Maybe I should post this in the Rotax section but I race a Rotax DD2 and wonder what the best RPM to shift is. The engine produces maximum power at 11,750 rpm. A competitor of mine short shifts and insists he goes quicker that way but so far I haven’t noticed a difference. Any thoughts on this?
“Maximum power” and “maximum torque” are usually quite different for our motors, be they Rotaxes, CR125’s, or Leopards. You should probably try to shift somewhere between max power and torque, and then gear the longest straight to reach just short (100-200rpm) short of your max power/redline (whichever is lower).
11,750 is where the engine is capable of producing the most HP. That doesn’t mean that it is producing max HP on the kart. It probably isn’t because the kart is moving at a pretty good speed and in low gear it probably doesn’t require max HP to make that speed. Idealy you should shift at an RPM that puts you at 11,750 after you go into high gear. At that point your in a lower gear and the kart is accelerating so the engine is fully loaded and producing all the HP it’s capable of at that RPM. It may not be possible to get to a high enough RPM to be at 11,750 after going into high gear so you probably want to shift at the higest RPM you can to come as close as possible. I had a DD2 and shifted around 13,000.
I feel when it comes to road racing the engine doesn’t matter. Iv ran almost identical times with three different brand kz engines and have seen stockers and mods run the same times. For sprint racing kz is faster over the stockers and mods. I was half a second faster in a kz. That’s just my two cents.
In road race engine size does matter along with the skill and balls of the pilot. Single car times can vary greater from event to event based on track conditions and weather due to how long a road course is vs any shorter sprint track. You might fight hard at the sprint track to find that half second lost with a green track, you might fight hard to find 2 seconds lost on a green road circuit. ICC @385lbs vs stock moto @400 (IKF standard) on a 2+ mile track will usually be about a 1.5 second lap time difference. Throw a big pack of dudes in with either style motor and expect the fast lap time to come from the best drafting person. If you want to really test road times you need the same guy with skill on both motor styles with no draft.