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  • Exhaust backpressure myth?

    Hello, over my years of talking with people regarding engine tuning in particular playing with exhausts i have heard many a time "blah blah blah you need backpressure". Now can someone please tell me if i am wrong but "backpresure" i thaught is a restricton in the exhaust (in regard to exhausts) meaning it is not allowing the exhaust gasses to be released freely, weather the exhaust diameter is too small or otherwise. Wouldnt the most efficiant a 4 stroke engine could run is with a compleatly open exhaust and intake causing no restrictions at all with the air/fuel ratio then set correctly for those changes? Or is there something i am missing to do with Bernoulli's principle of something entering a restriction its velocity increases and its pressure decreases in this case air thus allowing a better "flow" in and out of a engine. Cheers for any help recieved.

  • #2
    in a car, an exhaust with a muffler and a resonator on the end produces more back pressure. some cars have more than 1 muffler. my experience with exhausts is the ones with more back pressure provide more bottom end or torque, but arent that crash hot up top. and free flowing exhausts have far less bottom end value, but are great for top end. i guess it depends what you want to do with the motor.

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    • #3
      ^^ WHS agrees with what all Daytona 675 owners say when they disable their EXBV.

      Experts have trouble agreeing on this, I don't think PSB is going to agree either...

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      • #4
        Get a yamaha with a EXUP valve in the exhaust and disconnect the servo so you can change the backpressure.

        You will observe a noticeable increase in bottom end performance with the exhaust restricted.

        When an engine has to perform across a wide RPM range everything is a trade off. Race engine with a narrow power band is different kettle of fish.
        Being an Australian is not an excuse for being dumb and racist.

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        • #5
          Look what drag cars use...

          Open exhausts.. the only reason they don't come straight out the heads is you have to duct the heat away...

          But....that engine has been designed to use an open exhaust, I think you will find camshaft design and timing are the key...

          So to just open up the exhaust on a motor not designed for it will have issues...

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          • #6
            But shouldnt all engines be designed to be using a "open exhaust" as in be designed to run with the highest efficency possible? Other than muffling intake and exhaust noise i dont see the point... If bottom end power is the issue the vehicle in question should be geared better as to keep it in the power band even with part throttle thus keeping it out of this bottom end. Ehh i dont know just a crazy theory i know, im sure there are many vehicle/usablilty/mechanical dynamics my theory dosent take into account i was just having a beer and wondered thats all. Cheers for your responses.

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            • #7
              Originally posted by Daz View Post
              But shouldnt all engines be designed to be using a "open exhaust" as in be designed to run with the highest efficency possible? Other than muffling intake and exhaust noise i dont see the point... If bottom end power is the issue the vehicle in question should be geared better as to keep it in the power band even with part throttle thus keeping it out of this bottom end. Ehh i dont know just a crazy theory i know, im sure there are many vehicle/usablilty/mechanical dynamics my theory dosent take into account i was just having a beer and wondered thats all. Cheers for your responses.
              You are talking about bottom end power and gearing as if you are wide open throttle all the time.

              As mentioned before road engines are a compromise, and sometimes it is a requirement to use 10% throttle and have the engine respond without being in the performance part of the rev range. No usable low end power compensated by low gearing gives shit fuel consumption as well.
              Being an Australian is not an excuse for being dumb and racist.

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              • #8
                You need to define what you mean by 'efficiency' Daz as this will help answer your question. If you are talking about efficiency interms of maximum power that can be achieved... then you will get one answer. If you mean efficiency in terms of fuel, you will get another answer. It is all relative.

                You cant just say an open exhaust is best because there are so many variables depending on what out come you or the manufacturer wants to achieve. You cant just change one thing without changing something else, it just wont work. Unfortunately the answers to your question are not as straight forward as you would like.

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                • #9
                  Thanks for your replies, i understand it wouldnt be a straight foward answer, im not personally trying to achive anything personally, other than getting some feedback about the idea. When i say open exhaust i mean something with out a restriction to the gasses trying to leave the combustion chamber, but not only that i try to consider anything in the intake, valve sizes and any of those modifications people make to get a engine to breathe, flow or what ever you want to call it better. When i say efficiency, i mean having no enxernal or internal parts chocking it up. I understand fuel efficency will be altered to properly fuel a engine that has had these changes made but if i relate it to a vacume cleaner, if i use a skinny nosel on the vacume end and partially block up the air output from the fan and wish to vacume a bed room, lets pretend it uses less power(fuel) in this configuration, it may take me 10 min to clean the floor as its amount of work it can do is reduced. If i open it up so there are no restrictions we can pretend it uses more power( fuel ) note: not sure if it technically does or not but i could clean the floor in half the time because its a more efficient system at vacuming.

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                  • #10
                    You are right in some of your theory, and there is always going be some bottle neck in the system limiting how much air an engine can consume, and then how much fuel it needs and then how much power it can make, etc etc. As Rich said, valve timing has a lot to do with it and so does ignition timing... and the list goes on and on. Its quite involved but also very interesting.

                    The main reason there is exhaust back pressure is for emissions and fuel efficiency. There is always unburnt fuel that escapes after a power cycle so increasing the back pressure limits how much exhaust gas can escape from the combustion chamber and reusing some of that unburnt fuel. The downside being is that if you cant get the old gas out you cant get any new gas in so it limits how much new air can enter the chamber and because of this you have to limit how much fuel goes in so that the mixture is just right. Does that make sense to you ?

                    An open exhaust in theory will help so long as everything else is matched as well, like intake and exhaust port sizes, the volume of the plenumb chamber, the length of the intake stacks etc etc, again the list goes on. The downside to all this power is that fuel effieciency goes out the window which is why you dont see it very often (ie: the general public just wouldn't buy it)

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                    • #11
                      I don't know, I may be totally wrong but I just want to throw it out there.

                      Maybe this "myth" grew from 2 stroke engines, where backpressure IS required for the engine to operate?

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                      • #12
                        Not true...

                        A two stroker will run without an exhaust, just not very well...

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                        • #13
                          Originally posted by FRK View Post
                          I don't know, I may be totally wrong but I just want to throw it out there.

                          Maybe this "myth" grew from 2 stroke engines, where backpressure IS required for the engine to operate?
                          Performance 2 strokes don't rely on backpressure, they rely on pressure waves, resonance, pulses etc.

                          Try and find some old books on 'intake and exhaust design' its an amazing subject with a lot of interactions to account for.
                          Being an Australian is not an excuse for being dumb and racist.

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                          • #14
                            Google "exhaust scavenging".

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                            • #15
                              Scavenging is not the same as back pressure. Scavenging refers to the concept of the previous exhaust pulse pulling the next pulse along, increasing the efficiency of the motor. Back pressure generally refers to the concept of restricting the exhaust in some way to increase the performance of the motor in the low rpm range (at the expense of top end).
                              Originally posted by The Wolf
                              That's thirty minutes away. I'll be there in ten.

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