fast86z28

This friday I'm taking my car to the track for the first time, i just rebuilt the 350 its .060 over basically stock though, I really need exhaust, I still have the stock lg4 exhaust and I know its gonna hurt me, but anyways I live in phx and when I race the temps will prob. be about 106-7 with humidity around 20%...I'm not expecting to go out and run extremely fast but I just want to know how much faster my car could be if it were say 70% with no humidity....thanks

fast86z28

oh yea its also a 5spd if that matters

Synapsis

I've noticed that on my car (basically stock L98), for every 5-7 degrees increase in air temp, I lose around a tenth. It's pretty common for me to gain 2-3 tenths about 30 minutes after the sun goes down.

IHI

iTrader: (1)

I will say common sense would tell you as the night goes on the car should be quicker, but my buddies malibu actually slows with temp drops. I'm not kidding! Just like everything else, different cars respond differently, to different things.

AlkyIROC

Go to the Calgary Drag Racing web site. There are many good tech articles about racing including how weather will affect a car's performance.

Humidity does play a part but only a small part and it depends on temperature. 20% is nice and low no matter what the temperature is. 50% humidity at 60F is much better than 50% humidity at 90F. The hot air can hold more moisture because of the saturation pressure. It's similar to disolving salt to water. You can put more salt in hot water than in cold. Since the hot air can hold more moisture, there's less air to burn.

Phoenix is a high altitude track just like Calgary. We sit at 3500 feet elevation and most race days we're around 5000 feet density altitude. Last weekend we got up to 7000 feet density altitude. I had to jet down 3 jet sizes to keep the car from running too rich and even then I was .2-.4 seconds slower than normal. It was just too hot to get the engine and tranny down to a cold level where it will make more power. (new aluminum rad goes in this weekend)

There are a couple of reasons a car will run quicker as the sun goes down. First as it cools off the density altitude will decrease. If you watch the weather, the temperature will cool down and the humidity will rise. This is just how it works. If you actually watch the dew point it should hardly change. The dew point is a better indication as to how much moisture is in the air. You also need to track the barometric pressure. 31.30 is a much better day to race than 28.50 is regardless of the temperature and humidity but all three are important to keep track of.

The other reason that the car will pick up is jetting. When it's hot out and the density altitude is high your normal jets can be too rich or lean. As the air gets colder and the DA drops you have more air to burn and the jetting requirements should change. It's always better to run a little rich in bad air so when the air gets better it will run richer. If the air gets worse it will run even leaner and you'll have to jet down again. Although this will slow you down if it gets too rich, you won't hurt the engine by running it lean. Picking up speed usually means the car was running lean when the air was bad and now that the air is better it has more air to burn with the fuel thus creating more HP.

Clear as mud?

AlkyIROC

Knowing when to rejet is a bit of an art. You have a few options on how to know. The best and oldest method is by reading plugs. More modern methods is with a pyrometer (exhaust temperature gauge) or by using an O2 sensor.

A new carburator is always jetted for sea level. Once you find the optimal jetting at a specific altitude you use that as a benchmark. The old rule of thumb was to drop one jet size for every 2000 feet increase. I found that a little awkward and found a better formula that's used by ET prediction computers.

Search the internet for air density. All the calculations and formulas for converting weather data into usable numbers is out there. Air density is usually calculated into kg/M^3. Kilograms divided by meters cubed. It's how dense the air actually is. LAst Sunday our air was so thin it was 1.00 but a few months ago it was 1.08. Denser air means more oxygen to burn.

Keeping track of this number and finding the optimal jetting you change jets for every .03 change in air density. I did a bunch of testing at the track on the weekend and got my best mph when the air density was 1.02. If the next race the air density is 1.05 I increase my jets one size. If the air gets even worse and drops to .99 I drop down one jet size. The key is to find where the jetting works best for a good start point.

five7kid

iTrader: (14)

I have a few racing buddies who have those hand-held "weather stations" that compute altitude equivilent based on barometric pressure, temperature, and humidity. They're kind enough to give me a reading after a run.

My best time to date with the '57 came last Friday night when the altitude equivilent was 8400'. Earlier in the evening, with 9200' equivilent, it ran .08 slower. But then, it ran that same time later when the altitude was still 8400'...