Nice article
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| Exhaust Backpressure, The myth and why it's wrong. http://www.club323f.com/forum/viewtopic.php?f=3&t=840 |
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| Author: | troy_moto [ Sat May 29, 2004 3:32 pm ] |
| Post subject: | Exhaust Backpressure, The myth and why it's wrong. |
I found this when i was looking into exhaust systems. it is very informative and well worth a read - quote: -------------------------------------------------------------------------------- Originally posted on the www.mx-3.com stie One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Mazdas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion. II. Some basic exhaust theory Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream. III. Backpressure and velocity Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate. The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM. Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM. IV. So how did this myth come to be? I often wonder how the myth "Mazdas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity. V. So why is exhaust velocity so important? The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s). VI. Conclusion. SO it turns out that Mazdas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible. |
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| Author: | Ronin (Adam) [ Tue Jun 01, 2004 12:42 am ] |
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amen... |
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| Author: | maz323f [ Sat Jun 05, 2004 3:51 pm ] |
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Nice article
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| Author: | Dan [ Sat Jun 05, 2004 8:14 pm ] |
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very interesting.... got me thinking though! you always see the high powered 400 bhp+ skylines & supras with massive 5inch exhausts or whatever, and this also made me think of those massive trucks, they also have huge exhausts... does this mean the more power the engine pumps out, the larger the exhaust should be? so with this in mind, what would be the ideal size for our cars? |
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| Author: | maz323f [ Sun Jun 06, 2004 5:18 pm ] |
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The above article generally applys to the down pipes and cat back pipes rather than the size off the tail pipe. Size of the tail pipe doesnt make much of a performance difference, it mainly alters the noise and style. |
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| Author: | Mark.T [ Mon Jun 07, 2004 7:48 am ] |
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I'm still not fully convinced (no disrespect to troy_moto). I read an article regarding turbo'd engines This was written by a tuning specialist and published in Japanese Performance Magazine. They were saying that, as far exhausts were concerned, turbo'd engines differ from naturally aspirated ones because they (turbo'd) need no back pressure and must be as free flowing as possible (they can only generally go to 3" exhaust because of ground clearance). So they were implying that NA engines require backpressure. 
I can see the sense in what the MX-3 article is saying, but with so many people over the years saying you need back pressure - I'm not sure. Even more confused, Mark |
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| Author: | Mark.T [ Mon Jun 07, 2004 8:07 am ] |
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OK, I'm starting believe. Check out this web site for some good exhaust info... [url]http://www.cobbtuning.com/tech/exhaustdesign/index.html[/url] Mark |
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| Author: | Mark.T [ Mon Jun 07, 2004 8:20 am ] |
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OK, I'll go along with the MX-3 articel!! : [url]http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm[/url] [url]http://www.nissanperformancemag.com/july02/revengeofthenerds2.php[/url] On an NA car theres a tradeoff between flow and heat dissipation. You want the best possible flow you can possibly get WHILE STILL KEEPING THE EXHAUST AS HOT AS POSSIBLE. Quote from a post on another forum: "What that means is while you can just slap a 3 inch exhaust on there, the exhaust gas expands, cools off the air in there, dissipates and cools off faster. The cooler air loses velocity and pressure on the back end isnt equal to pressure on the front end and you lose power. On a different token, exhaust is too restrictive, exhaust gas velocity is HIGH and its HOT but the engine wants to push MORE through then the exhaust can handle. Thats the backpressure thing. Which is bad for the most part. So NA exhausts are custom tailored to the application per horsepower and engine size. There is an art to it, and there is a flow formula they do." Mark |
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| Author: | Dan [ Mon Jun 07, 2004 9:51 am ] |
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it would be great if someone could let us know the ideal size etc for our cars by engine size! however i dont know any exhaust designers! |
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| Author: | maz323f [ Mon Jun 07, 2004 6:10 pm ] |
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It depends on what mods you have, engine size, power and what revs you want peak power. So its always hard to say. Most companies just do 1/4 in increaments. |
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| Author: | YOZZA [ Tue Aug 02, 2011 4:56 pm ] |
| Post subject: | Re: Exhaust Backpressure, The myth and why it's wrong. |
thought id comment on this to get it nearer the top as its a good read |
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| Author: | MrKikker [ Wed Aug 03, 2011 7:50 pm ] |
| Post subject: | Re: Exhaust Backpressure, The myth and why it's wrong. |
THANK you i was looking for this one for ages |
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| Author: | Stuz [ Thu Aug 04, 2011 9:08 pm ] |
| Post subject: | Re: Exhaust Backpressure, The myth and why it's wrong. |
[quote="YOZZA"]thought id comment on this to get it nearer the top as its a good read[/quote] Epic thread resurrection  Think this may be the oldest thread resurrected 
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| Author: | Madmacs [ Sun Sep 04, 2011 10:01 am ] |
| Post subject: | Exhaust and intake resonance - why shape matters more than s |
Excellent article. Anyone want a teeny bit more technicality? Also consider that the escaping gas weighs something. If it's moving quickly, when the valve slams shut and the stream stops being pushed, that fast moving gas will want to suck what's behind it and if you take advantage of that for a given engine speed, when one exhaust stream stops another opens and the movement of the first 'charge' sucks the next one. Enter the use of extractors. They're just like the opposite of tuned intake runners, really. BMW make good use of them. Instead of one pipe going past all exhaust ports and then leading away, each exhaust port has it's own pipe of a specific length and they only meet after a certain distance. The main benefit of that is to make best use of the momentum the exhaust gas has when it enters the pipe to the catalyst, the one that the pipes from each cylinder meet at. ...so what's with the length? Again it's like the intake runners. Here's the difficult bit to get so don't feel silly if it doesn't make sense because I'm probably not explaining it well enough for you. When you consider that through a given pipe, the gas will flow at a rate mainly determined by the engine speed, obviously. I'm actually going to explain this using the example of intake runners because it's a lot easier to get your head around. For those who don't know why I'm calling them that, in multi-point fuel injected cars, they are pipes going to the intake ports of each cylinder at one end and at the other meet in a much wider pipe called a plenum chamber (plenum=meeting). When the intake port(s) slam shut a shockwave is going to bounce back up the intake runner and bounce back from the plenum chamber down towards the valves. >>The trick is to make them the right length so that by the time that shockwave gets back to the valve, it's time for the valve to open again so it carries more, denser air. That is resonance. Specifically Helmholtz resonance, picture blowing across a closed tube and hearing a sound. That will vary depending on how quickly that happens, i.e. Engine speed. Shorter runners resonate at higher engine speeds but most smaller cars have longer runners which improve torque at lower engine speeds. Porsche are masterful at this on their engines that aren't turbo. Thankfully they shared some of that with Mazda's K series V6s, at least as far as I know. The V6 engines in these 323Fs do a little magic trick called VRIS. Some manufacturers vary the length of intake runners, current Holdens in Australia for example. The KF, K8 and KL engines have two plenum chambers, one for each bank, and the firing order alternates between the two banks. A pair of butterfly flaps change the capacity of the combined plenum chamber for four different conditions so low end torque is pretty good for pistons that (except in the KL) really don't move very far and you get a nice little kick at 5000RPM when the system configures for maximum flow. I'm sure someone could do a better job at being more specific about VRIS than me. In case anyone's not asleep, I'm sure that if that lot makes sense you can picture the exhaust process working similarly and why velocity is so important. If something's not moving, it doesn't have much momentum and remember, being gas it doesn't have much mass. Toyota's 4A-GE in some variants simply close one of the two intake valves to increase velocity at lower revs and open it when flow is needed. The only difference being that the process is reversed. The source of the gas is what pulses as valves slam open and shut. That simply means that instead of a positive pressure shockwave, where a wave of gas being compressed moves between the valve and where all the pipes come together, it's a vacuum from when the valve slams shut and the force of the gas suddenly stops, it's a negative pressure shockwave. >the aim is that when one exhaust stream stops, at the right engine speed, it'll pull the next stream out of the next cylinder to open it's exhaust valve(s). |
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