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Discussion Starter · #1 ·
assuming these hump heads are stock heads (64 c.c.)...stock flat top with valve relief pistons... what kind of compression would these make ? ( providing rings etc. are good to go). Also...how much decking would be required to boost it to 9.5 / 10. to 1? ..considering using a voodoo cam..
 

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I was thinking with 64 cc you would be at 9:5-1 with what you spec'd already. If you have a piston part number it would help find a spec chart online. Google it. Lots of info on small Chevy.
Mario
 

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The shop that built my 355 SBC told me my setup with 461 64cc heads is about 10.25 compression. Mine are flat top 4 relief pistons, but may not be the same as yours. Some sit at different height in relationship to the head surface. So would need to measure from TDC to head surface to see how they sit in the bores.
 

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Discussion Starter · #4 ·
thanks guys...have to get out the dial indicator to see where its at...i have the "186" heads (don't know if that makes any difference)
 

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The heads wont make a difference in compression. A 64cc head will be the same as any other 64cc head for compression factor.
 

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Are we talking a 350 or 327 or ??? A 350 with stock 4 valve flat tops, .025 in the hole with shim head gaskets was rated at 10.25:1 by GM. I believe the flat top 327 was also rated at 10.25:1 but it`s not quite that high. If you can give me the bore, stroke, deck height, piston valve relief cc, head gasket bore and thickness and headcc I can calculate it for you. With around 10:1 watch your cam timing, too short of duration will create too much cylinder pressure for pump gas.
 

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Discussion Starter · #7 ·
vortech heads

decided to run some earlier vortech heads...already set up for .550 lift...still have the flat tops..( will get tdc in the a.m.)...but curious to the experience here as to what gasket thickness to run ( to help decrease compression, as I have read this combo may be a bit excessive for pump gas). I have read that with the right gaskets, valve overlap and ignition timing, I could run on pump WITHOUT dishing or replacing pistons. Any and all feedback welcome...thanks
 

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My .030" over 350 is running old camel hump 64cc heads, with flat top pistons. My compression ratio is 10.25:1 and I can run on pump gas, but has to be premium, and I have to watch the initial timing setting. I think .041" is the thickest you can buy, and I'm running .039" on mine.
 

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My .030" over 350 is running old camel hump 64cc heads, with flat top pistons. My compression ratio is 10.25:1 and I can run on pump gas, but has to be premium, and I have to watch the initial timing setting. I think .041" is the thickest you can buy, and I'm running .039" on mine.
Without knowing what size engine the OP has I cant help but I will tell you this, unless you actually measured your compression you may not even be close to 10.25:1. I know the manufacturer claims this, at least TRW/Speed-Pro use to for a 4 valve relief flat top 350 piston. The fact is it`s based on a .025 deck height and a .015-.020 shim head gasket.

To measure your actual compression you need to know what the bore and stroke is and then you need to know your deck height. This is the distance that the piston sits in the hole at TDC. Then you need to know your head gasket thickness and the head gasket bore size. You also need to know the type of piston, in this case a 4 valve relief flat top but you need to know what cc`s those valve reliefs make up. Then you need to know your combustion chamber size. Most oem heads that are spec`ed at 64cc`s usually are off some.

Most people who rebuild a flat top 350 will shave the deck a little to square it up. Then they usually use a rebuilder type head gasket in .039 or .041 thickness. There are thicker and thinner gaskets but not in "rebuilder" type. So lets say you take .005 off the deck and then use a .039 head gasket. you will have lower compression than the oem spec`ed .025 deck with .015 shim head gasket. so your "10.25:1" pistons could be closer to 9.75:1 at best. You need to measure to verify.

Once you figure that out and know what your static compression is you can work on picking a camshaft that will work for your application. The newer camshaft grinds have less advertised duration than they did back in the day and that will build cylinder pressure. You want to keep your dynamic compression under about 8.2:1 with iron heads to run on pump gas. The advertized numbers, opening and closing numbers and lobe sepreation and the amount the camshaft is advanced or retarded will effect the DCR (dynamic compression ratio).

Here is a excellent article on compression,

http://cochise.uia.net/pkelley2/DynamicCR.html
 

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decided to run some earlier vortech heads...already set up for .550 lift...still have the flat tops..( will get tdc in the a.m.)...but curious to the experience here as to what gasket thickness to run ( to help decrease compression, as I have read this combo may be a bit excessive for pump gas). I have read that with the right gaskets, valve overlap and ignition timing, I could run on pump WITHOUT dishing or replacing pistons. Any and all feedback welcome...thanks
Vortec heads have a very efficient combustion chamber so they need less ignition timing than say your camel hump heads. If you set your quench correctly and pick your cam accordingly you shouldnt have any issues with pump gas on a flat top 327 or flat top 350. The quench is the deck height and gasket thickness. OEM was .025 deck height, piston in the hole .025 at TDC, and a .015 metal shim head gasket for a .040 quench. A lower compression engine with excessive quench can ping more than a higher compression engine with a tighter quench. You want the quench between .035-.045 max. You will need to measure your deck height before picking your head gaskets.
 

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I am not savvy enough to calculate compression, so when I had my engine built, I left it to the experts to tell me. Larsen and King has been building hi performance engines for nearly 50 years, so I have to accept what Ted tells me, when they calculated my engine's compression based on their measurements, of the block, and my heads. I had them build up the short block, and degree the cam at the same time. I only had to bolt the heads on using the head gaskets they provided to me. My heads were all cc'd, and chambers matched. They were fairly new on my previous engine, so I only needed them looked over, and not rebuilt.
 
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