Compression
Race Director
Joined: Nov 2003
Posts: 10,696
Likes: 3,094
From: Cottonwood AZ
C1 of Year Finalist (performance mods) 2019
Trick question/answer.
The official factory rating is 11.0 however like all of the big three of the era the pistons were usually “down in the hole” from the factory and the true compression varied but in general for the L79 they were more like 10.0 to 1
Last edited by 68hemi; May 30, 2021 at 06:05 PM.
Le Mans Master
Joined: Jan 2007
Posts: 6,005
Likes: 1,510
From: Prescott Arizona
Joe - in addition to what 68Hemi said about the true compression of Showroom Stock L79 truly really was, if by chance your L79 has ever been rebuilt and they used composition gaskets which were a lot thicker than the thin flat steel gaskets, the compression is probably much closer to mid 9's or a bit more, depending on whether they stayed with domed pistons or not.
Mike T - Prescott AZ
Mike T - Prescott AZ
Race Director
Joined: Apr 2005
Posts: 11,293
Likes: 2,149
From: Bowling Green, Kentucky
He think wants cranking compression @ xxx psig. Service Manual says 150 psig, with max 20 psi variation between cylinders. Throttle wide open and engine warm (if possible).
Larry
Larry
Thread Starter
Drifting
Joined: Apr 2004
Posts: 1,717
Likes: 3,679
From: North Central Wisconsin & Shitcago
My engine has been rebuilt at least twice before. My engine is suffering stuck exhaust spring. The fellow fixing it said my compression is 9:1. I was wondering what it should be and what getting it to where it should be at would entail and cost. Also, does it make a big difference??
I like power, but certainly don't race and never will.
I like power, but certainly don't race and never will.
Last edited by Joemac8; May 30, 2021 at 07:53 PM.
Le Mans Master
Joined: Feb 2008
Posts: 6,536
Likes: 1,947
From: Folsom CA
Compression makes power and adds efficiency for sure and you can hear it in the exhaust. Too much will cause many problems, detonation and pinging so you want as much as possible without causing detonation or pinging when properly tuned and running 91 octane pump gas. I think most on here would agree the L79 is rated at 11:1 compression but in reality has around 10.5:1 compression. I have not checked but suspect the L79 would have around 180 PSI per cylinder in blue print form while the 300 HP and 250 HP versions would be around 150 PSI cranking compression. I am curious to learn what others know on this subject. Most rebuilders replace the factory gasket with a much thinker .039 thick head gasket which lowers compression a little.
Melting Slicks
Joined: Sep 2005
Posts: 2,727
Likes: 907
From: Warminster PA
How did he determine the compression? Did he measure the cylinder head chamber volume? You need to know the cyl head chamber volume - not by published spec but by actual measurement. Especially id the engine has been rebuilt a couple times. Someone may have opened up the chamber while doing some porting work which directly umpacts the chamber volume ans as such the CR. Once you have that info you can then determine the required piston (domed or flat) and head gasket thickness. If you know what pistons are in there then you can calculate using chamber volume, gasket thickness, deck clearance and piston volume using an on line calculator.
Although I do seem to recall a way that the tech inspector checked CR at the track using a "whistler" device? No experience with that so I can't comment on accuracy. Maybe the guy fixing your engine is using a device like that.......?
Tom
Although I do seem to recall a way that the tech inspector checked CR at the track using a "whistler" device? No experience with that so I can't comment on accuracy. Maybe the guy fixing your engine is using a device like that.......?
Tom
Last edited by thoyer; May 30, 2021 at 08:16 PM.
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,271
The primary variable was deck height. As the broach tools wore the decks got higher than the nominal 9.025" (small blocks). I've seen them up to .015" high and .010" difference side to side. Based on my measurements and data I've received from others who accurately measured all the relevant dimensions and volumes, the average CR of an advertised 11:1 engine was closer to 10.5 than 11, but no where near as low at 10 from the engine plants.
To the OP: search for threads STARTED by me and download the compression ratio article so you can learn how to manage it to a target range that you select. You may also find some other threads I started that are of interest when rebuilding an engine.
Duke
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