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Exhaust back pressure on a 4 stroke ICE?

  1. Dec 27, 2007 #1
    Would someone explain to me why back pressure is not needed on a 4-stroke engine and what exactly is exhaust back pressure?

    Thanks guys and gals.
  2. jcsd
  3. Dec 27, 2007 #2
    Well, I'm a physicist, not an engineer, and all I know about engines is what I learned in high school ... but since it's late at night and no one else seems to be answering, I'll take a stab at this:

    I'd guess that exhaust back pressure is needed in a 2-stroke engine because the compression stroke and the exhaust stroke are the same thing, i.e. the exhaust valve is open while you're trying to compress the next dose of air+fuel. Without some back-pressure from the exhaust (which presumably comes from the exhaust manifold and plumbing being sufficiently restrictive), you wouldn't get much compression - the fuel/air mix would just be forced out the exhaust valve.

    With a 4-stroke engine, the valves are all shut during the compression stroke, so you don't need any external pressure to get good compression - the cylinder is effectively sealed.

    Make sense?
  4. Dec 28, 2007 #3


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    Hi ‘stang,
    “Backpressure” is a fairly common term regarding the resistance to flow of some piping system (ex: exhaust pipe) when the pressure at some point downstream of this resistance is known. In the case of an exhaust pipe, the outlet of the pipe is open to atmosphere, so we know the pressure at this point is atmospheric pressure. But because there is flow through the pipe, which represents a resistance to flow, the pressure is not atmospheric pressure all the way up the pipe to the engine. If you were to measure the pressure at various points along the exhaust pipe, you’d find that the closer you got to the engine, the higher the pressure would be. This is simply because there’s a resistance to flow of any fluid through a pipe. So the term “backpressure” simply says that there is a pressure drop through a pipe and the pressure at the inlet of the pipe is higher than the outlet and we generally acknowledge that it’s the outlet of the pipe who’s pressure is knowable. This is true for the exhaust on a 2 stroke or a 4 stroke engine. Clear as mud so far?

    In the case of the 4 stroke, the desire is to eliminate ALL the burned gasses from the cylinder when the piston gets to TDC - so the more backpressure, the higher the pressure will be inside the cylinder, and the less exhaust gas we will be able to expel. We don’t want those gasses inside the cylinder, so we want the backpressure on a 4 stroke to be as low as possible. Here’s a decent discussion on 4 strokes:

    In the case of a 2 stroke, the thing that’s clearing the exhaust from the cylinder isn’t the piston going up to TDC because the piston is in fact only a little past BDC when the exhaust gasses are displaced. For a 2 stroke, it’s the sweeping of the cylinder by the fresh charge of air and fuel which is pushing the exhaust gas out. If there’s no back pressure, the fresh air and fuel will not only sweep out the exhaust gasses, but they will also begin to exit with the exhaust gas, resulting in a loss of fuel and air which reduces efficiency. If the back pressure is too high, the exhaust gasses can’t all be removed by the fresh charge. So for 2 strokes, there is an optimal back pressure to produce the most efficient fuel use while maximizing power. This optimal back pressure must be tuned to a certain RPM, because above and below that RPM, one of the two problems will surface. 2 strokes have a very dynamic gas flow which is difficult to optimize because of this. Most 2 stroke mfg’s simply tune the engine at a given RPM and throttle setting which results in the best overall performance.

    This is a rather simplified explanation of the 2 stroke, there are other considerations. A small amount of back pressure for example can increase the total air/fuel charge in the cylinder by raising the pressure at the point the exhaust port is closed off. If you’d like to read more, I’d suggest these:
  5. Dec 28, 2007 #4


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    If you want to get into engine/exhaust performance a little more deeply, look up "anti-reversion" exhausts. An unbaffled or lightly-baffled exhaust on an ICE like a Harley engine undergoes a series of pressure pulses during operation, including acoustic pulses which can result in exhaust being pushed back up the pipe toward the engine. Properly-designed crossover pipes or a collector system feeding a single muffler can help improve exhaust scavanging, but another, often more aesthetically-pleasing design mounts anti-reversion cones inside the exhausts. They offer relatively little resistance to the exhaust gases coming out of the heads, but help break up reversion pulses and improve overall exhaust flow.

    I tinker on Harleys, but have had no experience with AR exhausts for V-8s, though I'm sure that there is stuff available.
  6. Dec 28, 2007 #5
    turbo-1 - That's interesting. Is that what is involved with "exhaust tuning"? I always assumed that that phrase referred to adjusting the relative length of the header pipes so that when pulses from one cylinder worked their way back to another, the timing would be such that the optimal pressure (low, presumably) was achieved there.
  7. Dec 28, 2007 #6
    Great responses guys, thanks Q Goest! That is a great start!

    I am trying to digest it all, and may have some questions later.

    I see in many of the car forums I am on, the term exhaust scavenging. They sometimes refer to the exhaust piping exhaust scavenging, and I also see it refered to during a 4 stroke's cycle.

    So, can anyone provide an exhaust scavenging explanation in the collector, and also in the combustion chamber?
  8. Dec 28, 2007 #7


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    The relative length of the header pipes is a matter of design, as you say, but if you are dealing with limited space, then the resistance induced by the the bends of the closer pipes must be weighed against the lower resistance of the longer, straighter pipes. This resistance must be balanced against the ability of the exhausts from other cylinders to aid in exhaust scavenging, so that the next pulse of exhaust emitting from that cylinder will encounter the least resistance possible. Throughput=horsepower.
  9. Dec 28, 2007 #8


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    Try this page:

  10. Dec 28, 2007 #9
    even a 4 stroke needs some back pressure
    go too big on header pipes size and loose power [tork]
    esp at mid range and lower RPMs

    gas flow speeds and harmonic waves esp in tuning is a black art
    and get worst in two stroke racebikes as some make wild power levels
    but only over very narrow RPM ranges
  11. Jan 6, 2008 #10
    So it is possible to have to little backpressure on a 4-stroke?

    But you do not need near as much as you would on a 2-stroke...

    You guys provided great info for me to digest!

    I got a quick question...

    What is exhaust scavenging? How would you explain it?

    I read of opening up the exhaust valve near the end of power stroke to increase exhaust scavenging, and I also read how collector's on headers increase exhaust scavenging...

  12. Jan 7, 2008 #11


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    Exhaust scavenging is removal of combustion products from the cylinder after the exhaust valve has opened. Obviously this is important so you can get the right amount of charge into the cylinder, with only a controlled amount of exhaust gas left in the cylinder (internal EGR) ready for the next combustion. Too little and you don't get enough fuel/air into the cylinder; too much and you put fuel out through the exhaust.

    Opening the exhaust valve at the end of the power stroke is what's known as 'blowdown'. The cylinder pressure at this point is still way above atmospheric, but most of its work has been done. Opening the exhaust valve allows combustion products to begin exiting the cylinder earlier, and at a greater rate, to empty the cylinder more effectively.
  13. Jan 8, 2008 #12
    Thanks brewnog, that answers my questions!
  14. Oct 27, 2009 #13
    Gentlemen, I was having this very discussion with my brother, now i agree that there should be some back pressure in a 4 stroke,why else would race cars have exhausts that are the same diameter as the exhaust port,,
    My question is this, what are the implications on the valves being burnt, I was under the impression that if there was no back pressure the valves burn ??
    any info would be great
  15. Oct 27, 2009 #14

    Ranger Mike

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    I just got on this post..many good points are posted...

    There are several reasons why valves burn. One is normal wear. As an engine accumulates miles, the constant pounding and thermal erosion wears away the metal on the face of the valve and seat. The exhaust valve sheds most of its heat through the seat, so when the face and seat become worn and the area of contact is reduced, the valve starts to run hot. Eventually the buildup of heat weakens the metal and pieces of it start to break or flake away. Once this happens, it forms a hot spot that accelerates the process all the more. The valve begins to leak and compression drops. The result is a weak or dead cylinder and a noticeable drop in engine power, smoothness and performance.

    A bad exhaust valve will also increase exhaust emissions significantly because it allows unburned fuel to leak into the exhaust. High hydrocarbon (HC) emissions, therefore, may also be an indicator of a burned valve.

    An exhaust valve can also burn if the valve lash closes up for some reason (improper lash adjustment, cam or lifter wear, a bent push rod, worn rocker arm or cam follower, etc.). The lack of lash (clearance) in the valvetrain prevents the valve from closing fully, which causes it to leak compression and overheat.

    Valve burning can also be caused by any condition that makes the engine run hot or elevates combustion temperatures. This includes cooling problems, abnormal combustion like detonation or preignition, loss of exhaust gas recirculation (EGR), retarded ignition timing or lean fuel mixtures.

    An engine with short headers will run leaner than if it had longer exhaust tubes due to sucking back fresh air diluting the mixture. If the mixture is weak enough or the engine load great enough, it will go into detonation and/or pre-ignition. That will wreck valves for sure.

    A vacuum intake leak will lean out the fuel air mixture and could burn a valve. An exhaust header gasket can burn out and thus cold air may enter the exhaust port and warp the ex. valve and this will cause it to go bad.
  16. Oct 28, 2009 #15
    Thanks Ranger Mike, awesome info. Let me just confirm, there needs to be some back pressure in the exhaust system because the cylinder will run leaner due to sucking back of fresh air and diluting of the fuel mixture, right ?.

    Therefore, even a freeflow exhaust has to be tuned to the correct lenght versus diameter of the pipe, in order to produce correct mix/detonation/torque and engine wear. Correct ??
  17. Oct 28, 2009 #16

    Ranger Mike

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    The short answer is yes..exhaust should be tuned to RPM range for max performance. Too little back pressure kills off low end torque. Too much BP makes the engine labor too hard and kills off horsepower. I just went thru this with my diesel pick up truck..tried to take the cheap way out by running 3 inch pipe from turbo to the axle ,,i could feel the engine really laboring when going up hill..i swiched to 4 inch turbo back and things really came alive...a friend had gone to insane 6 inch turbo to axle and said his bottom end suffered but not high end performance. emperical data...yes the dyno numbers bear this out so some BP is good

    Attached Files:

  18. Oct 28, 2009 #17
    Fantastic, thank you very much, I will definately start fiddling with diameters and tuned length. So glad finally found a forum that helps me, Ranger Mike, You da man.

    look forward to disscusions in the future, thank you keep well
  19. Oct 28, 2009 #18

    Ranger Mike

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    most kind of you
  20. Oct 28, 2009 #19
    Holy necropost Batman!
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