While everyone is fully expecting the 125cc Grand Prix class to be axed soon (latest suggestions says 2012) and most are expecting that it will be replaced with a 250cc four stroke single cylinder-engined based bike, the latest rumour out of Assen this weekend is that the expectation that the preferred engine will be based on existing MotoCross technology, could well be wrong.
Basing the engine on an existing MX motor could have some limitations as I have already detailed below. A well-tuned 250cc MX engine can produce about 60bhp, approximately the same output as an existing 125cc two stroke Grand Prix bike. But the problem of longevity (making it last at full or close to full revs for half an hour) is a problem. MX bikes operate in short bursts, a totally different scenario.
Also, MX engines are comparitively low-revving, which would provide good torque, but not the high-speed that is required.
So, with that in mind, while DORNA is stressing that there will be no “control engine” as with Moto2, the replacement bikes will NOT be based on MX engines. Instead they will be based on a “specification-determined” engine whose main requirement will be a maximum bore of 81mm. This will mean that the engines will be distinctly oversquare and would thus be quite high-revving, an eminently more suitable scenario for road racing.
BUT…..hang on just a minute. 81mm. Where have I heard that before????
Oh, yeah. 81mm is the maximum mandated bore measurement for the new 1000cc MotoGp bikes….
