360 J heads vs 318

Engine, Trans & Driveline

  1. Sonny

    Sonny Well-Known Member

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    I have a 1978 360 engine I just put in my Belvedere. It has 318 heads on it. Is there an advantage to putting a set of 1974 360 J heads on it? There is a guy on craigslist selling a pair fresh from the machine shop for 200 bucks.
     
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    • 451Mopar

      451Mopar Well-Known Member

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      yes, the 360 heads have larger valves and ports than the 318, but the chamber is larger too, so it would drop the compression ratio a bit. Why does the 360 engine have 318 heads?
       
    • Sonny

      Sonny Well-Known Member

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      No idea. It was a low miles crate engine bought this summer. Runs great but looking at getting those heads. What’s the impact of reduced compression? Does it cancel out the larger valve upgrade?
       
    • PurpleBeeper

      PurpleBeeper Well-Known Member

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      Good question. Maybe look for Rumblefish on this site (knows a TON about small blocks). If you have a "340/360 intake port size" intake manifold, then 1/3 of your incoming air is hitting a "cylinder heads port wall" which is horrible for flow. My gut instinct is that any compression advantage of smaller combustion chamber (318) heads is nothing compared to the flow loss at both the intake runners & the smaller valves vs. the 340/360 heads. Also, if you're worried about lowering your compression, you could always use some "stock style" (very thin) head gaskets....just my 2-cents. I'd buy those heads if the guy can provide paperwork on the rebuild.
       
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      • Bill Monk

        Bill Monk Well-Known Member

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        I agree. buy the 360 heads, the lower compression will help with the lower octane fuel anyway and they already had the hardened seats.
         
      • GTXperience

        GTXperience Well-Known Member

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        I'm wondering which 318 heads those are (possibly the later 308 swirl ports?). If so. they may be on there for compression/velocity reasons. What's the specs on your motor? I know you said it was a '78 but you also mentioned it was a crate motor as well.
         
      • Sonny

        Sonny Well-Known Member

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        0A5F7CC6-BD2B-4A04-88F5-64A6166ECE6D.jpeg 2E114E87-AF34-484B-A585-98EA9D49C292.jpeg 77D46AC5-1FA3-4CEF-8624-77CF6A5D545C.jpeg EA5D4B0B-E5CE-4E9E-9810-EB57E5A55904.jpeg
        l don’t have any info on it. That’s what l was told. And that it came out of a ‘78 Karry-van. Here’s a few pics. Notice the tiny exhaust ports. lntake openings are small too.
         
      • Dibbons

        Dibbons Well-Known Member

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        Someone/somewhere on a forum said the intake manifold mis-match is no big deal because the intake charge is sucked in (like a vacuum cleaner) and not pushed in from the outside (like a blower). I don't know if that theory is more "correct" than what PurpleBeeper stated in post #4 or not. Just a thought.
         
      • mopar 3 B

        mopar 3 B Well-Known Member

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        I dont think there is a difference in 318 360 exhaust manifolds. From what i have read the swirl heads are preferd
         
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        • 451Mopar

          451Mopar Well-Known Member

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          If you could read the head casting numbers it would tell which heads they are. Should be on the head under the valve cover/rocker gear. As mentioned, if they are the 308 heads, they are supposed to be pretty good.
           
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          • toolmanmike

            toolmanmike Super Moderator Staff Member FBBO Gold Member

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            Yes those are either #302 or the #308 "swirl port" casting heads. You can tell by the plugged air ports below the exhaust ports. They have a closed heart shaped chamber and good flowing ports for a factory head. I wouldn't change them.
             
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            • toolmanmike

              toolmanmike Super Moderator Staff Member FBBO Gold Member

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              Ps, the 74 J heads are small valve open chamber.
               
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              • rumblefish360

                rumblefish360 Well-Known Member

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                Hey Sonny. I’m on vacation right now in Palm Bay over by Melbourne this week. I don’t have any books on hand to decode any numbers you may gleem from the cylinder head.

                I hate to say this but I truly don’t mean any harm by what I’m about to say, most of the info above is wrong.

                This site rocks for big blocks co knowledge which I sorely lack on. These guys got that down packed tight.

                As a note to everyone, I’m going to quote everybody and correct some of the information as a correction and nothing else, so don’t get pissed off at me when I quote and correct.

                I mean no harm in any way shape or form.
                 
              • rumblefish360

                rumblefish360 Well-Known Member

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                This is 50% correct or backwards. If the 360 heads go into a 318. It would be more accurate to view it as returning the engine to its proper ratio, which would be lower. How low? Depends on where the piston sits and the head gasket used.
                 
              • toolmanmike

                toolmanmike Super Moderator Staff Member FBBO Gold Member

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                Lots of variables and unknown parts.
                 
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                • rumblefish360

                  rumblefish360 Well-Known Member

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                  Generally speaking, in a low performance package, 1 pt. in ratio is 3% and probably less for your stockish set up, if it is stockish.

                  So, for example, IF your engine is making 300 HP, your down 9 hp. Larger valves question.
                  1.88 vs. 1.78 is an answer I can not calculate due to a lack of information. But I’d call it a worry move. I have felt a noticeable difference in moving from a 1.88 to a 2.02. It wasn’t a huge difference at all but one that was noticed.

                  Member IQ52 did do a test on Flow vs. compression and the clear winner was flow!
                   
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                  • rumblefish360

                    rumblefish360 Well-Known Member

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                    Super agree with this and thanks for the Kudos buddy.

                    While the 1/3 mention maybe off, the idea still holds true. I never measured the port sizes or did a measurement of actual amount of port wall interference. But it is something to avoid if possible.
                     
                  • 451Mopar

                    451Mopar Well-Known Member

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                    That was the quick, 5 in the morning answer. Since we really don't know any specifics about the engine, Normally the 318 head will have a smaller sized combustion chamber than the 360 head, which would result in a higher compression ratio than the 360 head with all other variables being the same.
                     
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                    • rumblefish360

                      rumblefish360 Well-Known Member

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                      318 swirl port heads are to have the “302” Designation. Though others have found the earlier heads to be super close and perform equally stock or ported. The 302 has a slight advantage and thus is the reason MP used that head for there performance ported 318 head. (Also IMO because it was the current head being casted.)
                      The ported 302 made 55 hp more than the 360 head on there little bit rod 318.But that’s a 318 & we are talking 360 here, so..... moving along.....
                       
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                      • rumblefish360

                        rumblefish360 Well-Known Member

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                        This has merit. Small amounts. The way it all works is a long explanation. It would take a while to explain it all in detail with science and pictures. David Vizard does an excellent job.

                        After the cylinder fires and the exhaust valve opens up the exhaust gas starts going out of the cylinder. There is a pressure wave that travels down the exhaust pipe and then back upstream to the head and then Back out. As this wave leaves it creates a negitive pressure area that helps create a wave that the air and fuel ride on like a surfer into the cylinder.

                        That was a simple explanation that is really dumbed down but provides a picture that everyone can see.

                        This is responsible for filling the cylinder more than anything and if tuned... OH BOY! Whatch out!

                        Keep in mind that the higher pressure from outside is pushing its way through the carb. This accounts for a little bit like the surfer paddling for the wave.
                         
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