Difference between "x" and "j" heads?

[Whitehatspecial]

Building a performance 340 to replace the 318 in my car. I'm now trying to decide what heads to use. I've got a pair of 1972 J heads with 1.88 intake valves that need to be done over. What is the difference between a set of "J" heads with 2.02 valves and "x" heads with the same sized valves.
Someone told me that J heads (with 1.88 valves ) are smog heads and won't flow the same as X heads even if 2.02 valves are installed.

[Rob C]

Same head, same casting number, smaller valve, bad freind poor info.

[Chryco Psycho]

I would use the 87-92 308 casting a pick up 40 HP over the X or J heads

[Whitehatspecial]

What kind of aluminum intake would you use with 308 magnum heads?

[moparnut]

I would use the 87-92 308 casting a pick up 40 HP over the X or J heads

Chryco,what did the 308's come on?How to identify?Is there a 308 casting number on them?thanks

[Chryco Psycho]

the 308 heads are Pre Magnum & all LA intakes fit from 66-92
 308 heads come on 87-92 Dodge trucks with the 360 engine & yes there is a 308 casting # under the intake runner on the heads
 I port them to maximize the flow , you can have 2.08 intake valves installed But the heads work well with the stock size intakes also

[moparnut]

thanks Chryco,so the 308's with stock valves will outperform a J head with 202's?

[Chryco Psycho]

I dynoed a 340 last year , we removed X heads & bolted on ported 308 heads , nothing else was changed including the timing , the rear wheel HP was up 41 HP

[moparnut]

thanks for the info,do you think the porting is what gave you the extra HP or the heads.I would be curious to see if there is any HP difference between stock 308's and stock J heads with 202's

[fishn4cuda]

What does a  "U" or "O" head belong on with the 2.02 valves? Was told TA heads. I just bought a rebuilt 340 with "U" heads and the LD340 intake.

[Follicly Challenged]

Different "short turn" on the intake side of the "X" head from the "J", but less swirl, Don't matter anyways, I vote the "308" deal these days.

FC out.

[moparnut]

What does a  "U" or "O" head belong on with the 2.02 valves? Was told TA heads. I just bought a rebuilt 340 with "U" heads and the LD340 intake.

Nope TA heads are W2 heads,good luck finding them

[moparnut]

And they have adjustable rockers

[fishn4cuda]

Not sure. I'll pull a valve cover and check it out.

[Whitehatspecial]

With all due respect to ChrycoPsycho, I would like this engine to look vintage since the car is really a time capsule itself. While the 308 heads may add a bit more hp, I'm having a mental problem bolting on 80's heads to a 60's motor and setup.

I'm sure the 308 swirl heads do add punch (and certainly a lot cheaper than doing the old x heads over) and I'd like to hear more about what other mods (rockers, cam, etc.) are necessary before jumping an board.

Do 308 heads need roller's?