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BackPressure Myth Explained

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  • BackPressure Myth Explained

    Originally posted by Certified BMW Engineer
    Some say that "an engine needs backpressure to work correctly." Is this true?

    No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

    The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

    The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

    The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

    Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

    Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
    I know it usually is debated amount the newbs and found it interesting so postededed-ed
    CSW Tuning or Facebook!
    - Stork

  • #2
    There you have it.

    Someone on Saturnspot said that torque increases with backpressure, and I told him to stuff a tennis ball up his tailpipe.

    I was also told to drive gently open header to avoid burning valves. HA.

    so, I guess this is a good scientific reason behind what I've been trying to argue with idiot reasoning.
    -2002 Saturn SC2 - Clean, slow, low daily
    -2008 Saab 9-7x Aero
    -1996 Suzuki Bandit 750 swapped
    -1998 VW Golf GTI VR6 Mid Engine LeMons car
    -1986 RX7 - 275whp @ 10psi


    • #3
      wow there you have it all in one post. I've stated so much of that shit before till I'm blue in the face.

      sadly, most noobs won't understand a fucking word of all that, and so it's lost, but great find Stork. I'm suprised he didn't go into velocity as well but then again that's for a different purpose all together.


      • #4
        Stickified and renamed. Good find hunny bunch!
        Jeff W.
        Old Fella Nobody Remembers


        • #5
          Originally posted by Schizzo97SC2
          wow there you have it all in one post. I've stated so much of that shit before till I'm blue in the face.

          sadly, most noobs won't understand a fucking word of all that, and so it's lost, but great find Stork. I'm suprised he didn't go into velocity as well but then again that's for a different purpose all together.
          Remember we were talking about that vin? I said that to the hotrod guys that are used to carbs and stuff and they told me i was still wrong about the newer cars that adjust fuel. Great find stork.
          2002 Saturn SL1- 5 speed, basic, slow, and staying that way.
          1993 Lexus SC300- Project turning into gold...

          Formerly knows as "Saturntuner05."
          Too many past turds to list.


          • #6
            you are my fucking hero for posting this!
            94 SC2, manual, the whore that won't die
            96 SC2, auto, 146k, wife's car
            96 Formula, LT1, T56, T-tops, gas guzzling whore

            Malo Periculosam Libertatem Quam Quietum Servitium


            • #7
              Originally posted by JazzyJeff
              Stickified and renamed. Good find hunny bunch!
              Yeah, I didn't know if it really needed to be listed as a sticky or anything because of the common sense factor, but I do remember it being an issue with newbs so there ya go and glad I could help [img]{SMILIES_PATH}/biggrin.gif[/img]
              CSW Tuning or Facebook!
              - Stork


              • #8
                Originally posted by Schizzo97SC2
                wow there you have it all in one post. I've stated so much of that shit before till I'm blue in the face.

                sadly, most noobs won't understand a fucking word of all that, and so it's lost, but great find Stork. I'm suprised he didn't go into velocity as well but then again that's for a different purpose all together.

                i remember when you and me debated it. it was great, basically arguing yet we both meant the same just saying it differently. at least it worked out in the end [img]{SMILIES_PATH}/lol.gif[/img]
                Ian---The Fat Ass

                1995 SL2-High compression engine FOR SALE
                2010 Chevy Cobalt SS- tuned with plenty of goodness
                2002 SC2-New daily
                2017 Kawasaki Ninja 650 KRT- got a stupid deal
                2003 Ford Taurus SES- free daily beater


                • #9
                  well "back pressure"is more of a general term.

                  Once you get into the fluid flows and dynamis and testin, you will see a
                  tendencey for the torque peak to be pushed around by different diameter intake and exhuast pipe sizes....all torque peaks being around a certain
                  fluid velocity(s) in the pipes. And it would kinda hard to double saturate
                  the air in a fuel injected car as the air can't go back past the injector and
                  pick up more fuel as it would with a carb system...the pipe size/velocities
                  holds true to vehicles that are fuel injected and has been proven time
                  and time again by professional race teams.

                  The sweet spot tends to be between 200-300 ft/sec for fluid flows...over
                  that and the exhuast is being forced to move faster then it want to and
                  you start to build up excess pressure in the pipe...think of a garden hose
                  with a jet nozzle on it, its a very simlar concept.

                  On on the other end, you may not have enough "back pressure" to make
                  torque...when in reality is that you don't have eough exhuast velocity fo
                  for a scanveging-vortexial (is that word?) exhuast flow to form to help
                  scanvenge the exhaust gasess out of the cylinder. Without the scavenging
                  effects, the there is less space for clean charge air to fill the cylinder to
                  combust....resulting in less cylinder pressure, and ......less torque.

                  Now...when is the "back pressure" the lowest? Generally the faster the
                  air/gasses move threw a pipe, the lower the pressure at the pinch of
                  a venturi in a carb, the lowest pressure is at the pinch and the highest
                  pressures are above and below the venturi.....but if you force the
                  fluid/gasses to move faster then they want, then it takes more power because
                  it starts to back up like the jet nozzle on the end of a there is a limit to how
                  fast the fluid moves to the amount of pressure in the
                  pipe/ happens to be best lowest pressure movements are
                  between 200-300 ft/sec....HOLLY SHIT!!!! RIght around the velocities
                  of peak torque preduction...Coincidence? I don't think so.
                  -6S Resident Mechanical Forensics member #001.
                  1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
                  1998 F-250 5.4L triton...stock.


                  • #10
                    Thanks sam, I was going to talk about scavenging etc. but ya beat me to it.
                    [quote author="shortbus"]Stage 2 is great. Its covered by warranty, better fuel economy, the eaton screams louder so deer wont jump onto the road, it makes girls get wet, and the emissions produced will not kill songbirds or melt glaciers.[/quote]<br />[quote author="retardpartol"]...who is sober and noton any kind of intoxication whether is be ibuprophin[/quote]<br />LSJ phun


                    • #11
                      scavenging and velocity are a must when talking about this topic. its all about bernoulli's principle. S.Bretz hit it right on the head.
                      <img src="{SMILIES_PATH}/stc.gif" alt="" title="STC Logo" />-NY


                      • #12
                        Wow, thats so awesome.
                        2000 Saturn SC2 DD... RIP
                        2009 Ford Escape XLT 3.0L 4x4
                        2009 Yamaha FZ6


                        • #13
                          nice ...very informative
                          [quote author="Saturndriver"] Imma have my car runnin like ''damn'' lol.[/quote]


                          • #14
                            I have a question for this. Why is that if you need zero back pressure that a car can run worse without it?

                            Also most of those pepboy civic driver that have the fart cannon dont have back pressure and that creates the pop pop pop sounds coming out of there exhaust. I have seen two civic hatches one with a straight through and a cannon and one with with a good header, high flow cat, manderl bent pipe, glass pack and fart cannon.

                            The one without any backpressure makes the loud ass popping noise where as the one with a nice exhaust setup is just loud. No popping noises it actually sounds good.

                            So can anyone tell me why this? Because honestly I know how to work on engines to some degree but I dont nearly as much as I'd like too. I have only been working on cars seriously for about a year.

                            And please dont give me any shit about the honda stuff. There both guys that I work with. The one with the crap exhaust is my friend and I'm trying to get him to redo it so it sound less like crap.


                            • #15
                              first of all, sound has absolutely no bearing on how well it flows, and honestly if one pops and backfires from reversion it probably outflows the other by a bit, at least in theory and at first glance.

                              your descriptions about the two civics in question don't really make much sense. straight through and a cannon? is that on stock piping and stock manifolds and just a canister style muffler? that would then make sense why it may make a lot of noise and pop but not really do a whole lot. the other sounds like a full exhaust system that has been pretty well thought out. the main reason it doesn't pop much if at all is most likely because the cat is still in there, OR because there's a good chance it's also a totally different engine to begin with.

                              sound does not equal backpressure, nor does "popping" or volume. if a car runs worse with a high flow, low restriction exhaust, then either it needs to be tuned to support the changes made, or there is another issue with the car all together. you want high flow, low restrction, and velocity in a usable engine speed area, all of which means as little backpressure as you can get away with.


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