Most engine builders will designate max rpm for your engine (that would be where the HP curve breaks, or falls off drastically.)
Yes, you are more concerned with the power band and staying in the meat of it, but it's easier to tell max rpm and rpm drop. You can calculate the rest if you have the data, but it's not necessary. Aiming for peak rpm and looking at lap times is a much quicker way to get the gearing correct.
Drops are key for determining peak rpm for a particular track. That is, if you are on a momentum track where you scrub less than 500 rpm in the corners, you want to turn the engine less rpm @ peak. If you are on a track where you scrub a lot of rpm (>500) in the corners, then plan on turning more rpm @ peak. Sometimes your builder will specify a different rpm for each application/track. That's the way I do it with our engines. The difference in peak rpm is generally around 500 rpm (less on momentum ovals, and more on tracks that put the engine in a bind.)
Building a positive relationship with the other dads of successful jr 1 drivers at your track will speed the learning curve up considerably on finding the proper gearing as well.
Another way to work on gearing is to take advantage of track practice days or extra track time.
Start with a recommended gear that's at least close, and make some 8-10 consistent laps. (I prefer to throw out the worst and best lap, then average the others.)
Come in, drop 2 teeth in rear gear, and go right back out on the track (before it changes.)
If your lap times are better, come in a drop two more teeth.
Repeat as necessary until the lap times start going the wrong way.
If you find yourself dropping 6 or more teeth in the rear, then go up 1 tooth on clutch sprocket and repeat this process.
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Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cut
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32 years of service to the karting industry ~ 1Cor 9:24
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bcarlson@CarlsonMotorsports.com