X heads, J heads...do they flow differently?

[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.

[whitehatspecial]

Anyone?

[hemihead]

I don't think that if both had 2.02's there would be any difference.The change they made on those engines were the compression not the heads, same with the 440.

[Chryco Psycho]

the flow will be very similar , if you want a quick 40 HP use the 308 casting off a 87-92 360

[Blown70]

the flow will be very similar , if you want a quick 40 HP use the 308 casting off a 87-92 360

Really, CHRYCO, could you tell more.  I may be building a 340. 

Thanks

Tom

[Chryco Psycho]

with mild porting & good cam choice , I have seen a 40 HP rear wheel HP gain this is without changing cams just having a good cam in the engiune to start with , these are are a LA head & take stock rockers & shafts & intakes & exhaust manifolds

[694spdRT]

Is there any advantage to doing the Magnum head conversion over using the 308's?

I already have a set of Magnums that I was thinking about replacing a set of stocK '72 318 heads with. I would just need to buy the pushrods and new intake as everything else is ready for the switch.

[Chryco Psycho]

I have not tried the Magnum swap yet to see what it will do but I doubt it will do much better given the magnum crate I worked with last year , if you have the Magnum parts go with htose , you will need a set of AMC lifters as well to make the magnums work

[hemihead]

The 308 comes with a smaller intake valve than the X, correct? Is the 302 casting for the 318 different than the 308?

[694spdRT]

I have not tried the Magnum swap yet to see what it will do but I doubt it will do much better given the magnum crate I worked with last year , if you have the Magnum parts go with htose , you will need a set of AMC lifters as well to make the magnums work

The 318 was built using a 270H Comp Cam and I was reading that most Comp lifters were already the oiling type. I won't really know until it is opened up. From what I have read the '72 318 heads are pretty much worthless from a performance standpoint so I am sure there would be some noticeable improvement with either head swap. I was wondering about the 318 pre-magnum roller heads as well. Are they any good because around here the #308 360's engines are hard to find in junkyards for some reason.

[owatajrkiam]

I've got a set of "J" heads on my car,ported,flow tested,and a good" blueprinting" they are just short of the edelbrocks.The "X"s are expensive and hard to find,they only flow a few cfm better on the exhaust while the "J"s flow about the same on the intake.Get a reputable machine shop to put in hardened seats if they don't have already.Get out the grinder and just clean up the floor of the "Dog Leg"floor on the exhaust,gasket match em..If you can't put in "good"stainless valves,as they are "pricey",have you're machinist back cut the valves and take that grinder and unshroud the valves in the combustion chamber.These are the two most bang for the buck things you can do to these heads to wake em up.If you have a heavy car and auto, you can stay with the 1:88s as they keep the velosity up at lower rpms.lighter car w/4gear or auto w/high stall go with the 2:02s.If you try and use the "308"s,you better know a good welder as many of these heads had a cracking problem under the exhaust seat.In all honesty the magnums are the way to go,with a bit of porting,"pro"valves and a good "blueprinting",these heads are in W-2 territory.Good luck.Glen.

[Chryco Psycho]

308 & 302 castings are different, the 302 has a much smaller combustion chamber & yes the have the 1.88 intakes but they still flow well or can b e upgraded to 2.02
many of the 308 castings crack between the seats but if you machinist is any good it is a very easy fix to install a pin under the seat & it willl never crack there again

[hemihead]

Thanks Chryco, time to hit the U- Pull - It.

[694spdRT]

308 & 302 castings are different, the 302 has a much smaller combustion chamber & yes the have the 1.88 intakes but they still flow well or can b e upgraded to 2.02
many of the 308 castings crack between the seats but if you machinist is any good it is a very easy fix to install a pin under the seat & it willl never crack there again

Wouldn't the 302's smaller combustion chamber be a good thing for compression or will it raise it up to high?

[Chryco Psycho]

yes but it comes with even smaller ports , the 318 heads do have the heart shaped chamber too