holley 4412 jetting

[mudcrasy]

Hi I am going to a race tomorrow and the high temp for the day is going to be about 48 degrees. I normally dont run when its this cool. I was wondering if someone could give a little advise on the jetting? Should I go up and how much?

Thanks

John

[sj valley dave]

More to it than just temperature...There are on line altitude adjustment calculators. They take in effect temperature, barometric pressure, humidity and above sea level height to figure out what your carburator "sees" for air...Just because it's 48* outside doesn't necessarily mean you should change anything with the jets. Willy's told us about a jet size up or down for every 1,500 foot of elevation change.

[dynoman14]

John,
Buy yourself and air density gauge, they are $100-200. This tool should be in every racers tool box and is the best way to tune your race engine.

If you run on a dyno you can check the air in the dyno cel with your gauge. Once you have a baseline you can adjust jetting up or down when the air gets better or worse.

It is on a scale from 60-120% so it is easy to calculate the % change in air, from day to day or hour to hour.

Another thing you need to get a hold on is the area of a jet if you are going to be doing a lot of jet changes for weather. The jet number is not always accurate to the pin size of the jet so when making % changes you should know the area numbers get accurate increments of fuel increase to air changes. In most cases it takes 2-3-4 jet sizes to increase fuel 1%.