Help with compression!
Thread Starter
Joined: Sep 2003
Posts: 813
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From: Harlingen, TX.
Help with compression! I have a forged 347 with stock unported but milled heads. My compression is 11.5 to 1 right now. If I were to get some Partiot stage 2 (64cc) heads how much would that drop my compression? I want to keep it as close as possible to what I have right now.
Staging Lane
Joined: Oct 2009
Posts: 78
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From: Scottsboro,Al.
Its according how much they were milled.If no more than .018 then you should stay around the same due to the stock being 66.7cc before they were milled.I was going to buy the 64s but since I decided to stay motor I went with the 59 instead.My numbers are with stock ls1 intake and t/b too.I havent had a chance to get back to the dyno since I put on the ls6 and 85 tb.Nice times btw.
Staging Lane
Joined: Oct 2009
Posts: 78
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From: Scottsboro,Al.
They add material to the com chambers on the 59cc and then cnc it to get their 11.0 1 on a stock ls1 Patriot use a 243 cast for their 59cc raise intake runners to 227cc and reduce exhaust runners from 75cc to 70cc........64cc patriot heads are same specs................stock ls1 heads are 190cc intake 70cc exhaust 66.7cc com........... stock ls6 are 210cc in. 75cc ex. and 64cc com............thats why if your stock heads were milled about .018 then you should come out with about the same compression with an untouched and just bolted on 64cc head.
Staging Lane
Joined: Oct 2009
Posts: 78
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From: Scottsboro,Al.
Figure on this is you used a stock LS1 head which is 66.7cc combustion and you had them milled .018 thats gonna drop you in combustion chamber to about 64.5cc and give you a bump in compression from the stock 10.5 to about 10.8ish along with your forged piston you have choosen to give you to give you what you have now which is like you said 11.5 now you take that set of heads off and bolt on the patriots 64s you are looking at right around the same UNMILLED.
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They add material to the com chambers on the 59cc and then cnc it to get their 11.0 1 on a stock ls1 Patriot use a 243 cast for their 59cc raise intake runners to 227cc and reduce exhaust runners from 75cc to 70cc........64cc patriot heads are same specs................stock ls1 heads are 190cc intake 70cc exhaust 66.7cc com........... stock ls6 are 210cc in. 75cc ex. and 64cc com............thats why if your stock heads were milled about .018 then you should come out with about the same compression with an untouched and just bolted on 64cc head.
His stock heads have been milled. Milled enough to land him at 11.5:1, which suggest they've been milled a LOT more than .018".
His question is about bolting on some 64cc 243 heads, which will undoubtedly lower his compression ratio from where he's at now.
The three unanswered questions (that could only answer his question), are...
1. what is the piston volume? (if not flat tops)
2. what is the current head chamber cc?
3. What is the head gasket thickness?
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Verdad Gallardo Staging Lane
Joined: Oct 2009
Posts: 78
Likes: 0
From: Scottsboro,Al.
You're misreading the question.
His stock heads have been milled. Milled enough to land him at 11.5:1, which suggest they've been milled a LOT more than .018".
His question is about bolting on some 64cc 243 heads, which will undoubtedly lower his compression ratio from where he's at now.
The three unanswered questions (that could only answer his question), are...
1. what is the piston volume? (if not flat tops)
2. what is the current head chamber cc?
3. What is the head gasket thickness?
His stock heads have been milled. Milled enough to land him at 11.5:1, which suggest they've been milled a LOT more than .018".
His question is about bolting on some 64cc 243 heads, which will undoubtedly lower his compression ratio from where he's at now.
The three unanswered questions (that could only answer his question), are...
1. what is the piston volume? (if not flat tops)
2. what is the current head chamber cc?
3. What is the head gasket thickness?
Last edited by frazierboy98; Oct 6, 2009 at 09:05 PM. Reason: more info
Thread Starter
Joined: Sep 2003
Posts: 813
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From: Harlingen, TX.
I dont exactly know how much was taken off my stock heads. As far as my gasket thickness I dont know that either...sorry. I had the build sheet but I lost it. I'll have to get with LME and have them check for me. How much would I have to mill the Patriot heads more or less to get compression to stay at 11.5? I'll post up info on my stock heads once I get it from LME.
Staging Lane
Joined: Oct 2009
Posts: 78
Likes: 0
From: Scottsboro,Al.
You can go by as listed above by predator z which is the most accurate or you can find how much they milled your heads.I could almost bet an slp mls gasket was used as used on just about all head swaps.The thing is you may not have to mill the patriots at all to keep the same compression.You can only mill a head so far especially without having to worry about ptv clearance.I see your dur do you know your exact lift?
Staging Lane
Joined: Oct 2009
Posts: 78
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From: Scottsboro,Al.
Why the hell does everybody want to argue?????????I didnt say it did but if he does have a bigger lift cam I tell u what you throw a .620 .630ish cam on an ls1 with a set of milled *** heads and tells us your results!The more you mill a head the less duration and less lift you can put in it.
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
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From: BFE
Many poeple confuse tappet lift and .050 lifts, again 2 different things. Take 2 cams same durations, 1 with .560 lift and the other .640 lift. If the first one clears by a safe margin, so will the second. The effect of lift on PTVC is negligeable at the durations we run in these motors.
Thread Starter
Joined: Sep 2003
Posts: 813
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From: Harlingen, TX.
2) 6.125 rod size
3) I believe the bore size is 3.903"
4) Dont the gasket thickness
5) Dont know the piston to deck height
Its your basic LME 347. Those are specs I can remember the rest I have to get with LME.
You can go by as listed above by predator z which is the most accurate or you can find how much they milled your heads.I could almost bet an slp mls gasket was used as used on just about all head swaps.The thing is you may not have to mill the patriots at all to keep the same compression.You can only mill a head so far especially without having to worry about ptv clearance.I see your dur do you know your exact lift?
Staging Lane
Joined: Oct 2009
Posts: 78
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From: Scottsboro,Al.
With that dur and that lift its a guarantee that the heads have not been milled that much probably no more than has been quoted .018 I just really dont think your c/r ratio is going to change enough for you to tell any difference with a set of unmilled 64cc patriot heads w/mls gaskets.I believe if anything you might get a very slight BUMP up in compression.I personally wouldnt recommend milling the patriots.Best thing to do is either find all the info predator listed or simply find out how much the heads were milled.If you can find out the exact cc on your combustion cham thats a dead giveaway as to what your gonna end up with.Just go back with the same type same thickness gasket.
here is a guesss, but like pred said u need to talk to lme. bore 3.903, gasket bore (this is a huge guess) 4.04, .04 (if they used a cometic) they more than likley decked the block for a flatter surface so the piston might set out .003, with all of this info that would be 10.624:1. more than likley less than that but it is safe to say u are around 10.5:1 compression.
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
With that dur and that lift its a guarantee that the heads have not been milled that much probably no more than has been quoted .018 I just really dont think your c/r ratio is going to change enough for you to tell any difference with a set of unmilled 64cc patriot heads w/mls gaskets.I believe if anything you might get a very slight BUMP up in compression.I personally wouldnt recommend milling the patriots.Best thing to do is either find all the info predator listed or simply find out how much the heads were milled.If you can find out the exact cc on your combustion cham thats a dead giveaway as to what your gonna end up with.Just go back with the same type same thickness gasket.
Please avoid all this missinformation you are dishing out.
With 2cc valve reliefs he would have roughly over .16 clearance with stock gasket and unmilled heads on intake.
2cc relief is ~.080 deep and that cam on unmilled stock valved heads will clear with over .080 intake.
So plenty of space for him to mill a substantial amount.
Assuming his piston is .010 out of hole (which is usualy the max they set it to be from a slight deck clean on the block to have everything straight)
With stock gasket to have 11.5, he would need ~ 57.5 cc chamber
With .040 gasket to be 11.5, he would need ~ 59.5 cc chamber
With a .018 mill, or 3 cc (64.7cc) , he would have ~10.78 (.040 gasket)
and ~ 10.5 (stock gasket)
So most of the info you are giving is not even in the ballpark, clearly you are mistaken on your assumptions.
To the OP, contact LME, get the info I asked for and I'll calculate what you need. You could definitely mill the Patriot heads but I am skeptical that your motor has that much compression as we speak, but possible. It would seem more like you have around 10.5 SCR (.018 mill)