Difference in X heads & J heads

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Waco Demon

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Is there a difference in the combustion chamber between X & J heads .I know the intake valve in the X head is larger . Harold
 
Nothing outside of production tolerances or casting irregularities. X (894) head ports are said to flow a bit better. But the difference is minimal. Considering how the much sought after, old, worn and remachined 894 heads are over the years, machining a fresh 2.02" intake seat in a 1.88 J (915) or newer 360 head will provide as good or better results.
 
The x head will flow about 12 cfm better at max effort porting level.
 
J has better exh port to start with.

they are about equal on intake side.

The best approach, as Loco mentioned, machine a set of 1.88 to accept a 2.02 with a clean throat cut. Much better than the factory 2.02.
 
Some J heads had 2.02" intakes from the factory... They switched from X to J heads in mid 1970 and they had 2.02" intakes through 1971....
 
Waco Demon said:
Is there a difference in the combustion chamber between X & J heads .I know the intake valve in the X head is larger . Harold
Click to expand...
There is a difference in combustion chamber between a J head and another J head, all in how they were cast. X heads had 2.02 intake valves, some J heads had 2.02 intake valves, some had 1.88.
 
FM 3 DART SWINGER said:
I did some search on the 915 J Heads int showed that they were only used in the 340 in 1970 is that right ?
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I believe regular '70 340's were 894, but 915 was the casting used for 340-6 T/A heads. Just machined for 2.02 intakes and the offset pushrod holes. However, there were some very early 894 T/A heads which may not have made it into production cars. Another member here has photos.
 
70 was the change over year from x to 915 (J) heads. Had a car that had an x on one side, J in the other... Early engines could have had either head. Whatever was on the line at the time. Late 70's would be J heads in all likelyhood.
 
The biggest difference I have seen between X and J heads, is about $300 a pair! LOL. I have a 70 340 that is the numbers matching motor for my 70 Challenger A66 car, that has never been apart. It has a J head on 1 side, and a U head on the other.....both 915 castings with 2.02 intake valves.
 
i had one of my spare 340 come with an x and j head , not sure it was fromthe factory this way as it was modified , had the old trw forged 12.5 pistons . still have the heads somewhere in the garage
 
Seems I remember the chambers were a little smaller on every set I’ve saw. could be they’re so old someone has machined on them?
I liked starting with the small valve j head, and putting bigger valves in. Fresh seat area, valve sitting up pretty flowing better than some wore out x heads with sunken valves.
Never measured port cc to see if there’s a difference.
 
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