X-heads vs J-heads: what's the difference??

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RzeroB

RzeroB

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X-heads vs J-heads: what exactly is the difference between the two?

I tried searching the tech forums for the answer but I'm obviously not very good at getting the most out of a keyword search (that's another story).

Anyway, I'm not sure what the difference is between these two heads. I use to know the difference by casting numbers but nobody seems to refer to them by their casting numbers anymore like they do with the big block heads. Can someone school me on the "X" and "J" heads and the differences between them, or point to me a thread that may have already beat that horse to death?
 
All the X heads came with 2.02 valves. Only some of the J heads did. Other than that, they are pretty much identical. Also, not worth a ton. I would say probably about 100 for J heads with 1.88 valves and maybe 200-250 for the X heads, maybe a hundred more if they are ready to bolt on and go. People think they are worth their weight in gold. They ain't.
 
im no head expert but from what justin used to tell me, unmodified X heads are the ****.
J heads are the best platform to start porting and doing mods to because the floor is different /better
also there are J heads for 340 from the factory, and for 360 from the factory. differnt intake valves ans valve springs.
you can modify 360's for 340 but it takes alot of porting and bowl work.
 
X heads were used on 340s from 68 to 70. They came with 2.02 / 1.6 valves. The J head was from 70 to 71. They came with 2.02 or 1.88 intake valves and 1.6 exhaust valves. There is some factory parts bin scrounging / emptying so one may find these on later years than whats the norm. As far as OOTB performance goes, both heads IMO are very similar when equipped with 2.02 valves. Some may argue the subtle differences between the two, but the reality is they are nearly identical.
 
Rusty nailed it, but I think the price for a fresh, un-ported, ready to bolt on set of x head are closer to $500.
There are much better aftermarket heads for not much more.
 
hardened valve seats didnt come around till fuel went unleaded.
unless building a race car or huge power, aftermarket heads are a waste of money in my opinion
 
I'll just add that the X's, "in general" may have a little tighter combustion chamber. But with the factory variances, you'd really have to measure any particular set you were looking to use.
 
All I will say is, you can get the new Pro Comp aluminum heads for a couple hundred more. That was civil, wasn't it?
 
I have a steel crank, '72 motor in my car. J heads, that already had hardened valve seats, and 2.02 valves installed when I bought it. I personally would not have spent the $$'s to do this. I would have purchased aftermarket's.
 
discharger1332 said:
im no head expert but from what justin used to tell me, unmodified X heads are the ****.
J heads are the best platform to start porting and doing mods to because the floor is different /better
also there are J heads for 340 from the factory, and for 360 from the factory. differnt intake valves ans valve springs.
you can modify 360's for 340 but it takes alot of porting and bowl work.
Click to expand...

please explain further.
 
RustyRatRod said:
All I will say is, you can get the new Pro Comp aluminum heads for a couple hundred more. That was civil, wasn't it?
Click to expand...

Yes! Rob. :D

I was trying to think of a way to let him know what I thought as soon as I saw the listing, but figured.....best to keep quiet for now.
 
Casting
#2531894 'X-Heads' {1968 thru 1971}....................................#3418915 {1970 340 T/A}

Intake Valve............................... 2.02"..................................... 2.02"
Exhaust Valve............................. 1.60"..................................... 1.60"

Intake Bowl Size......................... 1.75" {87%}......................... 1.75" {87%}

Factory Combustion Chamber...... 67.0 CC's to 69.0 CC's........... 69.0 CC's to 73.0 CC's
Recommended Minimum.............. 63.5 CC's.............................. 63.5 CC's

Intake Port.................................. 1.16" x 2.27"........................ 1.16" x 2.27"
Exhaust Port................................ 1.25" x 1.75"........................ 1.25" x 1.75"

Intake Port CC............................. 160 CC's............................... 158 CC's
Exhaust Port CC........................... 70 CC's................................. 72 CC's

CFM Flow @ .500" Lift.................. 225 CFM............................... 228 CFM
CFM Flow @ .500" Lift.................. 147 CFM............................... 146 CFM

Intake to Exhaust %..................... 64.5%................................. 64.1%

The 'X-Head' is generally considered the best 'Stock' Cylinder Head, as the Combustion Chamber is a little tighter than the 'J-Head'.

Both 'Ports' are near identical in volume. But the Intake Port on the '1970 T/A Head' has a slightly better {miniscule} better floor design.

The 1970 {340 T/A} have the Intake Push-Rod holes bored 'off-set, which are designed to allow for added Racer Modifications to open up the Intake Ports.
 
When I had the combustion camber checked on my stock X heads they were 74 cc. I thought they would be a little smaller like 70 cc.

Fred B
 
X heads are good if you own a 68-71, to stay original.. some 71's had the 2.02 valve head.You need to keep in mind large valve heads like higher compression, the mixture tended to swirl if the gas quality was lower. the j head came around as gas quality went down. Today, the X head won't perform any better because the gas sucks. Actually, the 70 AAR six pack used the first 360 type smaller valve, because it flowed better, because of so much fuel at a lower RPM. It's in the books. Now, a 69' with 100 octane and X heads, watch out! I beat a street hemi with it! ( back in 1975) Chrysler called it a baby Hemi for a reason. My racer guy always said: big valves high octane and high fuel flow, small valves low octane, . at least how the were built to run then. No technology, no chips, electronic Ignition, no big fancy sticky tires. Pure muscle, and keep it on the ground. It was all light to light, over and over. 0-60 ruled, as it was hard to find a quarter mile on a city street. When you got to the boonies, then things changed. It was all fun! the cops had the attitude: If I can't catch you today, then there is always tomorrow. by 1970 I was in the military, and racing was OVER!
 
The early 340s had a positive deck height piston. They were rated at 10.2:1, so the chambers couldn't have been too small, or compression would have been too high. I don't think the X heads were "supposed" to be any different chamber wise and if they are, I think it's a dead coincidence.
 
I'm guessing a 74cc "X" head must have been on a 68/69 magazine test car, that went 14.90's @ 93mph.....lol.
 
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