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Head gasket recommendation

December 3 2017 at 4:10 PM
Joe overholser  (Login exgi)
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Finishing a 352/360 hipo engine stock cr 10.75. 6090D heads 60 cc chambers I polished and cleaned up around valves, .010 off surface. Block .030 over and .009 off surface, crank is 10/10. The replacement Piston, sealed power spl-246ap30 flat top with 4 brow, the comp distance is 1.816 as opposed to the factory 1.844, not much difference. Nevertheless, they seem to be about .070 from the deck while the factory spec for the hipo was .036. I'm looking for about 9.5-10.0 cr, pump gas. Cam is comp 270s.
I have SEC 534174 small bore head gaskets which I understand compress thickness is .040. Question is, will this get me where I want to be, 9.5-10.0 cr? Advice much appreciated. Car is a 60 Starliner 3 speed OD.

 
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john vermeersch
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With those specs you should be

December 3 2017, 4:33 PM 

around 9.6:1....depending on valve relief volume

 
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Joe overholser
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Head gasket recommendation

December 3 2017, 4:55 PM 

Thanks John, appreciate you sharing your knowledge.

 
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Tom P
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Re: Head gasket recommendation

December 3 2017, 6:20 PM 

I think you would be better off with the small bore steel shims (70's truck 360 and 390's used them) and get a better quench distance. It might already be too far away to tolerate raising compression.

 
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Joe overholser
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Head gasket

December 3 2017, 7:52 PM 

If I use a .020 shim instead of the .040 composit, what does that give me in compression ratio? I thought the thinner gasket would increase the compression ratio. I was thinking it would tolerate 10.00 cr on pump gas? My goal is to run on pump gas, so whatever cr will get me in the range safe from severe detonation is o.k.
Thanks for your input.

 
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Ross
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Compression

December 4 2017, 4:00 AM 

In the end, any of your possible gasket combos are over .080 quench when you add the gasket, so at this point, you just want as much compression as you can without going too far with the cam you have. Anything over about .055 quench distance is the same

.030 352 with those pistons, assuming a 60 cc chamber and a .041 gasket, and guessing deck height at 10.160, would be about 9.1:1. That is .060 below deck and .101 quench (I know you said .070 below but the math doesn't work that it would be that much, you may be seeing piston rock)

The same combo with a .020 gasket would be 9.57-ish with .080 quench distance

Keep in mind that I used 60 cc, because most of the other heads I have seen come in a little big, and you cut them, but if it is at 58 cc it will be 9.30 with the .041 and 9.78 with the shim.

In either case, that is a lot of room above the piston and you are giving up the benefits of quench, so although it is a good compression ratio, you are losing the benefit of the quench.

Luckily, your cam is big enough that it should be fine on pump gas. Assuming the motor is together and you don't want to spend money on custom pistons, and the fact that you have two fresh sealing surfaces on the heads and block, for a little more performance I'd run the steel shim and you'd be between 9.57 and 9.78, then lash the cam tight (to make it act a little bigger for a little insurance) or if you want to be more conservative run a .041 gasket for compression between 9.1 and 9.3.

Either combo should be fine on pump gas.

However, 2 comments, not that I think you could get to 10:1 with your combo, but you mentioned it. It would not likely be happy with the cam choice you have. That's OK because more cam wouldn't match the car, so the 9s are a good number.

The second comment is, I wouldn't do this, but the Felpro blue gasket would get you to 9:1. You just don't need it and will be giving up power. However, it's likely the cheapest way out and if power wasn't your primary concern, it'd run on anything happy.gif





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This message has been edited by Bad427stang on Dec 4, 2017 4:01 AM


 
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Joe overholser
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Head gasket

December 5 2017, 8:50 PM 

Thanks Ross. Very good explanation of the technical aspect of my situation. Short block is in car, so I'll go with the .020 shim gasket.

 
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