From discussion going on in another thread it seems pertinent that we start a new one... so here it is.

Excerpts liberated from: Suck, Squish, Bang, Blow: Part 2
By Mike Kojima

Exhaust System
The exhaust system is, in the simplest of terms, the piece of pipe that directs the engine's exhaust stream from the exhaust manifold to the tailpipe. To get there, it must first pass through the catalytic converter (the emissions device that converts poisonous exhaust emissions to water vapor and CO2), through the muffler and out the back of the car. The purpose of the exhaust system is to contain the noisy, hot, toxic exhaust stream. With no exhaust system, a car would be incredibly, painfully loud and spew lots of potentially deadly fumes, poisoning our atmosphere. To prevent these two admittedly anti-social phenomena, the exhaust system must primarily quiet the engine's noise and remove its combustion byproducts before discharging the exhaust stream into the general air.
The exhaust system that comes stock on your car was not designed with power production and cool looks first on the priority list. For the engineers who designed your car's exhaust, their first job was to make the engine as quiet as possible, last throughout the car's warranty period and be as cheap to produce as possible. While these are the attributes that 95 percent of the car buying public deems important, the goals of the OEM engineer are not necessarily in line with someone seeking performance. The roar of a tuned engine is music to our ears. Since most cars are designed to appeal to what most customers want, by design the exhaust ends up getting optimized on the quiet scale and compromised on the performance end.

 

The key part of an exhaust system is the muffler. The muffler is the can at the end of the exhaust whose job it is to make the exhaust quiet. To be the whisper quiet most car owners demand, a typical stock muffler must have an intricate and labyrinth-like internal flow path to help slow and cool the hot vibrating exhaust gas. It contains baffles that cause the exhaust flow to reverse direction and intermix. These are great for reducing noise but are not so great for having good power-unleashing flow.
To produce the most power, an exhaust should have the least amount of restriction to the exhaust flow. Restriction hampers the burned exhaust gases from exiting your engine, causing some charge dilution with the incoming fresh fuel/air mixture. (In other words, exhaust left in the cylinder is mixed with the fresh intake charge.) This causes a loss of power. With greater restriction, back pressure is generated, making the engine work harder to pump the exhaust out of the cylinders. This is one source of pumping losses.
Some stock mufflers have up to 18 psi of power-robbing backpressure. A well-designed performance exhaust typically has about 2 to 6 psi of backpressure. For comparison sake, an unmuffled straight pipe usually has 1 to 3 psi of backpressure.
To get the least amount of backpressure, most of the good, high-performance mufflers available today have what is called a straight-through design. These type of mufflers quiet the exhaust by the absorption of high frequency vibrations in a heat-resistant packing usually consisting of stainless steel mesh and heat-resistant ceramic fibers. They typically have a straight inner core with no baffling at all, much like a straight pipe with many small holes. The pipe is louvered or perforated when it passes inside the muffler's shell, allowing sound energy to pass through the holes but leaving the exhaust gas flow unimpeded. You can see straight through these types of mufflers. The louvered or perforated core is usually wrapped with either Fiberglas wadding, hence the old-school term Glass-Pack or on the better mufflers, stainless steel mesh to help further absorb the sound. On straight-through mufflers, the longer the muffler, the quieter it is. The length usually has no effect on the backpressure, just the noise output. These mufflers work in the same manner as the silencers used on guns. If a silencer impeded bullet travel, you would definitely have problems!
It is best to avoid those types of mufflers with a louvered core. Many old-school glass packs suffer from this design flaw. The louvers generate quite a bit of backpressure; they stick into the exhaust stream and create turbulence. Even though these mufflers are a straight-through design, they can have more backpressure than a stock muffler. When buying a straight-through muffler, look for one with a perforated core to produce more power. A good, properly sized, perforated-core, straight-through muffler will add only about 1 to 2 psi of backpressure to your exhaust system. Mufflers like the Walker Ultra Flow, Thermal, A'PEX, Borla, Edelbrock or Magna Flow are examples of a good, low backpressure mufflers with an absorption design. Many premade exhausts like A'PEX, Tanabe, GReddy or HKS are also of the free-flowing absorption design.
Another old-school type performance muffler that has seen better days is the Turbo Muffler. This is a less restrictive version of a stock-like reverse-flow muffler. In the old days these were considered high-flow mufflers, but now they have been superseded by the perforated core, straight-through absorption types.
A more recent trend is the big exhaust tip. Big tips do nothing for power but can dress up the back of your car. Some big tips feature resonated cores which quiet the exhaust's note by a few dB. You can spot these by their perforated or mesh inner pipes. Just remember, function gets respect.
A disadvantage to the straight-through muffler is it is louder than a reverse-flow type of muffler. Usually a straight-though muffler needs a small sub-muffler or a resonator to keep the exhaust quiet. In short, a resonator is really just another small absorption muffler that helps avoid a loud irritating drone at intermediate rpm. This drone is an unfortunate trait many straight-through absorption type mufflers exhibit. The resonator works by using its irregular spacing with the main muffler to help break up droning resonance. Army recruits are trained to break from the march step on bridges to prevent overstressing the bridge due to resonance. The irregular spacing of the resonator works in the same way to prevent droning resonance from forming in the exhaust. A resonator is usually a small, perforated core glass pack placed somewhere between the catalytic converter and the main muffler. Like the main muffler, the longer the resonator, the better for noise reduction. A Walker Magnum Glass-Pack is a good muffler to use as a resonator. Almost all of the pre-made performance exhausts feature resonators.
When designing a custom exhaust, it is important to remember to make it as quiet as possible. Loud may be cool to some, but remember: A too loud exhaust is perhaps the number one harassment ticket given to performance enthusiasts by friendly local law enforcement officers.

 

To save cost, a typical stock exhaust uses small diameter, crush bent pipe. Crush bends are easy to make in mass production. However, crush bends can reduce the flow of a pipe by up to 50 percent. A typical exhaust system made by the local neighborhood muffler shop is also crush bent. The best exhaust systems, like most pre-made exhaust systems, come with mandrel bends. Mandrel bending is done by a special machine that uses a non-crushable insert or mandrel that goes into the pipe while bending to prevent it from becoming crushed. Making your own exhaust? You can buy pre-made mandrel bends from Burns Stainless, Kinsler or Bassini.
Some self-proclaimed engine gurus claim too large of an exhaust tube on a car can cause problems; engines need a certain amount of backpressure to run correctly. Although the statement about not running too large of a tube is correct, the assumption about engines needing backpressure is not. A vehicle needs the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially in low-end torque, because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging effect from tuned headers. (We will discuss this in more detail later.) In simple terms, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the four-stroke cycle where both the intake and exhaust valves are open.
If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be as good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous engines. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo's efficiency depends a lot on the pressure differential across its turbine. A turbocharged engine can have an exhaust gas volume of about 1.5 to 2 times more than an equivalent displacement, naturally aspirated engine. Engines using nitrous oxide also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for nitrous operation.
Some companies like HKS or GReddy make a pre-made exhaust system. These usually have a good balance of low backpressure, properly sized tubing and low noise.
The stock exhaust system on most modern sport compacts is often so well designed that just switching to a high-performance cat back exhaust often does not allow for huge gains of power. With the exception of turbo cars, you can usually expect only about a 2 to10 hp at the wheels on a stock car. The power gain will usually be the greatest from just below the torque peak to redline. On cars modified with headers, intakes, cams, headwork, etc., expect bigger gains with the addition of a cat back exhaust due to the higher flow volume these mods produce.
Because turbocharged cars are very sensitive to backpressure, expect much larger gains, especially if the boost is turned up. A free flowing exhaust usually allows the turbo to spool faster as well. A turbo car usually gains from 8 to 30 hp at the wheels depending on how bad the factory exhaust was and how high the boost is turned up over stock.
Whatever you do, do not remove or gut out the catalytic converter on your street car. The monolithic, straight through design of modern 3-way catalytic converters is usually quite free flowing on most modern sport compact cars, producing at the most, only a pound or two of extra backpressure. A gutted cat can actually hurt power as the empty box can cause flow stagnation, which effectively shortens the length of the moving gas column in the exhaust pipe. The empty box can also reduce important flow velocity. This can be felt as a loss in bottom end power.
Because of these factors, some of our project cars have actually gained power with the addition of a cat. As the number of vehicles on our highways grows every year, we must all do our part to manage pollution. If every last bit of power must be extracted, as in real, off-the-street sanctioned racing, then the cat can be removed and replaced with a length of pipe, not simply gutted.

 

Why headers work is an arena full of old hot rodder's old wives tales and myths. Headers can produce substantial amounts of power on a engine with very few negative compromises. Headers work so well in producing extra power that they are a rare, win-win modification with hardly any negative trade-offs. This makes them a mod that is almost essential for any serious engine build-up.
For space, cost and catalytic converter light-off reasons, most cars come with a crude, cast iron, log-type manifold stock from the factory. A log manifold is simply a tube with stubby legs connecting the exhaust ports to the main tube. This is good for conserving heat to quickly light off a catalytic converter during cold starts, and it is compact, preserving valuable under-hood space within today's crowded engine compartment. However, a log manifold is detrimental for power production. Some Japanese sport compacts come with crude cast iron approximations of headers. This is better than nothing but it is still far from optimal.
A header is an exhaust manifold fabricated from tubular sections of pipe. Full radius mandrel bends are preferred so the pipe's tight radiuses will not be crushed down. Each individual exhaust port is treated to its own separate primary runner instead of merely dumping into the shared main pipe of a log manifold. The equal length, or close to equal length primary pipes converge at a single, larger-diameter point called the collector. The collector then leads to the main exhaust pipe. An old hot rodder's tale is that headers produce more power by reducing backpressure and by the long individual runners preventing the exhaust blast from one cylinder from blowing into the next cylinder, contaminating the charge on overlap, While this is partially true it is not the primary reason why headers produce more power than a stock manifold.
Headers make more power by primarily using resonance tuning to create a low-pressure, reflected wave rarefaction pulse during the overlap period. (The overlap period, remember, is between the end of the exhaust stroke and the beginning of the intake stroke. At this time, both the intake and exhaust valves are open at the same time for a few degrees of crankshaft rotation. Now don't you wish you read Part One?) Engine designers take advantage of this overlap period to help the engine breathe better.
To work right, a header must first exploit the inertial force of the outgoing exhaust gas. This rapidly moving, high-mass, high-pressure pulse creates a suction in its wake to pull burnt exhaust gas out of the cylinder. This first negative pressure wave helps evacuate the cylinder of burnt exhaust as the piston nears TDC and slows down.
To get the best breathing and to help pull as much fresh fuel/air mixture into the cylinder as possible during the overlap period, it is best if a low-level vacuum or rarefaction can be created and maintained past the initial low-pressure wave in the primary pipe. A well-designed header can use acoustic energy to maintain low pressure near the exhaust valve during the overlap period.
The way a header is tuned is much like how an organ pipe is tuned. The optimal length used is the one needed for the primary pipe to have a fundamental note corresponding to the time when the exhaust valve opens. When the exhaust valve opens, a high-pressure pulse of hot, expanding exhaust gas travels down the exhaust port at approximately 300 feet per second. This wave of hot, moving, high pressure gas has mass and inertia of its own which pulls a suction or a low pressure rarefaction behind the pulse.
Depending on the engine, the pulse can have a positive pressure of anywhere from 5 to 15 psi with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure. As this low-pressure rarefaction is several milliseconds behind the initial high pressure pulse, it can be exploited to help suck residual exhaust gases out of the cylinder toward the end of the exhaust stroke as the piston approaches TDC. The build up of this negative pressure and its timing in the exhaust stroke is closely associated with the primary pipe's length and diameter, just like an organ or other musical instrument.
As the exhaust valve starts to close and the intake valve starts to open, the engine enters the overlap period. During the overlap period the piston is starting to slow down as it approaches TDC and gets ready to reverse directions. To maintain good scavenging, a negative pressure must be maintained near the exhaust valve to help continue to suck stale exhaust gas out of the cylinder to make room for fresh fuel and air. As the main column of high pressure gas is almost out of the end of the header's primary tube, the pressure near the exhaust valve starts to rise again. All is not lost, however.
As the pulse of high-pressure, high-energy gas leaves the end of the primary tube and is diffused in the larger diameter header collector, a reflected pulse of sound energy just like a musical note is generated, much like that of a organ. This reflected sonic pulse travels down the exhaust pipe at the speed of sound, which is usually around 1100 to 1900 feet per second in thin, hot exhaust gas, causing a slight rise in pressure at the valve. The wave is then reflected back toward the open end of the primary pipe. Just like the initial exhaust pulse, the reflected sound pulse has an area of rarefaction, or low pressure, behind it. If the pipe is of proper length and diameter, this reflected wave can be exploited to lengthen the amount of time the condition of low pressure exists around the exhaust valve.
These phenomena are harnessed by the smart header designer to tune the pipe to help get the maximum amount of burnt gas out and to help pull the most fresh fuel in. Of course, because a header is tuned like a musical instrument, a header can only be optimized to produce the greatest scavenge-improving vacuum in a band of several hundred rpm.
Without going into a lot of math, there are some general guidelines you can use for selecting a header. Shorter primary runners and/or larger-diameter primary runners are better for top-end power. This has to do with the tuning of the pipe's fundamental note for reflected wave tuning and the travel time of the main initial exhaust gas pulse. Just like a piccolo is a higher-pitched instrument than a clarinet, a shorter, fatter primary pipe is better for higher rpm. Conversely, a longer and/or smaller diameter primary tube is better for lower rpm for the same reasons as above. Camshaft design and the duration of the exhaust cam are a large factor in header design. Generally, the later the closing point of the exhaust valve, the shorter the header primary pipes must be.
The way the primary pipes gather together is important also. This area of convergence, called the collector, is critical for proper header function. It must be of larger diameter than the primary tubes because it must be large enough to acoustically represent the end of the pipe (this is necessary to get the reflected sound wave to help scavenge the exhaust), and it must be big enough to support the flow from all the cylinders without creating excessive backpressure. Usually the collector is just a junction where all of the pipes are stuffed and welded into a larger pipe that may or may not neck down into the final size of the exhaust pipe. A well-designed collector, pairs cylinders opposite in the firing order with each other so an exiting pulse from one cylinder will not hamper the evacuation of the next cylinder. Adjacent cylinders in the firing order are kept separate so the exiting pulse of one cylinder cannot contaminate the next cylinder that may be on the overlap part of the power stroke. In a typical inline four cylinder, that would mean paring cylinders 1 and 4 and 2 and 3.
The best collectors are called merged collectors. This is a collector where the two opposite cylinders are paired together in a smooth taper before being introduced to the flows of the other cylinders. Merged collectors usually produce a wider powerband and sometimes more top-end power. Not too many production headers are merged due to the difficulty in fabrication, except for those headers found on real race cars.
Many headers presently available for popular sport compacts are of the tri-y design. For street cars, tri-y's are usually the best as they are forgiving to camshaft design and other tuning factors that the header builder has no control over, unlike a real race car designer who knows exactly what is in his engine. Tri-y's also promote a wide power band. A tri-y design pairs the opposite cylinders in the firing order together in a short "Y" and then brings the two pairs of "Ys" into a single collector, hence the name tri-y.
When a pulse travels down the primary of a tri-y header to the collector, it mostly goes down the main branch of the primary. When it reaches the collector, the reflected wave also travels back up the main primary to the exhaust valve and back out again. However in a tri-y, the branch that goes up to the opposite cylinder, since the exhaust valve is closed for that cylinder, acts like an interference branch, creating a pulse and an assisting wave of its own, slightly out of phase with the main pulse and wave. This widens the bandwidth of rpm over which the additional scavenging is effective and makes the pipe less sensitive to rpm-induced pitch.
The tri-y pipe becomes "in-tune" for a wider band of rpm, widening the engine's powerband at the expense of slightly reducing peak power over a 4-into-1 design. Since some of the pulse's energy is dissipated in the interference branch, the main pulse is not as strong and the scavenge is not as complete for the tri-y. Peak scavenging efficiency is compromised for having good scavenge over a wider range of rpm. That is why many full race engines where peak power is important use 4-into-1 designs, while many headers that are designed for best driveabilty like street engines or rally engines use tri-y headers.
For the most part, the majority of street performance drivers are better off either with a tri-y or a 4-into-1 header optimized for midrange power with either long runners, small runner diameters or both.
For street cars it is essential that the headers you purchase have provisions for all of the vehicle's stock O2 sensors, EGR fittings and any other emission controls that the vehicle originally had fitted to the exhaust manifold. Most modern emission controls do not rob any horsepower at wide-open throttle. The common EGR valve, which reduces toxic oxides of nitrogen, closes and has no effect at wide open throttle. Most air injection devices only operate either on cold start or under closed throttle deceleration on most modern cars. Removing these controls does not help power and does pollute the air.
On a more sinister note, on the new OBDII cars (1995 and later), if any of the emission controls are not hooked up, the car's ECU will start recording the error codes generated. In many states, you cannot register a car if the ECU has uncorrected error codes in its memory. There are many in our government that would like to closely monitor these codes and restrict our driving privileges if these codes can be found in our ECUs.
Just because your headers have provisions for all of your smog equipment, don't assume it is street legal. Due to the intelligence of some of our local government agencies, unless an aftermarket part is CARB approved with a CARB EO number, it is not legal in some states, no matter how clean the gsses coming from the tail pipe are! If having no smog certification hassles are important, either check local laws before installing or make sure the part you buy has a CARB EO number. A parts dealer should be able to answer that question.
A power gain of about 5-to-15 hp at the wheels can be expected from a well-designed header on most cars depending on how bad the factory exhaust manifold was. If you drive at a regular speed, (not exploiting your new-found power too frequently) you can expect better mileage with a header due to the improved pumping efficiency it produces. In buying a header look for thick-wall mild steel tubing, at least 16 gauge, and preferably 14 gauge (the steel is thicker on lower gauge tubing). Rust- and heat-resistant ceramic coated or stainless steel primary pipes are preferred for longer life. Look for thick flanges also, as these will resist exhaust leaks and last much longer.

 

Voting for Sticky..
I learned ALOT from reading ur posts........!!
thanks!

 

Sticky!!!

 

God dayum.

 

The thread is now stuck.

Minimc,
Thanks for the info on this thread.
It's very complicated. There is much to consider.

I wonder with our MINI engine design, how much of a limiting factor that is towards designing a better exhaust system. Are there inherent bottlenecks due to the MINI engine that limit the benefits of a custom exhaust (no matter how well designed).

 

Minimc, great post!
Hopefully the, but you have to have backpressure myth will start to die here on NAM.

 

Lifting copyrighted mat'l aint cool.

 

Not lifted - media contacted first. Author wrote it for Sport Compact car... is no longer his property, but is still credited.

Check your facts

 

Good for you!

© tsukiji – used by permission

 

Is this information accurate relating to forced induction as well?

If you listen to the arguments here and in other forums there is the constant mantra of bigger is better in exhaust design.
Big header primaries, 3" exhaust systems, etc.
There seems to be no concern for tuning headers and exhaust systems for designated RPM bands.

One thing I didn't note in this great piece of info is collector length. Collector tuning used to be a useful tool. Don't know if it's still relevant.

 

If I can jump in here, I think minimc posted this as a general overview to help NAM members understand known results of why headers work and other points in their design.
As you probably know the items you have addressed are application specific for each engine design. Materials, coatings, diameter sizing, lengths, configuration and shapes at transitions ( did I leave anything out?) determine the scavenging potential. This also is directly related to firing order, cam timing valve overlap and exhaust port configuration.
This is a deep subject, maybe a new thread is needed where everybody goes out and does the research on this subject then posts links back from various header builders, designers.

 

Thanks for helping out Norm

My intention in posting this thread was to provide a source of technical information in laymen's terms regarding exhaust sytem function. I was motivated to relieve some of the pressure in other threads... disputes over basic exhaust design, function, purpose & theory.

I agree, this thread could be more exhaustive , but it does explain quite a bit and dispels several misconceptions.

BTW
Mike Kojima, the author, is a cheif engineer with Nissan. He know's his stuff! He's a serious techie-gearhead who currently contributes HERE

 

wow Minimc, didn't expect you to do this after our debate in the header thread

Didn't mean to come off rash, just didn't like Norm's condescending comments

 

Thanks for another awesome NAM post...that really helped me take a crash course if you will and explain a few things that had me scratching my head.

Awesome!

 

I was searching for this post again today after having read it initially... why isn't this stickied? It's AWESOME.

 

^ ^ What he said ^ ^

(bump)