350 chevy compression estimate please !!! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-17-2006, 10:20 PM
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350 chevy compression estimate please !!!

I recently had my chevy 350 rebuilt ,and im getting conflicting estimates on its actual compression ratio,i'm wanting to put a bigger cam and possibly aftermarket heads on it, but im unsure of its compression at this point. i tried the conversion tables on the kieth black site, but i had to be doing something wrong anyway, its a 350 bored 40 over with 75cc 993 heads .the block was not decked and it has flat top pistons that say compression would be 10.2:1 with 64cc heads but i have 75cc most quotes i see are for 76cc heads the pistons are speedpro 345np40 cast pistons also the headgaskets came in a basic rebuild kit from r o l ,i think.thanks in advance
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Old 08-17-2006, 11:00 PM
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Your a shade over 9:1.
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Old 08-17-2006, 11:12 PM
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thanks thats what the machine shop said but others told me as low as 8.3
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Old 08-18-2006, 12:34 AM
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i downloaded a virtual engine calculator, that helps with anything like this, so just google it
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Old 08-18-2006, 02:18 PM
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I'm thinking 8.7:1 cr with 76cc heads.

Cylinder volume = 730cc
piston dish volume = 6cc (4 relief, spec'd in summit catalog)
head gasket volume = 9cc
piston below deck volume 0.015 = 4cc
Cylinder head volume = 76cc

(730 + 6 + 9 + 4 + 76) / (6 + 9 + 4 +76) = (730 + 95)/95 = 8.68

And 9.8:1 cr with 64cc heads

(730 + 6 + 9 + 4 + 64) / (6 + 9 + 4 +64) = (730 + 83)/83 = 9.795


Summit says 9.72:1 with 64cc with a +0.030 bore. A +0.040 bore will increase the compression a little over a +0.030 bore.
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Old 08-18-2006, 03:27 PM
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But the heads are 75cc not 76cc heads ,and the pistons i have are advertised in jegs as speed pro cast pistons 345np40 ,not the hypers and it says in the desription 10.2:1 with 64 cc heads thats why i bought these instead of the hypers because the were advertised with slightly more compression. im hoping to have atleast a 9.1 motor because i want to upgrade to a bigger cam it has a lunati 00016 now with 218 dur 458 lift i was wanting to go with a magnum 270 or 280 by comp cams and eventually some aftermarket heads but for right now just a cam change.also the piston are supposed to be 5cc valve reliefs
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Old 08-18-2006, 03:53 PM
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Ok, 5cc piston reliefs and 75cc heads.

Now I'm thinking 8.85:1 cr with 75cc heads.

Cylinder volume = 730cc
piston dish volume = 5cc (flat top w/4 relief, spec'd in Jegs catalog)
head gasket volume = 9cc
piston below deck volume 0.020 = 4cc
Cylinder head volume = 75cc

(730 + 5 + 9 + 4 + 75) / (5 + 9 + 4 + 75) = (730 + 93)/93 = 8.85

And 10.0:1 cr with 64cc heads

(730 + 5 + 9 + 4 + 64) / (5 + 9 + 4 +64) = (730 + 81)/81 = 10.0

You could get 10.2:1 cr with 64cc heads if the piston was only 0.010" under the deck. The amount the piston is under the deck makes a pretty big difference in compression ratio. For example, if your block was zero decked, the compression would be 9.2:1 with the 75cc heads and 10.5:1 with the 64's. 0.020" below the deck is about the norm.
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Old 08-18-2006, 08:47 PM
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Thanks to all that replied, If in fact this is a 8.85 :1 motor is that enough compression for either of those cams, i thinking about putting some World Product heads on it to raise the compression a bit, for a bigger cam and added performance,But if what i have know will work, ill use that money for my posi and gears.THANKS
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Old 08-19-2006, 09:09 AM
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I like this one.
http://www.csgnetwork.com/compcalc.html
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Old 08-21-2006, 07:08 AM
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Should be able to run a 270H with 8.9:1 cr. Yes, spending money on a posi will be a good idea. A bunch of hp doesn't mean much if you can't stick it down on the street.
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