[roy381]

I recently picked up a 69 428 block with a set of 62 heads. The block and heads were done years ago and put in storage. All the machine work was done its 30 over. It came with a set of new old stock speedpro 2282p forged pistons I believe they have 12cc dish. The heads were also machined but came bare. Being 72 cc heads that would put me at 10.5 cr. My question is there anyway to lower the compression with these heads? Since the heads are machined and are original to the engine I would like to use them. I also have a set of 6x-4 heads to use but need to be redone.

Thanks in advance

[Icefan71]

Measure your combustion chambers first to see exactly what they are. I have a set of #62's. They're 73cc's. I've read some can be as much as 77cc's. Once you know for sure what you have, you can go from there if you want to lower your compression.

[PONTIAC LARRY]

Factory compression ratios are way over rated, the 10.75:1 rated GTO YS 350HP 400's are like about 9.8:1 for example. I had no trouble running around on 93 octane Super Unleaded with a 68 428 (030 over with TRW 2282 pistons and #16 heads) and a 69 428 (std bore OEM pistons reringed and #16 heads) , It wont be 10.5:1 unless you deck down the block and mill the heads down. NHRA minimum was 65cc (same as Royal Bobcat tune) and those with thin gaskets would get on up there but dont sweat it man. Also you can take a few CC out relieving the ridge on the intake side in the chambers. Probably around 75cc now if I was guessing some are as big as 78cc on the #62

[steve25]

What do you mean by done in regards to the heads?
If they did not get all new guides and valves besides a valve job then they are in no better shape than the 6x heads pretty much,unless the 62s where also decked to clean things up.
I would CC the 62s to see where your at,most of these are 74 to 75 CCs.
If you unshroud the chambers at the sides to match the fire ring and releave the reverse curve in that area on the intake valve side(this extends around to the spark plug somewhat) you will have a 78 to 80 CC chamber and a workable compression ratio, if not that by all means do up the 6X heads.

[roy381]

The 62 heads have new guides and new seats installed. Where would the 6x-4 heads put my CR at ?

[67GTO4SPEED]

Originally Posted by roy381

The 62 heads have new guides and new seats installed. Where would the 6x-4 heads put my CR at ?

Assuming your engine was not zero decked, those Speed Pro pistons will sit about -.020" in the block. With 93cc 6X - 4 heads you'll be at about 8.5 : 1 on a .030" over 428. You'll be at 10.1 : 1 with the 62's, if they are actually 72cc. If they are 76cc, it will be about 9.75 : 1 CR. If you deshroud them and get them to 78cc, they it will be right 9.5 : 1.

You need to know what your 62's actually are now to know for sure. After a valve job, the 6X's could actually be a little bigger than 93cc. Another consideration is the #62 has a 1.77 exhaust valve and typically flow a little better on the exhaust side than a stock 6X with the 1.66 exhaust valve. Based on the flow numbers on Stan Weiss' site, The 62's flow about 10 cfm better on the exhaust side than the 6X's and about the same on the intake side.

Butler has a calculator on their website for Pontiac engines that you easily find out what the CR will be with the different heads.

http://www.butlerperformance.com/pro...der_heads.html

[PONTIAC LARRY]

CC C ... (in bugs bunny gangster voice)

[roy381]

CC the 62's I shall. How much will I gain by using a thicker head gasket . Or is that not recommended.

[steve25]

You do not want to go that route to lower the compression, for the best detonation / ping control the deck to head clearance shoild be no more than.045".
The average Fel Pro gasket will provide .039" intsalled.
If the 62s have had a valve job done than that will add 1.5 to 2 CCs all on its own.
There is also another 1 CC or so lerking in rear of the two top compression ring groves that it no one ever adds into there compression figures it would seem!

[roy381]

Thanks for the help guys! I'm not an engine guy I only know from what I learned on this site and on the net. I would like to use the 62's as they are original to the motor. Then I could sell the 6x-4's with the 455 shortblock I have. I will get the 62 heads cc'd and if need be have them unshrouded. 94 octane is pretty much available in my area. and even have a station selling 100 a short drive away

[Skip Fix]

For a reference my 6X-4s were 93cc untouched. Unshrouding and a 0.050 mill they are still 93cc so unshrouding will definitely give you a few extra Cs

[PONTIAC LARRY]

Originally Posted by roy381

Thanks for the help guys! I'm not an engine guy I only know from what I learned on this site and on the net. I would like to use the 62's as they are original to the motor. Then I could sell the 6x-4's with the 455 shortblock I have. I will get the 62 heads cc'd and if need be have them unshrouded. 94 octane is pretty much available in my area. and even have a station selling 100 a short drive away

94 Sunoco ? Nice you will have no problems

[roy381]

yeah Sunoco