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ENGINE COMPRESSION CHECK- COLD?

Posted By: Labratt

ENGINE COMPRESSION CHECK- COLD? - 10/02/11 01:38 AM

Does anyone know what the difference in P.S.I. would be checking a cold engine...as opposed to one that's warmed up,like it should be? I did a check on my 440 this afternoon,without first warming it up! My average was 185psi. Thanks in advance! Randy
Posted By: Guitar Jones

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 03:20 AM

Quote:

Does anyone know what the difference in P.S.I. would be checking a cold engine...as opposed to one that's warmed up,like it should be? I did a check on my 440 this afternoon,without first warming it up! My average was 185psi. Thanks in advance! Randy



If it's still fresh there won't be much difference. As a general rule I only use a compression check to determine the state of health of a cylinder when compared to the others. Otherwise a leak down test is a much better tool.
Posted By: Sport440

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 03:28 AM

Randy, Dont you have around 10.2 comp and a 260* dur.@ .050 If so that 185 [psi. cant be correct. I think I told you before to check it with a second gauge.
Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:06 AM

Mike..you're correct on both the CR,and duration. I don't remember you telling me to use a different gage! That's my memory! I'll have to borrow P-Body's..and re-check it! What should it ACTUALLY be,given my compression,and duration? If it makes a difference...my intake valve opens 26* BTDC...and closes 54* ABDC. Thanks,Mike!

Attached picture 6852009-CAMSWAPPICTURES027(Small).jpg
Posted By: Dodgem

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:20 AM

I usually get 5 to 8 lb more cold than hot!
Supposed to be that oil on cylinders is thicker??
Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:48 AM

WHAT HAPPENED TO THE FOLLOWING POSTS???
Posted By: Sport440

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:49 AM

Randy, with those intake numbers your installed at a 104 ICL if that cams on a 110LSA, witch I think it is. Its a comp cam, they make 90% of thier cams with a 110 LSA.

Being a solid with lash its going to close about 4* earlier and with a 10.2 comp, I think it should be more in the 165 to 170 max psi cranking range.

Mopar 65, your cranking psi of 185 is in the ball park for a 11.1 combo with your roller cam 264 @.050 Maybe 5 psi high or so, or not. IMO

No software or math here, just some basic comparisons to past builds. I think 68 Lar has a software program to fairly accurately predict cranking PSI based off of Compression and a IV closing point.
Posted By: Sport440

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:52 AM

Quote:

WHAT HAPPENED TO THE FOLLOWING POSTS???




Mopar65, Bailed I guess! Maybe he didnt want to Hijack

He responded to you and then asked me a question.

I answered you both, in my last response
Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:55 AM

Mike..the cam is actually ground on a 108 LSA...installed at 105. I'm going to pick up Mike's gage early in the week,and see what I get with it! If I do have 11:1..maybe I shouldn't have Jeff mill them. Thanks!
Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 04:59 AM

Bobby..where'd ya go,Buddy? Thanks for your input on this subject!
Posted By: Sport440

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 05:11 AM

Quote:

Mike..the cam is actually ground on a 108 LSA...installed at 105. I'm going to pick up Mike's gage early in the week,and see what I get with it! If I do have 11:1..maybe I shouldn't have Jeff mill them. Thanks!



Randy I did a quick run on some estimated numbers of your engine and came up with a 10.1 comp.

I used
.001 below deck
4.34 bore
84cc combustion chamber
6cc PV
4.5 gasket bore
.038 gasket thickness
3.75 stroke

if this is close to accurate your comp is about right. 10.1 - 10.2
Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 05:33 AM

Mike,Todd's build sheet says the pistons are .008 in the hole @ TDC..Fel Pro 1009 head gasket,with compressed thickness of .039..volume as 9.9 cc's. The cam at that time was a Scott Brown solid with 252/244 duration,with a 112 LSA,and ICL of 107. He's got the CR listed as 10.211. How much,if any..would this Comp cam change the "original" compressional ratio,when he put it together? Thanks for ALL your responses,Mike! I REALLY appreciate! Forgot to mention that the bore is actually a .040 over...4.360"
Posted By: Crizila

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 02:59 PM

Quote:

Quote:

Does anyone know what the difference in P.S.I. would be checking a cold engine...as opposed to one that's warmed up,like it should be? I did a check on my 440 this afternoon,without first warming it up! My average was 185psi. Thanks in advance! Randy



If it's still fresh there won't be much difference. As a general rule I only use a compression check to determine the state of health of a cylinder when compared to the others. Otherwise a leak down test is a much better tool.

You can also use it to check the general health of your motor if you have a comp. test history - I do a test about every 30 - 35 runs. You gotta do the test EXACTLY the same each time. Cold, throttle open, 5 pumps, and cranking rpm is critical. I do one bank and then recharge the battery for about 10 minutes before I do the other bank. Also as stated, I have seen guages vary as much as 15 psi. Doesen't matter much as long as you use the same gauge. I also suggest you stick with a cold test. Hot just throws in another variable that's hard to control. JMHO.
Posted By: polyspheric

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 03:30 PM

You can't use the .050" IVC point.
Posted By: emarine01

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 03:40 PM

My small block runs around 200/205 cold 220/225 hot with 12 to 1 @ a solid roller 260@.050 106/106, Engine is 4.5 years old with around 500 passes, The only real difference I saw compression testing is when we went to 16 vdc, It makes pressure on the 3rd bump
Posted By: Sport440

Re: ENGINE COMPRESSION CHECK- COLD? - 10/02/11 06:58 PM

Quote:

Mike,Todd's build sheet says the pistons are .008 in the hole @ TDC..Fel Pro 1009 head gasket,with compressed thickness of .039..volume as 9.9 cc's. The cam at that time was a Scott Brown solid with 252/244 duration,with a 112 LSA,and ICL of 107. He's got the CR listed as 10.211. How much,if any..would this Comp cam change the "original" compressional ratio,when he put it together? Thanks for ALL your responses,Mike! I REALLY appreciate! Forgot to mention that the bore is actually a .040 over...4.360"




Cheeze, with those numbers the comp comes in even lower to about 9.75

There will be no static change difference in compression between the two cams. Just a dynamic change.

The diiference in the static compression numbers between mine 9.75 and Todds 10.2 can be just the use of a diiferent calculaters.

Jeff is right about not being able to use the .050 numbers to accurately figure cranking comp. But to have a true 185# with a 9.8 to 10.2 comp with a 260* @ .050 cam is far enough out that I suspect a guage problem.

Lets see what Mikes gauge reads. You do have a SQ cam with fast ramps, but I doubt they are closing soon enough to net you that 185* psi.
Posted By: BSB67

Re: ENGINE COMPRESSION CHECK- COLD? - 10/03/11 01:16 AM

I think they are SQ, IIRC. Probably puts actual closing at 71* ABDC, depending on lash.
Posted By: mopar65

Re: ENGINE COMPRESSION CHECK- COLD? - 10/03/11 02:55 AM

randy/sport440 yes i deleated my post did not want to high hack randys post.Randy i say have the heads shaved or sell them and get some indy EZ heads. randy i would send them to jeff or dawayne and them do there thing to them.Mopar65
Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/03/11 03:27 AM

Thanks,guys! I'll let you all know what Mike's gauge shows..sometime this week! Bobby..Jeff will be doing the CNC porting,valve job..and milling the heads. I stopped by the shop on Friday and had a good chat with him! He's doing REAL WELL!
Posted By: topbrent

Re: ENGINE COMPRESSION CHECK- COLD? - 10/03/11 03:56 AM

Switch from the .039 FelPro to a .020 thick steel shim head gasket and you can bump your compression about half a point.

10.2'ish to 10.7'ish

Compression Calculator http://www.kb-silvolite.com/calc.php?action=comp

Posted By: Labratt

Re: ENGINE COMPRESSION CHECK- COLD? - 10/03/11 04:46 AM

Brent,do these seal as well as the Fel-Pro's? What manufacturer? Thanks!
Posted By: topbrent

Re: ENGINE COMPRESSION CHECK- COLD? - 10/03/11 06:54 AM

Quote:

Brent,do these seal as well as the Fel-Pro's? What manufacturer? Thanks!



Great seal and an easy compression bump without the milling your heads.

A light coat of wet silver spray paint or copper gasket spray and you are good.

Mopar Performance $21.95
http://www.summitracing.com/parts/DCC-4286754/

There are also more expense Cometic and MrGasket MLS gaskets available in the ~.027 range

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