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Old 01-05-2013, 12:08 AM
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Originally Posted by blakethechop View Post
i hope to get 450 hp with head ported and milled new comp cam new edelbrock rpm intake 800cfm carb roller rockers.i have read that pontiac under rated the horsepower on the 455 during the fuel crisis is that true
The compression changes were made due to emissions standards being tightened. This meant no (or reduced) amounts of lead in the gasoline, which lowered the octane ratings, which led to lower compression. It had nada to do w/mileage.

Again- you need to CC the heads (both of them) first to try to determine if they've been milled before you got them. After that, mill the heads so the chambers are equal to one another (both heads match). When you're done, the compression ratio you will have depends on what heads you started out with and how much was milled. I don't cut them more than 0.050", personally.

If the heads have screw in studs, you may be in luck as far as the chamber size. Pressed studs are going to be the too-large 455 chambers. But screw in studs began to be used in more heads after mid '73, so that's not always going to help as an identifier. Pressed studs are a no-no, regardless.

Or just read up on them, instead of listening to me or anyone else: 4X Cylinder Heads - How to Pick the Right 4X Heads for your Pontiac - High Performance Pontiac Magazine

The factory ratings are not anything you need to concern yourself with IMO. Some were too high, some too low, some about right. But so what? What matters to YOU is how your engine is assembled, and it is not going to be the same as any factory engine anyway.
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