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Exhaust Technology -- Side Branch Resonators
- Thread starter Soldmy66
- Start date
A
Anonymous
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Were you going to put up a link? I'd like to read it. I have a lot to say about this subject, and a lot to learn. Or maybe you're suggesting we google the subject. I just did. It appears that the interested parties are all talking about side branches as a method for damping out exhaust drone. I own a '87 Dodge Colt Vista 4wd mini-wagon derived from the Mitsubishi Chariot/Nimbus/SpaceRunner, and it has a resonator of this type and purpose. But I'm interested in a rather different application for such a resonator (see below).
A
Anonymous
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I am constantly amazed at the great volume and variety of printed and on-line information available to motorsports hobbyists and professionals these days. When I got into this stuff fifty years ago there was little beyond repair manuals and short, basic articles in a few hot rod magazines.
But there were a few treasures, one of which was English engineer Philip H. Smith's book, "The Scientific Design of Intake and Exhaust Manifolds". The last revisions of this work were done in cooperation with another university engineering prof, John Morrison. The two did a lot of work on acoustic wave action in little dinky English car engines using some very clever apparatus of their own devising. The book can still be found, and is well worth your time.
Having checked out and re-read it a year ago, one of their design ideas got me thinking about some exhaust manifolds I want to fabricate (incl. one for the Colt Vista mentioned above, and one for a 300-six in a step van). Smith and Morrison came up with some conventional-looking headers of both equal-length and siamesed designs. But they also (and since I don't know how how to make drawings here, I hope you'll bear with me . . . and look for the book) offered some very short headers that they combined with a side-branch resonator, stating that this arrangement, given proper sizing, would perform quite similarly to the full-length headers.
Those short exhaust headers with the extra resonator attached and running alongside the downpipe for a foot and a half or so were certainly goofy-looking! But I realized they might offer a real tuning advantage, one which Smith and Morrison never mentioned, because they evidently were cutting and welding their pipes to fixed lengths.
(More in a minute; have to change computers . . . )
But there were a few treasures, one of which was English engineer Philip H. Smith's book, "The Scientific Design of Intake and Exhaust Manifolds". The last revisions of this work were done in cooperation with another university engineering prof, John Morrison. The two did a lot of work on acoustic wave action in little dinky English car engines using some very clever apparatus of their own devising. The book can still be found, and is well worth your time.
Having checked out and re-read it a year ago, one of their design ideas got me thinking about some exhaust manifolds I want to fabricate (incl. one for the Colt Vista mentioned above, and one for a 300-six in a step van). Smith and Morrison came up with some conventional-looking headers of both equal-length and siamesed designs. But they also (and since I don't know how how to make drawings here, I hope you'll bear with me . . . and look for the book) offered some very short headers that they combined with a side-branch resonator, stating that this arrangement, given proper sizing, would perform quite similarly to the full-length headers.
Those short exhaust headers with the extra resonator attached and running alongside the downpipe for a foot and a half or so were certainly goofy-looking! But I realized they might offer a real tuning advantage, one which Smith and Morrison never mentioned, because they evidently were cutting and welding their pipes to fixed lengths.
(More in a minute; have to change computers . . . )
A
Anonymous
Guest
The fixed-length of the resonators, and of any conventional headers, whether equal-length, 3-2-1, shorty, whatever, that is my point of departure. I have two small ideas on this.
In my long-ago youth, I raced outboards, and these included alky-burning 2-stroke exotics whose modification was limited only by displacement and natural aspiration. This began in the fairly early days of expansion chambers. Suspending expansion chambers on a racing outboard motor is far easier than trying to hang them on a motorcycle, kart, or sled; generally you have an exhaust stub coming sideways out of the powerhead, making a short 90 degree turn to the rear, and the megaphone(s) or expansion chamber(s) point straight aft, with some bracing arranged to support them. This made pipe-building much simpler for boats than for bikes, etc..
Well, since almost all of the exhaust was in a straight line pointing aft, smarter guys than I figured out that the pipes did not have to have a fixed length. You could make them with a sliding section, nearly always in the header stub immediately after it had turned aft and immediately before the first diffuser cone in the expansion chamber. You could get up to about 4" of travel, shortening and lengthening the tuned length of the pipe, which moved the torque peak up and down the rpm range a small but useful amount. By attaching a bowden cable to the pipe and to a lever in the cockpit, the driver could pull the pipe in (shorten the tuned length) or let it out. Coming off a turn, with the engine loaded down, he'd let the pipe go full-travel out, lowering the rpm of the torque peak. As the boat got going faster and faster down the straightaway, he'd pull the pipe in, raising the torque peak. Pretty cool, and something you couldn't do on a bike.
Do you see what I see? On a car, with possible exceptions of those with transverse-mounted 4-cylinder engines, you'd have a tough time devising any kind of conventional full-sized exhaust header with sliding sections. But if what you had was a shorty header, such as the EFI manifolds from the late 300s, in conjunction with a resonator (or two, in the case of those EFI manifolds), it might be easy to incorporate a sliding section in just the resonator. No, I'm not suggesting this be driver-controlled as in our hydroplanes (although you could do this for dyno testing). You would mark the sliding section in increments, clamp the resonator in place, go make your test run or your pass, re-clamp the resonator at a new point, and try again.
My contention is that this is a whole lot better way to get dialed-in than building a fixed exhaust with the guidance of some software program. When you hit the best all-around length of resonator, you could weld it in place . . . or just clamp it there, in case you make future alterations (cam changes?) that could call for re-testing. Obviously, this procedure is conceptually simple and takes many hours to actually do. For one thing, you'd have to chase the the exhaust alterations around with timing curve tests to get the full benefits. But if you're a real zealot, or a weekend racer, it might be more fun than golf.
My add-on idea: When Smith and Morrison were doing there cut-weld-and-tests with resonators, they terminated their resonators in flat plates. This flat cap on the end of the tube certainly gave them the strongest wave action and the most dramatic effects when testing the resonators. But, as any 2-stroke guy knows, a flat plate not only makes the effect strong, but peaky, that is, very effective over a very small rpm band. This might be okay in racing, but is terrible for a street car. So my idea is that after you do your testing while sliding the reasonator in and out, you take it off and cut out the flat end-cap, and weld a long-tapered cone in its place. This will weaken the effect of the resonator while greatly widening the rpm band in which it works, both of which are what you'd want in a street machine.
What do all you brainy guys think of it?
P.S. For SoldMy66, if all you want to do is put a resonator out in the exhaust pipe to damping a droning noise, the sliding idea would be real easy to do and test.
In my long-ago youth, I raced outboards, and these included alky-burning 2-stroke exotics whose modification was limited only by displacement and natural aspiration. This began in the fairly early days of expansion chambers. Suspending expansion chambers on a racing outboard motor is far easier than trying to hang them on a motorcycle, kart, or sled; generally you have an exhaust stub coming sideways out of the powerhead, making a short 90 degree turn to the rear, and the megaphone(s) or expansion chamber(s) point straight aft, with some bracing arranged to support them. This made pipe-building much simpler for boats than for bikes, etc..
Well, since almost all of the exhaust was in a straight line pointing aft, smarter guys than I figured out that the pipes did not have to have a fixed length. You could make them with a sliding section, nearly always in the header stub immediately after it had turned aft and immediately before the first diffuser cone in the expansion chamber. You could get up to about 4" of travel, shortening and lengthening the tuned length of the pipe, which moved the torque peak up and down the rpm range a small but useful amount. By attaching a bowden cable to the pipe and to a lever in the cockpit, the driver could pull the pipe in (shorten the tuned length) or let it out. Coming off a turn, with the engine loaded down, he'd let the pipe go full-travel out, lowering the rpm of the torque peak. As the boat got going faster and faster down the straightaway, he'd pull the pipe in, raising the torque peak. Pretty cool, and something you couldn't do on a bike.
Do you see what I see? On a car, with possible exceptions of those with transverse-mounted 4-cylinder engines, you'd have a tough time devising any kind of conventional full-sized exhaust header with sliding sections. But if what you had was a shorty header, such as the EFI manifolds from the late 300s, in conjunction with a resonator (or two, in the case of those EFI manifolds), it might be easy to incorporate a sliding section in just the resonator. No, I'm not suggesting this be driver-controlled as in our hydroplanes (although you could do this for dyno testing). You would mark the sliding section in increments, clamp the resonator in place, go make your test run or your pass, re-clamp the resonator at a new point, and try again.
My contention is that this is a whole lot better way to get dialed-in than building a fixed exhaust with the guidance of some software program. When you hit the best all-around length of resonator, you could weld it in place . . . or just clamp it there, in case you make future alterations (cam changes?) that could call for re-testing. Obviously, this procedure is conceptually simple and takes many hours to actually do. For one thing, you'd have to chase the the exhaust alterations around with timing curve tests to get the full benefits. But if you're a real zealot, or a weekend racer, it might be more fun than golf.
My add-on idea: When Smith and Morrison were doing there cut-weld-and-tests with resonators, they terminated their resonators in flat plates. This flat cap on the end of the tube certainly gave them the strongest wave action and the most dramatic effects when testing the resonators. But, as any 2-stroke guy knows, a flat plate not only makes the effect strong, but peaky, that is, very effective over a very small rpm band. This might be okay in racing, but is terrible for a street car. So my idea is that after you do your testing while sliding the reasonator in and out, you take it off and cut out the flat end-cap, and weld a long-tapered cone in its place. This will weaken the effect of the resonator while greatly widening the rpm band in which it works, both of which are what you'd want in a street machine.
What do all you brainy guys think of it?
P.S. For SoldMy66, if all you want to do is put a resonator out in the exhaust pipe to damping a droning noise, the sliding idea would be real easy to do and test.
Thanks. In retrospect, I should have included a link. I don't have an immediate use for a side branch resonator, I just find things like this very interesting. Same w/ David Vizard's no loss exhaust approach; very, very interesting.
I don't have a drone in my G35 or 1957 Chevy, but if I ever get around to building my dream truck it may include both David Vizard's approach and side branch resonators.
I don't have a drone in my G35 or 1957 Chevy, but if I ever get around to building my dream truck it may include both David Vizard's approach and side branch resonators.
A
Anonymous
Guest
On my '87 Colt Vista the resonater branches off from the tailpipe just forward of the muffler. It then makes a quick turn to run forward alongside the tailpipe. The resonator is made of the same tubing as the tailpipe. Total length including the turn might be around 30" (guessing). It is terminated by having been pressed flat and the flat end welded. This method has a similar effect to a (fairly short) cone, that is, there's a gradual reduction in area at the end, rather than a flat plate.
Curiously, only the 4WD 5-speed Vistas had this; the 2WD automatic versions, the same car with the same engine, show no resonator in the exhaust system drawings in the manual. Could it be that this resonator was not for altering the exhaust sound but to create an exhaust system that could be effectively a few feet longer without hanging out back of the car . . . for a lower torque band on a 4wd car??? I wouldn't seem to have anything to do with wave action, since that would surely be ended at the catalytic converter up forward.
Where are all our tech gurus?
Curiously, only the 4WD 5-speed Vistas had this; the 2WD automatic versions, the same car with the same engine, show no resonator in the exhaust system drawings in the manual. Could it be that this resonator was not for altering the exhaust sound but to create an exhaust system that could be effectively a few feet longer without hanging out back of the car . . . for a lower torque band on a 4wd car??? I wouldn't seem to have anything to do with wave action, since that would surely be ended at the catalytic converter up forward.
Where are all our tech gurus?
Well, in thinking a bit more -- if you decided you needed variability/tunibility re the length of the side branch resonator; you might have a primary side branch resonator of a determined length, which would be capped by an electric exhaust cutout. On the other side of the electric exhaust cutout, you could have an additional length of closed end pipe. Under certain conditions, the electric exhaust cutout would remain closed, so the length of side branch resonator ended at the electric exhaust cutout. Under other conditions, you could open the exhaust cutout, which would allow gases to extend into the additional length of closed end chamber, yielding a longer side branch resonator length.
A
Anonymous
Guest
You could do all of that, but I don't see it as having much effect or value if you're putting your resonator by the muffler. But a resonator at the end of a shorty header should have a marked effect, especially if you had "cammed-up" the engine a bit, or cranked up the compression a bit, etc.. An old-timer's trick, oh I mean an "Old School" trick, from "Back in the Day" when rodders had more ideas than money was to cut and weld the factory (inline six) exhaust manifold to make a "split manifold" with 1-2-3 and 4-5-6 having separate outlets. The late 300 EFI manifolds are an improved version of a split manifold. A split manifold is what those Brit engineers I cited above were adding resonators to, to make them work more like full-length or 3-into-2-into-1 headers. I like the adjustability of resonators. Full-length headers from Hedman or whomever are a one-size-fits-all proposition, probably geared to making power at "street performance" rpm levels and not so hot for a heavy truck. You could weld up your own full-length headers to your own specs, and probably they'd come out fine, but they are not adjustable. By using those EFI manifolds, a lot of the work is already done (and cast iron has advantages there); working with a pair of resonators from that point is at least comparatively easy . . . i.e. still a lot of fuss and bother and hassle, but maybe something that appeals to you.
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