Component manufacture and physical properties

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One thing which puzzles me is, if I am going to fit JS pistons and long rods, I would probably opt for 12 to 1 comp. to get more out of the methanol. But I suspect the piston crowns would he higher and heavier to cope with higher combustion temperatures. So I don't know whether I would be gaining. With the methanol, at higher comp you use more fuel to get more power. But most of the gain comes through latent heat of vaporisation which creates a supercharging effect. So I probably don't really need to go high comp.
 

Fast Eddie

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I don’t think there would be much extra weight Al. The dome wouldn’t necessarily add weight as I believe JS removes a corresponding amount of material from the underside. Whatever the specifics, they’ll be lighter than stock 850 pistons and MASSIVELY lighter than the high comp pistons used back in the day!

I run 11:1 in my 920 petrol engined road bike. I’ve never run methanol in anything myself, but from what I’ve heard, 12:1 does not sound at all over the top.
 
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Yes. A lot.
Methanol is much easier on a race motor than petrol, because the heat build-up is much less. I would not race an old four-stroke on anything else. My 850 is amazing with it. I've never had another motor which has run so well on methanol. The cast iron barrels seem to suit it very well. With aluminium barrels, it can be difficult to get enough heat into the motor.. Petrol is much more difficult to get right, methanol hides-up the tuning errors.
 
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My brother has run both an 1000cc 880 JAP and a 600cc two valve Jawa motors on 16 to 1 comp. with methanol, in sidecars . But he is not brave enough to raise the comp. on a 750cc methanol-fuel powered two-stroke. It can get very expensive.
I would never try to race using petrol in a classic motor where one of the limiting factors is heat build up. Commando motors on standard comp. love methanol. If you jet lean enough, they really fly.
 
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