Im real close to getting an exhaust put on my slant and ive read up on a few threads but haven't received a definite answer. Ive got a 71 Duster, 225 with a Super Six and im looking for better economy. What setup gives the best mileage? Also, whats a good quiet muffler with a decent rumble? I want something that sounds decent but isn't gonna wake up the neighborhood everytime I pull in.

the bigger the pipe, the better the flow.

Id suggest a flow master set of mini mufflers.

The pipes all the way out the back and the muffles under the trunk or as far back as you can get.

There is such a thing as too big. Oversized pipes to an extent can ruin the scavenging effects of the exhaust system. I'm sure someone will give the specifics between the 2 sizes and their pluses and minuses.

There definitely is such a thing as too big.
Bigger is not necessarily better.

The optimal exhaust pipe size is the one that is not so small as to restrict flow, but small enough to keep exhaust velocity up, especially near the front of the system where the exhaust is hottest. Velocity (higher is better, as long as you don't restrict) is one of the operating factors that go into scavenging (the tendency of the exhaust system itself to help pull the spent gas out of the engine). Excessive scavenging is generally not a concern with stock or approximately-stock exhaust system configurations, regardless of pipe size, but certain exhaust system configurations (some header setups on some engines) can create such a strong scavenging effect that some of the intake charge is pulled right through the engine, in the intake and out the exhaust during the overlap period. This can really do a job on mileage (lower) and emissions (higher).

Best mileage with a stock exhaust manifold? 2¼" headpipe into a muffler of your choice. Make your choice carefully. I find loud exhausts get annoying after a week or two, so I favour a conventional-type muffler specced for a larger engine, such as a 318; others have different priorities and prefences. Flowmasters get thoroughly mixed reviews, and whatever brand you pick, you want to try for the largest-volume muffler you can fit under the car. The stock muffler location is just about perfect; no need to re-engineer that.

Tailpipe size is likewise a matter of priorities, and of philosophy. As the exhaust cools, it occupies less volume (PV=nRT), so for any given level of flow, a smaller diameter will carry the cooler exhaust near the rear of the system. Sizing down towards the rear of the system significantly reduces exhaust noise. But, flow effects on the downstream side of the muffler don't affect exhaust scavenging up front (unless the tailpipe is crimped or otherwise severely restricted!), so there's no especial need to keep the velocity up downstream of the muffler, so if you want to run your 2¼" pipe all the way back, by all means do so. For some data points: Mopar engines 40 to 60 percent larger in displacement than the 225 were routinely equipped with 2-1/4" headpipes and 1-7/8" tailpipes from the factory, and the 1976 Feather Duster and Dart Lite max-economy cars used just such a system (2-1/4" headpipe, "318" muffler, 1-7/8" tailpipe). That doesn't necessarily mean that's the one and only best system you can make, but it's worth keeping in mind.

Also keep in mind the exhaust manifold outlet diameter of 2". A headpipe ¼" larger in diameter makes some sense because of gas flow dynamics at flanges, but not only does a 2½" headpipe not make sense, but it's also nigh on impossible to bolt to the manifold flange without a restrictive flange plate of smaller diameter, which would tend to defeat the purpose.

Best mileage with Dutra Duals? Ask Doug!

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the whole wealth of information guys. I think i'll go with 2 1/4" all the way through.

Clean bends and joints are very important. A 2" system with mandrel bends and a decent muffler will likely get better mileage than a larger system that is poorly made.

Having said that, I would go with the 2 1/4" as the largest and might even use a 2'' tailpipe behind the muffler, as long as it is cleanly swaged up to match the muffler outlet. 2 1/4" is big enough even for a high performance street car with this size engine. You would only see a very small increase in peak power by going larger. You don't need the extra weight of a huge system or duals either. If you get ten extra horsepower with a certain muffler, it's not like you would get a hundred if you put ten of them on the car.

Best mileage and maximum peak rpm horsepower are not the same thing. If you are driving in a manner to get best mileage you are not making max power and need the best part throttle torque instead. You just won't see any significant increase in backpressure with a reasonable sized exhaust than you will with a huge one, but you should see improved low end torque with the smaller system.

As a reasonable balance of flow and sound control with a nice tone and decent price, I like the Walker super turbos in the hemi sizes when they fit under the car. For max power the Hooker aero chamber is a good chambered muffler with much more engineering than the flowmasters and they sound good too, loud but good. If you really need flow and sound is not an issue, I don't think anything can beat the Walker Ultra flows for flow and lite weight. If you have good source or money is no object there are quite a few other good choices. I have yet to see the Flowmasters being the best at any one thing, except maybe marketing.

What about someone running a set of clifford shorties?? 225 bored .060. comp cam. 390 4bbl... i was planning on doing duals but would rather lake pipes, but will probably go duals... unless there is a better perf option for me...

thanks for the info guys

Chris

_________________
65 Coronet, 4 bbl, .060 over, mild cam.
66 Valiant, Black, 3spd, Hyperpak, headers.
works in progress...

I would say with the shorties and your approximate build, the best option is this:

- 2 X 2" head pipes running from each header to 4-6" behind the trans X member
- Flowmaster or similar Y pipe into a single 2.5" pipe all the way out the back of the car with a good turbo muffler (Super Turbo, Hooker Aero Chamber, Magnaflow is a bit louder). I would heartily NOT recommend Flowmaster mufflers. Way too boomy and resonance ridden. Super Turbo is most economical, good sound, but not too loud. All will provide enough flow. Magnaflow is more rappy and louder, but nice sound.

Duals will be no better for performance, but if you like the looks, then go ahead. X or H pipe should be used if possible.

The highest HP NA Slant on earth runs 2 X 2.25" into a 3 or 3.5" (?) Y and Hooker Aero Chamber diagonal tailpipe exiting in front of the L rear wheel.

I have 2 X 2.25" into a 3" Y, Flowmaster 50 muffler (yuk) and 3" all the way back on my 64 Dart (14 sec 1/4 mile, 24 MPG @75-80 MPH).

Lou

_________________
Home of Slant6-powered fun machines since 1988

Thanks Lou, I had 2" w/flowmaster on my 66 Valiant, stock with a 390 as well. it sounded pretty good, I will give the hookers a shot... I do like the look of duals... i was thinking x pipe too... cool. glad we agree... I feel better now...

what headers are you running Lou??

thanks for eh advice...

_________________
65 Coronet, 4 bbl, .060 over, mild cam.
66 Valiant, Black, 3spd, Hyperpak, headers.
works in progress...

I have the Clifford shorties on both 64 Dart and Project V, and Dutra Duals (front and back) on the 68 Dart.

Lou

_________________
Home of Slant6-powered fun machines since 1988

If you have a large A body, '67 or later' the annoying noise at cruise comes fromt he size of the pipe and the length behind the muffler. 2.25 pipes and bigger, BEHIND the muffler creates a really anoying din at about 2200RPM. If you like tht sound then fine, but my wife did NOT like to ride in my car when I had mufflers (now gone) and the 2.25 tail pipe behind the muffler. I now have the mufflers out of the system entirely, and it is way quieter at cruise than when I had the mufflers in there. It is louder under power. That rear pipe just seems to be a perfect rsonance chamber when it is 2.25 or 2.5. I have been in Lou's cars at cruise, and I don;t think we were paying any attention to the exhaust sound. I don;t know how it would sound to me after an hour of riding.

Sam

_________________

Same effect happens on '66-down cars (exhaust pipes are kind of dumb; they have trouble keeping in mind what year car they're installed in ). Tailpipe smaller than headpipe can in many cases cut or greatly reduce the drone, but not always. For a short while, I had a system on my '62 Lancer that consisted of a single 2¼" headpipe into a Magnaflow in the stock location, with 1¾" tailpipe. This system made completely delicious sounds at startup and idle, and when revved in Neutral, and when accelerating up through the gears. I used to go out of my way to use particular walled highway onramps and roll down the window just to hear the music as I accelerated up to freeway speed! The problem was a booming drone between about 54 and 68 miles per hour...exactly the speed range I needed to maintain most of the time! I tried adding as long a resonator as I could to the tailpipe. No good. That Magnaflow muffler, in that location, with that headpipe, on that engine, was bound and determined to drone. I wound up replacing it with an ordinary stock-type 318 A-body muffler. The drone disappeared, but so did the music.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

My 2-1/4" all the way out has a definite drone that wasn't there with the 1-7/8" tail.

Of course my wife won't ride in my Hooptie Valiant anyway and I have substantial hearing loss, so it doesn't bug me that much.

On the Positive side, it weighs more, so makes the car slower.
It also dries out more slowly, promoting internal corrosion of the muffler and tailpipe. Boy, the benefits never cease!

I kind-of like the sound, but it annoys some people. It's not loud, just always there. Sometimes it makes the trunk spring rattle going down the road.

It's an Harmonic.

CJ

_________________
Part of Tyrde-Browne Racing

Trunk torsion bar rattle fix: Slit fuel line hose lengthwise, push over one torsion bar where it passes closest to other torsion bar.

An harmonic(a): What's that from?!!!

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

I have 2.25 all the way with a 340 style muffler....stock location. Good tunes ...no drone. (The whole exhaust is right off a card on the exhaust machine where I get my work done. Sub in 340 S/hemi mufflers)

I always wanted to do a 6 cylinder version of the Formula S chambered pipes with resonator.

_________________
Yeah....Im the one who destroyed this rare, vintage automobile.....