I always wondered, how come as altitude increases, your ET tends to decrease?

 

air is not as dense, so less oxygen means less power..

 

I think you meant that as altitudes increase performance decreases (but ETs actually increase). The reason for this is that the denser air existing at high altitude doesn't allow you to make as much horsepower. We have several regulars here that race at very high altitude and I am sure they will chime in too.

Here is an article to read from National Dragster: Weather Tech 101 National Dragster - Find Articles

 

my track is at 5800ft elevation, the air is just too thin so the oxygen content is decreased substantially. An engine is just a big air pump...so more oxygen=more power and vice-versa.

 

I gotchya. I figured it had something to do with air being thinner, but I was never actually sure.

 

Track altitude is just a reference. Racers keep track of the density altitude. With a combination of altitude, temperature, humidity and barometric pressure, the density altitude can change drastically hourly or daily.

Denver is officially 5800' altitude however the DA during the summer can put the air at an equivalent of 8000'. Watch ProStock cars when they go to Denver. At sea level, they're now running 209 MPH. MPH is a direct result of HP. In Denver, to my knowledge, I don't think a ProStock car has broken 200 MPH yet during a national event. That's a big HP loss because the air is thinner to mix with the fuel to make power.

Having a 12 second street car at a high altitude track is a fast car. A car that runs 12.9 in Denver is faster than a car that runs 12.9 in Florida.

When you think of a power adder such as NOS, blower or turbo. The purpose of them is to put more air into the engine. More air means you can burn more fuel. More fuel to burn means more power. Now a naturally asperated engine can only pull in the available air. At high altitude the air is thinner so there's really less air to burn. Less air and less fuel (need to keep the proper air/fuel mixture) means less power.