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Old 07-05-2005, 07:55 PM
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383 Chevy Compression Ratio?

I plan on purchasing Speed Pro Forged Pistons for my 383 Chevy small block. The connecting rods are 5.7 Eagle forged steel and 4340 Eagle forged crankshaft. The heads are Dart Iron Eagle's Cast Iron with 200 cc runners and 72 combustion chambers. Speed Pro says that with connecting rods, heads and overbore I should be producing a 9.88:1 compression ratio. Is this compression ratio good for the street without using any special gas? Thanx in advance.

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Old 07-05-2005, 08:31 PM
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I have about the same.I have no problems but run prumium.
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Old 07-05-2005, 10:36 PM
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If those are flattop pistons, I think your static compression will be less. I've got flattops in my 383, with 64cc heads, and it was rated at 9.7:1.
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Old 07-05-2005, 10:41 PM
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You can't go by the manufacturer's specs. You have to do the math yourself if you want to know your compression. The manufacturer could be off by a point or two.
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Old 07-06-2005, 01:11 AM
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Quote:
Originally Posted by MI2600
If those are flattop pistons, I think your static compression will be less. I've got flattops in my 383, with 64cc heads, and it was rated at 9.7:1.
MI2600, that seems really low for a 383 with flattops and 64 cc heads. When I calculate your engine I get 10.79. That's using bore 4.030, stroke 3.75, head chamber 64, piston dome -4 (for valve reliefs), gasket thickness 0.058 (0.038 gasket plus estimated 0.020 deck height). With zero deck height it calculates to 11.33

For skunk, I calculate it to be 10.34 with zero deck height, 9.90 with 0.020 deck height.

There are plenty of online compression calculators. I just google it whenever I need one. I don't know which one is the best (most are the exact same). Check this one out:
compression calculator

Don't forget to include dished piston (or valve relief) as a negative value. And also consider the deck height; it makes a big difference in calculated static compression.

I recently measured an engine I'm rebuilding to have 0.027 deck height. I had it cut down to 0.002. I'm building a 385 similar to skunk's (72cc heads). I've calculated my compression to be 10.34 and I'm not worried about running pump gas. We have 94 octane around here so it won't be an issue.
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Old 07-06-2005, 08:07 AM
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ratio

Quote:
Originally Posted by Skunk
I plan on purchasing Speed Pro Forged Pistons for my 383 Chevy small block. The connecting rods are 5.7 Eagle forged steel and 4340 Eagle forged crankshaft. The heads are Dart Iron Eagle's Cast Iron with 200 cc runners and 72 combustion chambers. Speed Pro says that with connecting rods, heads and overbore I should be producing a 9.88:1 compression ratio. Is this compression ratio good for the street without using any special gas? Thanx in advance.

With a 9cc gasket
valve reliefs = 4 cc's
.020 deck = 4cc's
and a 72 cc chamber head

it will be 9.8:1 static compression.

Ok to run on pump gas but i would use 93 octane in it for sure.....

keith
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