Exhaust Pros/Cons for Slant 6

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Mopar Virgin

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Hi all ... My '68 Barracuda project is moving along and will start with a rebuilt '74 /6 comprising of a factory bore, mild cam upgrade, standard intake manifold with 2bbl carb and dual exhaust header) + A908 drive line. I was intending to build the exhaust system with the '74 motor as a dual exhaust, given it came with aftermarket exhaust headers.

I've watched several Uncle Tony's Garage YouTube videos breaking down the /6 engine. One of the things he advocates against is dual exhaust. In this video (start at the 28:32min point), he promotes a single 2.75-3.0" exhaust to avoid heat-velocity-backpressure performance and sound issues.



If anyone has thoughts/experiences on this please opine !!!

@RustyRatRod I believe I ran across a video you posted in this Forum in response to people shaming the sound of a /6, but yours sounded great. I am interested in your experience and advice on the topic, and if you've experienced any of the sound characteristics he mentions with your build?

Eventually, I want to acquire a '81-'87 /6 and build a more performant engine. But for the short term, I want to push that engine build spend out to another year or more, and simply get the car on the road this coming spring/summer.
 
Get ready for things to go off track, whenever anyone mentions UTG, it turns into a hate thread rather than a help thread...
As for your exhaust, as with anything else, it all has to be balanced. Assuming you're going with a 225, that translates to roughly a 3.6L (close enough) engine which isn't very big at all. And considering you're likely going to be driving it and not trailering it to the track, you probably won't be living close to redline all the time with your car. My guess is that a single 2.5" inch exhaust coming out of a quality header (either 6-1 or 3-2-1) will be both plenty for now and ready for any future (reasonable) upgrades. And even then, some might say that's too big...
Now if you plan on only building a track car or using forced induction then ignore me and go bigger.
 
Projectile Dart said:
Get ready for things to go off track, whenever anyone mentions UTG, it turns into a hate thread rather than a help thread...
As for your exhaust, as with anything else, it all has to be balanced. Assuming you're going with a 225, that translates to roughly a 3.6L (close enough) engine which isn't very big at all. And considering you're likely going to be driving it and not trailering it to the track, you probably won't be living close to redline all the time with your car. My guess is that a single 2.5" inch exhaust coming out of a quality header (either 6-1 or 3-2-1) will be both plenty for now and ready for any future (reasonable) upgrades. And even then, some might say that's too big...
Now if you plan on only building a track car or using forced induction then ignore me and go bigger.
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No track/racing with this car. I do want it to have some punch off stop lights and able to pass when necessary, but all-in-all, we are aiming the build as an eye-catching, unique, and sporty restomod cruiser that will be driven. Visually, the aftermarket rear valance has openings for dual exhaust. So wonder if I could go with a 3-2-1 through the muffler, then split it into a small dual exhausts out the back with larger chrome exhaust tips? Or, just dual exhaust all the way back in smaller exhaust pipes with wider chrome tips?
 
TTI makes a brilliant 2.5” single exhaust. I bought mine thru @HemiDenny . It looks and sounds great. I felt like it helped my launch a bit too. Several member have it, I’ve not heard complaints.
 
I used a fel pro exhaust flange gasket as a template to grind mine out, as it turns out when I lined up the mounting holes the 1-7/8 diameter hole that was the exhaust outlet was actually off center so I didn't have to touch it at one point but had to take the whole difference out on one side.... And it tapered down from there to the "thin point". I used longer burrs and tapered it in gradually as deeply as my carbide burr could reach.

And if you still have a leak, your flange on the pipe is probably warped
 
1 header collector, or 2? Either way, I would do single exhaust, decent muffler (though I don't have a rec. on that) then IF you want dual tips, split it. I don't see the point of split pipes just to look balanced, but thats me.
You gain scavenging and I don't think a 2-1/2" pipe is going to choke a slant, and I've never heard a 6 cyl with full duels that sounded good, V's or inlines. I've heard some good sounding jeeps with 4.0, but I wasn't very pleased with the Flow master I put on mine, But I never bothered to ask what others were using.
 
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I got the complete TTI 2.5" exhaust for mine and put a Dynomax Ultra welded muffler on it. It sounds good with no drone.
 
Mopar Virgin said:
Hi all ... My '68 Barracuda project is moving along and will start with a rebuilt '74 /6 comprising of a factory bore, mild cam upgrade, standard intake manifold with 2bbl carb and dual exhaust header) + A908 drive line. I was intending to build the exhaust system with the '74 motor as a dual exhaust, given it came with aftermarket exhaust headers.

I've watched several Uncle Tony's Garage YouTube videos breaking down the /6 engine. One of the things he advocates against is dual exhaust. In this video (start at the 28:32min point), he promotes a single 2.75-3.0" exhaust to avoid heat-velocity-backpressure performance and sound issues.



If anyone has thoughts/experiences on this please opine !!!

@RustyRatRod I believe I ran across a video you posted in this Forum in response to people shaming the sound of a /6, but yours sounded great. I am interested in your experience and advice on the topic, and if you've experienced any of the sound characteristics he mentions with your build?

Eventually, I want to acquire a '81-'87 /6 and build a more performant engine. But for the short term, I want to push that engine build spend out to another year or more, and simply get the car on the road this coming spring/summer.
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On my Slant I built my own SS 2.5" exhaust, and modified the floor to tuck the exhaust up tight.
If a 440 is satisfied with 2 2.5" pipes, my 225 should be happy with one 2.5" exhaust was my thinking.
 
I was thinking 2-1/4 since that's all the bigger my manifold outlet is
 
Projectile Dart said:
Get ready for things to go off track, whenever anyone mentions UTG, it turns into a hate thread rather than a help thread...
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It doesn't have to be that way. Uncle Tony is full of ѕhіt on this and many other points, so it's a good and helpful thing to warn people who don't know any better off of following his advice, but it really doesn't need to go any further along that road.

Thoughtfully-configured dual exhaust—according to people who actually know what they're talking about, from Chrysler's own engineers to Doug Dutra—brings real, solid, practical benefits in usable performance on a Slant-6 engine. You don't want to overdo it with too-big pipe diameters, and duals all the way back is past the point of diminished returns for many street builds (unless the point is to listen to the pipes), but a recipe like this is good. The Dutra Duals are now made in Australia, and the preferable setup for street use is Doug's original configuration: a Dutra front-3 unit with a cut/capped rear half of a stock exhaust manifold (keeping the central collector area for intake heat, and the automatic choke tower).

But a 2-1/4" headpipe coming off a stock exhaust manifold (the outlet can be enlarged a bit) into that same muffler and resonator as linked above is still a nice and much less expensive upgrade over stock miniature pipes.
 
I built my own exhaust (up to the muffler so far, still have to do muffler and tail)
One of the super 6 stacks I bought about 3-4 years ago had about a foot long, sawed off stub of 2-1/4, with a good flange already still there.
I hate to admit it but I went and raided a dumpster at an exhaust shop for the bends. I found a whole exhaust off of I don't know what that actually looked brand new, don't know why it was taken off..... "Upgrade" maybe? Bends looked mandrel bent, definitely not exhaust shop crimped and bent. And it was the same 2-1/4 as my stub on the super 6 manifold as well as OEM stainless.
But I put the truck on a lift, drug out my MiG and chop saw, eyeballed what I drug out of the dumpster, cut it up, tacked it all in place when I liked what I saw, took it down and welded it all solid ... Muffler is what came with the truck (it's been replaced some time since 1985) for the moment.... Originally I just did this so I could drive it to the exhaust shop and have them make me a new setup from manifold to tail but I'm rethinking that and may just put a new muffler and tail on what I built and run that for all I can. Thinking big oval almost round (what used to be called) RV muffler and see if I can still get just a stock replacement tailpipe for the truck. Mine is no racer but this exhaust configuration will definitely breathe better than the original one would have.....
 
volaredon said:
I was thinking 2-1/4 since that's all the bigger my manifold outlet is
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On my 2.5" SS exhaust, I used a tapered 1"? long transition right to 2.5". I opened up the CI manifold outlet as much as I felt safe, which was not a lot, and avoided as many curves as I could downstream. IMO there is a lot of restriction found by going to anything smaller than a 2.5" manifold back.
 
to take the advice of tony, would be akin to take the advice of Tim on tool time...both of which are shows made to generate ratings, and not necessarily to inform the masses

keep that in mind and you'll be just fine
 
If you want to watch a boring 20 part series on how I built my exhaust system, you can start here:

The cliff notes is that the system is 2 1/2" off the manifold and necked up to 3" out the back. It uses a pair of glass packs in series to eliminate any droning. The manifold itself was gasket matched to the gasket and the exit was opened up to 2 3/8". This makes for a fairly quiet and free flowing exhaust system. This engine will eventually get a turbo, so I didn't want any back pressure in the exhaust.

It would be nice if Summit/Jegs would just sell you half of their V8 kits, but they won't.
 
You want the 1-2-3 pipe to Y with the 4-5-6 pipe somewhere based on where you want the scavenging torque boost RPM to be. I put it around 50” back from the head which is around 2800 RPM. Doug Dutra put it back by the axle and he said it pulled hard off idle. After the Y 2-1/4” is plenty if you have no intention of winding the crap out of it.
 
Make you own pretty much 2inch to y pipe exiting 6feet back at 2.25. Head work porting upgrade valve springs milling head cam carb dizzy recurve and if you have a auto get a converter

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One other note is that 1-5/8 to 2-3/4” for the pipes from the Duals to the Y fives near identical performance as long as you have not done extensive head work with a huge cam and plan to make the power way above the RPM range where most 225s start to gasp for air due to the head. For a nice street motor with simple head improvements like oversized valves and just knocking out the lip right behind the valve 1-5/8” or 1-3/4” pipe is much easier to run down to the Y and has no performance impact and actually may make a tiny bit more torque down low due to the higher exhaust velocity and better scavenging effect. If you go smaller than 1-5/8" it will effect higher RPM performance and if you go larger than 2-3/4” the velocity drop will drop torque as the scavenging effect is reduced because of it.

This is based on a very accurate Performance Trends model Rusty and I have created. My Dart used 1-5/8” to the Y for an easier run and because I never run it past 4000-4500 RPM anyway.
 
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The stock exhaust manifold has a lot more material in it as long as you keep the opening centered and know what you're doing with a grinder. Which I do not. But here is what my friend Freddie did for me on one manifold and the one I have currently has been opened up to 2.5" and we feel sure there's room for 3" although that would be getting a bit thin. I'm just crappy with a hand grinder, but thankfully Freddie is a legend genius with one.

www.forabodiesonly.com

Exhaust Manifold Mod

Don't know how many of yall are interested, but here's what I did. Well.......here's what my friend Freddie did. LOL Since I am a walking retard with a grinder, I asked my friend Freddie about enlarging a slant manifold outlet. He took it out to 2 1/4" with meat left over. He said he was sure...
www.forabodiesonly.com www.forabodiesonly.com
 
RustyRatRod said:
The stock exhaust manifold has a lot more material in it as long as you keep the opening centered and know what you're doing with a grinder. Which I do not. But here is what my friend Freddie did for me on one manifold and the one I have currently has been opened up to 2.5" and we feel sure there's room for 3" although that would be getting a bit thin. I'm just crappy with a hand grinder, but thankfully Freddie is a legend genius with one.

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Since my experience was so different, I now wonder how many variations of OEM manifolds that might exist to account for this discrepancy.
 
RustyRatRod said:
The stock exhaust manifold has a lot more material in it as long as you keep the opening centered and know what you're doing with a grinder. Which I do not. But here is what my friend Freddie did for me on one manifold and the one I have currently has been opened up to 2.5" and we feel sure there's room for 3" although that would be getting a bit thin. I'm just crappy with a hand grinder, but thankfully Freddie is a legend genius with one.

[/URL]
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A good machinist friend of mine opened one up to 2.75” for me and commented that’s as far as he would recommend. This was on a 78 casting with the extra ribs. I only went to 2.5” after that, easier to run the head pipe vs 3” which I wasn’t using to its full potential anyway.
 
J-c-c said:
Since my experience was so different, I now wonder how many variations of OEM manifolds that might exist to account for this discrepancy.
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If you pay close attention to the enlarged manifold in that thread, you'll see it's a later one with the reinforcing ribs. I believe they have more meat in them than the early ones. THere have also been some examples of factory manifolds with a 2 1/4" opening and on first thought, you think "Oh that's the Super 6 manifold, but that's not the case either, as people have found the smaller 1 7/8" outlet on original Super 6 engines. So I don't know what the rhyme or reason was or even if there was. What I'm more thinking is the differences in sizes was simply discrepancies in castings. Maybe someone knows more than that, but that's what I've come up with so far. If someone knows for sure what the larger outlet manifolds were actually used on, I'd be all ears. Since I've only seen ONE since I've been around cars, I'm sticking with my theory. lol
 
RustyRatRod said:
If you pay close attention to the enlarged manifold in that thread, you'll see it's a later one with the reinforcing ribs. I believe they have more meat in them than the early ones. THere have also been some examples of factory manifolds with a 2 1/4" opening and on first thought, you think "Oh that's the Super 6 manifold, but that's not the case either, as people have found the smaller 1 7/8" outlet on original Super 6 engines. So I don't know what the rhyme or reason was or even if there was. What I'm more thinking is the differences in sizes was simply discrepancies in castings. Maybe someone knows more than that, but that's what I've come up with so far. If someone knows for sure what the larger outlet manifolds were actually used on, I'd be all ears. Since I've only seen ONE since I've been around cars, I'm sticking with my theory. lol
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I’ve seen one as well, it was sitting on a shelf so I’m not sure what it came from. I’ve personally pulled 6 super six set ups out of junk yards and every single exhaust manifold was the 1 7/8” opening.
 
There are a least 3 distinct manifolds, They deal with the choke housing differences and reinforcements along the manifold. In 1976 the manifold started to have a 5* cant to the outlet. I believe this was to allow for a larger exhaust pipe to clear the pan rail. A larger head pipe may have been used for cars that needed catalytic converters.

I have never seen a 2 1/4" pipe nor a manifold with an exhaust opening larger then 1 7/8". And I have 5 of them sitting in my workshop.
 
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