my Low dollar 318 hop-up project

318willrun said

I, personally, think open chamber heads flow better. If I can run the same compression with a open chamber head vs closed chamber head, I'll take the open chamber.

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They probably work better as far as flame travel if you think about what happens in the chamber as soon as the spark plug fires. Closed chambers only cover half the piston and the explosion starts pushing on that half of the piston for a fraction of a second. Flat top motors with open chamber heads usually run good.