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Why is the average piston speed of the 849 Sleeper important??
One of the biggest factors that engine builders use to predict engine reliability is “average piston speed”. In short, the peak rpm and stoke length are plugged into a formula to obtain the average piston speed in “feet per minute” (it’s a finer measurement than mph). Here are The numbers:
Stroke Length
68mm SuperJet
74mm Kaw750/800
SXR Setups
Stock OEM peak rpm
6150 rpm
6550 rpm
Piston speed @ stock RPMs
2742
3180
SXR stock
Piston peed @ 6800 rpm
3032
3301
Piston peed @ 7000 rpm
3122
3398
849 Sleeper
Piston peed @ 7200 rpm
3211
3495
Piston peed @ 7300 rpm
3255
3609
Piston peed @ 7400 rpm
3300
3592
Wet-Pipe
Piston peed @ 7500 rpm
3345
3641
Piston peed @ 7600 rpm
3390
3689
Piston peed @ 7700 rpm
3434
3738
Dry-Pipe
4000+ fpm – Completely unpredictable life span of crankshaft components
3700 fpm – Crank life can predictably be 20-35 hours
3500 fpm – Crank life can predictably be a full season of use
3300 fpm – Crank life is predictably 2-3 seasons of use
3100 fpm – Production unit range, predictably 4-5 seasons of use
It is common knowledge, among stand up racers, that modified SuperJets have considerably better crankshaft life than modified SXRs …. Average piston speed is the reason why. One of the best features of the 849 Sleeper is that it delivers the water-speeds of a high revving setup, but yields the significantly lower piston speeds that improve crank life. It’s true that the slightly increased weight of the 849 Sleeper pistons does slightly increase loads on the connecting rods. However that load increase is nowhere near the load increases subjected by the extra 400-700 rpms of the higher revving race pipe setups.
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Rich Belloff
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