[tom s]

Too many things can effect the driving.Quality of fuel where you live,if you drive in a lot of stop and go traffic where your engine temps get high.There is no hard firm number IMO.Tom

[Formulajones]

Originally Posted by geeteeohguy

What fuel are you running? I haven't been able to run #12 or #670 heads with an 068 cam and flat top TRW's for over 35 years here in CA with 91 octane gasahol being the best available pump gas. I was detonating all the time.

91 octane pump fuel for the last 20 years here in Arizona. It's all 10% plus ethanol here.

[Stan Weiss]

Originally Posted by Wanton Pontan

Interesting that there's still so much conflicting data on whether ~9.75:1 is too high or not....and these seem to be about the 2 most common heads I see cropping up. I'm working on resurrecting the 670 heads--which are in rough shape...and now thinking of cycling through the permutations.

Build plan is not for 0.040" over, spc-7 (4.10 lift) cam.
Now I'm thinking about going thru the paces with porting heads, trying 0.040 and 0.028" gaskets on the 6x4's. I was planning on keeping the 1.66" ext port on the 6x4....I'm starting to get a nice set of parameters.

Is there anything folks would want to see in the process?

You see this as conflicting data and I see it as accurate. There are many ways to get 9.75:1 CR and they don't all want / need the same octane. Also you then need to factor in the cam selection as another variable.

Stan

[Dragncar]

Run the 6X-4 heads. Just get the CR to 9.25 and you will be fine. A little insurance does not hurt. And the difference in power in 9.75 is not worth it IMO. Long time iron head pump gas guy here.
The 6x-4 is the best D port Pontiac head they ever made. So run it.
The Super Stock guys love that head.
Run a little more cam than you would like with Rhodes lifters.
Make some power with this thing and have some fun. Do a Jim Hand port job on those things !
670 heads are the worst 400 head. Very old school, closed chamber.
All those little cuts in the later heads were 10 years ahead of their time. Swirl is good, use them.
If it were me, there would be no question on which head to run. 670s vs 6X-4 ?
Just about all the other big valve, screw in stud D ports are better than a 670.
Get your power with air flow and a little less CR. Mild port job on the X heads, put a cam in it. Rhodes will tame it down and you will still have the top end.

[scott70]

Originally Posted by Dragncar

Run the 6X-4 heads. Just get the CR to 9.25 and you will be fine. A little insurance does not hurt. And the difference in power in 9.75 is not worth it IMO. Long time iron head pump gas guy here.
The 6x-4 is the best D port Pontiac head they ever made. So run it.
The Super Stock guys love that head.
Run a little more cam than you would like with Rhodes lifters.
Make some power with this thing and have some fun. Do a Jim Hand port job on those things !
670 heads are the worst 400 head. Very old school, closed chamber.
All those little cuts in the later heads were 10 years ahead of their time. Swirl is good, use them.
If it were me, there would be no question on which head to run. 670s vs 6X-4 ?
Just about all the other big valve, screw in stud D ports are better than a 670.
Get your power with air flow and a little less CR. Mild port job on the X heads, put a cam in it. Rhodes will tame it down and you will still have the top end.

My 455 has 93cc heads , ra4 sized cam with roads max lifters and comes out to a final 9.71 cr. Cranking cylinder pressure is right at 175 psi.

[Dragncar]

Originally Posted by scott70

My 455 has 93cc heads , ra4 sized cam with roads max lifters and comes out to a final 9.71 cr. Cranking cylinder pressure is right at 175 psi.

Not sure what you mean. I had a 455 that had 165lbs cranking pressure that would run past that thing.
I am sure you can run 9.75 CR on a 455 with iron heads and get away with it.
Its just the OP sounds like a bit of a newbie and a little insurance is a good thing.
No one wants a blown head gasket from a bad batch of gas on a hot day.
Make the heads flow and having just over 9-1 is a good street pump gas recipe for a Pontiac.

[lust4speed]

My vote with the 068 cam is for 9.1 compression for 91 octane or 9.3 for 93 octane (Funny how it works out). Larger cam, lower gears, cooler climate, or cutting a deal with the devil can allow higher compression ratios. Just not worth it to chance hammering out the rod bearings with a marginal tank of gas on a nice warm summer day.

Half a point drop in compression is a 2% reduction off maximum power, but having to cut back timing to avoid detonation might cost 10%.

[steve25]

Also note that not everybody’s fuel of a given octane level provides the same knock protection.
The blend of a fuel may work better at a lower or higher engine temp then another brand pump fuel of the same octane.

[Formulajones]

Mine likes 34 degrees total timing, that's where it dynod best and runs at the track the best.
Just 91 pump fuel with 10.13:1 and that little 068.
Tight quench and making sure the icl is where it needs to be is your friend.
If it were going to pound out rod bearings it would have done that 25 years ago. Ive put about 60,000 miles on it in just the last few years.

Things just have to be right. I do agree if someone isn't that tune savvy and/or don't pay attention to build details then by all means take some compression out of it