X or J heads

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nothingbutdarts

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I have a 340 bored .030 40 years ago as well as a set of ForgedTrue I believe 12.0 to 1 pistons that are that old and new as well. Just looking to put it together finally as just an occasional play toy. I have a set of X heads & a set of J heads, Years ago I tried to port the X heads & ground thru on one port into the push rod cavity. I believe it can be epoxied maybe?
Just as a play toy, which heads should I use?
I also live @ 8,800' It will be a winter project to put together. I am not looking to spend the money for a set of aluminum heads for a play toy.
 
The x-head has a really nice approach on the hump as in the floor where bumps up at the short turn before going down into the bowl.
The j head is a great head too..theres not much difference..but the difference is in the inake ports between the 2.
Hard to say which I'd pick. I have both as well..ported.. the x head is more stable. @271cfm @.550 and the j @278-284cfm @.500
None of that matters though.
Tube the push rod hole on the x heads and use those...the diff is marginal.
 
I forgot to mention..
You can lay the x head ssr back more than a j head before you hit water.

The little spec/pore...is a hole....when they are rusted from the inside..you hit water even quicker. Inspect the castings carefully for crust fallout with a pick

20211009_172740.jpg
 
MOPAROFFICIAL said:
I forgot to mention..
You can lay the x head ssr back more than a j head before you hit water.

The little spec/pore...is a hole....when they are rusted from the inside..you hit water even quicker. Inspect the castings carefully for crust fallout with a pick

View attachment 1715829296
Click to expand...
Ever mess with a “Z” head?
 
Just wondered if you ported a set. You did find a different between the X & J.
 
nothingbutdarts said:
I have a 340 bored .030 40 years ago as well as a set of ForgedTrue I believe 12.0 to 1 pistons that are that old and new as well. Just looking to put it together finally as just an occasional play toy. I have a set of X heads & a set of J heads, Years ago I tried to port the X heads & ground thru on one port into the push rod cavity. I believe it can be epoxied maybe?
Just as a play toy, which heads should I use?
I also live @ 8,800' It will be a winter project to put together. I am not looking to spend the money for a set of aluminum heads for a play toy.
Click to expand...
splitting hairs in my opinion. I'd run the J's means the X's have a hole to fill.
 
rumblefish360 said:
Just wondered if you ported a set. You did find a different between the X & J.
Click to expand...
Oh. Yeah the x was its own.
All the intake ports are roughly the same, the shape is the same, it's just they're cast sloppy and sometimes you have to grind out a lot more. The exhaust ports varied a bunch.
The 974 head was machined in two different ways. The first run of 974 was undrilled on the exhaust ports. Mid '74 cast for '75. Air/smog pumps came about and they were all drilled after that. 587, same story but the way they shaped the boss on the port floor for the hole varied 2-3 ways.
The taller the port floor was the closer the hole to the ssr. Some of the shallower ssr exh ports you'll see it's set back more.
So the idea of using those as extended ssr is dependent on that detail.

I can get 250-260cfm out of all of them, some even more. I think that goes for other head porters too. Everything has its traits, behaviors. Stable but less cfm vs more cfm and less stable. Quality of air etc
 
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epoxy a thin wall brass tube in that broken intake pushrod hole and forget about it.
 
With 12:1 pistons are you trying to run pump gas??

I would ditch the cast iron heads and use Edelbrock aluminum heads personally.

I have X heads on my 340 and to be honest, after 50 years they are nothing special (sorry purists). Mine both have issues and weird repairs on them. My only attachment to them is they were rebuilt, bolted to the engine prior to purchase, and run the engine fine for what my goals are. If ever removed they will be swapped to aluminum heads.
 
I'm running ported, oversized vavle "J" heads, that have been opened up on the gasket areas. An ol Mopar Racer here local looked at then, when I had em on a table at Big Daddys, he said , No, don't sell those, those are probably as good as heads, maybe better.. so I kept em.
 
nothingbutdarts said:
I have a set of X heads & a set of J heads, Years ago I tried to port the X heads & ground thru on one port into the push rod cavity. I believe it can be epoxied maybe?
Just as a play toy, which heads should I use?
I also live @ 8,800' It will be a winter project to put together. I am not looking to spend the money for a set of aluminum heads for a play toy.
Click to expand...

If the J heads still have 1.88 intake valves, they will provide a more efficient valve job & flow if enlarging to 2.02. Nice, fresh seats out in the chamber instead of sunken seats after who knows how many valve jobs on the existing 2.02's.

I think you can get away with higher compression at higher altitudes, but I don't know how to figure that. You can stay safe and cut down the domes, probably to flattops, while saving some weight off of those heavy pistons. (If the block cleans up at .030".)
 
70dart340 said:
I have a bunch of irons in the fire, Rob, but the "Z" heads are yours for the Rumblefish. I'll build some boxes and send them out in January.
Click to expand...
No worries. Nothing but time here.
Locomotion said:
If the J heads still have 1.88 intake valves, they will provide a more efficient valve job & flow if enlarging to 2.02. Nice, fresh seats out in the chamber instead of sunken seats after who knows how many valve jobs on the existing 2.02's.

I think you can get away with higher compression at higher altitudes, but I don't know how to figure that. You can stay safe and cut down the domes, probably to flattops, while saving some weight off of those heavy pistons. (If the block cleans up at .030".)
Click to expand...
There are formulas available on line that can also show the hard math as well as just a calculator to type in facts. They will spit back are density changes between elevations. At a 12-1 ratio, I figure a small-ish cam is not in the cards for play. I personally don’t know though.
 
The heads are the same. The difference in the performance was the compression ratio. If you check any old shop manual you will se tha 67-69/70 were 10.5-1 then in 70/71 10.3-1. If you ever went to put a Six pack on a 69 back 340 which I have done several back in the day. Getting those bolts started was a really tough job due to the heads being lower a tad. Surface a set of J's/ U's/ Z's /O's and you have a set of X's. We have clean bare heads and also one extra X. Bought 70 Dart years back and it had one O and one X head. motor was original. one 915 and one 894
 
Oldmanmopar said:
The heads are the same. The difference in the performance was the compression ratio. If you check any old shop manual you will se tha 67-69/70 were 10.5-1 then in 70/71 10.3-1. If you ever went to put a Six pack on a 69 back 340 which I have done several back in the day. Getting those bolts started was a really tough job due to the heads being lower a tad. Surface a set of J's/ U's/ Z's /O's and you have a set of X's. We have clean bare heads and also one extra X. Bought 70 Dart years back and it had one O and one X head. motor was original. one 915 and one 894
Click to expand...
The 894's I've cc'd were 70cc
All the j heads were 72cc
If you get the 2 set side by side... check out the floor approach, the hump between the 2. Its different. Good to see you're feeling better.
 
If the goal is to port them to a pretty high level, my preference is the X head.
The SSR can be shaped to a better form with less chance of ending up with a hole(it’s better to start with).

For something that might be getting a bowl blend, with budgetary concerns in place .... use which ever ones are in better condition from a guide and seat standpoint.
 
[QUOTE="Locomotion, post: 1973695401,I think you can get away with higher compression at higher altitudes, but I don't know how to figure that. You can stay safe and cut down the domes, probably to flattops, while saving some weight off of those heavy pistons. (If the block cleans up at .030".)[/QUOTE]

The new pistons weigh 524 grams, I don't know how much an original 340 piston weighs.
 
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