I have a set of 1971 #7 heads and to the best of my ability, have concluded (from published head spec's) that the combustion chamber volume is 64 cc's.

So using the data from all components of my 355 SBO engine and using a CR Calculator, I come up with approximately 9.59:1 compression..

My question is how accurate is the 64 cc number for a set of stock #7 heads??

Thanks,

Thirdcoast

 

They’re typically 68-70cc but need to be measured to be sure.

 

Ok, I was taking the Olds Junction specs for the #7 heads (64) and figured it was factory minimum to allow for manufacturing variables as NHRA specs 60.5 cc as minimum..

I recalculated the compression and using 68 cc's still have 9.21 CR.. I cannot measure the chambers right now as the engine is still in the car.. just an exercise to see how NON-Ethanol Gas with an octane rating of 90 will fare in this engine.. This fuel is readily available in my area due to the huge boat motor use.. $3.09 per gallon

What do you think CutlassEFI?? This ought to do OK until I get the new Eddies What compression would be too much for 90 Octane NON-Ethanol

Thirdcoast

 

At 9.2:1 you’re fine with 90 octane. BUT use a wideband to tune it right.

 

Absolutely, I user the AEM unit that you recommended..

BTW, the jet recommendations worked out great, my 64 is running better with the 355 in her than she ever has !!

Thanks for all the help you provide this forum

 

I ran 87, Olds 350 with 9 to 1 and a 214/214 cam. I ran aggressive timing, 22 initial, 38 in by 2600 with 30 degrees of vacuum advance on manifold vacuum. I did get low speed bucking, a 20 degree vacuum advance can fixed it. The cam used makes a big difference and you having it properly tuned will also make a massive difference.

 

First, any document that lists a single number for head CCs is wrong. These were mass-produced sand castings. There was 2 - 4 cc variability in chamber volume just from the manufacturing process.
Second, any machining of the valve seats or milling of the head surface changes the as-cast CC number.
Third, Olds typically cast heads on the large side to allow extra metal for head rebuilding (or assembly line reworking) without excessively raising the CR.
Fourth, the published numbers you do see are the NHRA Stock Class blueprint numbers. These are NOT the as-delivered numbers. These are the size chamber you are allowed to run and still be legal in NHRA stock classes. This is why you blueprint an engine. The blueprint numbers have nothing to do with what a particular casting will check out at.

 

My 7A-heads as an example are 66cc AFTER installing new bigger valves, and cutting the deck twice ( first guy did cut it too coarse for MML-gasket), and "polishing" chambers just enough to get rid of the highest ridges.