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Hey guys just as the title says I’m building my 6.0 with summit pistons and have bought 2 dif gasket sets.. the basic ls9 .051 ones will put me at 9.86... and I bought some mls .027 ones..the thinnest I could find..would put me at 10.44. All on E85.. 7875 gen2 turbo..any advice or opinions are appreciated!
The block has not been decked nor have the heads been milled.. my block measured out at a true 9.240 so using the .027 gaskets Versus the .051 ones would be like milling the heads .024 which is safe
Last edited by chinaoreo; Aug 1, 2019 at 04:54 AM.
Reason: Add text
The compression difference will hardly be noticeable when not into boost and virtually makes no difference at all under boost. I would go with the gasket that stands the best chance of holding under boost.
I'd go with the .027" MLS gaskets for 10.44 SCR. Give Cometic a call. They can usually make MLS gaskets in just about any thickness you want.
While you probably will see all of a 1.5-2.0% difference in power in boost, the tighter quench will also improve responsiveness when out of boost. I try to keep quench at .035-.045" and no more than .060".
Compression will hardly make a difference but a really thin gasket will have a much harder time staying sealed under boost when compared to the LS9 gasket. LS9 gasket, no question.
The compression difference will hardly be noticeable when not into boost and virtually makes no difference at all under boost. I would go with the gasket that stands the best chance of holding under boost.
I picked up 300 RPM spool and 40 ft-lbs from 1 point bump in compression switching from dished pistons to flat tops, on 93 octane..
So I just talked to Cometic and they said that the spring load on the 2 diff size gaskets will be the same.. and with a boosted application I could go down to as far as .010 with no sealing problems..so Looks like I’m gonna go with the .027 ones
I picked up 300 RPM spool and 40 ft-lbs from 1 point bump in compression switching from dished pistons to flat tops, on 93 octane..
That sounds good but can you really say with a 100 certainty that the gains are due compression change alone? Different pistons change more than just compression. It changes flow characteristics with the cylinder and flame burn. I'm not saying that's not the reason you saw the increase only that when changing pistons that there are other factors that can be attributed to your results. Hell even running the same engine combination on the same dyno under slightly different conditions can change the end results and net you a gain or loss with no other changes. If this was a max effort deal where you're chance every last ounce of power then yeah go for the highest compression that works with the fuel and boost capabilities your turbo can make and the engine can handle.
But in a street engine I would choose the better of the two gaskets first if there is a difference. A little more boost and/or being a little more aggressive with the tune can make up for a good part of the power loss attributed to the loss of a 1/2 point of compression.
Here’s something to munch on.. so I decided I would check my quench/squish tonight.. so I put a .051 ls9 gasket on the block..put some nice thick peices of solder on top of the pistons.. set the head on and torqued it down..turned the engine over.. took the head of and mic’ed the solder.. .085 was my quench..! Now the heads have not been milled or the block decked.. and my pistons are .005 below deck.. now the 1 variable over factory is that I did put a set of summit LS pro pistons in it.. they are flat top +2 because they have valve reliefs..and although I haven’t checked it with a factory piston in the Block it should by all rights be entirely the same height as the factory one.. which brings me to believe that the quench from GM when they built it was .085..!
Last edited by chinaoreo; Aug 3, 2019 at 12:30 AM.
Reason: Fix
That sounds good but can you really say with a 100 certainty that the gains are due compression change alone? Different pistons change more than just compression. It changes flow characteristics with the cylinder and flame burn. I'm not saying that's not the reason you saw the increase only that when changing pistons that there are other factors that can be attributed to your results. Hell even running the same engine combination on the same dyno under slightly different conditions can change the end results and net you a gain or loss with no other changes. If this was a max effort deal where you're chance every last ounce of power then yeah go for the highest compression that works with the fuel and boost capabilities your turbo can make and the engine can handle.
But in a street engine I would choose the better of the two gaskets first if there is a difference. A little more boost and/or being a little more aggressive with the tune can make up for a good part of the power loss attributed to the loss of a 1/2 point of compression.
I once got into an online heated argument with an automotive magazine editor (who should have known better) who swore up and down that you would see a hp increase with a rear gear change and that he had experienced it first hand.
That sounds good but can you really say with a 100 certainty that the gains are due compression change alone? Different pistons change more than just compression. It changes flow characteristics with the cylinder and flame burn. I'm not saying that's not the reason you saw the increase only that when changing pistons that there are other factors that can be attributed to your results. Hell even running the same engine combination on the same dyno under slightly different conditions can change the end results and net you a gain or loss with no other changes. If this was a max effort deal where you're chance every last ounce of power then yeah go for the highest compression that works with the fuel and boost capabilities your turbo can make and the engine can handle.
But in a street engine I would choose the better of the two gaskets first if there is a difference. A little more boost and/or being a little more aggressive with the tune can make up for a good part of the power loss attributed to the loss of a 1/2 point of compression.
Pistons were the only thing that changed. Swapped from gen 3 dished to gen 4 flat tops.
Your argument that a full point increase in compression in a 900 horsepower engine didn't result in my torque gains is extremely confusing.
Originally Posted by gsteele
I once got into an online heated argument with an automotive magazine editor (who should have known better) who swore up and down that you would see a hp increase with a rear gear change and that he had experienced it first hand.
You will see an HP increase with a gear ratio change on an inertia dyno....
Pistons were the only thing that changed. Swapped from gen 3 dished to gen 4 flat tops.
Your argument that a full point increase in compression in a 900 horsepower engine didn't result in my torque gains is extremely confusing.
.
That's not what I said at all, There are many things beside compression that change when you change pistons alone, I would be pissed if I didn't see a gain.
I know you know this already but maybe some don't.
Boost is completely different than NA builds when it comes to compression, With a NA build you have what you have when bolt everything together, Boost essentially allows you to control effective compression ratio. You can have any compression ratio you want up to the point you exceed the capability of the fuel or the mechanicals of the engine itself or the turbo/supercharger ability to make more boost.
Speaking of boost and compression, I have a 8.5:1 compression 408 that made over 900hp with a roots blower @ 15lbs on methanol. Obviously a turbo on my engine wouldn't be responsive on the street but would still make great power when on boost.
I don't recommend going by effective ratio. There's way too many factors at play for any chart of it to be even remotely accurate.
Just using it as an example, It has some validity but not a bible by any means. There are many other variables and now with computers controlling timing and fuel it's possible to get more power than you could have with the same combination pre-computer since you can control fuel and timing in individual cylinders.
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9.86 or 10.44 E85 compression ratio..? 
