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Old 12-30-2006, 04:06 PM
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what compression will this combo make??

350 engine bored .030 over to 355
flat top pistons w/4 valve reliefs
64 cc combustion chambers
just give me an estimate on what you think the compression might be.

thanks

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Old 12-30-2006, 04:43 PM
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You need to pick out a specific piston and give us the model number of the piston. Need to know some details about the piston. Not all flat tops are created equal.
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Old 12-30-2006, 07:59 PM
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They are speed pro hypereutectic pistons

edit

link

Last edited by 70ChvyNova; 12-31-2006 at 04:04 PM.
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Old 12-30-2006, 08:09 PM
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Still makes a difference, probably the biggest is how far down in the hole your piston is....along with thickness of head gasket ect. An example: my 331" sbc has 64 cc heads, .041 compressed head gasket, 4 eyebrow flat tops and is down .010 in the hole. After computing all the numbers it came out at 9.502:1 comp. Its not rocket science but you have to some figurin' if you want a true reading.
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Old 12-30-2006, 08:13 PM
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piston head volume is a key piece to the puzzle. Should be listed with piston info. As well as all the other things mentioned by 327Nut. A lot of variables.
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Old 12-30-2006, 08:26 PM
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thanks guys but I just figured it out w/ an online calculator. I entered in all the correct numbers and came out to be 10.0:1 but I know it isnt perfect.

compression calculator

Last edited by 70ChvyNova; 12-31-2006 at 04:00 PM.
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Old 12-30-2006, 09:07 PM
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Check the compression height, the factory height is 1.560, most of the "rebuilder" pistons are 1.540, they have .020 whacked off. They`ll still say it has the same compression but it don`t. These pistons kill quench, compression and power, I used some in a .040 350 with a real small cam with 64cc chambers, the pathetic mullet had 135 psi on all cylinders.
If your block is standard deck height, from the factory it`s suppose to be around .025 in the hole, if the lower compression height pistons are used, it`ll be around .055 in the hole.
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Old 12-30-2006, 11:04 PM
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The Speed Pro 345's used to have the 1.54" height, according to a 1998-or-so catalog, but recently their website was claiming 1.56". Anybody got a ruler?
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Old 12-31-2006, 02:52 PM
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does anybody know what the piston deck clearance would be w/these pistons?

Last edited by 70ChvyNova; 12-31-2006 at 04:14 PM.
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Old 12-31-2006, 03:29 PM
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70, you can't just show people a picture of a piston and expect people to help "estimate" your compression ratio. Many of the pictures Summit uses have a caption that says something to this effect, "Image is a representation. Actual part may vary". Now after saying that I went and found the actual info for the piston I'm assuming you were talking about based solely on the picture you showed us and matched it with the picture Summit shows us. Is this the piston you're talking about?

Does your block have its original deck height or has it been milled? Which head gasket are you using?

Assuming a standard deck height and a basic stock replacement Fel-Pro 4.125" bore and .039" thick head gasket I came up with approx 9.78:1. Of course using something other than a 5.7" rod will change it too. You have to be at least somewhat technical otherwise you may end up running 92 octane gas on 8.5:1 compression or, even worse, running 10.5:1 or 11:1 on 87 octane gas. Knowing your compression also helps you determine what camshaft options you have. You could assume you have 10.5:1 compression and run out and buy a large cam and wind up with a poorly performing engine because you really have 8.5:1 compression. You wouldn't want that now would you?
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Old 12-31-2006, 03:56 PM
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sorry about that picture, I meant to post that web page
and yes its standard deck height and stock replacement gasket

heres the calculator I used came out to be 9.99:1

Last edited by 70ChvyNova; 12-31-2006 at 04:07 PM.
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Old 12-31-2006, 04:10 PM
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You are looking at around 9.3-9.5:1 CR. That's pretty close with a .040 gasket,uncut deck and H345P pistons,NOT the 345 cast pistons.
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Old 12-31-2006, 08:52 PM
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Quote:
Originally Posted by 70ChvyNova

heres the calculator I used came out to be 9.99:1
What numbers are you using? That's the same one I used originally and I still got 9.78:1
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Old 12-31-2006, 09:24 PM
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I used:
1 for inches
bore size=4.030"
stroke=3.48"
gasket=4.125"
compressed gasket thickness=.039"
combustion chamber=64cc
piston dome volume=-5cc
I just left the piston deck clearance .016 b/c Im not sure what it should be.
Then I tried .025 clearance and got 9.78:1

Last edited by 70ChvyNova; 12-31-2006 at 09:36 PM.
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Old 12-31-2006, 09:38 PM
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Quote:
Originally Posted by Blazin72
70,


Of course using something other than a 5.7" rod will change it too.
Rod length does NOT affect static compression ratio,

which is the ratio of the volume with the piston all the way down compared to the piston all the way up.
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