340 J or X heads?

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    Author Topic: 340 J or X heads?  (Read 4767 times)

    Offline Tonefiend

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    340 J or X heads?
    « on: April 01, 2010 - 08:31:01 PM »
    I have a line on some J and X heads for a 340.   Both need valve jobs and are priced at $250 and $300.

    What is the better head for a street cruiser?  I would like to get around 400-450 hp
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    Offline 72hemi

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    Re: 340 J or X heads?
    « Reply #1 on: April 01, 2010 - 08:36:23 PM »
    For me I would go with the X heads, but I paid $350 for mine and they were fresh ready to go.
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    1972 Dodge Challenger 340 6 Pack 4-speed
    1996 Dodge Viper GTS Coupe

    Offline Moparal

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    Re: 340 J or X heads?
    « Reply #2 on: April 01, 2010 - 08:42:11 PM »
    maybe some 308 casting heads would flow better for you. The J and X's flow about the same. You will need to do a bunch of work to get them up to 450 hp with the 340
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    Offline 72bluNblu

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    Re: 340 J or X heads?
    « Reply #3 on: April 02, 2010 - 12:09:24 AM »
    X and J heads are basically the same, the X's had 2.02/1.60 valves stock, I think most of the J's had 1.88/1.60's but that's easy to change. Otherwise the castings are the same, so a J head with the larger valves will flow the same as a X head. 

    The 308's would flow better on the exhaust side, although they're pretty similar on the intake side. They do have the advantage of already having hardened seats, which neither the X's or J's will have. But they also have the 1.88 sized intake.

    Getting to 400 hp with either X's, J's, or 308's will require the 2.02/1.60 valves. You should be able to get to 400 with stock heads, but if you want to get over 400 hp I would say all of the above choices would need porting. At least to keep things reasonable with compression and cams. That's basically the build that I've done on my 340, which goes like this...

    .060" over, KB 234 hyper' pistons (~9.8:1 with my specs)
    308 heads, 2.02/1.60 valves, ported to flow 264 cfm at .500"
    Lunati 60404 (.513/.533 lift, 276/284 advertised duration, range 2200-6400)
    Stock forged crank
    stock rods, re-sized with ARP bolts
    Edelbrock RPM air gap intake
    Hooker super comp 1 3/4" headers

    Looking for the same 400-450 hp, should have the engine together shortly, hopefully in the Challenger with a 4 speed by the end of summer.


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    Offline Tonefiend

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    Re: 340 J or X heads?
    « Reply #4 on: April 02, 2010 - 02:57:14 AM »
    Thanks '72 that is some great info.  I think I will look for the 308 heads.  Your build looks like it's about exactly what I am after, my buddy helping me with the build recommended the hyper pistons. 
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    Offline Chryco Psycho

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    Re: 340 J or X heads?
    « Reply #5 on: April 02, 2010 - 03:19:18 AM »
    properly ported the 308 will eat a X or J casting & you can install the 2.02 intake as well
    the 308 is on trucks 87-92 [pre-magnum] engines
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    Offline 72bluNblu

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    Re: 340 J or X heads?
    « Reply #6 on: April 02, 2010 - 03:44:49 AM »
    Quote from: Chryco Psycho on April 02, 2010 - 03:19:18 AM
    properly ported the 308 will eat a X or J casting & you can install the 2.02 intake as well
    the 308 is on trucks 87-92 [pre-magnum] engines

    This is true!  :burnout:

    These are flow numbers for a stock set and a set of stage 3 ported J heads from the Shady Dell website.

    LIFT------ASCAST---------PORTED
    .100”----62.1/46.6---------62.1/51.8
    .200”----112.1/99.4-------135.2/103.5
    .300”----156.6/128.3-----198.7/135.2
    .400”----193.2/137.3-----239.8/158.7
    .450”----200.1/138.3-----253.9/169.7
    .500”----200.1/139.7-----258.4/182.9
    .550”----200.1/140.8-----264.9/186.3
    .600”----200.8/141.5-----254.6/186.3


    These are the flow numbers from my 308's (intake side), plus a reference set of 915's (J heads) that were flowed on the same bench as the 308's. This was done at 28" of water just like the Shady Dell #'s.

    Lift (inches)   Stock 915's   308's
    0.200"                            165.78
    0.250"         154.63         184.36
    0.300"         178.42         202.20
    0.400"         199.97         243.84
    0.450"         197.00         258.70
    0.500"         193.28         263.91
    0.550"         193.28         257.22
    0.600"                            252.76

    The stock 915's (j's) outflowed the stock Shady Dell J's down low, so that may be a bench difference. Or it could just be that set of heads, the J's have been know to vary pretty widely from head to head. But my 308's outflow the stage 3, Shady Dell ported J's all the way .500" of lift. Some of that is probably the different benches, but considering I don't have a stage 3 Shady Dell port on mine, I think its pretty darn impressive.  :grinyes:

    And Steve (71chally416), I promise I'll have that 340 and those heads on a dyno, hopefully sooner rather than later. But you know how these car things go sometimes.  ::)
    « Last Edit: April 02, 2010 - 03:52:50 AM by 72bluNblu »
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    Offline wally426ci

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    Re: 340 J or X heads?
    « Reply #7 on: April 02, 2010 - 06:47:16 AM »
    Would you have to use a roller cam then with 308's?  :clueless:
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    Offline Chryco Psycho

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    Re: 340 J or X heads?
    « Reply #8 on: April 02, 2010 - 09:47:53 PM »
    no you can use any cam even if you use the roller cam block
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    Offline Tonefiend

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    Re: 340 J or X heads?
    « Reply #9 on: April 02, 2010 - 10:03:32 PM »
    Wow cool info.  Do he 308 heads go by any other name? 

    What do I need to look out for in regards to the cracking issue?  If I get a set thats uncracked, will they last?
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    Offline Chryco Psycho

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    Re: 340 J or X heads?
    « Reply #10 on: April 02, 2010 - 10:08:53 PM »
    You can easily repair the crack between the seats by installing a plug , all the sets of heads I have found have been cracked
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    Offline Tonefiend

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    Re: 340 J or X heads?
    « Reply #11 on: April 03, 2010 - 03:31:46 AM »
    Quote from: Chryco Psycho on April 02, 2010 - 10:08:53 PM
    You can easily repair the crack between the seats by installing a plug , all the sets of heads I have found have been cracked

    Cool thanks again Chryco!

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