I read on an older post that some folks taper their exhaust as it goes back to reduce the "hollow" sound one gets from a 2.5" exhaust.

I'm running DD's down to a 2.5" Y and 2.5 all the way back. I'm using a Dynomax Super Turbo muffler which is a HUGE improvement over the flowmaster hollow droning empty tin can thing I had on there before. But, there are certain situations where the sound is more hollow than I'd like, I think, because the pipe is 2.5" all the way back. For example, you'll notice the slot car folks with their Honda's and such use big open pipes on the tail end to get that hollow sound.

Q: Is it true that the exhaust gases cool and contract sufficiently as they travel through the system such that one can reduce the pipe diameter and not create undue back pressure? My engine has a worked head, dual 2bbls, larger valves, DD's and breathes very well; I don't want to restrict the exhaust flow, but I'm wondering if I could say go down to 2" at the tail pipe, not restricting my flow, and maybe get less hollow sound.

Thanks all you exhaust guru's out there.

Brian

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The usual shrinkage is 2.25 before muffler to 2 after muffler.

2.5 to 2.25 would be the equivalent downsize.


Maybe 2.125 would be ok, but I think 2 would be too small.



If droning is your problem, do a web search for "1/4 wave exhaust"

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Absolutely!

I'd also say your 2½" pipe is larger than necessary or beneficial. If I were designing a system from scratch for a setup like yours, I'd have a wrapped 2" pipe coming off each exhaust manifold, wye into a 2¼" pipe to a thoughtfully-chosen muffler (probably not a "turbo" type from any maker) and either a plain 2" tailipe or perhaps a 2¼" with a resonator.

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

Dan,

Yes, I wasn't designing well in the beginning, but I have 2" off my DD's, then into a Y to 2.5. I like the turbo muffler actually, it has great flow but packing to quiet the system down over say a flowmaster with no packing. I don't have any drone now, which is great, so I don't need a 1/4 wave resonator side tube.

I did make a homemade "factory style" resonator at the tail pipe, I used a Smithy muffler, like a glass pack, and welded a mopar style exhaust tip at the end. I took a perfectly good SS exhaust tip and cut it off and welded the end onto the smithy. Here is the cut off remaining piece:

http://s785.photobucket.com/user/67dart ... n.jpg.html

And here is my creation:

http://s785.photobucket.com/user/67dart ... h.jpg.html

So, Dan, I agree, I've been playing with mufflers and such, and the resonator at the tail end made a big difference. I'm just down to the hollow sound at certain rpms. It's really quiet on the highway and at most cruising speeds, save certain situations where I get the hollow sound from the tail end. So, if I go down to a 2.25" resonator maybe that's the final piece of the puzzle...

Brian

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The 1/4wave exhaust is an interesting idea I have heard about recently elsewhere. Some guys say keeping the exhaust speed up is more important than reducing back pressure. Without back to back dyno pulls, it is kind of guess work, and speculation. Being a piano tuner, and a French horn player, I am very aware of wave charactertics in an open pipe. I cannot speak to the power changes, but feel strongly that the tail pipe on an A body must be 1 3/4" to keep it quiet with a slant. Any larger makes it the ideal size to resonate, and loudly at certain rpms. I currently have 1 & 7/8" stock 340 tail pipes, and think even that is "prone to drone". Not obnoxiously so,but keep in mind my new target is BMW quiet.

Sam

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Sam,

I started with 2" duals from my DD's and that had a horrible drone at 60 mph. I played with an x-pipe, H-pipe, didn't make any difference. I was going to try Helmholtz resonator (1/4 wave) designs but instead I went to single exhaust from a Y pipe. I played with resonator before the muffler, still had some drone, and now after the muffler, and the latter seems best so far. I really don't have what I call a drone any longer, the exhaust sounds pretty good overall. It's that hollow sound, that I'm hearing and only sometimes. At idle it sounds great, deep tone, and at highway speeds it is nearly silent.

I think you're right though, going smaller at the tail will likely take care of that last hollow tone. I'll try that someday, see if that is the final piece.

Brian

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The odds are fairly good you will wind up picking one or the other of those two things, but not both.

My own philosophy on exhaust systems is that even moderately loud ones are fun for about a week, then they get tiresome; I tend to get my quiet and have my more-than-adequate flow by picking a larger-than-stock, stock-type muffler rather than a "turbo" or other "high performance" muffler.

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

Dan,

I totally agree on the loud exhaust getting old really fast.

I quickly went to a muffler with some packing to quiet it down, though it is a "high flow" design.

Yes, I think if you use a larger than called for standard type muffler that should do the trick, i.e. not restrict flow and keep it really quiet. I'm not familiar enough with how big, what size or type I would use to achieve that end. The net has so many misinformation about how overly restrictive standard type mufflers are (mostly this comes from flowmaster and their ilk, and then folks who like loud systems propagating the fables). I'm happy to hear any suggestions.

As you might be able to tell, I'm not afraid to try new iterations on a theme. I have a little stick welder, my chop saw is loaded with a 10" metal cutting blade and I have lots of enthusiasm...also, I specifically designed my exhaust system to be modular, it comes apart into three pieces, 1) downpipes/Y/collector, 2) intermediate pipe/muffler and the 3) tail piece including resonator. I have a flange at the collector and one at the back of the muffler, just before the pipe goes over the axle. So, it's easy to disassemble and reconfigure the system. Also, that turbo muffler I have on there is pretty small compared to available space for a bigger muffler.

brian

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Most of the slant-6 cars got tiny little exhaust systems through '75: a 1¾" headpipe, a 3¼" × 7¼" muffler, and a 1½" tailpipe. Wheeze! The '76 Feather Duster/Dart Lite got a 2¼" headpipe, 318 muffler, and 1-7/8" tailpipe. All Slant-6 cars got 2¼" headpipes for '77.

Muffler selection? Can't really point to a make/model/year to buy for. Mostly just pick the largest stock-type muffler that can fit in the available space. I'd tend to use a 4¼" × 9¾" muffler, as long as possible, with 2¼" inlet, then a 2" tailpipe without or a 2¼" tailpipe with resonator. The major muffler makers (Walker Exhaust and AP Exhaust) put out cattledogs with dimensional listings of mufflers; spend some time paging thru.

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

Recommended muffler here a long long time ago (think it was Dan):

Walker Quiet Flow 21206


Here are the specs:

http://www.summitracing.com/parts/wlk-21206


Straight core (not louvered)
2" Resonator (my own research, I don't think it's too long ) :

http://www.summitracing.com/parts/wlk-24200



Bought these several years ago...............still haven't made the exhaust system.




Neither recommending or NOT recommending summit; just using them for reference.

I got the muffler from rockauto and the resonator from (I think advance auto).


Walker has a $40-50 rebate on the mufflers once or twice a year. Don't know if they've had the rebate already or not.........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Yes, except take a look at that Summit link: "NOT AVAILABLE". RockAuto has one of them left. There's one seller offering one of them on Amazon. Probably some more of them scattered here and there, but the part's been discontinued.

Now see whatchya done, you made me go look at the cattledogs! Try a Walker № 21489 instead. It's in Walker's QuietFlow SS premium OE line, all stainless steel (which is nice for durability), made for the '01 Jeep Cherokee 4.0 so it's tuned to keep an inline-6's exhaust quiet, has a 2¼" inlet and outlet (can easily step down to a 2" tailpipe). The inlet has a 2-bolt flange (it's meant to bolt directly to the outlet of the Jeep's catalytic converter) but there's plenty of length to the input pipe that the flange can just be sawed off. It's got the 4¼" × 9¾" housing, muffler shell 20" long, so it'll be a bit of a tight fit in an A-body, but those are very close to the size parameters of the discontinued № 21206, and we know that fits an A-body.

I don't like those cheap red "hot dog" things, even just as resonators. They don't last and they aren't built very well. If I'm rich, I like a Flowmaster HP2 stainless unit with 2" in and out. It's 18" long, which is a probable fit behind most A-body quarter panels. If I'm broke, I like a Walker № 17198 4" round can-type unit, 2" in and 2" out, with offset outlet next to a proper hanger bracket.

Speaking of that, whatever muffler you wind up with, make sure to use very soft hangers. The hook-and-O-ring type, for example. Today's popular "rod and rubber box" type transmits a lot of noise to the passenger compartment.

(Tailspout outlet 45° or greater downward.)

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

Almost the same size:

http://www.summitracing.com/parts/wlk-22070/overview/

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Yes, I use very soft hangers, the muffler shop guys use way too stiff of hangers 'cause they're quick and easy.

Dan, I did have an HP2 resonator at the rear, it fits fine back there, you're correct. I just wanted to try the Smithy because I wanted to see what it sounded like in combo with my turbo muffler. Though my HP2 was 2.5" in and out.

Thanks for all the great feedback, very helpful.

brian

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I do like Summit, they are quick to ship, good customer service, easy returns, prices are usually the lowest but not always. On the muffler front you can find just about any muffler you want.

brian

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I reversed my resonator and it's a lot better...

The flow direction arrow on the resonator indicated that I installed the resonator (basically a glass pack) correctly, but I noticed the louvers open end was facing away from exhaust flow, rather than catching it and directing it into the fiberglass packing material.

I did some searching and apparently many folks reverse their glass packs for more turbulence and quieter systems. It makes sense to me, if the flow is going over the louvers it can't be directed into the packing.

Anyway, since I have a 2.5" system, I have plenty of room in there, no undue restriction by turning it around, and it's much quieter. Now I have a deeper tone at idle and the "hollow" sound at 1200-1500 rpm is gone, on the highway it's really quiet.

The distance between the tops of opposing louvers is about 1.75" though the pipe's ID is 2.5". This means a lot of laminar flow is being disrupted when the exhaust gas passes through the resonator; the louvers are catching quite a bit of the gas and directing into the packing material. Also, on a Smithy muffler the louvers are in a spiral pattern; I don't know what difference that makes, if any.

FYI,

Brian

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