[69FIREBIRD76MM]

Asking for a friend, motor is 428, will Edelbrocks 72 cc raised compression too much for street on 91 octane? Thanks

[ponjohn]

http://www.wallaceracing.com/cr_test2.php

[Steve C.]

The compression calculator posted brings up the many questions that your friend needs to address. Example, is this engine going to be more or less stock or will it be rebuilt. Will the rebuild include a larger bore with different aftermarket pistons with different valve relief cc's. And will it be 'zero deck' or with the pistons down in the hole. Etc, etc.


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[Steve C.]

For interest, information found on the internet.....

"I just ran "numbers" for 670 heads on a standard 428 (4.12 bore, 4.00 stroke) and assumed a standard factory compressed gasket thicknetss of .042 with the pistons .020 "in the hole" at TDC - which is what Pontiacs "usually" are from the factory unless the block has been decked. I also assumed flat top pistons that have about 6 cc's worth of clearance volume in the valve reliefs. Factory specs for 670 heads state that their chambers have 72 cc's of clearance volume. However Pontiac heads have been known to vary from factory specs quite a bit in either direction, and also we don't know if your heads have ever been milled/surfaced, so the only way to know for sure would be to pull the heads and measure the chamber volumes directly. With all the "disclaimers" out of the way, that combination works out to 10.549:1 static compression using nominal factory measurements. That's probably too high for any pump gas."

Note this was based on iron heads. The obvious, aluminium heads can run more compression.


( Information provided in this post does not represent any endorsement. And unless specified it is not based on personal experience and is offered for general interest only )


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[67gtospud]

Dished pistons, and the correct cam choice it'll be fine. I run my car on 91 octane no problem. My 428 build is in my signature

[steve25]

It will be fine with 72 CCs as long as the Cam used has at least 222 to 225 Intake duration @ .050" and the chambers get polished out and any edges from the deck to chamber interface that hang out passed the fire ring of the head gasket get rolled over by a bit.

[roy381]

67gtospud, do you recall what the dish is in your pistons?

[67gtospud]

Originally Posted by roy381

67gtospud, do you recall what the dish is in your pistons?

Icon 10cc dished pistons

https://butlerperformance.com/i-2445...tegory:1234832

[Cliff R]

I've been running 76cc KRE aluminum "D" ports on my 455 for over 10 years now and zero issues on pump gas. It specs out at 11.3 to 1 compression. Not recommending that deal to anyone but wit tight quench, flat top pistons, the right camshaft, and full control of timing/fuel curves no problem at all.

I consider the higher compression your friend with these things as it opens the door for larger camshafts and more power at every RPM plus good idle quality and street manners..........Cliff

[steve25]

Yup! All things being equal, small chambers equates to higher vacuum levels and less tuning headaches, as does the minimum amount of intake port volume for a given power band.