Dear Ralph: Help me understand something; I’m fairly new to the sport, but know a thing or two about motorsports in general. Why can’t...

Dear Ralph:
Help me understand something; I’m fairly new to the sport, but know a thing or two about motorsports in general. Why can’t I just increase the operating RPM of my sled a couple hundred RPM and go faster? Is each sled set intentionally “low” to provide engine durability?
Randy Nastrould

Your general premise is fairly accurate in that more RPM would provide more MPH, assuming you had the horsepower to do so and all other things were equal (like gearing).
The basic problem here is, when we’re talking two-strokes, is each specific engine and exhaust system combination produces its peak power output across a fairlynarrow RPM range. As long as your changes would allow the engine to operate within this narrow window, you’re going to be correct. However, most sleds are tuned fairly well to allow the engine to operate at its peak power RPM, and they are really not “de-tuned” as you suggest.
We could get into a very long discussion about gearing and clutching, but the key point to take away from this is that every two-stroke has a very specific “target RPM” for maximum power output. Tuners MUST know this number for their specific engine and exhaust combination. Many engine, and just about all exhaust changes or modifications, will change this number, and adjustments to the clutching and possibly gearing will need to be made to compensate.
If your engine makes its power at 8300 RPM, but your clutching and gearing at your elevation only let the engine run at 7800 RPM, you’re missing out big time.
Or, if your sled was originally set to spin at 8000 RPM and you install some go-fast parts so the engine now makes its power peak at 8500 RPM but do nothing to the clutching and gearing to compensate, you’re again missing out big time.
With four-stroke engines, this is really much less of an issue. You still need to know your target operating RPM, but being right on the money is less of issue. Their power bands are typically much wider, and they produce similar power across well over 1,000 RPM in many cases, so if the engine is spinning a few hundred RPM lower than ideal it doesn’t make nearly as much of a difference. Here we could successfully entertain the argument that more RPM will provide more MPH, but HP is going to be the limiting factor. That is, do your have enough power to “pull” the higher speeds? If not, the CVT clutching system will compensate by shifting into a lower ratio, so then what have you gained? Also, as engine RPM rises, CVT transmission efficiency drops, yet another consideration.

No comments so far.

Be first to leave comment below.

Your email address will not be published. Required fields are marked *