x heads vs. j heads ?

[72cudaguy]

Hello everybody,  I have a 72 cuda with a numbers matching 340. I'm just getting started with the motor tear down and rebuild. I'm looking to rebuild fairly hi po, is there that much of a difference in the x heads to warrent trying to track a pair down or should I just stick with the stock j heads ?  thanks for any input

[Chryco Psycho]

There is virtually no difference between the X or J heads but if you 40 more HP use the later 308 swirl castings  I generally port these heads to get closer to 60 hp gain

[HP2]

 

In stock form, the 308s flow better than X or J heads.

[mrbill426]

I've got stock J heads on my '72 too, they have small valves.  Wonder what it would cost to install larger ones?  I hear Mopar now sells bare castings for the 340 with larger openings.

 

[Chryco Psycho]

I would hit the wreckers & get 308 castings for $100 or less of a truck instead of putting $$ into X or J castings

[mrbill426]

Okay, so the 308 heads are truck heads that have the larger valves?  Are they open chamberd?

[dimples]

They have the standard 1.88/1.60 valves, and are open chamber. The main difference is intake port design, and higher % flow of the exhaust.

[HP2]

...or since you are going through a rebuild anyway, just put on magnum heads, intake, rockers, and pushrods in it. Then you get high swirl, fast burn, high flow, and small chambers all in one.

[Chryco Psycho]

& the awesome chevy ball stud rocker system 

[dimples]

Well, it's a bit tougher than the chev............they go over 200,000 miles in a stock engine.

But real engines have rocker shafts.

[Dustu]

308's must be a good increase ,I have a set that was on a Super Stock Cuda 340,how did he manage that ?Legal ?