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12/10/15 7:48pm - Original Message: 'Determining compression ratio with several know values'
apesa63
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Hillsboro, OR - USA

Vette(s):
1975 L48 4sp. Coupe... Rebuilt Motor

Joined: 7/20/2015
Posts: 44
Greetings, 
Happy holidays to all.... I have an opportunity to buy a set of aluminum SBC heads that are 202/164 with a 64cc chamber volume. I have a 1975 L48 with 8.5:1 compression with the standard 333882 heads with I believe a 76cc chamber volume. I am looking to figure out what the end compression ratio increase would be if I went to the Aluminum heads.

A Little background. I have a bad exhaust valve guide on the number 6 cylinder and have the choice to just replace the 333882's with a refreshed set with 202/164 valves but same chamber volume or go with the aluminum set with 64cc chamber volume. My main concern is the additional compression and its effect on the short block.. rings, bearings etc... 

Any advice is appreciated.
Thanks,
Pat


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Daytona Yellow '75 350 Muncie 4 spd
'73 454 on the engine stand.


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12/10/15 10:26pm - Reply: 'Re: Determining compression ratio with several know values'
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

Joined: 11/8/2003
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The small difference in chamber volume between the two heads is not enough to make a noticeable increase in compression. Definitely will not create any issues with the rings or bottom end. Won't hurt to use them at all, and you'll loose some weight at the same time. Thumbs Up

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12/10/15 11:15pm - Reply: 'Re: Determining compression ratio with several know values'
apesa63
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Hillsboro, OR - USA

Vette(s):
1975 L48 4sp. Coupe... Rebuilt Motor

Joined: 7/20/2015
Posts: 44

I read on a pro touring forum that roughly 1cc equals 1/10 of a ratio, if you make the chamber 10 cc smaller you will end up with an extra whole number of compression ratio which would put me about 9.5 plus. Not sure but it seems about right. I am definitely thinking about it. I took a comp reading on all 8 and they are all at 150 +/- 3 so it could be worth the extra money.


Thanks for the response,
Pat


|UPDATED|12/10/2015 9:15:10 PM (AZT)|/UPDATED|


____________________________________
Daytona Yellow '75 350 Muncie 4 spd
'73 454 on the engine stand.


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12/11/15 1:49am - Reply: 'Re: Determining compression ratio with several know values'
Joanne Gold Member
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Joined: 11/30/2006
Posts: 441
if you go to jegs or summit ,look at heads and they have a guide to compression and head vol.
what are the ali heads you can get??
ok I took a look for you,and that will take you to about 9.5/1
which is a good compression for most cams.
  for the record this is not joanne,but her hubby.





|UPDATED|12/10/2015 11:49:26 PM (AZT)|/UPDATED|


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1979 Corvette
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12/11/15 1:58am - Reply: 'Re: Determining compression ratio with several know values'
daveo76
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Vancouver, WA - USA

Vette(s):
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Joined: 8/25/2005
Posts: 831
I have 64cc vortecs that I installed when I rebuilt and I think I calculated my ratio at 9.4 ish.  

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1976 Silver/Firethorn.  L48, 4spd.  Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
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12/11/15 6:10am - Reply: 'Re: Determining compression ratio with several know values'
Sarge81 Lifetime Member
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WEST SENECA, NY - USA

Vette(s):
1981 Blue Ice Met. w/pearl ghost flames TKO500 5speed. LS1 Swap 3.45 Dana 44.

Joined: 12/3/2001
Posts: 2376
The last small block I had in my vette had 64cc Vortec heads. The CR was about 9.5:1. The CR will vary slightly between castings and whatever thickness of a head gasket you go with. I would go with the aluminum 64cc heads if it was mine. Makes no sense not to take advantage of the benefits of bumping the compression up, losing a few pounds of weight and quicker heat dissipation.  

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12/11/15 11:52am - Reply: 'Re: Determining compression ratio with several know values'
apesa63
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Hillsboro, OR - USA

Vette(s):
1975 L48 4sp. Coupe... Rebuilt Motor

Joined: 7/20/2015
Posts: 44
Thanks for all the replies. I am heading out to pick the Aluminum 64cc's up in a few. Hope to get them in and running tomorrow.

____________________________________
Daytona Yellow '75 350 Muncie 4 spd
'73 454 on the engine stand.


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