J-head question

[ValiantMike]

Would J-heads be a worthwhile improvement to my 318? Its a stock lower end with a Performer intake and a too large Holly, and headers. Car is 4spd with 391 suregrip. Thanks Mike

 

[340 Dart]

My "J" heads were 72cc. Left alone, NO. It will kill your compression.

If you can get your compression ratio up with new pistons or if you can shave enough off of the heads without screwing up your geometry, Yes.

I have a set of "302" casting number heads sitting on the shelf for the future 318 in the Duster. If I were you I would look for a set of these. Great head for a 318.

 

[GoodysGotaCuda]

My "J" heads were 72cc. Left alone, NO. It will kill your compression.

If you can get your compression ratio up with new pistons or if you can shave enough off of the heads without screwing up your geometry, Yes.

I have a set of "302" casting number heads sitting on the shelf for the future 318 in the Duster. If I were you I would look for a set of these. Great head for a 318.

Absolutely. If you do not touch the j-heads it will kill your engine.

Oh the other hand, i have milled j-heads with a performer rpm intake. (56cc chambers). My car runs well enough for me at 280rwhp, and gets out of its own way very well with a 3500rpm dynamic converter and 3.91. :thumbup:

has no lack of grunt (though its not 'fast', it does ok) at 14.0 for 5,200ft. Should be able to dial it in here it florida to mid 13s without much trouble i'd imagine. Stock port J's.

Shameless video adding. lol I was having fuel cutout issues at about 1000ft that night of racing :angry4:

 

[rumblefish360]

If you cam it well and mill the heads some, it'll perform well.
Better off with 302 heads or small valve, small chambered 360 heads. (The "J" may fit the latter well after milling.)

 

[408duster]

The best gain in performance on my sons 318 a few years ago was to replace the J heads that were on it with some 318 castings. They really help on the low end responsiveness. Too much port volume and cumbustion chamber area for a slightly warmed over 318.

 

[ValiantMike]

So to do any good with these I would probably have to mill heads or raise pistons in the hole? Cam is going to be changed for sure.

 

[ValiantMike]

I just want to add that I have these heads already. Got them on a complicated deal. They were supposed to be all freshened up. Numbers on heads are 3418915c and 3418915h. Does this mean anything to anyone?

 

[340 Dart]

That is the same casting number "J" head that I have on my 360.
Just 2 weeks ago I wanted to get my compression ratio up and I had stock dished pistons .094 in the hole. I either had to shave over .0100 off of the heads or go with new pistons. I chose the pistons.
Great head, but there is no way I would put them on a 318 without some of the suggestions above.

 

[GoodysGotaCuda]

So to do any good with these I would probably have to mill heads or raise pistons in the hole? Cam is going to be changed for sure.

My short block is just .030 not milled, just replacement flat tops. I have 9.7:1 Compression how it is :salute:

 

[ValiantMike]

I hear what you guys are saying. As is, too much head for the motor. I'm not ready to tear this motor apart at this time. So I guess I'll shelved them for now, until I can get some pistons. I've been checking the numbers on heads and heads come down to either one or the other of the below listed. Does anyone know how I can find out which one? Mike
3418915 1970 340 T/A 6bbl 2.02" 1.60"
3418915 1971-1972 340 J-heads 2.02" 1.60" 65cc - 73cc

 

[djwhog]

So how much do you guys think he should cut off the heads to get the cc's down into at least the 66cc range, this should get into the range of decent compression correct? If he were to advance the cam 2-3 degrees would that also help the low end for him? Trying to find a way to work with what you got..

 

[ValiantMike]

Thanks Dave. Is there a way I can check the cc/s myself, or do I need to take heads to a shop? Mike

 

[340 Dart]

http://http://www.csgnetwork.com/compcalc.html

Here is a CR calculator for you.

http://http://www.hydrostream.org/ArticleArchives/CCHeads.htm

And this will tell you how to cc your heads. I used a syringe instead of a beaker.

I actually advanced my cam 3 deg. to help bottem end. :yawinkle:

 

[ValiantMike]

Thanks 340, I'm heading out now to find this stuff to check cc/s. I feel like Mr Wizard. Mike

 

[djwhog]

Unless you have the correct tools I would take them to the shop.

Mike the issues are:
1 Larger cc chamber = less compression on a 318 bore.
2 Larger ports valves = less low speed (RPM) air flow speed, this results in a sluggish bottom end less torque etc.


So if you eith have the 318 setup with enough cam, carb, intake and exhaust to utilize these heads you will go backwards in terms of power VS stock heads that have have a great cleanup IE good 4-5 angle valve job, maybe clean up the ports a bit etc. Add better high flow valves as an option too.

Also you need to up the compression to compensate for more camshaft. The reason is that with a bigger high durration cam we end up with more valve overlap. So the valves are open longer, this causes the piston to have less time to compress the air/fuel in the bore = less compression. Is very noticable at low RPMs. So have to get more compression to get the low end back.

I just am not sure on how much to cut these heads with a 318 bore? I think every.015 = 2 ccs. So I think you would need to cut a bunch, then you will need to shave the intake or the intake side of the head, and if you have a hyd cam you will most likely need adjustable 273 type rocker arms.

It sounds like you have good heads, but it were me and I was staying with the 318, and going to get new pistons, I would get a basic cast crank stoker kit for only a few $$ more and then these heads would be a good match.

Hope this helps.. Remmember it is all about bottle necks and part matching. Not only the engine but the trans, clutch or convertor and gearing to get the engine in the proper power rpm range and how you drive it as well..
:headbang: :headbang: :headbang:

 

[340mopar]

I hear what you guys are saying. As is, too much head for the motor. I'm not ready to tear this motor apart at this time. So I guess I'll shelved them for now, until I can get some pistons. I've been checking the numbers on heads and heads come down to either one or the other of the below listed. Does anyone know how I can find out which one? Mike
3418915 1970 340 T/A 6bbl 2.02" 1.60"
3418915 1971-1972 340 J-heads 2.02" 1.60" 65cc - 73cc

Mike, if they are the T/A heads the intake pushrod holes are offset away from the intake ports for porting and the other version has the pushrod holes in the standard location like your 318.

Chuck

 

[ValiantMike]

DJW, thanks for making it clear to me the big picture. I'm going to mull over all of this and come up with a plan.
Chuck, I'm going out now and pull the valve covers on the 318 and compare the intake pushrod holes to the new heads. I'll let you know what I come up with. Thanks guys, I appreciate the help. Mike