People are having problems answering you as you have asked a small question that has WIDE implications.....with a built in misunderstanding.
Of COURSE your engine has an area in which it works best. You already MUST know that it producs MORE power at 7000rpm than it does at either 10rpm or 50,000 rpm, surely?
I suspect you are therefore asking a different question.... "how do I FIND the best rev. range and how do I set my kart to PERFORM best in that range!"
Basically, there are two methods. One is SHOCKINGLY simple.
1) Go to the track
2) Warm the motor
3) Take the lap times with a sprocket that you KNOW is too small (that will NOT reach 'peak revs' by the end of the straight)
4) Bring him in and change to a sprocket one tooth LARGER and send him out a gain, taking the lap time.
5) Keep changing the sprocket to larger and larger ones and keep taking and comapring lap times.
6) The lap times SHOULD improve each time you change sprockets up to a peak, when the lap time starts to get WORSE you will know that you are no longer making BEST use of the engine.
7) Now use the sprocket that gave you the BEST lap times!
It's THAT simple and, no matter WHAT else happens, that's the BEST method!
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Now... the chosen rear sprocket will probably be DIFFERENT for each track. Those tight twisty tracks will (probably) need a LARGER sprocket than a LONG open circuit.
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The second method uses more thought but is OVER-RIDDEN by the better, first, method. It's just quicker to GET the right choices.
a) Buy a Mychron 3 or 4 and fit it
b) Learn to use it
c) Get your lad to drive the track with almost ANY sprocket.
d) Display the data showing the bhp/torque curve (your Mychron CAN do it!) and note the shap and scale of the power. You may find that your engine produces MOST power between 8000 and 9000 rpm
e) get the display to show you the Percentage of time in EACH thousand rpm band (Your Mychron does that TOO!). What you might see is a display that shows that for any lap, you are spending 10% between 5000 and 6000, 20% between 6000 and 7000, 40% between 7000 and 8000, and 30% between 8000 and 9000 and 10% above that.
f) You may now notice that you are NOT spending much time (only 10%) in the range where your kart IS most effective and that you are spending too MUCH time below that range. That SHOULD tell you that you need to run with MORE revs.... Top do that, you change to a larger rear sprocket and test it AGAIN..... while checking that you lap times IMPROVE!
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Setting up your kart CAN be helped with the right tools (e.g., the Mychron) but it STILL comes down to checking your lap times and WATCHING how other karts behave around yours.
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Finally, if we assume that most engiens are VERY similar and your lad knows how to drive and your kart and tyres are pretty good... then you can tell what you need to do by watching how he comapres to OTHER karts. If they are significantly faster than you out of corners and you don't get past them at the end of the straight, then you SHOULD fit a larger rear sprocket to get that additional KICK out of the corners.
However, if he is QUICK out of corners (and leave them for DEAD) at the START of the straight but he then gets SWAMPED by them at the END of the straight, then you can be pretty sure that you are UNDERGEARED and have a too LARGE rear sprocket... change it!
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Finally, the conditions ON the DAY make all the difference, too. If it's wet or cold, your exit speed from the corners WILL be slower... you therefore will NEVER get to the same top speed by the end of the straight as you STARTED slower. You therefore SHOULD be fitting a LARGER rear sprocket on siuch days. Similarly, a cool day in September often produces more power from you motor than does a HOT, sunny July day (the 'air' is 'thinner' on hot days)... so, your power curve WILL have changed and you need to react TO that change with your ratios.
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Finally, don't get the idea that the power curve has 'cliff-shaped' edges: it does NOT! The edges trail-off as you go up/down the revs. Therefore it is NOT an 'exact' science.... you can only do your BEST...there is no such THING as the PERFECT set of ratios... only a best compromise.
Put simply, the Larger the rear sprocket, the BETTER the acceleration but the lower to TOP speed is available a 'max' revs. Git a SMALLER sprocket to get higher rop speeds but be prepared to lose accelleration in the process!
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Phew! I hope that lot helps!
ian
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