Site Supporter
A lot of that will be dependent on the build as well as the carb builder, then you can throw in the weather factor as well.


I bought an air density gauge to throw in a more precise jet choice.
Total waste of a hundred bucks. The air density in my area never
changes enough to warrant a jet change except when it's bad
weather and then the race is called off.
I have leaned out when my head temp is too low for my liking.

alvin l nunley

Site Supporter
If you go to Longacre's website, and read their articles on the air density gauge, you'll learn that a 1% change in air density calls for a 1% change in jet size area. Now in a clone, if you have a .037" jet in the engine now, and the next size jet in your box is .038", that would give you a 5.5% change in jet size area. Way too much for a 1% or 2% change in air density. You would need, for a 1% increase in air density, a jet that has a .0002" bigger hole. Not exactly, but very close. In my area, a little south of Austin Texas, I've seen the air density change 12 points over a years time. From day to day, sometimes during the same day, it can easily change 5 – 7 points. Air density refers to barometric pressure as affected by temperature. You're not going to be able to feel it so you have to measure it. Of course I'm not saying that a 1% or 2% change in air density is going to give you a big boost in power, but it is going to give you a boost, or loss, depending on whether it goes up or down. Knowing the air density, when you 1st start measuring it at your track, is not going to tell you what jetting to use. It's a learning process. You know how there are days when you're really flying, well one of the things that could be happening is your jetting is matching that moments air density. Write it down, use it for a reference. If it goes up, go richer, if it goes down, lean it out.