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post #1 of 3 (permalink) Old 05-24-2014, 04:19 PM Thread Starter
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head question

I have a set of #17 heads from a 68 350. They should be 72cc and have a compression ratio of around 8.5 on a 350. I want to put these heads on a 400.
I don't know how much that will change the compression ratio. I am not building it to race but want decent performance on pump gas. Will these heads do the job?
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post #2 of 3 (permalink) Old 05-26-2014, 12:38 PM
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Any 72cc iron head with stock flat top pistons will yield over 10:1 compression: too much for pump gas. You need a head with 87-90cc chambers to run on pump gas with stock flat top pistons. You can run your heads if you install new, dished pistons to lower the CR to 9-9.5:1. If you run your current heads with flat top pistons, you will need to run 100 octane fuel. BTDT, and it is a drag.
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post #3 of 3 (permalink) Old 05-26-2014, 08:17 PM
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#17 heads on a 350 is shown to be 9.2 compression, but the Wallace web site states 8.6 (which may be the true number and not the inflated Pontiac number). A 72CC chamber head, such as the #48 found on the 1969 350HO is rated at 10.5 compression, but the same head on a 400CI is rated at 10.75. If we theoretically applied this difference to your head, then on a 400CI it would be about 9.5 compression. I could not find an exact CC for your head, but my guess is it may be closer to 78CC's. The Intake supposedly flow well. However the intake valves are the smaller 1.96" and the smaller 1.66" exhaust. If you use them on a 400CI, I would have your machinist fit the 2.11" valves to the head. The 1.66" will work, but the 1.77" exhaust valves will help your flow numbers. The added size of the valves will lower your overall CC's. The heads most likely have pressed in studs, so you want to add screw in studs and get the correct push rod guide plates.

As stated, you want to keep compression around 9 - 9.5 maximum for the street, with 9.2 a good number to shoot for. If you use the heads, you should have them cc'd after you do any work to them. This will them become a number to use in the formula that will calculate your compression. This will tell you what type of piston to use, ie flat top, 2 valve notch, 4 valve notch, or dish, to adjust for the amount of cc's each style of piston provides to the over all formula in figuring your actual compression ratio with your new heads. You machinist should be able to help you on this on. Online calculators can also be played with to give you an idea of what you are looking for in compression ratio.
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