I'm pretty sure that is the difference on the ProMAXX 63 and 65 CC heads. The 65 have a little more room for the positive height pistons. I think the Edelbrock do the same, hence the 340 version?
So the 65cc heads are an open chamber design to clear the piston. The only pics i could find of them looked like closed chambers but maybe that was a generic pic?
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One phone call will give you better answers than any of us can give you. He kindly answered every question I had for him and if I was still in the head porting business I would have grabbed a few sets.
