Compression Question.
Thread Starter
Burning Brakes
Joined: Feb 2004
Posts: 1,069
Likes: 8
From: Sanford NC
Compression Question.
I bought an 05 LS6 (CTS-V) for my 02 Z06. I am running 59cc AFR's but have not picked a head gasket yet. It is between the stock MLS .054 gasket and the Cometic .040's. Does any one know what my compression ratio will be with both. I am guessing 11.2 with the .054 and 11.5 with the .040. I still have to check piston to valve clearace as well. I am running a comp XER 228/232 .588/.595 114+4. Would 11.5:1 give me too much cranking compression for pump gas? Thanks.
Race Director
Joined: Dec 2004
Posts: 13,760
Likes: 75
From: Haymarket VA
I believe the MLS compresses to .052. Using that gasket I come up with 11.43:1 SCR and 8.81 DCR. Using a Cometic .040 gasket it is 11.8:1 SCR and 9.09 DCR. The DCR is much more important than the SCR.
9.09:1 DCR is awfully high for 93 octane. I'd go with the GM MLS and a very good SD tune. Even that is going to be high if you have to run 91 octane.
9.09:1 DCR is awfully high for 93 octane. I'd go with the GM MLS and a very good SD tune. Even that is going to be high if you have to run 91 octane.
Thread Starter
Burning Brakes
Joined: Feb 2004
Posts: 1,069
Likes: 8
From: Sanford NC
Thanks for the info. I have 93 here where I am. I am trying to keep the cranking compression to under 200 psi. I ran the stock compression in the first motor with a 224/228 cam and had 205-210 cranking compression putting 10psi of boost to it. I don't know how that lasted. I wanted to run the MLS any way. It is cheaper and will give me more P to V clearance.
Burning Brakes
Joined: Nov 2006
Posts: 1,071
Likes: 0
From: Goose Creek SC
I bought an 05 LS6 (CTS-V) for my 02 Z06. I am running 59cc AFR's but have not picked a head gasket yet. It is between the stock MLS .054 gasket and the Cometic .040's. Does any one know what my compression ratio will be with both. I am guessing 11.2 with the .054 and 11.5 with the .040. I still have to check piston to valve clearace as well. I am running a comp XER 228/232 .588/.595 114+4. Would 11.5:1 give me too much cranking compression for pump gas? Thanks.
The .054 gasket gives you a SCR of 11.36:1 and a DCR of 8.76:1
The .040 gasket gives you a SCR of 11.79:1 and a DCR of 9.08:1
Based on those calculations, the .054 gasket is what you want to use...
In reality, comp grinds big and the seat the seat duration of the cam will be slightly larger than advertised, therefore I expect real DCR to be about 8.60:1 with the .054 gasket and should be safe with pump gas....Plus, that cam may be on the edge of P/V issues with a 59cc head and .040 gaskets...so use the .054"
Hammer
Race Director
Joined: Sep 2005
Posts: 11,060
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From: Pasco WA
I've read, and been quoted from GM, that their MLS gasket is .060 compressed, not .054. On my application, with 61cc chambers, I figured 10.95-1 SCR, and 8.5 DCR with my 228/232-.600 @112+5 cam. It runs fine on 92 octane with no KR. You should have no problems with PTV on your cam.
Safety Car
Joined: Sep 2003
Posts: 3,558
Likes: 2
From: Cape Coral FL
The big question here is quench height. How far out of the hole is the piston? I've found you can't just assume that it's going to be .008", or .004". When I had my heads off I measured and found them to be .004" out. The .004" will change your DCR by ~0.15.1. Not much but you are above the DCR range for 93 octane. Putting a .060" gasket in to lower the DCR isn't going to help you much either because it will trash your tight quench, which is what helps knock. 59cc heads run fine with G5X3 cams because of the duration and overlap, it brings the DCR into range. Your current cam is too small for those heads. Of course you can stock up on Torco, or be prepared to pull a lot of timing. Here is a thought... You said your cam is degreed at +4, if you retard it to +2 your DCR will be in range with the .040" gasket and you can take advantage of those big AFR quench pads.
Le Mans Master
Joined: Nov 2003
Posts: 8,003
Likes: 249
From: Tampa Fl
St. Jude Donor '12-'13-'14
When I was figuring my numbers, I used the GM stock gasket and figured my quench was .051. I saw numbers from .051, .054, and .060, and my AFR 205s milled .020 off. I dont have my numbers in front of me, but I think I figured my compression would be about 10.85:1. I wanted to be under 11:1 just in case I moved to a 91 octaine state.
I figured mine for the .051 so either Id be on target. But if the gasket ended up being .054 or .060, then I had a little loss of power and room for error.
If you figured it .054 or .060 and it was actually .051, You'll have smilie faces on your pistons ( more likely with the .060).
My choice was to have a couple less HP and some room for error. My numbers turn out to be 5 RWHP more than I estimated during the build and that was with a bonus when I added 3:90 gears on the dyno. I was shooting for 420 RWHP and I hit 425. Another dyno with really the exact came set up as me except his AFR heads were milled slightly more (59cc), ended up a couple HP less that the 420 RWHP mark.
Just some food for thought
ARK and WKMCD know more about motors than me.
I figured mine for the .051 so either Id be on target. But if the gasket ended up being .054 or .060, then I had a little loss of power and room for error.
If you figured it .054 or .060 and it was actually .051, You'll have smilie faces on your pistons ( more likely with the .060).
My choice was to have a couple less HP and some room for error. My numbers turn out to be 5 RWHP more than I estimated during the build and that was with a bonus when I added 3:90 gears on the dyno. I was shooting for 420 RWHP and I hit 425. Another dyno with really the exact came set up as me except his AFR heads were milled slightly more (59cc), ended up a couple HP less that the 420 RWHP mark.
Just some food for thought
ARK and WKMCD know more about motors than me.
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Thread Starter
Burning Brakes
Joined: Feb 2004
Posts: 1,069
Likes: 8
From: Sanford NC
Thanks for the info guys. If I retard the cam two degrees will I be loosing power? I haven't installed the cam yet. It has the advance ground into it. That would put it on a 108 centerline instead of the 110 perscribed by comp correct? So who sells a gasket with a .052-4 compressed width? I talked with Scoggin Dickey today and he said the same thing on the .060 for the GM gasket. I want to run about 11.2-4 compression. Is dynamic compression the "NEW" cranking compression? It used to be all about cranking compression when it came to octane tolerance.
Brett
Brett
Last edited by PRAY; Feb 16, 2007 at 09:02 PM.