Slant Six Forum :: Compression ratio

.015 difference works out to be a .21 of static compression change.
For me, the issue is always about maximizing compression with-out pre-ignition (pinging) for the fuel being used... in all cylinders.

All your tuning work will be based on the first cylinder to detonate. (ping)

Generally, a SL6 with a factory 1 or 2 BBL intake manifold, with compression totally equal in all cylinders, cylinders 2 and / or 5 will usually be the first cylinders to detonate because those are leanest, from a fuel distribution standpoint.
#1 and 6 tend to run the richest and therefore, are more detonation resistant because of manifold shape. ("wet" intake manifold boundary effect)

Following these observations, if I had a .21 compression increase, I would want it on cylinders # 1 & 6.
If I had a .21 compression decrease, it would be nice to have it on #2 & 5.

In reality, this type of compression difference is usually the result of a "front-to back" mis-alignment between the deck and the crank mainline (line bore) so you do not get the compression difference in the cylinder(s) that could benefit from it.

Long story short, it is best to get the compression as even as possible in all the cylinders, to help final engine tuning
Doing so allow for leaner fuel mixtures and more aggressive timing curves then you could run with an engine that has one cylinder detonating way sooner then the others.
DD

Author:	CoyoteDan [ Mon Mar 22, 2021 1:47 pm ]
Post subject:	Re: Compression ratio
Ok thanks that's good to know. I have a question about the HEI though. I saw a good article about retrofitting one on this forum. My distributer uses a pick up coil, I can use that distributer and follow the steps in the article to create my own HIE? I don't need to get a new distributer for that correct? I assume it's a good method over purchasing a system?

Author:	volaredon [ Mon Mar 22, 2021 6:37 pm ]
Post subject:	Re: Compression ratio
Who was it here that was the "resident expert" on distributors?

Author:	CoyoteDan [ Sun Apr 04, 2021 10:23 pm ]
Post subject:	Re: Compression ratio
Ok not sure if this is to late and I should start a new thread. I'll give it a shot and if I get responses I know it worked. I installed my #1 and #6 piston, and measured my piston to deck clearance. #1 measures .177 and #6 measures .161. So I have a Fel pro gasket someone was saying compresses to .035 and a combustion camber volume of 54.5cc (I only measured #1 combustion chamber.) I belive that gets me 8.3 to 1 #1 and 8.5 to 1 #6. So is the differnce between the two nessary to worry about? Is it worth trimming a thousand off and bumping the compression slightly? Is that splitting hairs? Also now that I installed the pistons and rings would it be bad to remove them and reinstall the same rings again? I only turned it over a few times. Thank you

Author:	Joshie225 [ Mon Apr 05, 2021 6:45 am ]
Post subject:	Re: Compression ratio
.016" is a pretty big variation. I would use the #1 piston and rod in both #1 and #6 to measure the deck clearance to see if the block is even front to back. You can take the rings off and put them back on, but use a ring expander. You're close to your target so it's more about deciding what is an acceptable variation.

Author:	Rob Simmons [ Mon Apr 05, 2021 7:02 am ]
Post subject:	Re: Compression ratio
Quote: Who was it here that was the "resident expert" on distributors? That DI feller what posted just b'fore you did...

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Author:	DusterIdiot [ Mon Mar 22, 2021 5:10 pm ]
Post subject:	D
D

Author:	CoyoteDan [ Tue Apr 06, 2021 5:18 pm ]
Post subject:	Re: Compression ratio
So I repeated the test with only the #1 piston in both cyclinders and came up with the same results. .160 #6 and .175 #1. I called the machine shop and they said not to worry about it, variance of .020 all over the block happens. Then he said install all the pistons measure and call back, its probably not a gradient from front to back.

Author:	Joshie225 [ Tue Apr 06, 2021 5:26 pm ]
Post subject:	Re: Compression ratio
I wouldn't accept that much difference in the deck clearance, but it's not my engine. I would have the machine shop measure the main bearing bore to deck dimension on both ends of the block, but once again, that's what I'd do.

Author:	emsvitil [ Tue Apr 06, 2021 6:36 pm ]
Post subject:	Re: Compression ratio
Use the same rod and piston on all the cylinders to see if it's the block or the crank.

Author:	Rick Covalt [ Wed Apr 07, 2021 3:06 am ]
Post subject:	Re: Compression ratio
My block in the race car was out about the same amount (.019 I think) front to back. I'm not sure it is any problem as long as the block is flat. My was straightened when the engine was built.

Author:	Dart270 [ Wed Apr 07, 2021 7:31 am ]
Post subject:	Re: Compression ratio
With your buildup and desires, I would not worry about that much variation. I bet you are OK using rings again since only a few turnovers. If you want to be persnickety, then go ahead and have them angle mill the block to bring them all close. Of course, you do not know its the block or crank unless you measure deck height to crank centerline distance. Many shops can do that right. I have never done it. I have seen piston-deck variations of 0.005-0.010" across the cylinders (not smoothly varying front to back) due to crank stroke variations and have not messed with it. If you want maybe the last 1-3 HP and 0.2-0.5 MPG, then maybe it's worth fixing yours. Lots of other things (carb, intake, ign timing....) can affect things much more than this. Lou

Author:	Doctor Dodge [ Wed Apr 07, 2021 5:23 pm ]
Post subject:	Re: Compression ratio
.015 difference works out to be a .21 of static compression change. For me, the issue is always about maximizing compression with-out pre-ignition (pinging) for the fuel being used... in all cylinders. All your tuning work will be based on the first cylinder to detonate. (ping) Generally, a SL6 with a factory 1 or 2 BBL intake manifold, with compression totally equal in all cylinders, cylinders 2 and / or 5 will usually be the first cylinders to detonate because those are leanest, from a fuel distribution standpoint. #1 and 6 tend to run the richest and therefore, are more detonation resistant because of manifold shape. ("wet" intake manifold boundary effect) Following these observations, if I had a .21 compression increase, I would want it on cylinders # 1 & 6. If I had a .21 compression decrease, it would be nice to have it on #2 & 5. In reality, this type of compression difference is usually the result of a "front-to back" mis-alignment between the deck and the crank mainline (line bore) so you do not get the compression difference in the cylinder(s) that could benefit from it. Long story short, it is best to get the compression as even as possible in all the cylinders, to help final engine tuning Doing so allow for leaner fuel mixtures and more aggressive timing curves then you could run with an engine that has one cylinder detonating way sooner then the others. DD

Author:	Jase [ Thu Apr 08, 2021 5:58 pm ]
Post subject:	Re: Compression ratio
Quote: .015 difference works out to be a .21 of static compression change. For me, the issue is always about maximizing compression with-out pre-ignition (pinging) for the fuel being used... in all cylinders. All your tuning work will be based on the first cylinder to detonate. (ping) Generally, a SL6 with a factory 1 or 2 BBL intake manifold, with compression totally equal in all cylinders, cylinders 2 and / or 5 will usually be the first cylinders to detonate because those are leanest, from a fuel distribution standpoint. #1 and 6 tend to run the richest and therefore, are more detonation resistant because of manifold shape. ("wet" intake manifold boundary effect) Following these observations, if I had a .21 compression increase, I would want it on cylinders # 1 & 6. If I had a .21 compression decrease, it would be nice to have it on #2 & 5. In reality, this type of compression difference is usually the result of a "front-to back" mis-alignment between the deck and the crank mainline (line bore) so you do not get the compression difference in the cylinder(s) that could benefit from it. Long story short, it is best to get the compression as even as possible in all the cylinders, to help final engine tuning Doing so allow for leaner fuel mixtures and more aggressive timing curves then you could run with an engine that has one cylinder detonating way sooner then the others. DD I'm going to be reading this post and thinking for a good while... Can this post go into the engine build matrix? So many followup questions that don't necessarily belong in this thread..

Author:	CoyoteDan [ Thu Apr 08, 2021 10:56 pm ]
Post subject:	Re: Compression ratio
Wow Doctor Dodge chimed in, pretty cool. Now I feel like a chump deciding to accept the variance and put it together as is . I haven't had the time to test all 6. So I'll see. If it is the crank alignment I assume embarking on correcting that is alot more work and another few weeks at the machine shop. I think Lou's perspective is a little more realistic for me. I'm starting to think I am split hairs with a build this mild. Also I'm starting to wonder how many times these bolts can handle being torqued. Thanks PS. If someone could PM me instructions on how to use the building matrix that would be great. I tried to use it getting into this build originally, but I couldn't find it or couldn't figure it out. There is something I'm not understanding.

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