need to raise compression w/o new pistons

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OUTLAW

OUTLAW

Master of All Mopars
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Here is the senario: My buddy scored a brand new rebuilt never been fired up 71 360 motor complete for $400. The guy had it rebuilt to stone stock specs and everything is new and showed us reciepts for all of it to a total of somewhere around $2400. But due to having a baby and cut back at his job he needed cash bad. His loss our gain!
So the problem is that a 71 360 motor w/ flattop pistons only has a compression ratio of 8.7 (probably closer to 8.0 really). He wants me to figure out a way to raise the compression as much as possible without replacing the pistons (doesn't want to re balance the motor again) My machinist is on vacation so I can't ask him about this.
What I need to know is, if we have the block zero decked and mill the heads to the safe maximum, does anyone know what we would end up with? Also what is the average cut needed on the block? What is the safe max cut on the heads? What is the stock cc on the heads (J heads)? I also figure on using a .029 head gasket and re camming as necccessary even possibly fly cut the pistons if need be.

edit

I found this burried in the archives on my site but I don't think it is accurate ...plug in your numbers and tell me what you think about its accuracy

http://moparsa2z.com/web/archive/auto%20converters/Engine%20Compression%20Ratio%20(CR)%20Calculator.htm
 
The typical measurement of a 360 head is around 65cc, but that would have to be verified. As would the measurement of the piston at TDC. The problem with all this milling, is that camshaft limitations will be apparent. With more compression, means more cam, ie lift and duration, which will bring the valve dangerously close to a piston with no valve releafs.
 
if you deck the heads about .010 then you can gain about .75 of a point on compression...I am in the process of doing this to my 318. you should have no problems running your stock pushrods also.
 
The amount to mill for a specific CR increase depends on the head. Larry Sheppard's, How to Hot Rod the Small Block Mopar, gives the amount to mill for the specific combustion chamber size. I'm running the Mopar .024 - .028 compressed head gaskets and have had no problems, so that's a cheap way to increase CR. The only way to really know what cc your combustion chambers have is to cc at least one chamber per head and then do the math required to figure out the correct amount to mill. I say this because cc size varied even on the same casting heads and someone may have milled the heads in the past. Also, as mentioned above, you must check how much your piston is down in the hole at TDC. Once you have all this info you can go to the Keith Black Piston website and use their compression calculator to give you the true static compression ratio. As an example, after I got all the info I used their calculator and came up with an 8.66-1 C/R for my 318. I used a short duration/high lift cam (267/485) and have a very responsive little 318. Keep the camshaft duration in-line with the final actual compression ratio and it should be a good running engine, even with 8.5 - 9.1 compression. Let us know how it all works out.
 
I find out that the head cc is 65 to 71 (seems like a large variation ) but there is a .2 cc reduction for every .oo1 milled from the head. I can always shim the rocker shafts to bring the rocker geometry back in line and to keep from preloading the lifters too far. I have access to the tool to flycut the pistons if needed (most likely will). Have no idea how much is the average when zero the deck -if I use .015 and plug it into the formula I posted it gives me an approx compression ratio of 10.9 which seems to be a bit much for (approx. .020 cut on the head and .015 on the block --which may be way too much )
 
lenweiler said:
The typical measurement of a 360 head is around 65cc, but that would have to be verified. As would the measurement of the piston at TDC. The problem with all this milling, is that camshaft limitations will be apparent. With more compression, means more cam, ie lift and duration, which will bring the valve dangerously close to a piston with no valve releafs.
Click to expand...


A 340/360 open chamber heads have a a volume in the range of 68-72cc's and most are closer to the 72cc number. The only heads with a volume down in the 65cc range are the small port 318 heads and the closed chamber 302, 308 and magnum heads. The magnum heads on my 360 measured 64cc's.
 
DBrown said:
if you deck the heads about .010 then you can gain about .75 of a point on compression...I am in the process of doing this to my 318. you should have no problems running your stock pushrods also.
Click to expand...

0.010 on the deck will raise the compression on a 4" bore about .2.
 
OUTLAW said:
He wants me to figure out a way to raise the compression as much as possible without replacing the pistons (doesn't want to re balance the motor again) My machinist is on vacation so I can't ask him about this.
What I need to know is, if we have the block zero decked and mill the heads to the safe maximum, does anyone know what we would end up with? Also what is the average cut needed on the block? What is the safe max cut on the heads? What is the stock cc on the heads (J heads)? I also figure on using a .029 head gasket and re camming as necccessary even possibly fly cut the pistons if need be.
Click to expand...

Likely cheaper to buy new pistons and and have the rotating assembly balanced again then getting involved in decking the block. Certainly should be checked into before doing anything.
 
Starting with the cylinder heads,you will need to have them milled .050 inches to get the compression up.You will need 2.02 inch intake valves with the intake and exhaust bowls opened up to the valve seats and ported out to the gaskets with a good three angle valve job.Use Mopar 284/284 degrees .484-.484 inch lift cam package #P4529958 with valve springs #P4120249,along with 1.6 ratio adjustable rocker arms#P4529692 and there special pushrods for adjustable rockers #P4529556.Next Mopar single plane manifold #P4452891,with Holly 750 cfm Double Pumper carb#0-4779.Mke sure to have the intake milled to match the heads as the geometry will be different.Use wind tray kit #P4529790 and HD oil pump #P4286589. This set up will be good for 380+ horsepower according to Dick landy.Hope this helps.
 
Yep magnums or 302's with some work done, the 302's are smaller chambers yet, like 60 or 62 cc's.
 
outlaw, I've got a set of 302's I just took off a 85 318. Let me know if you need them. I'm in Waterloo Mike
 
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