Exhaust Plumbing 4.0L Head w/ Dual Manifolds
Printed From: TheAMCForum.com
Category: The Garage
Forum Name: AMC 6 Cylinder Engine Repair and Modifications
Forum Description: AMC-made I-6 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: https://theamcforum.com/forum/forum_posts.asp?TID=95125
Printed Date: Apr/16/2024 at 5:31pm
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Topic: Exhaust Plumbing 4.0L Head w/ Dual Manifolds
Posted By: knobbler
Subject: Exhaust Plumbing 4.0L Head w/ Dual Manifolds
Date Posted: Jul/05/2018 at 9:48pm
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I've been working on the exhaust for my Gremlin after switching to a 4.0 head with dual manifolds. I pulled a Y pipe from a Grand Cherokee to hack apart for the two 2" pipes, though have a couple details I'm not totally clear on. This may all be basic stuff (maybe even nonsense), but that would be because I've never done any work designing an exhaust setup. The main thing I'm uncertain on is regarding equal length of pipe segments, and how important it is (or isn't) in my situation. I intend to run the 2" pipes from the manifolds as true dual exhaust with each stepping up to a 2-1/2" pipe, which will run to the mufflers. I understand that the general idea is that, when plumbing exhaust with headers, the collectors and primary pipes should be as close to equal length on each side as possible. Even though these aren't true headers, I expect that the same idea would apply. Is that the case? If so, would I be able to offset having different lengths of the first sections of pipe (the 2" pipes from the manifolds), with a longer run of 2-1/2" pipe on the side with the shorter length of 2" pipe? I ask because I'm not only trying to set this up as a true dual exhaust system, but I'm also running the exhaust to side pipes (probably just a bit advanced for someone that's doing their first exhaust design). Getting the exhaust plumbed to both the driver and passenger sides while using equal lengths of each pipe diameter looks like it would be a challenge, to put it mildly. So what do you say? Am I overthinking this when I should be welding pipe? Or are my suspicions correct in thinking that doing this the right way is going to involve a whole lot of measuring, bends, and material? |
Replies:
Posted By: billd
Date Posted: Jul/05/2018 at 10:49pm
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With only 232 or 258 (don't know what engine you have) cubes, why go to 2.5" pipes? You could end up losing scavenging effect with pipes TOO LARGE. That's a small engine to begin with but cut the exhaust in half and you aren't pushing very much out each half of the exhaust if you actually made a dual exhaust manifold setup. (4.0 had a single exhaust manifold going to a single head pipe - so apparently you bought headers or made something for exhaust manifold) You are running two exhaust manifolds on the 4.0 head so you are exhausting two 3 cylinder engines. 2.5" is overkill. Heck, I'm running almost 400 HP and running that size on my Javelin 360 with at least 120 cubes more than you (my 360 is bored out so is more like 373 or so) - anything larger on it would be overkill. I'm running 4 cylinders per side, you are talking three. I'm running at least 120 cubic inches more - assuming you have a 258 under that 4.0 head. Also if you are running 2" from manifolds, stepping up would be a total waste because your restriction is 2". If anything you want the exhaust to go into large pipes and then they can be smaller at the end when the gasses have COOLED and shrunk. Anyway it's a bit confusing because if you have two MANIFOLDS already, forget anything about equal length pipes - it's meaningless really. The equal length is in the headers. But you say you have dual manifolds. So just run pipes from the manifolds, 2" is plenty on dual exhaust on an AMC 6. IF you are building headers, that's a totally different animal and you will NOT have manifolds because headers take the place of exhaust manifolds. -------------  http://theamcpages.com" rel="nofollow - http://theamcpages.com http://antique-engines.com" rel="nofollow - http://antique-engines.com |
Posted By: knobbler
Date Posted: Jul/05/2018 at 11:30pm
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Thanks for the reply! I had totally lost track of the fact that there's no point of stepping up between the 2" pipe and the mufflers. I was paying a lot of attention to the 2-1/2" inlet at the mufflers when trying to plan everything out and. . . Either way, thinking of this as a pair of 3 cylinder engines instead of a single 6 makes a lot of sense and gives me some much needed correction in perspective. My engine is a 258 with an early 00's 4.0L head (03, I believe. I'll have to check my record book to confirm. Either way, it's one of the "TUPY"-marked heads from after the cracking issue with the 00-01 castings was corrected). The manifolds are from a Grand Cherokee of the same period, which had cast dual manifolds, like shown on the lower half of this picture:  So to clarify, I do have manifolds and am not building headers. I've seen these referred to as "header-style", or sometimes "cast header" manifolds, which was probably part of what was causing me some mix-ups. As an aside, I had originally intended to use a set of aftermarket headers before installing these. After two different sets I'd bought failed to clear the underside of the 4.0L intake, I opted to go for the OE manifolds until I'm ready to go the custom header route. |
Posted By: farna
Date Posted: Jul/10/2018 at 6:42am
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You might want to reconsider a true dual setup if you're intent on using that 2.5" muffler. Personally I'd make a Y pipe to tie the pair of 2" pipes into one 2.5" just before the muffler, or use a one in/two out muffler to save a lot of trouble. If you want it to look like duals split the tail pipe the same as the inlet -- except Y from the 2.5" outlet to a pair of 1.75" pipes. 2" will be fine, but exhaust gas cools and "shrinks" as it goes back, so no real need for 2" tail pipes. Probably won't make much difference though. If you still want true duals I'd make an H pipe somewhere near the front and use a two in/two out muffler. Would greatly simplify things, and reduce costs a bit -- only one muffler, no H pipe needed. The H equalizes pressure and makes it sound a bit better. I don't think it makes a difference in performance, but equalizing pressure should make all cylinders scavenge the same, so may make a difference if one bank is running a little different from the other. ------------- Frank Swygert |
Posted By: knobbler
Date Posted: Jul/10/2018 at 1:12pm
| Well, as it is it's a pair of mufflers that I've already got, and I've got some time constraints that makes it a big challenge to go with anything other than what I've got at the moment. I'm using a set of patriot sidepipes. I've been getting the pipe routed and welded together they past couple days and should be done by this evening. I've got it set up as true dual, so isolated runs from each bank, though if any problems come up with this arrangement, I can just notch the pipes and weld in an "H" with how I have things positioned. |
Posted By: farna
Date Posted: Jul/11/2018 at 6:20am
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Side pipes change everything! I don't think there will be any problems without an H pipe. AFAIK it really just balances the banks mostly for sound. When they are both out the back you cans sometimes tell the difference between them. Side pipes are separated enough you can't. I'm sure there is some other theory/reason for an H pipe, but it wasn't used in factory 50s and 60s cars, at least early 60s, don't know when it started to be used.
------------- Frank Swygert |
Posted By: 1982AMCConcord
Date Posted: Jul/11/2018 at 6:36am
Agree. For an I-6... it doesn't really need a lot but if I was going all out... I think running a set up like this to a Monza style muffler and a 2 tail pipe outlets would be pretty darn cool! I fully realize that this particular version would be way over kill but something like this hanging out of the driver's side of the car would cool on a Spirit or a Hornet... even a Concord... or any of the larger cars with a 6 cylinder too.
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farna wrote:Posted By: knobbler
Date Posted: Jul/12/2018 at 5:56pm
One of the reasons I was attached to the idea of a true dual setup for the sidepipes was actually one of laziness: if a cylinder isn't firing, I shouldn't even have to pop the hood to find out!  Seriously, though I'd spent a good amount of time trying to find some good info about pros/cons related to true dual setups with isolated banks, versus x/h-pipes, and so on, but specific to I6 engines. I couldn't really find much for this kind of application, probably because of search engine prioritization (if it doesn't point to something that can be advertised, Google doesn't want anything to do with it anymore. At least, that's what it seems like). That's actually part of the reason I'm going with the exhaust setup that I am, at least to start with. I want to document the process and results, that way I can get something put up to share, even if the methods and information aren't exactly empirical or practical to think of as directly repeatable. Public tinkering. "Wonder what this'll do?" |
Posted By: tyrodtom
Date Posted: Jul/12/2018 at 7:03pm
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I had true duals on my 69 232 American.
Viper tuned, equal length headers, dual collectors. Collectors adapted to two stock exhaust pipes running side by side to the location of the stock muffler, right in front of the left rear bumper. Rear muffler was a Austin Healey 3000, dual in dual out muffler. I never had that muffler apart so I don't know how it was constructed inside. The exhaust then exited right under the left rear bumper, dual pipes side by side. A Austin Healey 3000 was pretty much what the car sounded like. I liked it. And it did result in a lot of people asking " What do you have in that thing ?" When that exhaust finally rusted out, I had the car 18 years. I repaired the headers, replaced the pipes again with 2 stock exhaust pipes running under the car to 2 Hush Thrush mufflers mounted on their sides where the stock muffler mounted. These were too loud, I tried installing a H-pipe about a foot behind the collectors, it helped. But I never again got that mellow Austin Healey sound I had with the dual in dual out muffler.
------------- 66 American SW, 66 American 2dr, 82 J10, 70 Hornet, Pound, Va. |
Posted By: farna
Date Posted: Jul/13/2018 at 7:25am
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knobbler, try your search with V-6 as the engine instead of I6, or simply six cylinder. Most V-6s are constructed the same as the I-6, with the "front" half" of the cylinders on one side "rear half" on the other. The exception is the old Buick odd-fire V-6 (62-mid 77), maybe some others are like that since the odd firing is due to shortening a V-8, but a change in the crank eliminates that. Buick just took a short cut and made the shortened V-8 crank work... changed it in mid 77 with a new crank. The Kaiser Jeep built V-6s (used only in CJs, 66-71... AMC sold back to Buick who started using it again in 74) are odd-fire also, but a heavier flywheel dampened vibration better. ------------- Frank Swygert |
Posted By: Jmerican
Date Posted: Aug/07/2018 at 11:59am
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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. |
Posted By: g-man
Date Posted: Aug/07/2018 at 12:32pm
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 |
Posted By: knobbler
Date Posted: Aug/10/2018 at 1:40am
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! |