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Discussion Starter · #1 ·
I built a 360 a year ago: 0.040" over, #596 heads ( 68 cc from the books) that I didn't cc after a moderate port job. El cheapo stock replacement pistons (so they are way down "in the hole" now, but ???how much exactly). Heads were not milled. So now, my question, as you might tell, my C/R is way down (never bothered to calculate that either ::) ) and I want an easy way to get a little higher. How much can I take off the heads/intake, in general, and still be safe (yeah I'll use clay and check, it's a way mild RV cam) and how much can I expect to gain? I want the most w/ in reason, cause later I'm goin to propane.

People told me that I should take the time to measure EVERYTHING, but I obviously didn't.
Any help?
Mike

P.S. (((( I needed a few more parentheses)))))
 

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Take off at the least 30 do the intake side of the head so you can get the intake back on easily
 

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You mention in your post that the pistons are prensently "Way down in the bore..." The first thing that we need to figure out what your static compression is to begin with. (Not going to go into all that right now.) To assume that "any" amount should come off of the heads until you fully what your current compression is, could cause other problems, like the intake does'nt fit, etc.

Is it possible to do a current compression test and find out how much psi the current combination is producing? You need to work this one step at a time in order to spend as little $$ as possible while achieving the desired end result. I wouldn't "assume" anything just yet, (relative to your fix), until you know more about your present situation.

I't's going to be important to tell a competent machine shop these (4) things, prior to having the heads milled, (If that's the way you decide to go: 1) How far down in the bore are the pistons? 2) Was the block ever decked? 3) What compression ratio were your pistons designed to develop in your motor using a prticular rod? and 4) What is the present cc volume of the heads you're running?

All this will help you make the most "informed" and economically ($$$$) smart choice.

Just my .02.

Dave
 

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Runin' With The Devil said:
To assume that "any" amount should come off of the heads until you fully what your current compression is, could cause other problems, like the intake does'nt fit, etc.

Dave
That's why you mill both sides, if the machinist doesn't do it i would never use him again
 

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Providing the Machinist knows the values of all the "variables," (ie; approx. piston down in the bore dimension, piston mfg. and design / compression height, engine rod length, crank stroke dimension, head combustion chamber cc volume, and compression reading), he can mathmatically "Back into" the would be / should be static compression ratio....

Dave
 

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Discussion Starter · #7 ·
Well, I get 138 psi averaged, last time I checked. No, the block was not decked. Stock crank and rods. The el cheapo pistons' only advertised C/R was "stock replacement" (that's why I say the pistons are deeper, they have to be deeper to get the same C/R with 0.040" bigger diam. right?) Heads weren't cc'd.
 

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Stock replacement pistons that i've seen for Mopars are .020" further in the bore. For a 360 the stock piston has a pin height of 1.59 " The Stock replacement Federal-Mogul pistons are 1.57". So you could mill the heads .020" to be back at stock plus the slight gain you get with an overbore or you could go ahead with .030" since the 360 is pretty low compression anyway. The heads could probably handle .040" but .030" is a safe limit because i'm not sure all heads could take .040".
 

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Bummo said:
Is there some sort of math that can be done from a compression test (say he gets 150 psi) that will tell him the aproximate comp ratio he has?
Static cylinder pressure is HIGHLY dependent on cam duration and cam timing, not so much on compression ratio.

E.G. My 10.1:1 (calculated) slant only has 155 psi cranking pressure. Bone stock (approx 8:1) it had 150 psi.

The only way to know compression ratio for sure is to take the measurments and calculate it out.

-SM
 

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Discussion Starter · #10 ·
Yeah, SM, I realize the only true way is to measure......Let me find the cam card and I'll post some more #'s. I'm just lookin at how much I can gain while leaving the bottom end alone. Like I said, I wanna eventually run propane, so, 10:1 wouldn't be so bad. (Don't think I'll get there, but anything is better)
Thanks for the help so far,
Mike
 

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Well, check the parts list on the site....mopar makes a head gasket that is only 24-28 thou compressed (stock is 40 thou)...which will boost you compression quite a bit (.3 - .4 points).

9.5 - 1 is about the limit on pump gas...10 - 1 will give you fits on pump gas.

YOU NEED to rotate the engine until one of the pistons is at top dead center, then put a straight edge across the bore, and use feeler gages to find out how far the piston is down in the bore at TDC. Make sure you take your reading at the middle of the piston between the intake and exhaust side of the deck. That way, piston rock won't affect it as bad.

Do this at least 3 times, rotating the engine each time. BY doing it 3 times and averaging the readings, you will get fairly close. The best way to do this is with a dial indicator to determine when the piston is at TDC. IF all you have is your eyes and fingers, do it at least 3 times and average them.
You also need to know how deep the dish or notches are in the piston top. If you can find out who made them, and have a part #, they should be able to tell you.


If I assume your piston is a true flat top (most likely not) and that your piston is .100" in the hole (deck height), with the rest of the data, and a .040 head gasket, you are at 8.7 - 1...with a .026 head gasket, you are at 9 - 1.

If you assume .090 deck height, you are 8.9 with a .040 gasket, and 9.1 with a .026 gasket.

.080 deck height gives 9.3 (.040 gasket) & 9.1(.026 gasket)

With closed chamber heads, you will decrease the chamber volume by roughly 1.8cc per ten thou cut from the heads, but on chrsyler heads, those numbers are a bit different, because there is no quench pad in the head.

So, (assuming .100 deck height) if you cut the heads 10 thou you get either 8.9 or 9.1, 20 thou gives 9.0 & 9.3, and 30 thou gives you 9.2 or 9.5.

I wouldn't cut the heads more than 15 thou, as it will make them hard to use with any other piston combo, and if you cut them 30 thou, and then get new pistons, you will be well over 10.5 - 1 with any deck height less than .060 unless you buy cutsom pistons.



Bottom line is, you need to..must know, your current deck height (how far in the hole the piston is when at TDC) and the volume of the notches or dish on the pistons. THEN, you can determine how much you need to cut the heads, if any.

Trying to do this without knowing these numbers, could make the engine un-economical or even worse, break it !!!

Right now, I have a deck height of .110 on my engine, and 65 cc heads. With the thin gasket, and cutting the heads only 10 thou, I will be at around 9.1. If I cut the heads anymore to get 9.5, then when I put pistons in with a better deck height, I will be over 11-1 unless I order custom pistons.

Cutting heads and blocks to gain compression are really only for a specific combo...and when you change one element of that combo, you may not be able to use the other elements.

Lets say you cut the heads 30 thou to get 9.4 - 1. Then, you decide to get new pistons. If those pistons have a different deck height, you may not be able to use the heads anymore, as you can't put back what has been milled.

I would recommend using the thin head gasket for now, and then, at least you can go to the thicker one later on if needed. The thin ones are only 30 bucks a set.

If you can get me the deck height within 5 thou or so, the volume of the dish or notches, and a measurement on how deep the heads arerom the machined deck area (the flat area opposite the valves that is not machined--how much deeper is it than the machined deck??), I can give you some fairly accurate numbers to chew on.

Without this data, we are all really guessing, and that may not be good.
 

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I found a compression calculator on Keith Black-Silvolite pistons website http://kb-silvolite.com . If the pistons are the standard replacement pistons i'm thinking of then the piston compression height is 1.57" and the dish in the piston is 7-8 cc's. Figure in .040" gasket. 68 cc combustion chamber and .040" overbore and it comes up with 7.879:1 compression ratio. Mopar Performance says that for every .0048" milled off it decreases volume by 1 cc on 340 and 360 heads. Milling the heads .030" would take 6.25 cc's off the chamber and that would move the compression ratio to 8.28:1. This is all assuming he has pistons with stock shaped dish.
 

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But none of this takes into account the deck height. That makes a huge difference.

I also know from my experience, and several machine shops around here, that number may not be correct for all heads...it depends on which one you have, and how deep the un-machined cylinder head area is opposite the valves.

Using my calculator, and assuming a .100 deck height, it comes to around 8.23 with a 68 cc head and a 6-7 cc dish, but if the deck height is only .080, it is 8.5 or so.

What did you use for deck height ??? The only way I can come up with a CR that low, is using a .125" deck height.

With the thin gasket it is 8.7 with no head milling.

Actual math comes to .64 cc's per 5 thou cut.......until you get rid of the un-machined part of the chamber, then it goes up drastically. Until you get rid of the un-machined part of the chamber, you are taking a full .005 of the bore area. MP may have averaged it to get that number.
 

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I used .117 for the deck height. The distance from crank centerline to the deck is 9.6" (suppose to be anyway) Then subtract the pin height of 1.57, rod lenght 6.123 and half the lenght of stroke 1.79". The calculator is only a ball park figure. The correct way is to measure everything.
 

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ok..that explains why you had such a low cr.....I used a diff cr calculator...it also lets you figure out the cc's needed to get a desired cr in addition to the cr using the measurements
 

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Discussion Starter · #16 ·
Thanks for all the good info to chew on. I'll get some good numbers within the month and then decide how to get close to what I want.
Mike
 
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