Tbirder74

If I have a set of number 6 heads portered and polished and the intake match ported and install it on my 74 block without doing anything to the pistons as of now, will I see a performance increase?
I plan to change my pistons out this winter but kind of anxious to get those heads on the car

jmos4

Hi,

All depends upon the cam and what size dishes are in your pistons.

I had thought I read somewhere that 73-74 #8 heads had larger compressions chambers yet smaller dish pistons, vs #6 and older heads all had the same size chambers although and 4v had differentpistons. Not sure how the #7/#7a heads/ chambers are but they were the first under 9:1 compression ratios

Reference:

https://oldsjunction.classicoldsmobile.com/oldsfaq/ofeng.htm#ENG 1964 - 1990 Small Block CIDs

Regards,
Jim

rocketraider

You'll see some difference because of the improved airflow, but you have remember Olds changed compression ratio with the pistons. Doing much to the combustion chamber may actually drop CR.

What's your end goal for compression ratio?

Tbirder74

thanks Jim
So many factors... I'd like to do most of the build myself, but with big money being spent I want to gather the facts

Fun71

The #8 heads have larger combustion chambers (~79cc) than the #6 heads (~68cc), so installing the #6 heads will give a bump in compression. How much of a bump depends upon the actual volume of the combustion chambers - milling the heads, valve job, polishing, etc. will change the volume, so you'd have to measure to see what they are now.

The #8 headed 350 engines used pistons with 14cc dishes. Toss some numbers into an online compression ratio calculator and you get around 8.5:1 if the #6 heads have 68cc chambers and you use .041" thick Fel Pro head gaskets.

olds 307 and 403

CC them, they may be closer to the incorrect 64cc number usually quoted because they have probably been milled. Your motor will have 14cc dish pistons, if stock. My #6 are around 64cc that but only because they were milled. You will probably gain .5 to 1 compression, even with thicker head gaskets. You can go to .027" more expensive MLS head gaskets since those pistons sit .025" below deck and be around 9 to 1 if near the 64cc number. Unless you do a cam swap, even something like the generic 204/214 cam, you gain very little. You probably need a new timing set if it hasn't been done and degree the cam as well. Also seriously consider a 2000+ stall torque converter. The affordable Jegs 2300 stall is decent quality.

Fun71

I agree with the cam swap. The factory cam is very mild:
.400"/.400" lift
186°/202° duration @ .050"
250°/264° advertised duration

Install a new timing cam with .450" lift or so and 204° duration @ .050" and you'll see a noticeable increase in performance. Don't go too large on the duration or you can "overcam" the engine and lose low RPM power and driveability.

joe_padavano

The #8 heads used on the 1973-76 350 have about 80cc chambers. The #6 heads have about 64cc chambers.

olds 307 and 403

I found on two different pairs of #8 heads that still had the shim head gaskets were around 77 to 78cc. Guys on here claim 68 to 70cc on the early heads stock. My two pairs were both milled, so I can't say from experience.