Making a muffler

[ptuomov]

166.gif

D.V. artwork. Posted for reference of straight through muffler design.

I'd think that it would depend on the case size and porosity of the perforated tube inside it. I'm hoping that a design such as MagnaFlow race muffler or Borla XR-1 with the largest possible case will look like a really big, wide angle version of the second type in DV's graph to the pulses, but look like a straight pipe for the flow. Of course, that graphic by DV is a gross simplification -- but that's why it's useful, presenting 80% of the truth in a format that is easy for people to understand when the remaining 20% of the truth is highly application specific.

In any case, my car has turbos. I've come to the conclusion that given the pressure ratio and flow velocity at the wastegate and turbine, the pulse tuning downstream of the turbine doesn't impact power at full load and high rpms. I've come to this conclusion that many other people would call obvious via a torturous and long route! . What I care about in my exhaust and mufflers is low back pressure at high rpms and loads and great broadband muffling at low rpms and loads. For that purpose, a straight thru design with perforated tube(s), packing, and biggest possible case should be the winning ticket.

[j-c-c]

Does this temperature, density, and SOS computation look ball park correct?
0.75 Temperature decay coefficient per m

How was that derived?

[ptuomov]

Does this temperature, density, and SOS computation look ball park correct?
0.75 Temperature decay coefficient per m

How was that derived?

I eyeballed some graphs from a paper by a Korean research group. I also looked at some of our data logs from the dyno. Then, I decided to think about the problem like I did about the two-stage intercoolers on the car, and just said that per each meter of exhaust tract we're going to lose x% of the temperature differential between the exhaust gas and ambient air, which has no deep scientific basis but also doesn't strike me as completely stupid thing to do. With this decision made, I proceeded to eyeballing everything to round numbers and fitting a decay coefficient in Excel. It's not perfectly accurate (shocker there!) because I was eyeball-fitting data, dyno data is mostly from short pulls, the dyno fans don't accurately replicate the flow under the car at various speeds, etc. But when one has to make decisions, I'm thinking that educated guesses are better than pretending that not articulating an opinion means that the design doesn't imply an opinion.

[ptuomov]

Here's a question: Does it help or hurt power to keep the exhaust hot inside the exhaust pipe?

I can see why it very much helps to keep the exhaust hot up to the point where you'd normally leave the headers open. That is, if you have a race car with open headers and no mufflers, there's more energy in hot exhaust and if everything is sized correctly then hotter should be better. However, how about after the "termination box" in a full exhaust? That is, after the crossover and the first set of big a$$ chamber mufflers, is it beneficial to cool the exhaust as quickly as possible or to keep it as hot as possible? My logic says that one is better off cooling the gas rapidly in the rear section of the tail pipes, because that makes the exhaust gas much denser and allows one to muffle the exhaust with less restriction. But is my logic correct?

[Birdlab]

I have been wondering the same thing - can I get by with smaller diameter, more restrictive mufflers if mounted at the end of a long, large diameter exhaust system with termination boxes? Would the cooler, more dense exhaust gas be less negatively affected by smaller, quieter mufflers?

[ptuomov]

I have been wondering the same thing - can I get by with smaller diameter, more restrictive mufflers if mounted at the end of a long, large diameter exhaust system with termination boxes? Would the cooler, more dense exhaust gas be less negatively affected by smaller, quieter mufflers?

Yes, we both wonder.

One thing that points in that direction is that pretty much all factory cars step down the exhaust pipe diameter as they get further away from the cats, except then the huge visual exhaust tips for us suckers err paying customers.

[pdq67]

Please consider using, "static mixing", technology here if you want to make your exhaust pipes straight through.

About 6 or 7 static mixer flutes in a 20" or so long 2.5" dia. straight pipe down each side and go!!

If I had the needed welding equipment, I would have this under my car now!!

Otherwise, I like the way, "old-school", WDT, smooth perforated inner pipe REAL glass packs!

Suckers are straight through so no back pressure loss at all, imho. Just like DV has shown us.

If they are too loud, just add short ones in your exhaust system to act as resonators OR parallel double up on the long ones..

I love their way old school "cackle"!!!

And you haven't lived until you have heard a pair of, "smitties", on a way hopped up 261" 6-banger with split manifolds going up and down through the gears!!

It will raise the, "hair on the back of your neck"!!!

AND you will never forget the sound!!

F**, talk about F-1 crap, no a split-6 turns me on!

pdq67

Put the, 6 or 7 or so "static mixer", "flutes" inside a smooth perforated inner pipe and weld solid.

Wrap everything with either long strand high temp fiberglass or steel pack it if weight isn't an issue!

You will have a straight through, "fluted", muffler so go.

pdq67

[TBART1970]

Not to hijack, I have a question. I have on my Camaro with a 548 ci. BBC, 2 inch headers, a 3 inch exhaust with 4 SpinTech mufflers. Two up by the headers and two all the way at the back. My engine builder thinks it is way too restrictive. I didn't want it unbearably loud or have a drone that would make me nuts. I think this adds to the discussion, what is the better placement, near the headers or at the back? Or a set of mufflers and then more open mufflers at the rear, or vice versa? Do you think it is that restrictive how I have it? What about bigger headers? I will find a pic and post it soon.

Thanks,
-Tom

[ptuomov]

Not to hijack, I have a question. I have on my Camaro with a 548 ci. BBC, 2 inch headers, a 3 inch exhaust with 4 SpinTech mufflers. Two up by the headers and two all the way at the back. My engine builder thinks it is way too restrictive. I didn't want it unbearably loud or have a drone that would make me nuts. I think this adds to the discussion, what is the better placement, near the headers or at the back? Or a set of mufflers and then more open mufflers at the rear, or vice versa? Do you think it is that restrictive how I have it? What about bigger headers? I will find a pic and post it soon.

I am the opposite of the expert, so don't put too much stock into what I'm writing. I read some lecture notes on the web, etc., so these qualify at best as opinions.

That all said, I think one needs to have separate muffling strategies for high-frequency, medium frequency, and low-frequency noise. Placing the mufflers as far back as possible is the best, but since there's not an infinite amount of space at the back, some choices have to be made. Because the pipes and mufflers can generate whistling sounds themselves, the high-frequency muffler should go to the tail last. The medium-frequency and low-frequency mufflers can go further to the front. I think (but don't know) that "glasspack" mufflers take out high-frequencies, simple chambered mufflers take out medium frequencies, and complex chambered mufflers with Helmholtz resonator chambers take out low frequencies; so that's the order in which I'd place them starting from the tail pipe. I would try to not use two similar type mufflers in the same exhaust pipe. Also, my logic says that exhaust cools and becomes much denser as it flows in the pipe. Therefore, I think that the pipes should be big close to the engine and the tailpipe muffler(s) right at the rear of the car can be quite small. In terms of the pipe section lengths, short pipe sections that are interrupted by mufflers are better than long pipe sections, and if one can to combine both sides to a single pipe for a foot or two that allows you to do it. The magic number seems to be 2/3 in terms of keeping the sections close to 2/3 of each other lengths and not close to low-order integer multiples of each other.

[Kevin Johnson]

Here's a question: Does it help or hurt power to keep the exhaust hot inside the exhaust pipe?

I think this has been discussed before and an issue is the increase in viscosity of high temperature gases.

[ptuomov]

Here's a question: Does it help or hurt power to keep the exhaust hot inside the exhaust pipe?

I think this has been discussed before and an issue is the increase in viscosity of high temperature gases.

So what's the answer?

In terms of viscosity, I'd think that the reduction in density and the resulting reduction in velocity would be a bigger deal in flow-loss calculations.

[Kevin Johnson]

https://www.speed-talk.com/forum/viewtopic.php?f=1&t=43873

[ptuomov]

https://www.speed-talk.com/forum/viewtopic.php?f=1&t=43873

So was this the conclusion on viscosity?

"In turbulent flow conditions that are well beyond the laminar flow regime [like in car exhaust pipes], the viscosity is not a significant factor."

[Kevin Johnson]

I don't know if it has been suggested already but you are in a propitious position to test. The test(s) I suggest are to obtain a large quantity of durable exhaust wrap tape and to progressively tape sections of the system with intervening dyno tests and IR or thermocouple temp measurements. Wrap front to rear and then unwrap front to rear. That doesn't cover every possibility but it should give you plenty of data to determine trends if you want to then apply expensive coatings. Then you can publish an SAE paper.

[MadBill]

How about a pre-test, wherein you baseline it as-is, then wrap the whole works, to see if you can measure any difference in power or tail pipe gas temperature? If yes, then parse the details.