still only get 750 CFM through. Since there's less oxygen to burn in hot air as compared to cool/cold air in the same volume, the engine will make less hp.

Add to that the high humidity. All that extra space in the hot air is now filled with water vapour. The engine is sucking that in and trying to burn it. High water vapour and less oxygen to burn can dramatically decrease hp. That's why blower and turbo cars have an advantage when the air is bad. They force the air into the engine and don't rely on natural conditions as much.

The head wind will also slow a car down. Even an aerodynamic car like third and fourth gens will slow down but not as much as the older cars with the aerodynamics of a brick. Wind direction will change an ET so much that NHRA will not allow a national record if the tail wind is greater than 10 mph (I can't find the exact referance but I know it's around 10 mph). Being a good bracket racer means keeping an eye on the weather conditions. Doing time trials in the morning when it's cool and no wind and having eliminations start around noon when it's hot and a wind has picked up gets to be a real guessing game to see if the car will be faster or slower in the new conditions. Keeping lots of logs of each run helps. Record everything as a reference. I've seen guys miss time trials. They'll check their logs, write a number on the windshield for first round and run it.

 

Exactly what I was going to say. My dad actually has a computer in his trailer now with a program called "Crew Chief." The first thing he does when he gets out of his car after a pass is log it into the computer. He's one of those guys that can miss time shots and look back in his log to see what he should be running and he'll run it.

Weather and wind greatly effect your times.