Welcome to the Corvette Forums at the Corvette Action Center!

Exhaust question for "Redbob"



Redbob, your exhaust expertise is VERY informative and inpressive ! Maybe you can explain something thats been puzzling me. If you look at attached photo below, you will see a 3rd gen camaro muffler with a "h-pipe" connected between the muffler inlet and the r/s tailpipe. I got this off a website recently. Since I own a 3rd gen IROC with a stock muffler like the one shown, I'm wondering, will this improve performance/scavenging effect/sound good/lousy/etc. My car has no cats. Also since you were in on GM exhaust/muffler engineering, how restrictive is stock 3rd gen "cross-flow" muffler really. Aftermarket makes them seem like garbage. Your response would be much appreciated. Thankx.
Welcome to CACC pauls

I am sure you will enjoy the friendly people here at CACC. I don't have an answer for your question, I only wanted to welcome you.
(Is it just me, I haven't heard of "Redbob"?)
To: 78SilvAnniv
"Redbob" here! (Must be just you :)

That’s an interesting photo – I’ve never seen that particular mod before, but then I’m not quite as familiar with Camaros as I am with C4/C5 Corvettes.
My opinion would be that this short-circuits the muffler to a certain extent; that is, some of the incoming exhaust bypasses the muffler altogether. In so doing, it will a) make for lower backpressure, and b) make for a noisier, louder exhaust sound. Not necessarily “good” sound, but I couldn’t judge without listening to one.
However, since the connection is not a very efficient one, from a flow standpoint, most of the exhaust gases will still pass through the muffler. That is, this little bypass tube won’t make things as noisy, nor will it lower the backpressure as much, as if the two pipe joints were ‘Y’ connections. By simple inertia, much of the gas flow will pass up the the first connection and proceed directly into the muffler.

In summary, I’d say this: You’ve made a major step in the right direction in removing the cats – at least, with regard to reducing backpressure and improving performance; doing that is easily the equivalent of the best aftermarket exhaust system you can buy, and better than most. What you have probably sounds pretty good, without waking the neighbors (or the sleeping cop outside the doughnut shop!) or overpowering your stereo. I’d try that ‘H’-pipe you showed in the picture (great photography, BTW) if it’s not obscenely expensive, and if you can easily reverse it if it turns out too noisy for you. Looks like almost any muffler shop ought to be able to ‘gin one up for you. See if you can’t get one made out of 409 stainless, or better yet, a piece of aluminized 409 off a late-model car. Wish I could tell you an easy way to tell aluminized carbon steel from aluminized stainless. Plain 409 is quite good, though.
Sorry I can't give anything more than generalities; I know it would be much more helpful if I could say, 'Yeah, that thing'll give 4.5 HP at 7200 RPM," but I've just never run across that 'H'-pipe in a Camaro exhaust before.
I’d be curious to know how it turns out.

Regarding the stock muffler: it's pretty restrictive, since it's not very large and the internal construction is relatively unconventional, having one of the outlets one the same end as the inlet. If you found a way to fit two single-in/single-out oval mufflers, e.g. Flowmaster 3-chambers, it would probably get you a reduction in overall system backpressure. Whether or not that would be a significant reduction, I can't say. That is, how much power would you gain, if any, versus how much would it cost? A Camaro specialist might be able to give you some help here.
One more thing: Is that pipe in the photo COPPER? Hope not; copper is not much suited for automotive applications, for anything other than wiring!

OK, two things: Note also that the diameter of that 'H'-pipe has a pronounced effect on how it works, both from the standpoints of backpressure and sound attenuation. That would be one way to "tune" the effect you get.
If not too expensive, you might play with that a little.
You're saying you can use the H-pipe as an attenuator and adjust it for cacnelling resonance ? I'm curious on that. Also when looking at an H-pipe compared to an X-pipe it would seem the X-pipe would provide the best balance in flow, but maybe not in resonance? I've seen people put an H-pipe behdin the headers to balance the exhaust. This doesn't seem effective to me. The flow out of the engine should take the path of least resistance and the H-pipe is at 90deg to the flow. Once back pressure is created from the mufflers then it would appear the H-pipe chamber would start to fill and provide some balance. But and X-pipe causes an immediate 2 into 1 into 2 and at the intersection the exhaust is forced together. Wouldn't that be a significant improvement over an H-pipe. No matter where it is located. Could you explain this a little more please on the pro and cons of an H-pipe versus and X-pipe. ;help :confused
You're right, the exhaust flow will certainly take the path of least resistance, but a lot of what an 'H'-pipe accomplishes has to do with acoustics and wave effects. That is, an 'H'-pipe provides acoustic cancellation by acting as a form of quarter-wave tuner; this depends on the engine RPM, and the location of the 'H'-pipe, and is governed by wave theory. This quarter-wave tuner will provide attenuation of certain frequencies in the exhaust, depending on pipe lengths and ‘H’-pipe location. In essence, pressure waves are communicated via this pipe, but there is little actual flow.

If properly located, it can also provide an exhaust tuning effect.

You can think of a pressure wave or pulse, initiated by the exhaust valve opening, proceeding down an exhaust pipe to an "end", and being reflected back up the pipe as a vacuum pulse. This pipe "end" can be a true end, or it can be where the pipe goes into a muffler or resonator - anything that provides a high impedance. If the reflected vacuum pulse going back up the pipe arrives at an exhaust valve while the valve is open, it assists in scavenging the exhaust. If it arrives during the valve overlap period, when both the exhaust & intake valves are open, it greatly assists in cylinder filling. This is what produces the "tuning" in a tuned exhaust.
In fact, a similar effect occurs in the intake tract.
These pressure & vacuum (or 'rarefaction') waves travel at the speed of sound, and the speed of sound depends on the gas temperature. What an ‘H’-pipe does for acoustics is the same as what it does for scavenging; the pressure wave initiated by valve opening is a sound wave, and the reflected vacuum pulse serves to cancel that sound if it arrives at the right place at the right time
What all this means for you is that an 'H'-pipe will give an acoustic benefit at a specific frequency, and therefore at a specific engine RPM, due to its' location; and also that it will give a performance boost at a specific RPM. This RPM may or may not be within your normal operating RPM range. (There are formulae available to help in locating an 'H' for maximum performance) There will also be a reduction in flow resistance or backpressure since the exhaust is provided with an alternate path. It’s important to keep in mind that the gas flow in an exhaust pipe is not continuous flow, like water in a garden hose; it is a pulsed flow, with pressure peaks caused by the opening of each cylinder, and when I speak of “backpressure,” I’m referring to an average pressure over time, not to the peak pressures. The ‘H’-pipe communicates one pipe from one bank of cylinders with the other pipe, and the pressure peaks from the two banks don’t coincide, so there will be opportunity for flow from one pipe to the other. Obviously, this flow would work a lot better with something like an ‘X’-pipe, where the connections are smoother. Unfortunately, I’ve never studied the acoustic or tuning effects of an ‘X’-pipe, so I can’t tell you how they compare to an ‘H’-pipe. If you know of someone with dynamometer comparisons between the two, that would be the place to look – assuming of course that the person making this dyno comparison isn’t in the business of making, say, ‘X’-pipes!
Hope this has been of some use to you; if I’ve been unclear on any point (which I suspect), feel free to ask again.
thanks for the explanation. very interesting. in my earlier days i worked on radar and then at a research facility on sonar buoys and monitoring/testing them. some of what you describe sounds exactly like playing with RF, microwave waveguides, and sonar. attentuation is a big deal and doing it properly is the key.
i'll look for some sites that explain the H-pipe and X-pipe locations in more detail to see if i can find anything useful. what you said makes sense to me. just arbitrarily placing an H-pipe or X-pipe would have different effects, but placing it in right place in the stream would give you some control over power and sound.
thanks, Graham
More exhaust comments/questions for "redbob"

Thanks again "redbob" for your info. having worked the technology end of exhaust, your info is no doubt quite "valid". As for my 85 305 TPI, the sound & performance improved quite a bit with cat removal, pulls stronger at upper rpm, sounds richer/ throatier,etc. It actually sounds aggressive at WOT, and I doubt it would wake the neighbors, or the sleeping cops at the doughnut shop. This cat had both 3-in inlet & outlets, then pipe funnels down to 2 3/4 or 2 1/2 into muff. Others who have heard it are surprised it's actually stock exhaust. This is why I thought it "sounds" like this muffler might not be too restrictive, and I know that louder usually = less back pressure. I would think this "cross-flow" style of muffler would be less restrictivew than conventional 1-inlet/1 outlet on each end, just because the exhaust has 2 openings to "escape" from. If I understand you right, I would be better off with 2 standard flowmaster-type mufflers than 1 crossflow style because a) aftermarket" crossflows "aren't that much less restrictive than factory "crossflows"due to design, and b) 2- one inlet/one outlet flowmaster-types would outflow any "crossflow" style muffler . Is this correct?.Could you explain difference in crossflow design? Is it true that the farther "downstream " a muffler is the less "backpressure" it sees?

Whats your opinion on "flowmasters" as far as flow design and sound? Ive heard that Dyno-max "ultra-flows" flow better.What do ya think?

Yeah I might try the "h-pipe". Do you think larger than 1-inch would sound trashy/ crappy since its unmuffled? My thouight is that it might only be noticeably louder at WOY, because thats when high exhaust flow would seek " path of least resistance"? Please comment.

Thank you for your time and input, I hope this does not sound too redundant, .I feel like you know your stuff, I hope I dont "bend your ear" too much. I'm sure others out there like what you have to say,too! Thanks again for your patience!
As someone said, 'Flattery will get you anything.' Thanks for your kind remarks. I'm just a little bored in my current job; maybe 'bored' is too strong a word, but it's just not as much fun as working on Corvette exhausts, running tests at the Arizona and Milford Proving Grounds. So I don't mind talking/writing about the little I know about exhaust.
About mufflers: First, with basically a stock 305 engine with the cats removed, you may be into the point of diminishing returns for exhaust mods. From this point it might involve massive transfusions of cash to get small power increases, and you might be unhappy with the overall sound you end up with. Does your 305 use the same muffler as a 350? If not, changing to the 350 muffler might be your cheapest and overall most satisfactory way to make another improvement. I'm sure you can get one cheap that someone took off to put on a Flowmaster ;)
To answer your questions about mufflers will require a short discussion of what goes on inside one: there are two primary types of passive silencers for cars and trucks. The first, the one we use most of in this country, and what is found on Corvettes and Camaros, is the 'cancellation'-type silencer; the other, found mostly in Europe on luxury cars, is the 'absorption'-type. The cancellation silencer for passenger cars and light trucks works usually by providing multiple flow paths, and will have multiple internal chambers, three or four in production mufflers, often just two on a high-performance muffler.
What happens is that these chambers and paths provide opportunities for sound wave cancellation, since by taking different paths, sound waves of a given frequency will arrive at the same point out of phase with each other, and cancel out. Unfortunately, there's just not enough space under a car to cancel ALL frequencies, so the muffler AND its position under the car is selected to cancel out the most significant frequencies, the low frequencies that are reinforced or amplified by the exhaust system pipe lengths. Beyond this, perforations and small absorption silencers will often be incorporated into a muffler to provide some reduction in the high-frequencies.
What this all means to you is that the Camaro muffler is handicapped in its' internal layout by the need to have one of the tailpipes exiting the same end as the inlet - you can't get as many 'passes' as you'd like to inside that muffler. An aftermarket muffler maker is going to be faced with the same problems, and probably won't have either the sophisticated tools nor the inclination to spend a lot of time solving those problems. He *might* reduce the backpressure by simply giving you a large mostly-empty can, but my guess is it'll sound pretty awful. I recall working on a Camaro, and it was difficult for us to come up with any major improvement over the production system, which we considered to be unsatisfactory in its' overall performance. The Camaro seemed to be designed without any regard for an exhaust system - except for that huge catalyst hump under the passenger's feet!
If they could be fit into the car. a pair of well-designed low-restriction mufflers would work better - sound better and have lower restriction. My guess is the Flowmaster 3-chamber design would be OK, 3 chambers being better than two. The only Walker mufflers I've used were a pair of Dynomax (same as Ultraflow, except not stainless steel) jobs on a Mustang GT (don't ask!). That car then sounded more like a 15-year-old truck than a sports car; it certainly rumbled, but it would set off car alarms as you cruised through a parking lot! That's not to say they would sound the same on your car, of course, but they are VERY 'open.'
As far as muffler location goes, you don't want the muffler ALL the way at the rear of the car (like a Corvette!) since you'd like to a) have a little acoustic pressure to work with in the attempt at cancellation; and b) since you want to 'break-up' the long pipes in the system into shorter ones of unequal lengths to avoid the (organ-pipe type) pipe resonances that plague, oh, the Corvette, for example! The only advantage, backpressure wise, to locating the muffler further toward the rear of the car is that the exhaust gases passing through it are at a lower temperature, and therefore have a lower flow rate, since they have a greater density. Flow resistance effects are dependent on volumetric flow, not mass flow - i.e., they depend on cubic feet/minute, not on pounds/minute - so lower temperature gases passing over a sharp edge or through an orifice will have a lesser pressure drop than high-temperature gases. This, BTW, is a great argument for NOT wrapping your headers with insulation, unless they're getting so hot they set stuff on fire under the hood!
Let's see now: 'H'-pipe diameter: How about putting in a larger-diameter 'H' in the location shown in your photo, maybe 2" or even larger, but with a flange connection in the middle? Would there be room for that? Then make several stainless steel restrictor plates or shims (.030"?) to go in the flange joint. If a straight-through 2" was too large, you could put in, say, a 1.75" restrictor, or smaller. Worst case, you could go all the way down to 1", or you could go back up to full size for a run down the strip. I'm going to try to attach a sketch,don't know if it'll work.
Remember to design the joint with the shim in place, and to always run with one, even if it's the same as pipe size.
With regard to backpressure, like anything else an 'H'-pipe would only make an improvement at WOT - that's the only time you have any significant amount of backpressure, because that's when the exhaust flow is up, and the exhaust gas temperature is up.
And speaking of 'H'-pipes, if you were to go to dual mufflers, I'd strongly recommend devising a way to have an 'H'-pipe in that system, too.
Have I missed anything? If so, try me again.
Hey rebob, thanks again. Your info is appreciated. Yes, my car is basically stock with typical free/low cost mods, (maf screens & fins removed, 160* t-stadt with cooling fan on toggle switch ,base timimg bumped up to 12*, 92 octane, airbox gutted with k&n's & homemade forced/ ram air, airfoil, bosch 4's, good wires, basically good tune-up, nothing radical, etc). The difference I felt & heard with "modified cat" was enough that I hate to remove a otherwise perfectly good stock exhaust for almost no gain...recently got car, bone stock, with 45,000, actual from original owner who is a good friend, car never saw winter, and never will. I am NOT an advocate for over-polluting the atmosphere, however for the sake of added performance and limited driving (4000-5000 mi. annual) I guess I justified cat "modification", in my mind anyway. MY daily car runs very clean, if that helps!

Restrictive cats are why I've always doubted most aftermarket exhaust companies claims of enormous h.p. increases due to the cats giving a substancial cork effect upstream of muffler . High flow cats are no doubt better, though none probably flow best. In general, on anycar, how can a low restriction muffler reduce back-pressure THAT much behind an already larger restriction upstream? Rebob, you can respond if you like, though you've already been very helpful.Thanks.

P.S. If you attached sketch, it never made it.
You've got it exactly right: much of the exhaust restriction - say, 50% - is in the catalytic converters ("cataclysmic perverters", as a friend liles to call them) and diddling with the mufflers will get you relatively little gain - unless you've got something like an L98 that does truly have a corked-up exhaust. There's a bunch to be gained there, 10 -15 HP or so with less restrictive mufflers according to a fairly knowledgeable source at GM.
As far as clean air: I think fuel injection and electronic engine control give about 98% of the emissions reductions from modern engines, and that's good enough for me. Taking the cats off is cheap horsepower, and you can always save them for resale time!
My experience has been (standing on soapbox now) that the so-called engineers working at the EPA are a frustrated bunch of auto-industry rejects trying to avenge themselves on the companies that they weren't good enough to work for.
Rebob, I've read on several message boards , mostly at www.thirdgen.org , that it is NOT uncommon for a well-tuned , properly running , fuel-injected, computer monitored , modern vehicle to "squeak" past emissions without cats. I think cats probably assure the automakers that there vehicles will, even in a moderately diminished state of tune ,meet/ exceed emission laws ,there-by keeping the feds and the consumer off their backs...they no doubt pass the added costs of these parts on to us,anyway. Thanks again,Rebob
Yep; where cats especially help is in the cold-start mode, when cold cylinder walls promote unburned HC's and CO (once the cats light-off, that is), and at idle, when an over-rich mixture is usually required to insure smooth running.
That little EGR valve takes care of most of the oxides of nitrogen, although I can't help but think a *little* nitrous would be alright.:)
Happy New Year!
- R
H-Pipe placement.

Hello All ! :w

Graham, I don't know if this actually works, however I will pass it on. In answer to your question of redbob on how to find the most efficient location of your H-pipe, here's what I've been told. raise your car, slip under it with a can of spray paint, and paint about a two foot section of pipe on each side of exhauxt pipe. Let your car down, drive it around the block or until it has reached normal operating temperature, then raise it back up, slide back underneath, and look for the area on each side/pipe that has the paint burned off. This will tell you where the "hot" spot is, in turn also indicating the most beneficial location for a cross over for the exhaust. Hope that helps, I haven't tried it, so you may want to check with RedBob. Good Luck with your project though !

Graham, P-ColaVette:
The paint idea has been known to work pretty much like you described, except for one thing: the place where the paint is burned off the pipe will be correct for the light-load condition experienced in driving around the block.
To get the optimum location for best performance, you'd want to make for example a pass down the drag strip at WFOT ("wide fully-open throttle" is what the geeks at the EPA think that stands for!).
What the burnt portion of the pipe is showing you is the location of an acoustic node, and this node changes position with the speed of sound, which in turn varies with gas temperature.
One approximation of a good 'H'-pipe location in C4's is as far to the rear as practical, but just ahead of the rear axle.
- R

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors


MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Advertise with the Corvette Action Center!

Double Your Chances!

Our Partners

Top Bottom