So what about that exhaust?

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I was at the local Dennys when whom else pulls up loud and blaring into the parking lot but the local Subaru boys.(As you should all know I drive a Mitsubishi and Mitsubishi and Subaru just do not get along....on the strip that is). While I was amazed at the rather low exhaust tones the flat 4's were emitting, I noticed that one STi had an exhaust the size of a fecking catelope. Wich brings me to this article.

The Relative Exhaust Theory

Now you may have heard of this before and yes I've read extensive articles, books, and forums about the theory. So what you read here you may have read somwhere else before.. So lets get down to it.

There are too many people with the "bigger is better" mentality. It is'nt true with engines, and it certainly is'nt true with exhaust. Some people like to talk about backpressure and how its necisary for an engine to function. Others speak of exhaust wrap and ceramic coatings. Some people believe that the larger the exhuast the better a turbo will function. Well all of this information has its relivancy to some extent but to the newbie all of this information can be quite misinforming.

In order to understand how to better a system, we must first understand the system. So what is an exhaust, what does it do, and what is it composed of.

An exhaust is a system created to transfer the hot gases away from your engine while reducing the noise that your engine creates. Yes exhausts are made to make your car QUIETER. Oh yeah, and reduce dangerous pollutants to the atmosphere. Well for informational purposes lets talk about some of the polutants. You have your hydrocarbons wich are un-burnt fuel, carbon monoxide(CO), nitrogen oxide(NOx), carbon dioxide(CO2), sulfur dioxide(SO2), phosphorus(P), and as I have been informed occasionally heavy metals such as lead(Pb) or Molybdenum(Mo). All of the elements leaving an engine are extremly hot and under a constant pressure due to the exhaust stroke of the piston. After leaving the cylindrical containers of the engine block the exhaust gas is then pushed into a header.

Headers serve many purposes and are probably the most import part of the exhaust system. The exhaust manifold serves as a funnel to combine all of the exhaust ports on the engine block into one straight pipe, so you don't have 4,6,8.10, or 12 pipes sticking out the back of your car. However, exhaust manifolds restrict air flow and waste prescious power because you pistons have to push harder to get the gas through. Good thing though. There is an alternative to the exhaust manifold and that would be what we in the performance world like to call "headers". Whats that you say? Are'nt exaust manifolds and exhaust headers the same thing? FU(K no! Headers involve smoothe flowing tubes that connect into one. Gas, just like any moving matter works best when it is slid along flowing paths. Thats why express ways are faster than streets. They flow. Lets say a header is like an express way and a manifold is like folowing that expressway on streets with no stop lights. Even though there are no stop lights on the streets you still have to slow down to make 90 degree turns where as your express way gently bends those 90 degrees over a longer distance.

Meowww. Yep you guess it. Next up on the exhaust list is your "cat" or more professionaly know as the catalytic converter. The catalytic converters job is to remove harmful chemicals in your exhaust and more often than not tends to quiet your exhaust and give it that lower mellow tone. You may have heard of a straight pipe, or fake cat that all remove the catalytic converter all together in attempts to raise horsepower. Well..... thats a very touchy subject and is open for discussion. In short terms, removing the cat from a car won't give you anymore horsepower numbers than cleaning your air filter. From here we move onto the muffler.

Gas from your exhaust has lots of pressure and as we all know pressure in confined spaces make loud noises. The same reason cannons go boom, and without a muffler exhaust sounds like a large ravenous stampeding herd of 4 ton elephant through a skethoscope. Well maybe not that loud, but you get the idea. Mufflers work by 3 forms of sound "lowering"(for lack of better workds).


This is a design where the pipe goes straight through but has holes in the pipe and insulation such as fiberglass or steel wool in the sourround chamber. The gas enters the muffler and dissapates through the holes in the pipe and into the insulation in the surrounding chamber. Insulation deadends sound... this system works. NEXT


This design uses some of the same principals from absorption and a little more complex for or silencing. All of you audio buffs out there will know exhaust gas creates sound, and all sound(that I know of) comes in waves. Waves are like bouncing balls, they loose enertia as they hit more and more objects. Well when you have balls that bounce fast enough and a limited ammount of space to make those balls stop what do you do? Well in a reflection design muffler, chambers are created to bounce the waves and make them hit each other so they cancel. As we all know, when waves hit each other they simply cancel out. Cool eh?


Like tightey whites 2 sizes to small, restrictions attempt to restict movement of the gas via chanmbers, corners, and other engineering designs. Not much to know...

One important thing about exhaust is that they come in pulses. Just like your engine spins in pulses, your engine has valves that open and close thus giving a stop go, stop go pulse to your exhaust system. What we must keep in mind with exhaust is that the gases operate on a pressure system. In order for exhaust gas to move the tip of the pulse must be of a higher pressure than the air surrounding it. The other end of the pulse has a low pressure. Think of the pulse as a snake. The mouth has a high pressure and the tail has a low pressure. Now if the ambient air outside of your tailpipe is low the high end tip of the exhaus pulse is automaticly attracted to the low pressure, BUT the low end tip of that pulse will attract the high end tip of the next pulse. So the pulses suck each other along, kinda cool huh?

This is how headers are created. The runners in the headers are engineered to make the exhaust pulses of each other meet up so they suck each other down the pipe. As you may be thinking, won't the exhaust pulses differentiate as the engine increases or decreses in revolutions. Well of course, but thats not my job to design arround now is it?

Turbo setups generally follow the same rules as naturally aspirated engines do. With the excpetion that the mufflers on turbos work alot better. The Turbine of a turbo allready greatly reduces exhaust noise, so the muffler can handle the rest of the leftover noise. Not to mention turbos create a significant ammount of back pressure in your exhaust that also tend to silence the exhaust greatly.

With all of this taken into mind, there is no such thing as a "perfect exhaust". The perfect exhaust would be to run no exhaust, however even I am not sure what kind of negative effects running an enging with no exhaust on it whatsoever does short of being fined. Its generally accepted to upgrad an exhaust to 3-3.25 inches in diameter within reasonable horsepower. Now if you have a 180HP Civic and add a 3 inch exhaust to your car you'll notice that you may loose horsepower. Why is this? Well simply because the larger exhaust throws off the pulse matching of the headers/manifold. So unless you're pushing a minimum of 250-300HP it would be in your best interest to keep your stock exhaust. Anything larger than 4 inches does'nt belong on a street car and if you're pushing that much horspower to where you need a 4 inch exhaust you should probably consider buying a driveable street car.

posted by  DSMer

once again, DSMer has outdone his self. great article!

posted by  b_DuB13

Nice. Of course, that whole emissions thing is entirely unimportant in a number of cases. And, as many people throughout the '70s and '80s found out, removing the cats certainly did improve power, sometimes quite a bit. More modern cars don't have as much problem, as modern catalytic converters tend to flow better (until they are plugged). Some cars used multiple cats and no mufflers at all...

Most stock exhaust is pretty restrictive. It has to be. Especially on cars with larger engines. Ultimate power isn't a goal as much as running quiet and clean is.

As for the rule about turbos and exhaust, it still stands. After the turbo, there's no such thing as too big, as far as engine power is concerned. The more boost youre making, them more that's true (after all, at 28 psi, you're running three engine's worth of air through the exhaust). So a car like an STi can definitley use all that cantaloupe sized exhaust. Or at least, isn't harmed by it.

posted by  ChrisV

Yeah you read my mind. I meant to say that removing the cat on newer cars won't really give you much in the power category.

But I'm stilly a little hazy on the "When you have a turbo, nothing is too big". I think thats not very well thought out "rule". While turbos could use nice non restrictive air to breathe. I don't think the biggest exhaust is'nt going to better the turbo. Maybe on a race car with a turbo, but on a street car you're not always going to be running anywhere near 28psi(damn that almost 2 bar). Hell most asian imports would blow under 28psi. I can only think of a few engines that will hold under 28psi. When I think of that scenario I think of a 350Z with GReddy twin turbo kit that should'nt be ran at any more than 8psi.

So is a turbo car running at 8psi max going to be helped by 4inch exhaust or hurt by it? I'll have to say running the 4inch would'nt do you any better if not affect performance as if you were running a 3inch or 3.25. Again this is a touchy subject so if you know something that proves the "No Exhaust is to big for a turbo" Let us know....

posted by  DSMer

In the turbo world, you can hurt it by having too small of an exhaust, but not by having too large of an exhaust. 8lbs? That's low factory boost. Stock WRXs run 13.5 lbs (which is nearly twice as much air moved through the engine as an N/A version would). Chipped ones run to 17-18 psi. I've seen a lot of turbo cars running 18-20 in street form, and some larger turbo cars running upwards of 30 lbs (mostly in cars like the Mustangs and GNs). You ever see a GN with a 6 inch exhaust? I've seen a Mustang with an 8 inch exhaust on the track... Hell, even the SRT-4 factory upgrade kit runs more. Even the stock turbo is good for 18-19 lbs of boost, but the Mopar Stage II runs to 21 lbs. Take a car with a big turbo and big intercooler, and 28 lbs is pretty easily achieved.

True, on the street you're not always going for that, but still, a 4 inch exhaust, or even a 5 inch tip, isn't going to hurt, while a 3 inch exhaust might if you want to run higher boost at times. (no they don't run that boost all the time, but they might sometimes, especially in a dual purpose street/track car).

The basic point is in an N/A car, the exhaust size is critical to drawing out the following exhaust pulse. A turbo car draws nothing out: it forces it out under pressure. The less in the way of that, the better. We're REQUIRED to have a car quieter to be legal, but that's not optimum for performance, merely keeping your wallet intact.

posted by  ChrisV

Aha. The last line explains it. Well the car I spoke of, the Nissan 350Z, does'nt come stock turboed. But you can add a Twin turbo kit to it for about 6 stacks and most people usually run that kit arround 6psi. The VQ35DE[TT] is'nt exactly the strongest of Nissan Six-Clyinder engines, but it still puts out over 400 to the wheels with just 6-8lbs of boost.

I think of alot of cars like civics, integras, and S200'0s that have bolt on turbos and their blocks, stock or modified are not really able to hold as much boost as a factory turboed car will. So what do you do in their situation.

Does the ammount of boost your car will be able to run directly reflect and corelate with the size of your exhaust on a turboed car?

Also to set another thing clear. What about exhaust on Supercharged cars. Should they be treated the same as N/A exhaust or Turbo?

posted by  DSMer

If you are bolting it on, you're talking about using the stock compression ratio, which won't allow high boost levels. But even at 6-8 lbs, you're talking another half an engine's worth of air movement, thus being then equal to a 5+ liter engine in total air volume moved. 400 hp from an N/A 5 liter is pretty easy (I've done it), so saying it makes 400 hp with only 6 lbs boost is not as impressive as you might think.

You "think?" Have you looked at modern engine blocks as the added webbing? An example that I already know of is the Ford Duratech. No factory turbo versions, but the stock bottom end is good for 400 hp, and boosted ones have gone to 450 hp wth only rod bolt changes. Civics and Integras, on the models most comonly turbocharged, are also very stout. They can hold a lot of pressure (and even at 28 lbs, we are only talking 3 atmospheres, not too hard for the metal to handle). Gaskets are the main problem areas, as well as con rod and main cap bolts, which usually get changed out. Blocks are o-ringed to keep from blowing out between the block and head, and lower compression forged pistons are usually installed. Again, bolt on turbo kits are usually held to low boost due to the compression ratios of the stock pistons. Too much boost and detonation would occur. But that has nothing to do with the BLOCK.

Once the car is boosted, the boost is pushing the air out, so the exhaust doesn't have to provide backpressure to draw out the next exhaust pulse. Period. The ONLY considerations are legal noise and emissions levels, if applicable.

Same as turbos, as turbos are merely exhaust driven superchargers. The exhaust pipes aren't drawing out air, the air is being forced out of the car. Look at top fuel and funy cars with superchargers. What is the exhaust like on them? .jpg

One thing to remember, the exhaust situation we are discussing is AFTER the turbo. BEFORE the turbo, you want the exhaust pipes to be short, and no more than the diameter of the exhaust valve area, to keep the speed up and to keep the exhaust charge from cooling before it reaches the turbo. You want the exhaust charge to do it's expanding in the turbo, so it speeds things up and maximized turbo efficiency. The turbine will provide more exhaust backpressure than you want, you don't need or want any more backpressure from the rest of the exhaust.

posted by  ChrisV

So then I have a question. If I have a rather low-HP N/A car, would getting an exhaust help me or hurt me?

posted by  y.e.c.

Well if you've picked up a magazine or ImportTuner, you'd see that a majority of the time when they add a larger exhaust to a stock N/A car it generally looses horsepower more often than it gains it. But as you keep upgrading the car the engine will eventually itulize the spaze within that exhaust thus helping your engine achieve greater performance.

posted by  DSMer

Well... no. Remember that the exhaust gases will compress therefor you won't empty your cylinders of exhaust gases effiently.

posted by  DodgeRida67

Please explain. I can't really understand your grammatics.

posted by  DSMer

I haven't bothered reading the followups, but that is a good article DSmer.

I would like to mention a few things that I think are general public misnoamers too:

a manifold does not necessarily confine itself to a log style cast iron POS found on production cars. Headers are still part of a manifold;

headers,or more specifically extractors have a significant role in increasing exhaust rate of extraction and were shown to significantly increase venting of the exhaust side way back in US secret air craft tests of the 30/40's and probably before that by the Germans. Rarefaction relies on a conduit. There is a direct corellation between this controlled extraction and horsepower/torque.

the issue of backpressure is as DSmer implies is a misinterpretation of static regain and rarefaction. It is important however to have sufficient static pressure to produce the desirable control of gas flow during valve overlap. Contrary to popular belief the so call pulse does not produce a sonic reflected wave to suck air from other cylinders otherwise we would have cars running around with antilag style flames shooting out the back of the car; instead we have tuned primary pipes that are sonically nodal to that pot at a desriable engine speed. Too often engines have untuned big bore pipes added to the exhaust and wonder why the engine goes into det, the exhaust valves burn, EGT & fuel consumption rises and torque production moves up the rev range;

For turbo's, up pipe selection and manifolding is just as important as the infuser & inducer characteristics of the turbine volute as indicated by A/R. Likewise the down pipe and back pipes also play an important role. This is often disguised by the noticeable improvement from reducing friction losses by increasing cat back bore sizes (or even a bigger cat) ;

the exhaust note naturally increases in sound power, sound pressure and audible lower octave bands as the pipe size increases because the amplitude of the wave is allowed to increase. Point source becomes more obscure.

Of course I'm talking about engines that aren't built to have a drum of aircraft fuel dumped down their throats over a quarter mile.

posted by  Wally

I have a question. what about turbo manifolds. They come into one to spin the turbo but there is another pipe that goes somewhere else. can anyone explain this

posted by  CarEXPERT

Dude seriously. I could explain it but i think DSMer would do a better job :mrgreen:. How about it DSMer?

posted by  GreekWarrior

I think the pipe you are referring to is the turbo outlet pipe that flows into intercooler. I'll elaborate.The turbo has 4 openings. An Inlet, Outlet, Ambient air, and Exhaust opening.The Turbo bolts up to the exhaust header through the inlet. That leaves 3 openings left.One of those openings connects to a turbo up pipe that connects to the exhaust. The exhaust opening. This leaves 2 openingsAnother opening connects to what is usually a series of pipes that go into an intercooler. The outlet opening.(Radiator like object that allows air to flow through it and cool it off).The last opening is where the intake would connect to the turbo. The Ambient air opening. This is where the turbo "sucks" in ambient air. I'll write another article on how turbos work soon. Once I get some time...

posted by  DSMer

Just for interest this is how the GTR's RB26DETT works with its twin turbo setup.

posted by  GreekWarrior

No thats not what i mean. For non turboed cars the exhaust manifold is long and connects to the uppipe. When you bolt on a turbo you need to do a lot of stuff like intercooler and exhaust manifold. The turbo manifold has to be shorter so it can spin the turbines right.
IS a radiator for to cool the engine down and intercooler to cool the air. Do you know what is the water-to- air intercoolers. The intercooler is in the front of the car right and so is the radiator but how can they both be there because they are both big? Damn im confused can you help me DSMer Sir.

posted by  CarEXPERT

Do you know about the sequential turbos, where the turbos are diffrent sizes? How do they work with the exhaust? And why are twin turbos better than single ones. It just makes the engine hevier.

the supra 2jzgette twin turbos are sequential do you have a picture of how that works?

posted by  CarEXPERT

For non turboed cars the exhaust manifold is long and connects to the uppipe.
Its not really long. More often than not it will be significantly shorter than that of a turbo manifold

When you bolt on a turbo you need to do a lot of stuff like intercooler and exhaust manifold.
The term "bolting" on the turbo is a mileading statement. Turbos require more than just parts to be bolted on. Lots of tuning and possibly streghtened internal components are needed.

The turbo manifold has to be shorter so it can spin the turbines right.
Well the turbo manifold is pretty much the same thing ass a regular exhaust manifold. It just has an ending bracket that bolts up to the turbo.

IS a radiator for to cool the engine down and intercooler to cool the air.
The radiator cools engine coolant, an intercooler(or technically known as "aftercooler" cools incoming air to the engine after it passes through the hot compressor end of the turbo. It has to be cooled because it could pre-detinate the air/fuel mixture within the combustion chamber.

Do you know what is the water-to- air intercoolers.
It uses both oncoming air and water to cool the air within the chabmers.

The intercooler is in the front of the car right and so is the radiator but how can they both be there because they are both big?
Radiators are mounted just infront or behind the frame. The intercooler(wich is significantly smaller) is usually located behind the front bumper in open car space(able to be touched without opening the hood), in the wheel wells, or on the top of the engine as commonly seen in Subaru WRX's

posted by  DSMer

Do you know about the sequential turbos, where the turbos are diffrent sizes? How do they work with the exhaust?
The same way that twin and single turbos work. One turbo is just smaller so that it spools faster than the larger one reducing turbo lag.

And why are twin turbos better than single ones. It just makes the engine hevier.
Thats up for debate. Engine weight is no problem so long as the added weight boost performance.

posted by  DSMer

:thumbs: :thumbs: :thumbs: :thumbs: Damn thank you so much DSMer. You are my car hero and I want to be just as smart as you.

posted by  CarEXPERT

Pfft, I'm not smart. I'm a car novice. Barley even that... There are far smarter guys here.

posted by  DSMer

No i mean if there is 2 diffrent sized turbos and only one works at a time, for example a supra has 6 cylinders so 3 of the exhaust pipe will go into one and the other 3 will go into the big turbo. So how will the exhaust gases go out of the turbo that isnt working?

posted by  CarEXPERT

When the larger turbo spools up the smaller turbo simply closes the wastegate and diverts air away from its turbine. If the car has 2 exhaust manifolds then the exhaust from the 3 cylinders just exits as it normally would.

However this is'nt always the case. In the Bugatti Veryron that has 4 turbos all 4 are still working during acceleration irregardless to size.

posted by  DSMer

I've read about wastegates but i dont really know much about them. for the external wastegate, it has sensors that sees if the boost is correct right? and then if the boost goes to high it diverts the gas right into the air or into the up pipe?

posted by  CarEXPERT

A wastegate simply diverts exhaust gas away from the turbines when you stop accelerating so the pressure does'nt build up in the exhuast headers. Wich is dangerous for your engine. If the turbine is'nt taking air it only has one other place to go... through the exhaust..

The thing that releases air into the atmospehre is a BOV(blow off valve). Wich also opens when you stop accelerating to relieve the pressure in your intake pipes.

posted by  DSMer

You guys serious or acting the goat? These are pretty basic queries and seriously wrong answers.

For anyone else that really wants to know here is one of those pics that probably originated on howstuffworks and has been plagiarised by every other wannabe:

If any of you are dead serious I will assist in answering.

posted by  Wally

I think you might be wrong on some of this. This is what i read from a mag.
"the wastegate's task is to vent excess exhaust pressure on the turbine side, so that the turbine blades dont over spin for desired boost levels."

But i dont think the wastegate has to do with anything when you stop accelerating. As for the blow off valve thats the one that works when you stop accelerating.

posted by  CarEXPERT

Hey look at that picture. When it says the exhaust manifold/header goes to the uppipe and then to the turbo. Thats strange I thought the exhaust manifold goes right into the turbo. Hey can you explain why the header goes into the uppipe first before the turbo.

posted by  CarEXPERT

Because when an up pipe is employed it always goes between the manifold and the turbine housing infuser...that's what an up pipe is. It is used for several reasons, not least when the turbo is physically located away from the manifold, also to make the air more laminar, etc. The down pipe (or dump pipe) is the one after the turbine connected to the diffuser.

The wastgate is a bypass that reduces gas flow through the turbine, thus slowing the compressor wheel and mass flow.

A blow off valve measures manifold pressure and vents the charge pipe when the manifold pressure drops into a vacuum region. Its purpose is to reduce stall flutter on the compressor blades so that pumping continues. Contrary to internet folklore the compressor wheel does not physically stall because of high surge pressures during throttle closure between gear changes. The compressor is slowing because the volumetric efficiency of the engine has dropped due to the throttle plate closing and reducing exhaust flow.

posted by  Wally

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