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Old 04-24-2003, 07:39 PM
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Post Pontiac Comp ratio for pump gas

I am ready to put my 455 pontiac together, the only problem is I am putting on 400cu 670 heads. The block is bored .030 and have pistons. I was told I needed the pistons milled down in order to drop the compression to 9.5 to1 in order to run pump gas. The engine guy I talked to estimated .075 off the pistons. (I dont want to mess with the heads) I dont want any estimates when it comes to my engine. Is there any techs out there who could tell me how much to mill them down. 670 heads are 72 cc's

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Old 04-24-2003, 08:01 PM
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I'll be the first to admit that this may not be the "percise" answer you're looking for but I rebuilt my Olds 455 with 81cc heads and "medium dished" pistons and the compression ratio is 8.9 to one, according to Keith Black. I can run the auto on regular but I have to reduce the timing a bit but it still has good power. I think that you need to get dished pistons to keep the compression reasonable. If you cut the present pistons tops down then sure, it will lower compression but your quench is going to go out the window. I think that if this auto is a daily driver then you have some decisions to make. Do you still have the bigger chamber 455 heads. I had a "special performance" 1972 Pontiac 455 at one time and it had 8.5 compression but had big valves. The engine came out of a "SJ" Grand Prix. That engine flat boogied! I really don't think that you need mega compression in a big block to make them run. A little head porting a medium performance cam and good exhaust is all you really need for "bristling" street performance! Good luck
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