340 with Magnum heads

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71valiant

71valiant

Make fast, break, repeat.
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I'm thinking about going this route. 1970 block. What kind of problems do you guys think I may run into? If any. I know the comp. ratio will be up there, so I'm sure race gas would be a must. Please let me know what you guys think. All suggestions welcome.
 
I am doing that very thing, but I used a 360 crank for a few more cubes. I have KB 372 pistons with a dish made to be used with magnum heads. Problem is finding uncracked cores, I have one good out of three. If I could do it over, I would buy new ones. The heads cc'd at 67cc's.
 
Shouldn't be any problem at all. Many guys do this conversion all the time. Depends on what piston you use as to what compression you will end up with. If you choose your pistons right you won't have to worry about running race gas. Like Bob said his cc'd at 67. Most I've seen go from 62-67 cc. I've heard some guys talk about having problem finding good ones but I've had 2 sets and none of them were cracked.
 
My motor was built at zero deck. With a .040 head gasket and the magnum head it should have an almost ideal quench.
If you use stock replacement pistons they might be to far down in hole to get the quench benefit from the heads. If you use flat tops at zero deck you'll have to much compression. Of course final dynamic compression depends on your cam choice. Bob
 
Im using a Comp xe268H cam and .020-.028 head gaskets. I'm trying to do this on a budget. Any insight to what all I would need internal-wise?
 
You don't want to get the piston any closer than to the head than .040 as stock rods will streach and since your using a closed chamber head, it might kiss it. Not in a good way! The .028 head gasket you want to use is to small. If its the one I'm thinking of, the mopar performance, it has a 4.060 bore and you have a bore size of 4.070. Comp Cams dosen't give the intake closing point on their cams so you can't figure out the dynamic comp. ratio.
 
The intake closing point on the XE268 cam is 38 degrees ABDC at .050 and 60 degrees ABDC advertised so now you can figure your dynamic compression.

BTW: What Mopar head gaskets you got. I was getting ready to buy some. If there the ones I'm looking at and you can't use them maybe we can deal. Send me a PM if you don't think you can use them.
 
I thought stock 340 pistons were above the deck. I would be concerned about interference with the closed chamber magnum head. If you go with an aftermraket flat top piston at zero deck and a typical 0.040" thick head gasket you will have about 10.6:1 compression. This is the set up I have in my 360 and I too am using the XE268H cam. My 360 is a strong runner.
 
bobscuda67 said:
Do you run your car on pump gas?
Click to expand...

Yes, with the ideal 0.040 quench on the zero deck pistons the engine runs with no audible detonation on 89 octane (though for only $1.5 extra per fill up I noramlly run 93) even with the 10.6:1 compression.
 
DGC, piston height is year dependent. Later years are low comp. engines while earlier years are positive deck - high comp pistons/ratio's.
If he goes to a surplier of pistons and doesn't make sure of the piston, he could get a low ratio piston, way in the hole.

71valiant, what bobscuda67 said and what DGC said.
If you go with an aftermraket flat top piston at zero deck and a typical 0.040" thick head gasket you will have about 10.6:1 compression.
Click to expand...

This is a good way to go about it.
If you have a positive piston in your 340 now and intend to keep it, you may have to find a much thicker gasket to allow the closed chambered Magnum head. Look to Cometic gaskets for a custom thickness and proper bore size.
Remember, no closer than .040 for saftey and long life.
 
The 340 has never been apart. So I'm not really sure as to how far the piston is down a tdc. I do not have the engine in my possesion yet, so I'm kinda trying to get all this figured out beforehand. I'm planning on TRYING to use the stock rods. So that'll give an idea on the rod length.
 
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