6x8 or 7m5 for a 400 (461) stroker?

Pontiacpower

Pontiacpower

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Hello. Been a while since I’ve posted but I have my 400 together and my compression ratio is too high for pump gas
Current combo:
1972 400
4.25 stroke crank
4.155 bore
1968/69 #62 heads. (75cc chambers
Scorpion 1.65 rockers
Isky custom cam 472 lift
Can’t remember duration or overlap
(If you need these numbers tell me and I’ll get the card and post it)
anyways. I bought a collection of parts off a gentleman and among these parts is 2 sets of 6x8 heads and 1 set of 7m5 heads.

MY QUESTION
What would be a better choice for this setup?

6x8:
100 cc chambers
2.11 in
1.66 ex

7m5
114 cc chambers
2.11 in
1.77 ex

I believe both are screw in studs..... however both need to be tanked and valved. Machinist said I should be able to swap everything from the 62s over to the new set. With exception of valves.
Which set do you guys think I should Use? The 7m5 has bigger exhaust valves but the 6x8 would have a little more compression.

Idea is #1 to run pump gas. Reliability is #2. I feel like price between both sets would be comparable.Thank you for posting your experience with this topic
 
What is the octane of available pump gas?
Not knowing all the information on your set up the 100 cc heads will drop compression about 2 points and the 114 cc will drop compression about 4 points which would be too much.
If you Google compression ratio calculation you will find all the factors needed to calculate it.
 
I did do that and I got 9.86 for the 100cc chambered 6x8s and I got

8.83 to 1 with the 114 cc chambers

would it be better to have more compression and smaller exhaust valves or less compression and bigger exhaust valves?

most gas stations around me carry 89 91 and 93 octane. 89 is cheaper so yea that would be cool but I have a tendency to put 93 octane in the toys
 
What did you get for the current 75 cc heads?
I would go with the higher compression that helps through the entire power curve, the exhaust would only effect top end.
Get a multi angle valve job.
6X is on the list

Another thought, your current heads with a thicker head gasket.
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With the 62s I got 12.6 to 1 compression which I believe is too much for 93 octane. 10.5 or 11 to 1 is as high as my internet search shows I can go without aluminum heads. And I have to limit my timing at 36 degrees btdc. Again correct me if I’m wrong. I’ve never built a motor before so all my information is off the internet
 
Ok how about 1970 455 heads with 87 cc chambers
 
I don’t have any 70 455 heads. All I have is 2 pairs of 6x8s and a pair of 7m5s

but if I did have some 87cc chambers it would be 11.09 to 1
 
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Go with the 6X
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Will do thank you for the input I’ll go drop them off at the machinists place when I get some spare time
 
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