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Discussion Starter #1
Hi. I have a 351W with flat top Keith Black pistons, and I want to run a set of GT40p heads on it, replacing my E7TE heads. I'm not sure how many CC's are taken by the valve reliefs in the pistons, but the compression ratio would be over 10:1 (if I calculated correctly).

Is there a way that I can safely lower the compression to a more pump fuel friendly ratio? I'm trying to avoid putting in new pistons. Would thick head gaskets do the job reliably?

Thanks for any help.
 

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are those heads aluminum? if they are, you can get away with more compression, with aluminum heads i wouldnt hesitate to run 10.5:1 maybe even 11 with 93 octane, if their iron u could run pump gas with anything under 10.5 i would think, but if u still want to lower compression, you could get thicker gaskets but ive read about how they throw of the swirling, or some technical term of the combustion process and arent good for making power (not just because of the loss in compression) but i dont believe its anything major
 

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I don't know much about those heads, but if they are closed chamber you don't want to hurt your quench (min. distance between top of piston and cylinder head) by using a thicker gasket.

I think the rule of thumb is try to keep it under 0.045 (which isn't much more that a typical gasket thickness). You will most likely increase the chances of detonation by using a thicker gasket and thus increasing the quench height.

You might consider taking the heads in for a little deshrouding work to gain some chamber volume since they are off the car. Make sure you polish the chambers too.

What compression ratio are you looking at?
 

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Discussion Starter #4
Thanks guys.

The heads are cast iron that were used on more recent 302s in Mountaineers. I think they are 60cc combustion chambers, with 1.84 intake and 1.45 exhaust. With a 3.5" stroke, and 4" bore, and a gasket thickness of .041, I calculated compression in the 10.5:1 range. I may be way off on my numbers, but I think they are in the ballpark.

Would that be too much to run on 91 octane (the best grade available in my area)? I just don't want any detonation, especially when summer rolls around again. My current E7TE heads are 64cc, so maybe there wouldn't be too much of a difference, anyways.
 

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Four cc's is quite a bit of difference when you are cutting the line real close like that ... You may want to have the heads cc'ed to be certain of the chamber size ... Then you can calculate and "know for sure" what you have.

"Guessing" on the 60cc number might be risky on this assembly


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Larry Madsen
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<font size=-1>[ This Message was edited by: monstermach on 1/3/02 2:29am ]</font>
 

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cool

Our 351w is 10.35-1 with the numbers I found to enter in the fields. Thats real close to what I had figured.
 

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Discussion Starter #11
Thanks everyone! You have all been a big help.

I will use a graduated cylinder to measure the exact volume of the combustion chambers, and go from there. I'll also check the above link to the KB pistons.
 

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My guess is 88cc. My interchange guide quits at '74- so the best bet would be to cc a chamber when the heads are off.

JL
 

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Hey guys, remember MetulHead is up around 5000-5500' altitude and 10.5:1 compression can be run on 86 octane gas. I run over 10.5:1 compression in my '78 Fairmont wagon with stock cam, 51CC chambers, fully ported 289 heads and ported intake, 3.08 gears, stock converter, and single exhaust and I run 86 octane with only having to reduce my total timing 2 degrees to stop any kind of pinging or detonation. You can actually run over 12:1 compression on super unleaded up here at this altitude. His 10.5:1 or more compression is alright and the GT40P heads are much better than those crappy E7TE heads he is currently using while bumping compression up some which is needed at this altitude. NM is mostly high plaines at or above 5000'.
 

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yup, even if you do the math by hand, that is around 11.6:1

So did you expect less, more? Also, is it a popup piston for a cleveland? who makes it?
 

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I mainly relied on a internet calculator while doing my build ...

That one (which I can't get to any more) figured 11.1:1 ...

I may well be .5 higher than I thought ...

Still runs on 92 octane though.

My pistons are KB flat tops ... 2cc is an additional 2cc (valve relief) not a minus 2cc (as in dome).

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Larry Madsen
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<font size=-1>[ This Message was edited by: monstermach on 1/4/02 9:36am ]</font>
 

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MonsterMach

if you have flat tops with a 2 cc valve relief...you would enter -2 for piston volume. Giving you 11.09-1 comp
 
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