I have done both 6x-4 and 6x-8. I can’t think of any reason that you would want -8 heads starting with a clean slate and can pick pistons out to put the compression where you want it. I think Skip alluded to that.
There really isn’t a close to enough info on the plans of this “4.25 6x street engine” to nail down where the compression should be. It is a bit like asking a tailor to make you a 3 piece suite and only telling them what size sweat pants you wear.
Most people with 450-470 Cid combos using stock cast iron heads for pump gas have the best results 9-9.5 scr, and the cam will be either size of 230*@.050”. If you want a very mild cam with less than 220*@.050” I suggest staying down close to 9:0.
I currently do not have a cast iron headed Pontiac pump gas combo with less than 10:0 compression. One is up at 11.0:1, and it is a 4.25” 6x-4 combo . Higher compressions can be done, but most street builds are better served by lower compression than we run, especially when the heads are stock with no porting.
Last edited by Jay S; 10-16-2021 at 05:54 PM.
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