another piece of the puzzle

[bb71challenger]

I sent a money order off today for a set of 915 heads. Should be tons easier to get good quench now. They are stock except for the 214/181 valves with some unshrouding done in the bowl for flow. Basic throat cut for the bigger valves. I heard somewhere that the old MP porting templates dont work for the closed chamber heads, is that right?

[Chryco Psycho]

the templates will work on the iron heads
congrats on the purchase , I do not touch the floors of the ports but work the sides & roof

[bb71challenger]

Hey Neil. On a low comp 400, what would the increase be just by swapping the 915's? Would be killer if I could get 9:1 with just the heads, the engine is fine and the only reason I am breaking it down is to put in pistons to raise compression.

[Chryco Psycho]

you would be close to 9:1 but you still would not have the advantage of quench

[femtnmax]

I heard somewhere that the old MP porting templates dont work for the closed chamber heads, is that right?

David Vizard did a nice detailed 2 part article on big block mopar heads in Mopar Muscle Magazine, including use and discussion about the porting templates.  IN summary he says:
With a little experience, the templates can be dispensed altogether when using the 75-degree bottom cut, since most of the same material is removed by the machine operation.

here's links to the articles:
http://www.moparmusclemagazine.com/techarticles/5115_cylinder_heads/index.html
http://www.moparmusclemagazine.com/techarticles/5118_cylinder_heads_ii/index.html

[bb71challenger]

So using the 915 heads with the stock compression would increase detonation potential? Are the Keith Black KB-240 pistons a good choice for both quench characteristics and for compression increase while using the 915 heads?

[femtnmax]

So using the 915 heads with the stock compression would increase detonation potential? Are the Keith Black KB-240 pistons a good choice for both quench characteristics and for compression increase while using the 915 heads?

For the quench type 915 cylinder head to work, you need the "matching" lower portion of the combustion chamber...meaning in your case a flat top piston.  Yes, the KB-240 would work well with proper machining of the deck surface of the engine block.
Typically to get the quench set up correctly requires extra machining.
In your case, the crank, block, piston and rod dimensions are as follows:
1/2 of crank stroke:  1.69
con rod length:  6.358
piston compression height:  1.908 for KB-240
add these three together = 9.956
compare that to block deck height spec:  9.980
So with crank, rod, piston the top of the piston would be 9.980-9.956=0.024 inch below top of block.   For good typical quench clearance between top of piston and head gasket surface of cylinder head you want 0.040 which is the typical thickness of head gasket.  This means the top of the piston should be at the head gasket "deck" surface of the engine block. 
So you would need to cut 0.024 off each head gasket surface of the block. 
Before you jump in you need to calculate the static and dynamic compression ratios based on this combo including your cam.  Neil would probably know the results off hand.

[bb71challenger]

Hey Phil, if I used the factory style .020 steel shim head gasket, wouldnt that pretty much take care of the material I would need to mill from the head mating surface of the block? Could I get away with doing that or am I just looking at it wrong? If this were to work then I am probably raising my compression to a very uncomfortable 10:1 or more. Can I do a little more unshrouding of the valves and maybe some combustion chamber polishing to add a little more volume to the chamber without getting rid of the quench characteristics that I am looking for?

[UKcuda]

I don't know about the big blocks but on the small blocks there is also a 27 thou gasket made from the same sort of stuff as the 40 thou stocker. 

To get the quench on my 360 Magnum engine I got the deck milled but not right down to zero, then I used a 27 thou gasket - I figured I could change to a 40 thou if I later decided I had milled it too far.

A 20 on yours should be close enough but measure the deck height with a depth guage as Mopar specs are not always a reliable indicator of what's really there (if you know what I mean).

It's a mystery to me why they used closed chamber heads and then put the piston so far down the hole 

On the Magnum the stock deck height was 50 thou, so quench was 90 thou with the stock gasket 

[femtnmax]

Hey Phil, if I used the factory style .020 steel shim head gasket, wouldnt that pretty much take care of the material I would need to mill from the head mating surface of the block? Could I get away with doing that or am I just looking at it wrong?

You are correct, your quench would then be 0.02+0.024=0.044 which is right in the ball park.  Best to measure how far your pistons are below the head gasket deck surface of the block.  The 0.024 is just a specification, and the actual number could be different...no problem if the correct head gasket thickness is applied.

If this were to work then I am probably raising my compression to a very uncomfortable 10:1 or more. Can I do a little more unshrouding of the valves and maybe some combustion chamber polishing to add a little more volume to the chamber without getting rid of the quench characteristics that I am looking for?

All correct again.  Unshrouding is an excellent way to drop compression while increasing mid-upper rpm port flow.  The chamber polishing is doing the same thing.
For the steel shim head gaskets ask if they will seal correctly on head or block surfaces that have not been freshly cut, do you need any sealer, etc.   
On the final compression ratio, I suggest using the calculator that KB silvolite offers on their website.
http://www.kb-silvolite.com/calc.php?action=comp
For your cast iron heads and living near sea level, yes the 10:1 would be about the maximum for pump gas.   I run 9.6:1 with 0.040 quench, cast iron heads, and use the cheapest, lowest octane gas available.  I have NO detonation, but I"m also at 4500 foot elevation which helps reduce detonation.
For street driving, remember the tendency to detonate will increase over time as combustion residue develops in the combustion chamber, so D. Vizard suggests to error on the conservative side with slightly lower final compression when you put the engine together.  You can move the camshaft intake closing point by 2 degrees or more to alter the final dynamic compression ratio, although I would tend to set the intake closing point for best performance and not a band aid fix for too much compression.

I don't know what all your doing to your engine, but if your putting fresh heads with valve job on old tired piston rings you can easily turn the engine into an oil burner because the extra sealing of the valves will push more pressure past the tired rings.

[bb71challenger]

For your cast iron heads and living near sea level, yes the 10:1 would be about the maximum for pump gas.   I run 9.6:1 with 0.040 quench, cast iron heads, and use the cheapest, lowest octane gas available.  I have NO detonation, but I"m also at 4500 foot elevation which helps reduce detonation.lite.com/calc.php?action=comp


I don't know what all your doing to your engine, but if your putting fresh heads with valve job on old tired piston rings you can easily turn the engine into an oil burner because the extra sealing of the valves will push more pressure past the tired rings.

I live at around 2800 ft
I will have to get the 240 pistons and most likely have the block bored since the ring lands are going to be higher up. I dont know if 20k miles will put a ridge on the cylinders but I would think it might. I have not torn the engine down, its high on my priority list now that I have a bit of cash for parts.
Thanks for taking the time to respond