Ken's corner #6: Exhaust

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Ken's corner #6: Exhaust

Postby ken.lagarec on Fri Jun 06, 2003 3:22 am

Well, until I finish my little thing on engines, I'll quickly discuss exhausts, as was requested by Axelrod regqrding pipe diameters, high flow cats and such.

I guess the first question to answer would be why do cars have exhausts? Simply put, gases produced during the power stroke need to be scavenged from the cylinders so they can fill with fresh air and fuel and go kaboom again. So an exhaust system is primarily just a way of scavenging the gases. Now one would think any old pipe, and the shortest possible would be perfectly fine. But no. See, just like you want to have a smooth high-flowing intake to let your engine breath air in, you also want the exhaust to be optimized for getting it out.

The exhaust system is made of 4 parts:
. a header or exhaust manifold, that bolts onto the head;
. the piping
. a catalytic converter
. one or more mufflers

The header is the most important part of the exhaust system. The piping must be of the correct size such that you get good laminar flow, with results in high gas velocity and maximum scavenging. As the pulse of exhaust gas flows out, it actually creates a vacuum behind it, so next time the valve opens, that vacuum will suck out the gas and help it out of the cylinder. A header of a given length will optimize exhaust scanvenging at a particular rpm (pulses coming from each cylinder will "help" each other).

No in addition to this "header" effect, you also want the gas to flow well. Like I said, a smooth laminar flow is best. If the pipe diameter is too small, at high rpm, when the amount of gas flowing out of the engine is high, the exhaust system won't be able to scavenge properly because of turbulence. Without good scavenging, more exhaust gas will remain in the cylinder, reducing the volumetriv efficiency of the engine. So small piping might be good in the low end, but it will kill in the top end. Notice I am NOT saying backpressure is good. Backpressure is ALWAYS bad. People say it's needed because an exhaust system that is optimized for the low end with small piping will cause back-pressure in the top end. Because American engines produce a lot of torque in the low end, people assume back-pressure is needed for torque. Not true (in fact, there is very little or no back-pressure at the rpm where the torque is made!).

The converse is also true. If you put large piping to get good scavenging at high rpm, then gases won't flow well at low rpm: in studying fluid dynamics, you find that you get turbulence when the flow is too high as well as when it is too low. That's because the large diameter causes a large drop in exhaust gas velocity, which causes turbulence and poor scavenging at low rpm. So a large exhaust will produce more power in the top end, at the risk of killing the bottom end. The ideal exhaust has variable geometry: a smaller pipe coupled to a second pipe that is only active at high rpm. That way you get good scavenging at high and low end. BTW, that's the same concept as variable intake geometries but for the exhaust. The Maxima is a car that has that type of system.

The catalytic converter is necessary to reduce many of the toxic and polluting gases produced by a gasoline engine. Because of the way it is made (see http://www.icat.dtu.dk/facultystaff/resasco-presentations/lecture7.pdf for an interesting presentation), it does produce an obstruction to flow. However, a properly functioning cat will, because of the way it is made, laminarize the flow, which can be a good thing. Although not wanted for race cars, keep yours on cause you really don't need the extra few horses you'll get from removing it. Driving skills alone will be far more important. And your children's children might thank you later if we haven't found another way to destroy the planet by then.

The muffler(s). Last but not least, this is meant purely to reduce the sound the exhaust produces. Although the sound will be stringly dependent on the piping diameter, the use of headers instead of a simple manifold (a manifold is like a header, but it's largely compromised by space considerations so it's not really optimised like a header: it just collects the gas as conveniently as possible), the muffler is key in reducing the sound your car makes. They work in various ways, typically with baffles (gases go into chambers and exit by some odd route) or louvered cores (gases transfer from one pipe to another by passing through small holes in the pipe and through acoustic deadening material) hereby reducing their velocity and the noise it makes. Because they reduce exhaust gas velocity, they will cause hp losses, but because they are far from the engine, it's not as bad as it seems.

I hope this answered most of your questions. The unfortunate answer to the obvious question, what piping diameter should I use is, like everything else in engine tuning: it depends if you want better low end or top end response. The following table provides an answer, resulting in a compromise between the two.

Image

Now, another question might be Can piping be too big? Of course! 3" piping might be OK (or optimal) for a 2.4l engine reving at 6-7krpm but it will kill the bottom end, so 4" piping would actually start killing the top end too by slowing down exhaust gases too much: unless the engine produces extremely large quantities of exhaust gas (by displacement, for induction or revving very high like a race car), you don't need a large exhaust (Mark, where's that Only one of these two really needs a 4" exhaust picture when I need it?..)

Hope this helps. As usual, questions and coments are welcome.

Ken
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Postby Axlerod on Fri Jun 13, 2003 2:02 pm

Thanks Ken!

So if youre using a system like the Maxima has... two different pipe sizes... well first I have no clue how the one in the Maxima works, but I have heard that people use a pipe that steps in size. So my question is, if you do that, does the big end of the pipe come first or second (from the engine)?
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Postby ken.lagarec on Wed Jun 18, 2003 1:45 pm

Axlerod wrote:Thanks Ken!

So if youre using a system like the Maxima has... two different pipe sizes... well first I have no clue how the one in the Maxima works, but I have heard that people use a pipe that steps in size. So my question is, if you do that, does the big end of the pipe come first or second (from the engine)?


The Maxima (IIRC) has two pipes, one of which has a flap that opens under high throttle conditions. The second pipe doesn't need to be bigger because having 2 small pipes will still flow much more than 1 small pipe.

With respect to steps in pipe size, because exhaust gases cool as they go through the exhaust, in order to keep the exhaust velocity up, you should reduce the diameter as you move away from the exhaust manifold. Keep in mind that the steps should be gradual with small angles to reduce the turbulence caused by the step.

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Postby Axlerod on Wed Jun 18, 2003 2:15 pm

thanks Ken
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Postby red98s14 on Wed Jun 18, 2003 2:41 pm

As always Ken, a well written article. Thanks for taking the time.

Are you a writer by trade?

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Postby ken.lagarec on Wed Jun 18, 2003 5:13 pm

Most of the info I provide is not exclusive to me. Some of it I read, some of it I think but none of it is private... That's the whole point of writing this: spread the knowledge. And if somebody takes credit for it elsewhere, well so be it. As long as he doesn't make money off of it...

As for being a writer, no I am not. I was actually a pretty average student in English. I write much better in French, but going through university, writing many papers and two theses made me a slightly better writer. Thanks for the compliment though. I always find I have a hard time staying at the same level, sometimes writing stuff as if the readers know nothing and sometimes assuming they are quasi-experts.

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Postby red98s14 on Wed Jun 18, 2003 5:27 pm

Communication is an art. I've picked up a modicum of knowledge over the years, but am not always the best communicator (just ask JZ)
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Postby Keo on Wed Jun 18, 2003 6:12 pm

Great read! As usual, Ken, your technical knowledge helps me understand the way our cars work :) Car exhausts kinda remind me of chimneys. You have to calculate how much smoke your fire makes and then have the right diameter for the chimney to have the right pull to get that smoke out of the house. If it's too small or too big, the smoke will either stay there or backdrop in the house :/ So it's the same principles.

Anyways, when I upgrade my exhaust, I would like something that would give me better performance, but that doesn't kill my ears hehe For example, Chris has an apex'i exhaust and I just find it a bit too loud for me. I would like to have something like Iketani in initial d... a nice full sound... but then again, I don't remember if he's running a stock exhaust or not hehe
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Postby Axlerod on Wed Jun 18, 2003 7:15 pm

Im with you Kev...

I liked the sound of Biz's exhaust :)
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Postby soundcatz on Sun Jul 06, 2003 1:43 pm

I just have 2 quick questions about that chart... what does Engine CID stand for? (my guess would be cubic inches displacement but I am pretty sure I am WRONG!). Secondly, with the single/dual chart, is the dual the combined diameter, of the diameter of each pipe?

Awesome article though! Another quickie - what's the stock diameter of the 240sx?
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Postby ken.lagarec on Sun Jul 06, 2003 3:29 pm

You are right. CID is the displacement of the engine (240SX is 146 cu. in.). And for dual, it's the diameter of each pipe. The stock piping for the 240sx is 2" if I'm not mistaken. 2.25" to 2.50" (~60 mm) would be a good improvement that won't sacrifice low end torque.

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Postby soundcatz on Sun Jul 06, 2003 4:23 pm

Cool ken, thanks! You should submit all of your "kens corner" as well as a bunch of other literature to a publisher... make a book about 240sx's. If that fails, just go to a printing shop, have a few copies made and sell them on ebay. There is money to be made out there. Spreading information is one thing... but getting paid for it is something completely different ;)

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Postby Keo on Thu Aug 07, 2003 4:36 am

I've found an interesting exhaust. It gives you more performance, less headaches and less attention from the police :)

The yashio factory super silent exhaust

A guy made a review on it with video clips on FA
http://forums.freshalloy.com/ubbthreads ... o=&fpart=1
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Postby philip_240sx on Wed Sep 03, 2003 1:05 am

ken.lagarec wrote:The Maxima (IIRC) has two pipes, one of which has a flap that opens under high throttle conditions.


Are you referring to the variable capacity muffler found on the prev gen Max?

From http://www.nissannews.com

The variable capacity muffler system improves both engine horsepower at middle and high speeds by reducing exhaust pressure and reducing exhaust noise at low speeds. The system uses a special muffler flap valve – which uses high temperature aerospace spring technology and materials – that opens and closes according to exhaust pressure inside the muffler, switching the flow of exhaust gases from one tailpipe to two.

At low exhaust pressure, for example, while idling or coasting, the control valve remains closed. Exhaust is emitted without passing through a secondary bypass, through a single exhaust pipe. As the pressure becomes higher, the control valve (using aerospace industry spring and composite technology) opens, and the exhaust gas flows through a bypass – reducing exhaust resistance and exiting through the car’s dual exhaust pipes. The valve opens at 2,000 rpm and is always open at wide-open throttle.
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Postby boMex on Sat Oct 04, 2003 6:37 pm

Keo wrote:I've found an interesting exhaust. It gives you more performance, less headaches and less attention from the police :)

The yashio factory super silent exhaust

A guy made a review on it with video clips on FA
http://forums.freshalloy.com/ubbthreads ... o=&fpart=1
You could prolly build something like the Yoshio(which has really fugly welds for what it costs) out of any 3" JDM canister exhaust. Just slap a tinty 3" SS Magnaflow muffler somewhere on the mid pipe. Cops will still love you beacause of the ricer can hanging off the ass of the car.
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