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Backpressure: The myth and why it's wrong

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  • Backpressure: The myth and why it's wrong

    tought this might help ye out

    Back pressure: The myth and why it's wrong.

    I. Introduction

    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 "Hondas 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 "Hondas 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 Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

    Turbo Exhaust Systems:

    Some of you asked for a better explanation about restriction in a turbo exhaust, so here 'goes. To sum it all up, on a turbo car, the tighter and more restricted the exhaust housing of the turbo is, the faster you're going to spool your turbo... because the restricted gasses escape through the exhaust housing with more velocity (much like the garden hose description quoted above)... but with this restriction comes the downside. Less exhaust volume will be able to fit through that turbine housing once the turbo is spooled and starts squishing more air through the intake.

    This is where the wastegate comes into play. The wastegate is actuated BEFORE the exhaust wheel in the exhaust housing. When it opens, whatever the diameter of your wastegate is gets added to your exhaust piping. Effectively increasing the capacity of the exhaust provided that everything downstream is large enough in diameter to handle it the extra flow. The main reason it does this is to prevent over-spooling the turbo. Once the turbo reaches its efficiency, it doesn't need to flow all the extra gasses through the turbine wheel, so the wastegate allows you to route the exhaust around the turbo... if it can't route enough exhaust around the turbo (restricted wastegate) then too much exhaust gas will be forced by the exhaust wheel and BOOST CREEP will occur as your turbo over-spools.

    One way to prevent boost creep is to port the wastegate housing (if you have an internally gated turbo) or to replace the wastegate with a bigger unit. If that still doesn't work, then the problem is likely going to be a restriction in the exhaust downstream from the wastegate. Many overboosted car owners prefer to vent their wastegate dumps to the atmosphere. Not only is it illegal to bypass your catalytic converter, but it's loud as hell, gets your engine bay filthy, and clogs up your K&N like nobody's business; however, it nearly eliminates boost creep. It's a cheap and easy solution that fixes boost creep on a race car.

    The best solution to boost creep is to route your wastegate dump past the catalytic converter and back into the exhaust. It will be a custom setup. Nobody makes this. Make it look clean and you'll pass emissions because they don't run your car hard enough to open the wastegate when doing emissions testing. You have to reach full boost for the wastegate to open, and since the exhaust runs through the cat until the wastegate opens, it all gets "cleaned" before it reaches the sniffer. With this setup, the wastegate will also be much quieter because it still runs through the muffler, and you won't trash your engine bay with black caustic funk.

    Putting a separate catalytic converter on your wastegate dump is a stupid idea because you'll never get it hot enough to "light off" and start converting the carbon monoxide... so don't get any crazy ideas and create unnecessary exhaust restrictions.

    Once compressed air comes into the mix with an engine, exhaust tuning has much less to do with making power. So what if you can milk another 3-5 hp out of a car with a tuned exhaust... the benefits of making an engine sustain an insane final compression ratio (boosted air x compression in the combustion chamber) has much more affect on making power if you can just get rid of the extra gasses it produces. Bigger is almost always better on a turbo setup. The only place where it isn't good is on the exhaust wheel where too big can = no chance of spooling your oversized turbo any time this year.

  • #2
    That was really informative, well done. I always wondered about the 'bigger is better' argument for exhausts, things are never as simple as just putting something bigger on a car.

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    • #3
      Thanks you for sharing this, but If you can edit it to say where you got this from it would be appreciated. The original writer would appreciate being credited for his/her hard work.
      Jay
      GTAusMotive

      2008 Honda Civic Type R, Honda Accord Euro CL9
      1991 Honda Civic Gli, 1977 Honda Civic 2 Speed Hondamatic Auto, Mini-Me Ed Civic

      Comment


      • #4
        Awesome post.

        I'm not sure on the sciences here though I'd be curious, (semi-rhetorical questions)

        - Is there any math / formulas to calculate flow / velocity / pressure / optimal pipe size?
        - Does pipe length play a factor? Since a pipe being made longer or shorter will increase or decrease it's volume as a chamber, can this have an effect on backpressure / velocity, and would diameter compensate?

        I think Turbo's cars can be equally affected. I remember Marty's old RS Liberty had an exhaust leak, he noticed a definite improvement when it was fixed. Also, a friend bought a Skyline R34 Turbo Coupe, like mine. Same engine, same turbo, virtually the same everything, except his has a stock exhaust and mine a 3" catback. He swears mine is quicker (and it probably is) however his feels to have a lot more push / torque down lower.

        I like the idea of routing the wastegate until after the cat, though I'm not sure what the authorities would say if they saw it. That said, in a test where the engine is revved up whilst stationary (no load), many engines won't actually build up much boost, if any at all. Strange phenomenon.

        I think the same physics can apply to intakes. The Carbs on my motorcycle have a sliding thing in the venturi, which constricts and opens the carb depending on engine load. Apparently it helps. They are vacuum controlled.


        Originally posted by milkchicken
        Thanks you for sharing this, but If you can edit it to say where you got this from it would be appreciated. The original writer would appreciate being credited for his/her hard work.
        Assuming he didn't write it, but yeah, it does look very copy paste, presumably from a Honda forum

        +1 on link if from elsewhere, sharing is caring, stealing isn't

        Comment


        • #5
          Originally posted by Rake View Post
          Assuming he didn't write it, but yeah, it does look very copy paste, presumably from a Honda forum

          +1 on link if from elsewhere, sharing is caring, stealing isn't
          I've read it on a few Honda forums before.. can't remember where though. But i always like it when someone else's hard work is shared around, just nicer when you know where it came from.
          Jay
          GTAusMotive

          2008 Honda Civic Type R, Honda Accord Euro CL9
          1991 Honda Civic Gli, 1977 Honda Civic 2 Speed Hondamatic Auto, Mini-Me Ed Civic

          Comment


          • #6
            Quick google search of first paragraph: http://www.hondacivicforum.com/forum...ts-wrong-3064/

            Dates like 6 years ago. Could be it?
            Originally posted by djzayas
            I picked Up a cleen stock as a rock s14 from a old grandma the other week
            Originally posted by NL_Taylor
            what did she upgrade to ? a GT-r ?
            My N14 SSS

            Comment


            • #7
              Originally posted by Sm1l3y View Post
              Quick google search of first paragraph: http://www.hondacivicforum.com/forum...ts-wrong-3064/

              Dates like 6 years ago. Could be it?
              Close... But no.

              First appeared on the internet 01/07/2002 on 4 different websites (3 related, 1 blog), but I believe the source may be from http://www.stainless-works.co.uk/
              If at first you don't succeed, add duct take.

              Comment


              • #8
                just the other day the new apprentice (smartass little prick of a kid) thought he was smart talking about "backpressure" in his girlfriends 323. i gave him a bit of a serving about the use of that word, i usually don't like cutting people down like that but he was giving me the shits that day

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                • #9
                  Some turbo vauxhalls (GM) Need back pressure. Only because they used poor quality turbos and if you stick a decent flow exhaust system on the oil seals in the turbo give up! Mega design fail :]
                  Me on the book of faces
                  http://www.facebook.com/hollands
                  sigpic

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                  • #10
                    nope i didn't write thought i said it at the beginning but must not have my bad, yup got of a forum which it was copied and pasted to so dont know where it originally came from

                    Comment


                    • #11
                      Back pressure can be generated by minor and major head losses in pipe. As far as math goes, generally I would assume that the velocity profile would be turbulent, as I can't see the exhaust being all nice and laminar as it snakes its way out of the cylinder and through the cat, if you still have one . If you use the commonly used math and assume a 1/7th power law profile and then integrate around a pipe, the results are close enough that you can assume a uniform velocity when doing momentum and energy calculations. I don't have the equation typed up at the moment, but you can calculate the pressure differential for a section of round pipe, it just takes a bit of math. There are 2 ways you can get headloss in the flow, which you would see as back pressure when you took measurements. One is the frictional effects from the fluid in the pipe its self and the other is from restrictions. The first needs to be carefully balanced with the momentum of the exhaust for optimum performance at the desired engine speed. The later, which comes from things like mufflers, catalytic converters and poor bends has no benefit whatsoever with the exception of keeping your car legal for road use. So in the end you can take out all the restrictive components and then balance the size of the pipe, you probably need an expert for this, so find someone you trust or buy a pre-made system. In the end you cant raise velocity without raising one source back pressure as well, but people confuse cause and effect often so it is an understandable mistake.
                      ok found some of the equations i was talking about. Not all of them though. :/
                      Originally posted by MOOG
                      Volvo is turbo and proper mad.
                      Originally posted by Jenga
                      You see, when a piston, block, crankshaft, and some fuel and air all love each other very, very much.....

                      Comment


                      • #12
                        I have a book with the formula for individual header lenghts. When I get back home I will try and find it so you guys can start calculating. I haven't found any formula's for cat-back systems. However, I do know that a venturi near the end of your exhaust right before the last silencer) helps keep up exhaust gas speed on N/A cars. This because the gasses cool down and "shrink" on their way out. Spoon N1 exhausts have this system too, if there where any doubts as to its effectiveness.
                        Originally posted by deadcell96
                        It's like Underground 2 all over again.
                        Originally posted by jmacman12
                        ...but, but he's got such a good star rating!!

                        Disclaimer: Under NO circumstance should anybody replicate any actions I did. Seriously. Don't.

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                        • #13
                          this is amazing!!!
                          Originally posted by OPTIMUS PRIME
                          no.

                          My girlfriend just caught me blow drying my penis and asked what I was doing. Apparently "heating your dinner" was not the right answer.

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                          • #14
                            Here is the formula I was thinking of. (it looks daunting but lots of things cancel out) you can use this to calculate the momentum of the flow. Also, luckily at speeds below .3 times the speed of sound, mathematically you can treat the fluid as incompressible. I think I just got waaaay of topic though.

                            Edit: Crap, that is not the one I wanted either, I'm bad at finding formulas on the internet, and I left my fluids book at home. I give up.

                            Edit: F**k this S**t I have a pen!
                            Last edited by FortKid; 18-06-2011, 08:23 PM.
                            Originally posted by MOOG
                            Volvo is turbo and proper mad.
                            Originally posted by Jenga
                            You see, when a piston, block, crankshaft, and some fuel and air all love each other very, very much.....

                            Comment


                            • #15
                              Can somebody explain me all of these ? hahaha

                              I´m not an engineer so, somebody can explain me this in human words?

                              Question: If i have a 2.4l 4cil turbocharged engine, can I install a 3" straight exhaust system?

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