J (915) vs. Magnum heads

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straightlinespeed

straightlinespeed

Sometimes I pretend to be normal
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Im currently running a set of magnum heads on my 360. I have a lead on a set of 915 J heads.

Can someone please tell me the differences of the two heads and if one is better than the other? Also what is a fair price for a set of bare J heads that have been magnafluxed but need a valve grind.
 
Magnum really doesn't compare to j. Magnums flow better and have a smaller combustion (closed) chamber.
 
Justin always argued the J heads were much better. He has a flow bench.
 
2 different beasts if you really want to get into it.
magnums are modern, and in stock form will probably do better than J's
Js have way better longevity, and porting possiblity. you know the Js need a valve job, so hardened seats mill you will have alot into Js.
But if you are building a motor the Js nice.

rusty you are correct he hates magnums. lol i would never want to run them myself either, unless maybe they came with another fresh set to swap in when they crack. lol
 
Figure on completely rebuilding the Js if they at the point of needing a valve job. Valves, guides, springs, locks, retainers, valve job, milling to equal out the compression loss, ect. Not to mention needing the LA valve covers, rockers, shafts/hold downs, head bolts, intake, ect.
Even once this is all done, you still need a fair amount of port work to get the Js to out flow the Magnums. Just swapping the Magnums for the Js in near stock form would result in a performance loss.
 
Agreed.

J's are open.

The T/A cars had the pushrod holes moved. Those would be ideal, but you'd need the rockers that go with. Also, if you have those heads, start looking for the rest of the car.

If you start tinkering with cylinder heads, look into a set of Engine Quest heads to start with. Your parts will swap into them and they are better castings than stock magnums. Or look into a set of aluminum heads.

The only way you'll see a gain on a set of J heads out of the box is if you are running negative cc pistons (positive deck height) like the T/A cars ran.

Have you checked cylinder pressure?
 
The only way you'll see a gain on a set of J heads out of the box is if you are running negative cc pistons (positive deck height) like the T/A cars ran.
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All 340s had a positive deck, not just the T/A's

I'm not a big fan of the mags BUT, if i had them on already, i wouldn't go to a J. Why? Because, i would loose compression, and a Quench pad is not a bad thing :thumrigh:
 
Cudafever said:
All 340s had a positive deck, not just the T/A's

I'm not a big fan of the mags BUT, if i had them on already, i wouldn't go to a J. Why? Because, i would loose compression, and a Quench pad is not a bad thing :thumrigh:
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Yep.

Depending on late/ early X casting or all J casting, the piston was different to fit the chamber shape. They were looking for 10:1.

If you've got flat tops and anything close to zero deck, you want tight gaskets and closed chambers.

The reason I would suggest Engine Quest heads, should you ever have to change or have cracks, etc., is because all of your current running gear will swap over and they have better quality casting.

Other than that, you'd gain more from RHS or Edelbrocks, but if you go that route, you'd still be better off with Indy's reworked Edelbrocks. The guide material sucks and CNC quality control is shaky, out of the box on the Edelbrocks.

If you're looking for more hammer, spend your money on a converter. If it's a street car, go with a Dynamic. If you like it loose all the time with a fair ratio and wanna save a little more, go with a Hughes.
 
Was the late 340 8.5 motor positive deck height? I sure would like to see evidence backing that up.
 
RustyRatRod said:
Was the late 340 8.5 motor positive deck height? I sure would like to see evidence backing that up.
Click to expand...

Hoping some one was going to argue that point.

I had a 73 340 but never built it,.......i thing even that one, was above the deck, but I'm just not sure.
 
skrews said:
Figure on completely rebuilding the Js if they at the point of needing a valve job. Valves, guides, springs, locks, retainers, valve job, milling to equal out the compression loss, ect. Not to mention needing the LA valve covers, rockers, shafts/hold downs, head bolts, intake, ect.
Even once this is all done, you still need a fair amount of port work to get the Js to out flow the Magnums. Just swapping the Magnums for the Js in near stock form would result in a performance loss.
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s
Unless he is using stock mag springs, which in that case he would need new springs if putting a real cam in anyways, he could swap them over.

Head bolts, what I think its only a couple that are shorter?

According to everyone on this site, minus you, the valve covers interchange.

.020 would even them up, thats about $65-90 for the pair.

Other than rocker gear in which the j's shaft mount is better, and chamber...they arent much diff.

If you do your homework you will find that the mag has a few more cfm in stock form in the .200-.300 lift range, otherwise the j heads flow more and are better suited for a higher lift camshaft.

In the performance world where every part is pretty much messed with to make more power, I dont see enough return investment in messing with Magnum heads.
I do think that if the op is cruising a stock/next to stock mill that the magnum head is fine.

Unless they are ported, its almost apples to apples but with one of the apples having crack indusive worms.
 
I think both heads are good if you already have them. as mentioned above there's a lot of things need changing when swapping from magnum to LA or vice versa.

Different intake manifold. Different valves, retainers and keepers. Different rocker covers. Different rocker design. I think maybe the springs can be transferred from one to the other and that's about it.

If you're after increased performance, there are more productive ways to spend your money.
 
Hebrews10:26 said:
s
Unless he is using stock mag springs, which in that case he would need new springs if putting a real cam in anyways, he could swap them over.
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So your point is ?

[/QUOTE]Head bolts, what I think its only a couple that are shorter?[/QUOTE]

Try buying just those 2 bolts alone. Good luck.

[/QUOTE]According to everyone on this site, minus you, the valve covers interchange.[/QUOTE]

I've got a Magnum head and an LA Mickey Thompson cover right here in front of me. They bolt on but, the gasket flange on the ends doesn't align. You can see right up into the head through the upper corner of the cover. They they might work with a wide and thick enough gasket, but questionable at best.

[/QUOTE].020 would even them up, thats about $65-90 for the pair.[/QUOTE]

To even up the chamber volume 72ish cc for the J, 64ish for the Magnum one would need to mill .048 at 1cc per .006" milled. Another expense to gain nothing.

[/QUOTE]Other than rocker gear in which the j's shaft mount is better, and chamber...they arent much diff. [/QUOTE]

The OEM LA rockers suck. They are weak and ratio inaccurate. The shaft mounting is a real plus though, but unless turning some high RPM with lots of spring pressure the advantage is insignificant. The chamber in the Magnum light years ahead of the LA.

[/QUOTE]If you do your homework you will find that the mag has a few more cfm in stock form in the .200-.300 lift range, otherwise the j heads flow more and are better suited for a higher lift camshaft.[/QUOTE]

I've yet to see a stock J head that flowed better than a Magnum. The difference isn't drastic though.

[/QUOTE]In the performance world where every part is pretty much messed with to make more power, I dont see enough return investment in messing with Magnum heads.
I do think that if the op is cruising a stock/next to stock mill that the magnum head is fine.[/QUOTE]

This is my point exactly. Why spend a bunch of money and time to gain nothing or likely go backwards. To swap to the LA heads would wind up costing him over a thousand $ by the time the things I stated were addressed.

[/QUOTE]Unless they are ported, its almost apples to apples but with one of the apples having crack indusive worms.[/QUOTE]

Take it from the guy that does the Magnufluxing, both heads have an equal tendency to crack.
 
Both heads are capable of making the same power. Just pick one and go.
 
I think he should leave the gaskets sealed and get a Dynamic converter for the dollar bills burning holes.

The late 340s do not have positive deck pistons. Flat tops, with J heads and 1.80 valves.

Open chambers with metal shim gaskets give it an optimistic 8.5:1. Closer to 8.0
 
Depending on your eventual goals, RHS and edelbrock heads are available in magnum configuration.

There's also EQ heads which are a very impressive unit.

fwiw, I run J heads on my engine and I truly love them. BUT I bought them when I was 18 for $500 AUD and got new guides fitted and a performance valve job done on them, I have recently had the bowls ported for more performance.

They're good heads, but if I had known then what I know now I would have paid a little more, been a little more patient and started with something better.

That's why I have a set of edelbrocks sitting on a shelf, still in their boxes waiting for my next engine build.

Hmm actually maybe patience still isn't really my thing after all.:D
 
I had no choice, being a Magnum s.b. When I yanked the stock Magnum heads,the E.Q"s,were .100+ taller onthe roof. Stock Magnum,"suppoesdly153 .c.c intake runner. The E.Q.'s a true 170-ish..These heads ,are awesome for the money.
 
Abodybomber said:
I had no choice, being a Magnum s.b. When I yanked the stock Magnum heads,the E.Q"s,were .100+ taller onthe roof. Stock Magnum,"suppoesdly153 .c.c intake runner. The E.Q.'s a true 170-ish..These heads ,are awesome for the money.
Click to expand...

Oh-so that engine IQ talked about is yours? :)
 
No TX. Spent so much time ,researching this before I even became a member. Mine has 150k plus on it. The cam went flat TX. The info's here,some people forget their own posts.
 
Ah-my mistake, I saw your other post about building an engine to fit the car...a 400-ish 360 Magnum etc etc etc. I know I'm definitely guilty of forgetting my own posts
 
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