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Discussion Starter #1
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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Some Punk Kid
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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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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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Some Punk Kid
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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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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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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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You got a leaky spark tube...
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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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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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Discussion Starter #14 (Edited)
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
 

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Save a horse, Ride a Cowboy.
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Blazin72 said:
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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Discussion Starter #16
If you changed the rod length then you would have to have your pin in your piston set different right? or something like that
 

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xntrik said:
Rod length does NOT affect static compression ratio,
You're saying that if he went from a 5.7" rod to, say, a 5.565" rod he's not going to have any change in compression? If he uses the same piston with a shorter rod it's going to send his compression through the floor.

If he uses those pistons with a 5.7" rod he's going put the piston .025" in the hole. If he uses the same pistons with a 5.565" rod he's going to put the piston .16" in the hole. According to the same calculator that 70ChvyNova used that's 9.78:1 with the 5.7" rod and 7.55:1 with a 5.565" rod.

I backed this up by doing it longhand and came up with 9.79:1 and 7.55:1 when changing nothing but rod length. If I'm missing something here then please explain.

70ChvyNova, the standard block deck height on a SB Chevy is 9.025". To determine the height of the piston in the block you add half of the stroke (distance from the crank centerline to the centerline of the rod journal), the rod length and the piston compression height which in this case is 1.56". Or 1.74"+5.7"+1.56"=9.00". This leaves you with .025".

70ChvyNova said:
If you changed the rod length then you would have to have your pin in your piston set different right?
Yes, if you were to use a shorter rod you would have to use a piston with a higher compression height. If you used a longer rod you would need one with a shorter compression height. Or like you said, a piston with the pin higher or lower on the piston.
 

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Discussion Starter #18
Thanx alot Blazin72 for all the info. So its safe to say my compression ratio is 9.79:1? (rounded to the nearest hundredth)
 

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Back in the day before the aftermarket started producing affordable stroker cranks and 400sb cranks were plentiful, the way to build 383's was to get a 3.75 stroke 400sb crank and have the main journals turned down .200 from 2.650 to 2.450. Doing this allowed the 400 crank to fit the 350 block. Most everyone used the 5.565 length 400 rods and off the shelf 350 pistons. This works because half of the increase in stroke is exactly the difference between 5.565 rods and the 5.7 rods.

I think what xntrik was trying to say was...wether you use a -6cc flat top piston with a 1.56 comp height for a 5.7 rod or a -6cc flat top piston with a 1.26 comp height for a 6 inch rod the CR will be the same.
 

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GMC boogie said:
I think what xntrik was trying to say was...wether you use a -6cc flat top piston with a 1.56 comp height for a 5.7 rod or a -6cc flat top piston with a 1.26 comp height for a 6 inch rod the CR will be the same.
That I agree with. If this is in fact what he meant then he should clarify that. Just going out and saying it won't change anything can be misleading.
 
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