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Exhaust Plumbing 4.0L Head w/ Dual Manifolds

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Jmerican View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jmerican Quote  Post ReplyReply Direct Link To This Post Posted: Aug/07/2018 at 12:59pm
Some seriously erroneous info on exhaust. Particularly in the six forum. True dual exhaust. Non - tuned manifolds. No secondary collector tuning. Just run the pipes. Equal length not important. Balance pipe will radically alter the tone, and help mufflers share the work. Using 2.5 inch mufflers is good, as most are necked down a bit and most flow 200 ish CFM, maybe more. Great. Side pipes should be 45 degrees down or slash cut. 2" pipe seems fine, on average. Healthy even. A short separated low restriction inline six exhaust such as this will be raspy.

The late 4.0 head with dual outlet cast manifolds has a small exhaust port. Very small. It can be made to flow well with work. This setup was designed for close coupled catalytic converters to be lit off quickly so the engine can acquire closed loop operation. The manifolds are decent but a bit small. I bet they work well. And they won't introduce as much heat into the intake manifold as the tubular 4.0 manifold does.

Tuned systems, scavenging, flow, back pressure, lengths, cooled gasses, comparison to other engine configurations and cubic inch sizes, pipe sizes on other engines, etc are so maligned on these six cyl threads. It's a mess. No offense to all.
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g-man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote g-man Quote  Post ReplyReply Direct Link To This Post Posted: Aug/07/2018 at 1:32pm
Originally posted by Jmerican Jmerican wrote:

Some seriously erroneous info on exhaust. Particularly in the six forum. True dual exhaust. Non - tuned manifolds. No secondary collector tuning. Just run the pipes. Equal length not important. Balance pipe will radically alter the tone, and help mufflers share the work. Using 2.5 inch mufflers is good, as most are necked down a bit and most flow 200 ish CFM, maybe more. Great. Side pipes should be 45 degrees down or slash cut. 2" pipe seems fine, on average. Healthy even. A short separated low restriction inline six exhaust such as this will be raspy.

Tuned systems, scavenging, flow, back pressure, lengths, cooled gasses, comparison to other engine configurations and cubic inch sizes, pipe sizes on other engines, etc are so maligned on these six cyl threads. It's a mess. No offense to all.

Amen to that.
No offense taken here.
Exact approach taken on my modified 258 Gremlin.
Definite improvement in performance.

g-man
AMC only daily driver
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knobbler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote knobbler Quote  Post ReplyReply Direct Link To This Post Posted: Aug/10/2018 at 2:40am
Originally posted by Jmerican Jmerican wrote:

This setup was designed for close coupled catalytic converters to be lit off quickly so the engine can acquire closed loop operation. The manifolds are decent but a bit small. I bet they work well. And they won't introduce as much heat into the intake manifold as the tubular 4.0 manifold does.


First off, thanks for the response! There's a lot you detailed that helped put some lingering thoughts at ease. I'm still working on getting this all finished up, but it's mostly getting the pipe hanger placement/fabrication done (with the aim to make as few new holes in anything as possible. Front's easy, but the sidepipes have required some thinking).

The part I quoted stood out to me, because when I pulled this head at the yard, I noticed that the downpipes were straight through from manifolds to the main 2-1/2" run, and they appeared to be that way from the factory. Other 4.0 engines with dual manifolds I've come across have all had the cats right up by the manifolds, like you described. Not really a concern to me, just something I thought was strange.

I'm also guessing that the lower amount of heat vs tube headers would be due to the decreased overall surface area, as well as the actual shape of the manifolds tucking right down instead of running parallel to the underside of the intake. Is that the jist of it? I'm still working on figuring out when and where to insulate/reflect heat, so I don't end up with everything looking like a fiberglass-stuffed baked potato, haha

Thanks again!
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