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11/17/15 4:21am - Original Message: 'Raising compression.'
KL80C3
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Oakland Gardens, NY - USA

Vette(s):
1980 Corvette L48 manual transmission.Dark blue with oyster interior.

Joined: 1/10/2015
Posts: 8
I have a zz4 350 that recieved from a friend that is going to go in my vette. The engine doesn't have the original cylinder heads, they are Edelbrock performer 6089. The original heads had 58cc combustion chambers the sbc 10:01compression while the edelbrocks has 64cc giving the sbc 9 to 9.5 compression I believe. What is the best way to raise compression, mill the heads or change the pistons? Milling would be easier and less expensive! To change pistons requires machine work, boring an honing which at this point might as well stoke it. Any would be much appreciated. Thanks.
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11/17/15 8:28pm - Reply: 'Re: Raising compression.'
Sarge81 Lifetime Member
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WEST SENECA, NY - USA

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1981 Blue Ice Met. w/pearl ghost flames TKO500 5speed. LS1 Swap 3.45 Dana 44.

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Have the heads shaved down a little. Cheapest and easiest route. You will probably have to have the intake manifold shaved down to accommodate the angle change depending how much material is shaved off the heads.

Another thing I was thinking about. If you can find out what head gaskets are on the engine now, see what the compressed thickness specs are on them. I believe ZZ4 gaskets are .051" thickness. I know Felpro make a gasket that has a compressed thickness of .041" which will bump up the compression slightly. 


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11/17/15 11:30pm - Reply: 'Re: Raising compression.'
KL80C3
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Oakland Gardens, NY - USA

Vette(s):
1980 Corvette L48 manual transmission.Dark blue with oyster interior.

Joined: 1/10/2015
Posts: 8
I will have to look up what type of head gasket is used on a zz4, but the head gasket that I am going to use is a GM performance gasket that has a compression thickness of .028. I don't remember the part # off hand but it is in there performance catalog. I guess this will raise the compression by .5 I think.
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11/18/15 12:40pm - Reply: 'Re: Raising compression.'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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KL80C3 said: To change pistons requires machine work, boring an honing

Milling the heads also requires machine work...Tongue

The only machine work needed to swap pistons is pressing the pins out and back in. Honing would be optional, depending on how the cylinder walls look. The only reason you would need to bore the block would be if you went with an oversized piston, or the cylinder walls were severely worn...in which case raising the CR would be counterproductive. Wink
Not sure simple gasket change would change the CR by .5, unless the original gaskets were really thick. By milling the heads, and using a quality, thinner gasket, you might get a .5 increase....depends a lot on how much is cut off the heads. You can only go so far before you have issues with the intake sealing properly, as Darryl mentions. The more the heads are cut, the farther away the intake ports are(wider) across the intake. This requires a thicker intake gasket, and milling of the bottom of the intake at the front and rear, where the end seal is. Get into angle milling, and you've opened a whole new can-o-werms.

Personally, I'd go with a piston swap....you can get a set of pistons that are designed to work with the chambers and valve sizes of the new heads, without all of the machine work involved in cutting the heads/intake. But that's just me. Big smile


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11/18/15 9:44pm - Reply: 'Re: Raising compression.'
KL80C3
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Oakland Gardens, NY - USA

Vette(s):
1980 Corvette L48 manual transmission.Dark blue with oyster interior.

Joined: 1/10/2015
Posts: 8
 Thanks for the info food for thought! I would have to measure the bore to see if in spec . If in spec I can replace the pistons. If not then full rebuild. If the later then I would go with a stroker kit because I think building a 355ci and a 383ci are close in price! with the 383 making more power. Am I correct thinking that way? I like to get  like 425 hp or more!

|UPDATED|11/18/2015 7:44:00 PM (AZT)|/UPDATED|
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11/20/15 2:27pm - Reply: 'Re: Raising compression.'
nosal1 Gold Member
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Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.

Joined: 3/21/2006
Posts: 238

Good afternoon, You have an abundance of suggestion regarding trying to increase compression of you ZZ4 engine, all of which are good to very good,however if I may make a suggestion, and that is look into using steel gaskets and copper spray normally used to help seal the contact areas of the heads to block.

These steel head gasket are .017-.019 thickness that will create a higher compression. I have had steel gaskets on my engine for over 20 years and no problem.
 Less work,expense,problems,and less expensive.
          Have a great day and good luck.
            Sal C               nosal1


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11/21/15 9:23am - Reply: 'Re: Raising compression.'
nosal1 Gold Member
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Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.

Joined: 3/21/2006
Posts: 238

Brand:Fel-Pro

Manufacturer's Part Number:7733SH1

Part Type:Head Gaskets

Product Line:Fel-Pro Head Gaskets

Summit Racing Part Number:FEL-7733SH1

Bore (in):4.100 in.

Bore (mm):104.140mm

Gasket Material:Shim steel

Compressed Thickness (in):0.015 in.

Compressed Volume (cc):3.245cc

Lock Wire:No

Quantity:Sold individually.

In-Store Pickup:Choose In-store pick-up (OH, GA, NV) on our web site.

CalculatorCompression Calculator



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