308 casting or new la - x iron heads???

[woody9512]

I'm going to replace my J-heads that have been on the car since I got it about 15yrs ago. I have to take one off eventually due to a exhaust stud issue. I'm not sure if they have the smaller valves or the bigger 2.02 valves as I have never had them off. I have a set of 360 heads "308" castings that I have had for a couple of years now and they are all cleaned and were crack checked and ready to use.
My question is am I better to just have these 308 heads done up with 2.02 valves / springs and all that good stuff as I read that they flow well and have very good ports or am I better to go with the newer LA - X heads "RHS" that I read about??   I like the stock look of the engine and plan on keeping my original stock manifold / valve covers / TQ Carb. Just looking for maybe a head with a bit more performance that what I'm taking off.
Car now has:
Engle Hyd Cam K58.......514 lift.........TTI headers......Magnaflow 2 1/2 stainless exhaust....3:55 gears!!
Thanks
 

[woody9512]

Forgot to add. Its a 340 4 speed

[Chryco Psycho]

I have no experience with RHS heads but doing a simple swap X to 308 with the smaller 1.94 intakes we gained 41 RWHP

[dodge freak 2]

If the 308 are crack free USE them. They also have the exhaust heat passage for the intake, I run the exhaust heat year round up here but night temps in the summer are in the mid 60's and very rarely above 80. I  block the heat in the past but the intake seem to get too cold in like 50 degree temps-which happens alot up here in Mich.

Sure right now with this 95--100 degree heat I be better off blocking the intake heat but I just don't drive it much and it be in the low 80's during the day before long.

I have those heads but the MP castings-forgot the number. Mine has 2.02 intakes and the exhaust ports never been drilled for A.I.R ports I think its called. Towards the bottom of the exhaust flange there be these 1/4--1/8 inch tiny holes, I would tap and plug them up.

I got no experience with those china heads 

[dodge freak 2]

I have no experience with RHS heads but doing a simple swap X to 308 with the smaller 1.94 intakes we gained 41 RWHP

Never dyno-track my car but it can go it seems, Muscle Motors told me 5 years ago they thought the 308 design heads are the best factory LA heads you can get.  The newer magnum hd heads can top them but then I pointed out I rather have the shaft rocker design. Mine are milled .030 to increase compression

 

[woody9512]

One thing I do notice is that when I put the 308 head next to a J head it looks like the exhaust port holes (where the headers attach to) looks a bit smaller on the 308 head and the holes are  even shaped a bit different as well than the holes on the J-head??
But that being said a 41hp gain is pretty impressive

[Chryco Psycho]

the 308 head has the floor raised in the exhaust port which helps power even if it is smaller

[woody9512]

I noticed that when I put my finger inside the exhaust outlets that the bottom / floor of the 308's curves downward quite a bit more than on the J heads. Would I be better to leave the smaller valves in them or go to the bigger 2.02 size??

[dodge freak 2]

Well the car only has 3.55 gears, the larger intake valve will mostly help top end power. I'm guessing it will not lose power by going with the larger 2.02 intake but I'm just guessing.

I do know some folks told me not to go with the 2.02 on the 318 but I swear those heads pick up power everywhere after I did the swap.

Its just how much $ you want to spend, I'm guessing a professional port job would help as well--that was too much for me to spend.

Personally if the heads are ready to bolt on, I would not upgrade to the larger valves but if the heads need a valve job, then I would look-price how much more new valves would be. Don't get cheap valves, if the stem breaks and drops the valve, good bye piston and maybe cylinder wall

[Chryco Psycho]

I would lean towards the 2.02 , it will bring the valve seat back upo to stock height & will not hurt so it is more of a budget concern

[woody9512]

I had hoped to get a new set of heads all ready to be bolted on either the new RHS style or my spare set of 308 castings all done up so I just take the old J heads off and put my new set on. I am told that the new LA-RHS heads will only work if my 340 has low compression pistons in it due to the difference in the chamber size / type on the new RHS heads?? I have never had these J heads off the car so I have no idea what type of pistons it currently has??  Apparently the high compression 340 pistons come up above the block and will not work with the smaller chambers in the RHS heads?? I was told if it does have the high compression pistons maybe a thicker gasket could be used for height to allow me to use this RHS heads with the high compression pistons? Or I could just get my 308 castings done up / maybe bowl ported (I'm told make them flow better) and add bigger 2.02 valves and just use these 308 castings as apparently these will work no matter the piston type that is in the car?
Any of this make sense as to the compression thing
I know now my car run good on 94 octane fuel (Ultra 94) that I get at Petrocanada but if I use fuel with less octane that the 94 I get spark knock??  I think maybe my pistons now are the higher compression ones??
Thanks
"Tim"

[dodge freak 2]

You really believe the pistons have never been replaced yet ? Chances are they have been

The 340 piston does come up .018, the rhs heads are close chamber, yes a .050 head gasket might be enough but then again if those are 42 year old pistons they will be rocking at tdc

Again I would build the 308 heads if they are not crack

[woody9512]

I have no idea if the pistons have been changed or not as I've never had the heads off. Probably will just go with the 308 heads I have!

[jimynick]

I stand to be corrected, but I believe the 1.94 valve heads came on the lower compression 72 on 340's, so the piston crown may well be lower. In the final analysis, you should check the valve clearance on ANY head you put on it. It also seems to me that one of the hot rod mags did a test on the RHS heads, so maybe google it and see. A little modellers clay goes a long way in a deal like this. Good luck 

[72bluNblu]

The RHS heads will outflow the 308's in stock form by A LOT. Even with 2.02's, the RHS LA-X heads will outflow the 308's.

In out-of-the-box form with 2.02/1.60 valves the RHS heads typically flow around 255 to 260 cfm on the intake side at .550-.600". I've seen higher numbers, as much as 273 cfm, but obviously that depends on the bench, valve job, etc.

308's in stock form will not flow nearly that much. On the intake side they'll be similar to X or J heads with 2.02" valves, typically these flow around 200 cfm, but usually peak around the .450" mark. The big gain on the 308's is the exhaust side, which most people don't test. The exhaust port on the 308's is similar to the later magnum head ports, and flows quite a bit better than the stock X or J heads. Since the exhaust port is really the bigger bottleneck for LA heads, the 308's can be a substantial improvement over the earlier heads.

I have set of 308's that I had ported, basically a stage 3 port, with 2.02/1.60 valves. They flow 264 cfm at .550", or a maybe a TINY bit better than the out-of-the-box RHS heads depending on whose bench your using. 

Biggest thing will be your pistons. RHS heads are closed chamber heads, and are usually around 62-64 cc's. 308's are open chamber heads, and usually between 65 and 72 cc's. But the open chambers have a different shape and give a lot more clearance for the pistons. If you have an early 340, that's important. In fact, that's why I went with ported 308's on my 340, because it's a '68 and the pistons are .018" proud of the deck. Hard to make that work with a closed chamber head without a really thick head gasket.

If money is no object, the RHS heads will outflow the 308's hands down unless you do some significant porting on them. In which case the RHS heads are probably a better deal, you might be able to get them for LESS than it will cost to have the 308's ported to that level and rebuilt with 2.02's.

But even with just the 2.02/1.60 valves, the 308's in unported form should support up to around 400 hp. If you're not planning for more than that, the 308's will be fine, and you already have them.