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...musical exhaust manifolds...

1629 Views 45 Replies 15 Participants Last post by  Canadian country boy
Ok folks - for my mild 360 magnum 'truck' engine...I'm looking at two versions of center dump small block manifolds (for both sides of my engine), and I've narrowed it down to these two: '71 340 p-side, and the later 70's 318/360 truck manifolds.

Both manifolds have similar port sizes, and the same 1-7/8" i.d. exhaust outlet diameter. I was going to use the magnum manifolds that were on the engine, but they aim way way back and frankly make 'getting in there' to access the transmission linkage and such pretty difficult, plus making things pretty toasty and requiring more shields, etc. The magnum manifolds also have the same outlet size of 1-7/8", and with that in mind I'm looking at center dump exhaust manifolds, so back to the 340 or later 318/360 units. The 340's flow good, and I think the 318/360 units probably flow the same, so I doubt there's not much of a flow difference that I would ever notice, and I don't care about gaining/losing 5 hp at 5000 rpm. The later units have more 'cooling fins' and a bigger internal core...but I don't know if that has any bearing.

My brain tells me these (below) both flow about the same, and will flow plenty well enough for my mild 360, again built to be 'done' by 4500 rpm - I'll just be adding gears - not for high rpms - for low rpm grunt work.

So, that said, am I missing something, or would either of these suit me just fine - I'm leaning towards the 340 units...and unless someone convinces me otherwise I'll be picking up a set of the 340 manifolds from a local club buddy tomorrow morning. But if the later truck units would be better...well tell me why.

71 340 -

70's pre-smog small block (318 and 360 I think) -

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I'd use the 340 manifolds simply because I'll bet they are lighter. (I assume)
IMO I like the look of the 340 ones better than the later stock style. It probably would come down to that in my position if they both measure out nearly identical. I don't really see how the "cooling" fins would help much in a low speed setting unless the newer style are thicker proportionally on top of the fins.

Just my .02...

To me, the later ones just say Mopar small block. You do not see that style on any other make.
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The 340's do flow better than any of the truck manifolds for sure but so do the Magnum manifolds. Actually the best manifold combo is the drivers side 340 manifold and the passenger side Magnum manifold. The 340 manifolds on both sides exit in between cyl#'s 5 and 7 and 6 and 8, so they are in between the Magnum manifolds and the 318/360 manifolds. Why not just get some shorty block hugger headers from Summit?

Or you could go with the Magnum manifolds and run a Lokar cable kickdown....

If the truck manifolds flow equally to the 340 manifolds, would it make sense to use the truck ones and sell the 340's for $$? I imagine they have way more value than the truck manifolds. Just my 2 cents......
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If you are after flow, I know where I could get a set of the 2" 83 Magnum manifolds.
If the truck manifolds flow equally to the 340 manifolds, would it make sense to use the truck ones and sell the 340's for $$? I imagine they have way more value than the truck manifolds. Just my 2 cents......
They don't. Only the Magnum manifolds flow close to the 340 manifolds but only the '68-'70 340 manifolds. The '71 and later had a small exhaust port on the passenger side.
92/93 360 manifold- 2 1/8" exit hole
96/97/98 Jeep Grand Cherokee 360 manifold - 2 1/8" exit hole
Stock 318 manifold - 1 5/8" exit hole
70s 340 passenger manifold - 1 7/8" exit hole
All 340 driver's manifolds - 2 1/4"
60s 340 passenger - 2 1/4" exit hole


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Shorty headers flow better than 340 manifolds for torque and horsepower by only 10 ft lbs and horsepower which can be hard to notice sometimes.
well like I said in my first post, because of potential clearance/heat/access issues the magnum manifolds are a non-starter - otherwise I'd use them; I even tried a set of the Grand Cherokee manifolds but they weren't any better. Nope, my goal is to find a set of center dump manifolds that will flow plenty well enough for a truck engine. Headers are out for several reasons, all of which have nothing to do with flow. Sounds like the 340 manifolds are a perfect blend of everything I want, and need, and my Mopar buddy offered a set to me for the same price as what a set of the truck 360 manifolds are going for on ebay, so "340 manifolds" it is.

Thanks guys - always a pleasure :) (y)

- Sam
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IDK. For the minimal difference in flow, I think I'd go with the truck manifolds.

They are bulkier and could probably hold up better in high load high heat situations. Not often a car is run full throttle for multiple minutes at a time like a loaded truck might.

If you're after the last bit of power, the manifolds can have their outlets bored.
Is Extrude Hone still a thing?
Seems so. There's a place near me in Charlotte NC that does it.
Is Extrude Hone still a thing?
I think so, but I'm not after 'that' kind of flow...
The 340 was Chrysler's small block performance oriented engine. So it would be logical to believe that the factory manifolds flowed pretty good for their time. The magnum manifolds are pretty good too, but lack your central dump requirement. I would avoid standard LA exhaust manifolds.

Is Extrude Hone still a thing?
I think so but not worth the money unless you're dumb like me. I paid over $600, 10 years ago. Wonder what it would cost today.
well shoot - my 'center dump exhaust' idea is just not gonna work - too much interference with the magnum engine mounts. I thought about going with LA mounts but that juice isn't worth the squeeze, so I'm back to using magnum manifolds, and I'm looking at the possibility of using a set of Grand Cherokee manifolds I have with the bigger 2-1/8" outlets. Problem is the d-side exit aims pretty close to the starter, and while I can alter that I'm weighing the bigger picture of pro's vs con's between having a 2-1/8" outlet or a 1-7/8" outlet. After lots of off-line discussions I'm convinced neither size will really 'matter' - both will work just fine and that the bigger size won't 'hurt' my bottom end torque...maybe even improving it way down low in my 'primary range' between 1500-2500 rpm. Again, pro's n con's.

I've already cut the GC manifold's outlet and had it professionally re-welded to aim away from the starter (now it aims in front of it instead of 'at' it, and I'm now simply wondering about the long-term effects of running a welded cast iron manifold. It was done right using nickle rod at a very good and reputable shop, so if it were to be done I'm near certain it was done right, and the mod indeed now does clear the starter...kinda-sorta like a center dump I'm leaning that way. Otherwise it's good ole OE manifolds with the d-side exhaust tube squeezing between the frame and bellhousing...and yah I can work with that but I'd sure like to avoid it. Hmmmmmm..... 🤔 🤔 🤔
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I have the larger dumps on my truck using the Magnum manifolds. Other than having had to use a different shaped exhaust pipe, it fit without a problem. Realize that this was stock in the '92-93 years and the factory had it fitting.
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