EXHAUST Theory can someone expain it to me..

i really don't know much about the theory behind a vehicles exhaust system..

usually in the hotrod/ custom car arena the goal is to make it fit, and get exhaust out the rear of the car..

however.. can someone please explain the following to me?

what happens if the muffler is further forward / further back?

how can I get a deeper tone or louder exhaust?

if I were to wrap the headers (or coat them) what benefits can I reap?

what if I have a cross over pipe, where is the best place for it?

what if headers are used, how does the length of the header tubes effect performance (torque etc?)

anyone out there have background knowledge on this stuff?

 

the farther back you put the muffler.... the slower the exhaust will be traveling thru the muffler.. so it gives it a richer almost straight pipe sound...

if you put an H or X pipe in... it balances out both sides of the exhaust and helps it travell faster out... but it gives it more of a mono tone sound... i believe it adds HP tho... but it kinda makes it sound like a ford...

ive got 2 1/2 inch pipe and 40 series flomasters on my 35.. small block chev....the mufflers are mounted just about even with the rear U joint... i think it sounds great... people ask me all the time if it is a big block...

 

First, you need to understand fluid dynamics a bit. (yes, gas is a fluid in this case) Push fluid through a tube of a given diameter and it meets a certain amount of resistance. The longer the tube, The greater the resistance, or restriction, in this case. Simple concept takes a minute to digest. To understand it more, look at a fuel jet chart for Holley or Mikuni carbs, etc. You can have two jets with the same orifice DIAMETER flow two rates of fuel by changing the LENGTH of the orifice. Same applies to exhaust plumbing. Food for thought.

Now, consider the two main characteristics of exhaust flow, VOLUME and VELOCITY. Say your engine moves 600 CFM at a given rpm. That VOLUME of gas will move through a 2" diameter pipe at a certain VELOCITY, but faces a certain RESTRICTION. Now flow that same volume through a 3" pipe. The restriction drops, but so does the velocity. Remember, you are still moving 600 cubic feet per minute. A larger pipe can deliver it in the same time with a lower speed.

Now what are the effects of volume vs. velocity? Velocity comes out of the tail pipe with a greater differential pressure to the atmosphere, so it is louder, Like a big air hose wide open. Volume comes out quieter because there is less pressure differential at the exit.

The other considerations of velocity and volume are this. Increased velocity means that gas sent down the pipe by one cylinder will maintain momentum and suck gas out of the other cylinders or head pipes in a header application. This is called scavenging. Scavenging helps restricted motors move more air at lower rpm, increasing torque at that rpm. As rpm increases, the restriction becomes too much for the increased volume and velocity drops, so high end power suffers. The remedy is larger pipe to handle the volume of higher rpm. This allows velocity to again provide scavenging at higher rpm and promote torque. (HP is calculated directly from torque, so is irrelevant in this converstation) The trade off is a loss of velocity at lower rpm, which causes reduced scavenging down low. In a nut shell, exhaust flow needs to match the rpm at which you want to make power. On a drag car, it is 6000 rpm or more, so large pipe and short runs are the order of business. On the street, you need the engine to make good torque at 2500 rpm, so pipe tends to be smaller. Mufflers and longer runs are needed to quiet the pop from increased exit velocity.

To sum up, decide where your priorities are and what your physical limitations are. Choose a pipe that gives good flow for your usual cruise rpm, race or street, and put any mufflers as far away as possible so the velocity has already dropped before the gas enters them. If you have limited space, consider a big single over smaller duals. That promotes velocity and conserves installation space.

 

Smoke goes down pipe and out the back of the car.

 

Just to throw another kink in it, you don't have to run the same diameter pipe down the entire length of the system. Smaller pipe at the front for scavenging and bigger pipe and the exit to soften the sound (remember the giant megaphones that were popular a few years ago?).

Case in point:
Many years ago when I still lived in an apartment the mullet-headed Metallica t-shirt wearing dude on the other end of the building also had a 70 Chevelle. His was an SS396. He used 2.5" pipe up front to 12" glasspacks and had 1.5" tailpipes. It still sounded like a big block but with a sharper tone. You could hear the damned thing a mile and a half away.

 

I knew it!!!! Elegant,yet so simple .

 

Beyond it being just smoke and velocity of the gas itself, you have to take into consideration the sound wave structure of the noise. When the engine fires and the cam overlaps, you get some of that 'bang' coming down the pipe as well as the 'pop' as the exhaust exits the tailpipe.

If you run straight pipe (meaning no mufflers as opposed to just a straight piece of pipe) you take the whole length of the pipe including all bends and you'll end up with a standing wave in the pipe. All pipes have a certain resonant frequency that they want to emit sound (see: blowing over a moonshine jug for an example) as you change this length, you change the frequency or pitch of the sound.

If you put the muffler exactly in the middle, you really won't affect the standing wave much because it is at a node. Imagine a wave on a spring looking like this ~. The two ends and one point in the middle really don't move if this is a standing wave. Putting the muffer at the front will help break this up BUT you still have a long pipe in the rear to have a new length wave AND you will sacrifice power because you are bogging down the faster velocity gasses causing back pressure.

Like an air hose, a small orifice will give a much higher frequency because of the great difference to the outside atmosphere. Think of these fart can tuner cars, a 1.8L can sound huge by going to a 4" can but it really doesn't affect the power more than a titch (scientific word) besides, everybody knows its the wing and stickers. BUT like pumping water from a 5/8 garden hose and then going to a 2" pipe, the same volume of water seems to just fall out so it becomes a waste of money to try to increase it past a certain point because the pump (your engine) can't get it out fast enough.

For up to about 350-400 HP, 2 1/2" pipe is all you really need. You can go to 3" if you want to feel good but you'll get a more mellow drone and very little pop out the back.

Additionally, in the OEM exhaust world, we use a rule of thumb of 10:1 ratio. If you've got a 5.0L engine, 50L of exhaust silencer (mufflers) is the goal. A Lincoln town car runs around there. A Vette runs around 6:1 but these factory cars must meet a certain federal noise regulation. This 50L also relies heavily on placement. With a roadster, you've got no cabin so you'll get wind noise and everything else. A coupe will have a small cabin that has some interior padding to absorb some noise but it will have it's own resonant frequency that will 'drone' if hit. A sedan is bigger, a sedan delivery or wagon is even bigger and suburban...you get the picture. The larger the cabin space, the lower the resonant frequency so the deeper the drone. Splitting up the muffler volume before and after the rear axle or moving the muffler around or decreasing the size of the pipes will help keep the car's exhaust from actually producing this same frequency, limiting the chance of setting this frequency off. Sometimes altering the tailpipe cut or diameter is enough to get out of this as well. If you look at the rear of a minivan you almost always see a small can muffler in the rear, this is why. It keeps that rear pipe from having a standing wave that matches the interior resonant frequency or one of it's harmonics (like 40 Hz, 80 Hz, 120 Hz are all harmonics of one another).

The H pipe helps mellow it out because it doubles the exhaust volume and helps scavenging from one side of the engine to the other. This can improve power but I've heard those say that they want to know which side of the engine has the problem so they won't run an H or an X pipe. The X gives better flow, so a path of lower resistance than a 90 degree H.

I've heard the old racers used to take their dual exhaust and run a grease pen down both pipes, run the motor for a bit, see where the grease stopped burning off and put the H there. Seems to have worked. We've played with them a bit at work but it is usually a SWAG where it goes.

Wrapping headers will keep the heat in the exhaust which makes it want to run faster out of the pipes. As the heat is dissipated, the gas will slow down. This might have a point in the H pipe discussion but I just don't have the experience.

The header length will affect the peak torque band of the engine. Where the introduction of each cylinder into the exhaust stream affects the positioning of the scavenging in the scheme of things. Where it takes place depends on the length and RPM because you can imagine little packets of pressure points of power being in line to get out of the pipe. When they come together they jostle for position. At a certain RPM they will fall into a nice line and actually fold together like a good card dealer. If they are out of phase enough, they will actually pressurize another cylinder's port so when the valve opens it sees backpressure and the next packet of power from that cylinder has to 'fight' it's way out. That is why 2x2 into 1 collector makes a different power than 4 pipes into 1 collector. Equal length pipes are the dream for max torque because each cyl should see the same amount of pipe, and theoretically a common pressure from the other cylinders on its bank every time the valve opens.

Lots of variables. It really is more than just a pipe. Mounting makes a difference for the vibration you feel coming into your seat and your car that can affect the resonance in your interior as well but that is a-whole-nother story.

 

Kev You are a regular Fuckin Einstein

 

Bigger is not always better. Applying racing technology based on WOT for all out performance may or may not be affective on a street driven hot rod.
I personally believe that in the lower RPM ranges more commonly found in street use that the 2 1/4 to 2 1/2 exhaust system keeps the gas velocity up for a better throttle response than a huge 3" sewer pipe.

 

Scotty and Danima and Tommy; Outstanding information. Thanks.

Sawzall, like Danimal said, if you want some "quiet" out of it, go with “volume”. And, if you want it toned nicely, go with a muffler packed with an acoustic absorbing/dissipating material such as a ceramic or glass or basalt fiber. St. st. packed mufflers (Porter Steel Packs for example) clip the edges off the sounds but the st.st has a very low sound absorbing characteristic as compared to the others (ceramic or glass or basalt).

Based on Danimal’s information, I suspect he may work for one of the large muffler manufacturers so he hopefully will not care if I throw some part numbers in here of things we learned over the years.

One of the best all around mufflers for sound out the rear and inside a car is Walker’s Dynomax 17749. It is large case muffler (20” by 4.75” by 10.25” (lots of volume), it has 2.5" inlets and outlets, offset-offset configuration and even fits under tight 32 Fords if you do it correctly. It flows a lot of air and has virtually no resonance out the rear or inside the car. You could hardly ask for a better driving or performing muffler than this one. The shorter Dynomax 17734 is slightly louder, flows slightly less, and has slightly more resonance on some cars. Their UltraFlo is a bit quieter under light throttle but a bit louder at WOT but flows a lot of air/exhaust gases.

Magnaflow and Goerlich Xlerators both use similar muffler designs to Walker’s UltraFlo; straight through with a lot of acoustic absorbing packing material around the perforated tube core. Both make 18" long case varieties which give you a bit more room but a bit more noise. Both are smooth sounding like packed mufflers tend to be.

If you want it to cackle and roar with more resonance inside and out, go with the chambered mufflers that use no packing material but instead rely on plates and "V’s” and other impeding devices. These will give you that terrific sound out the back if you can tolerate more inside, assuming you might not have much sound deadener.

A larger engine almost always is louder than a smaller one for reasons given by Scotty and Danimal. Higher compression pounds harder out the pipes. Longer tailpipes will almost always cackle more than shorter ones. Mufflers to the rear tend to smooth the sound more. Larger tail pipes sound lower and meaner. Large tips on small tailpipes can create a nice sound without as much interior noise (for example MOPAR of the late 60s used a big volume chrome tip to lower the sound tone at the tip). Smaller pipes tend to fart and cackle. Small diameter glass pack shells make the sound louder unless they are long (volume, again: it is key).

The Dr. Gas X pipe (and some others as well) makes more power up high and down low and makes a larger cam sound smaller. It also helps get rid of resonance from chambered mufflers. We saw about a 25% reduction in back pressure at the engine when we tested an X on 27 different mufflers compared to an H. This means you can run more restrictive, quieter mufflers with an X crossover and still make smaller power to not using one with better flowing mufflers. We also saw sound reduction most of the tie with an X over an H or nothing at all. For power and sound reduction, always use a crossover if you can.

We did a huge muffler test several years ago and made a CD of their sounds and created a large EXCEL spreadsheet showing how loud mufflers were, what size they were, and how much air they will flow as compared to a straight pipe. Email me at thand@kc.rr.com if you are interested.

And, when mounting the pipes and mufflers, look at the OEMS. They know why and how to mount. We use some mid 80' Chrysler four cylinder mounts to support our 32 system. Avoid any of the hangers that affix the pipes to the floor or the frame with hard rubber or urethane bushings if you do not want vibration and pipe resonance inside the car. When in doubt, look to the best of the OEMs like Lincolns or Cadillacs or high end imports or even MOPAR minivans. Duplicate how they mount the system if you can and you can avoid a lot of unwanted interior vibration and the resonance it might cause.

Tom

Tom

 

Very good info, guys. Just want to throw a couple thoughts/questions out there. First, not long ago I did a job for a dentist friend who's mom had given a '75 Camaro w/350, all stock(tune-up) but Torque Tech 2@1/2 tubes, sounded like shit down low, but got sweet when it reved, so we welded in a cross-over, nothing elaborate, and it cured it, sounded healthy even down low. A case of to much exhaust capacity for the motor, I'm sure. Another; My old '55, 9.8 compression, camal humps, Crane ss350 solid cam, full length headers but not 2", I guess 1@3/4", then 2@1/2" pipes back to some cheap replacement mufflers for a 455 wagon w/single exhaust, loud as hell so to the muffler shop for tail pipes, wound up with 2@1/4" but they sounded great! even seemed to have a sweet spot up around the time it had just come up on the cam(somewhere just shy of 4grand) I mean that it really sounded happy. My Ford buddy said that the exhaust cools some by the time it enters the tail pipes and the slightly smaller pipes keep the velocity up, I tend to agree, but I'd like to hear what you think.

 

Exhaust cools on it's way out. The pipes act as a radiator and dump the heat to the surrounding panels (try for about a 1 3/4" to 2" clearance to keep from melting stuff, heating your pants). As the gas cools, the acoustical waves slow down because it is harder to transfer the motion from one molecule to another when there is less room between them (cooler). With the restriction of the volume, you can help keep the gas speed up. (Bernoulli's Principle states an increase in velocity results in a decrease in pressure and the inverse works as well)

On the point of glass packed vs. V baffles, remember that high frequency required smaller mass to knock it down where low frequency requires much more mass. To knock out these high freaks, glass wool (fiberglass or basalt) is typically used. Bi-pass and tri-pass mufflers are usually stuffed somewhere with them. Basalt and standard fiberglass erodes over time and reduce their effectiveness. After a while, you'll have an empty can that won't do squat for the high freaks. The best stuff uses continuous glass strands. It looks like cotton candy (see Miraflex home insulation R-25 by the Pink Panther guys). It is similar stuff and it is super long strands so it doesn't come out of the muffler.

I stay away from the V baffles because I want to hear the exhaust on the outside NOT the inside of my car. I don't like the drone and I personally think there is as much PR in the exhaust side of the business as there is raw science. If I see one name brand at all of the car activities I do that's what the general public will tend to learn to talk about. Perception becomes reality. My personal experience lies with A/B studies on stand engine and on car dyno testing and I have made a personal preference but you can decide on your own.

The part numbers above are very good for most of our applications. I did LOTS of testing on those once upon a time (10 years ago) when I worked for one certain exhaust company. I do OEM exhaust now with another company and have moved from the Thrush side to the Cadillac OEM side of the industry. It is always a challenge to tune a car and they definitely aren't the same. Simply flipping a muffler around end for end can be the difference between a pass and an absolute failure in performance, back pressure, and acoustics.

If you're looking to make a solid type mount for your exhaust, look into decoupling the vibration from the engine with flexible decouplers (like on most FWD cars) so you don't get the shake to start with. When you do this and hard mount the exhaust you can go with thinner pipes. You also need to take into consideration the thermal growth of the system so you need sliding or rotating mounts. I like the slip mount that is on the 88-98 Chevy trucks between the box and cab for this.

 

This is the most informative thread. You guys rule!

 

Reading all this was just exhausting.

 

Sorry, didn't see your post last week. Was busy dreaming of Billetproof. Hope we made up for it! Should we offer this up to the Tech Archive?

 

Sure did.And yeah that would be a good idea.Thanks Danimal.I've read a lot of your replies to technical ?s & you're always pretty darn knowledgeable & helpful.

 

That's funny stuff!

 

I'll have to change my password then because someone must be logging on as me. Thanks, though.

 

Yep.. I like it. Better than reading about radials verses bias ply..

 

It's a little off the question but pressure plus velocity equal to a constant in flow. The system will be ideal somewhere, less than ideal everywhere else, as far a flow is concerned.

 

Big Pete, you are correct and that is why it is such a PITA for exhaust because you alter the pressure as you change the RPMs (assuming you ignore the fact that the air pump is multi cylindered to start with) AND you never can have a true straight flow unless it is coming right out of the head. The change in exhaust gas temps cause all kinds of difficulties as it goes from the hot engine to the relatively cool tailpipe.

Pick what you want to tune for/against and love/hate the rest. A standing boom in your cabin may also be caused by a structure issue. Use sand bags and let the exhaust lay on it instead of having any of the mounts on the car. If it goes away, you've got a structure borne issue. If you can find it at a certain RPM, you can back calculate your frequency and try to kill it but something Stacy Smith (now of Harley Davidson in exhaust) once told me, it is all Jello. Push it down here, it comes up somewhere else!

 

While all the exhaust experts are here, I'd like to divert for a moment and ask a semi- related question. What is the best tailpipe configuration to get exhaust away from the car so as not to enter the cabin. The car being a Model A coupe. No exhaust leaks, but can still smell fumes inside. Some guys have told me to extend the tail pipes out further. Currently they are straight out and end angle cut at the end of the body. I see alot of OEM tailpipes have a turn down. Of course these are streamline bodies so aerodynamics wont be the same. Any advise?

 

You smell fumes while under way, or sitting at a light? Under way is probably a result of the air flow around your car. If you happen to exit at a high pressure area, for instance, where air coming over the top is drawn under the rear bumper, you will need to consider a noticably longer tip to get the gases into the passing air stream, or a new exit point. Side exit usually works well because the air is passing by enough to draw any exhaust away from the car. Many cars have a dead, tumble zone at the rear. Ever notice the exhaust stains on the back of a van?
While sitting still? Could just be the way the wind blows...

 

I'm with Scotty on this one. You might take a peak at what others are doing and ask around or you can start by simply clamping on an extension and driving it for a day or so. If that works, then you can go for doing them up right. A simple turn down may do the trick.

 

Thanks. It's mostly while driving. I think I'll try the temporary extensions first and see if there's a difference.

 

Weeehaw, There's a tone of theory out there.

I researched for about a year till I build a set of headers and full exhaust for my jeepster with a 340 buick

Here is a few neat linkie I found.
http://www.headerdesign.com/extras/design.asp
http://www.headersbyed.com/bildbetr.htm
http://www.engineersedge.com/engine_formula_automotive.htm

 

Ok, I read through the thread, but, with two kids constantly distracting me, I'm a little confused and need some clarification.....

I am building my grandfather's 51 ford club coupe as a traditional custom. He installed a 350 chevy engine. It has no exhaust, other than the manifolds I installed.

If I want that ultra quiet smooth "vvvvvvvvvv" sound, like an old cadillac or it's ilk, do I want a smaller diameter pipe and a larger muffler placed farther downstream in the exhaust system?

Or am I mixing it up?

Can someone make a general recommendation for getting that ultra quiet kustom sound?

Smaller pipe, larger muffler - mounted far downstream?

Larger pipe, larger muffler - mounted closer upstream?

Other?

Help me out, please. I'm really confused.....

thanks
dv

 

I've been told that I'm hung like Einstein and as smart as a horse. Wait, that wasn't a good thing, was it?

Excellent post. The X is obviously superior to the H, but either is a noticeable difference over no type of crossover. The improvement in low and mid range torque and response is definite and something you'll feel immediately. I don't believe that the fact that it does offer an improvement indicates any problem with the engine per se. I would guess even if you were using 360 degree headers and/or had optimized the firing order, a crossover would still be of benefit when running a full exhaust.
Measuring the temperature by any means does indeed seem the way to optimize placement, from all I've ever read on the subject. Believe it or not, measuring temps on the system will isolate an area of only several inches long that will be at least 100 degrees hotter than the pipes just ahead and just beyond it. This is where I've always put them and with excellent results.
I would not recommend header wrap for a street car. It traps moisture and will even decay coated headers after a time.

 

If you want that quiet, big car murmur going down the road, use 2.25 pipe, duals I'd say. Put the biggest multi pass mufflers you can as far back as possible. The Dynomax super-turbos are a good example. Use an X pipe and don't be afraid of a few extra bends in the run. Up and over the axle, etc. You want to slow down the exhaust stream as much as possible. Now don't expect to go drag racing with this set up. It will have lots of low end torque, but it won't like anything more that 1500 rpm or so above cruise rpm.