>A friend of mine who is into national level competition in motorcycles
>claims that one important ingredient in an engine like mine is to
>minimize the piston crown to head clearance in the squish portion. He
>says that motorcycle people assemble engines so close that they 'click',
>and then shim apart. On the other hand, there are some pistons available
>for Triumphs that achieve higher compression ratios with a pop-up
>section in the open area of the combustion chamber.
>
>What's the current state of knowledge on this subject?
>
The most I've ever done, on an A-series engine (998cc), is to have the
pistons about .005 proud of the block. I figure a compressed gasket
thickness of about .030, so I have .025 piston to head clearance. When I
opened up that engine, I found that I had little cylinder head impressions
sort of etched onto the tops of the pistons, so that if I wasn't touching at
high rpm's, I was close :-) I suspect that once you get beyond say 14:1,
the return from higher compression starts to get expensive in terms of the
bizarre fuel you have to buy, and the risk of detonation. I would focus
instead on combustion chamber shape to get both better flow and better
combustion. Some engines are harder to get compression in than others, tho...
Cheers, Brian Evans
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