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please explain how exhaust really works

Discussion in 'Performance and Tuning' started by TacomaMtnMan, Jul 14, 2010.

  1. Jul 14, 2010 at 6:34 PM
    #1
    TacomaMtnMan

    TacomaMtnMan [OP] those who seek fear shall be followed by fear

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    Hey guys,

    I have been checking on youtube and dont really trust the info on wikipedia...

    I am curious to know and understand how exactly does exhaust work? I am sure most of us know the basics. I am more interested to know how to get more performance from exhaust, not just replacing from the cat-back, but the entire system from the headers...

    I guess I am more curious on why I should do this, does every part of a factory exhaust cause some kind of back-pressure?
    Including headers, cats, muffler, tubing...

    Why does a true-dual out better than dual from the muffler?


    Thanks for the info!
    ~ Chuck
     
  2. Jul 14, 2010 at 6:41 PM
    #2
    HomerTaco

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  3. Jul 14, 2010 at 6:46 PM
    #3
    NumNutz

    NumNutz One of the original 7928

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    I'm not a engineer but basically with a CBE/HBE system you are lessening the back pressure. Meaning that it will allow more air to flow through the engine. In other words it opens things up. It doesn't really do much without having more air drawn into the engine through say a cold air intake.

    IMO a dual system is a little much for our trucks w/o forced induction. I don't think with a cold air intake you can draw in enough air to make lessening the back pressure that much worth it. The dual system is more for looks. Like I said this is my opinion.
     
  4. Jul 14, 2010 at 6:47 PM
    #4
    HBMurphy

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    Well. this will be a long one...

    Headers are designed to help scavenge the combustion chamber (clean out the spent gases from the combustion chamber so that you have fresh O2 from the air, to mix with the fuel). Theoretically you want each tube length to be the same and for the collector to be as efficient as possible. If you were to picture the volume of one compression stroke to be one tennis ball going down the header tube, you would want the tennis ball to get in line and arrive at the collector at the moment just after the prior cylinder (in firing order) and just before the next.

    Now there is also a desire to have back pressure (resistance) in the system too. When you design an engine for all out power like a drag car or race boat - you want the engine to be winding (reving) nearly as fast as it can in the highest part of it's power curve - internal combustion engines have their highest power and torque at the higher revs ~75-90% of top engine speed.

    Interesting to note - electric motors are the opposite - their highest torque is at lowest RPM - great for jumping the line.

    Back to IC motors...

    On a truck you may want low end torque (rock crawling) or higher end power (desert pre running). With that in mind, designers try to optimize the torque/power curve to suit the application. The first thing an engine designer thinks about is what engine speed am I designing for? Then he builds an engine from there.

    In our case (all though I do not have headers, so if you would like, call me full of shit :)), headers clean up the opium tube length and are sweet. I don't know if they hurt the lower end (they could) and as long as you are not using your truck to pull stumps out of the ground at very low speeds, are a nice addition to your rig.

    One other thing to note is that our trucks have o2 sensors (lambda sensors) that dynamically detect the fuel air ratio and then corrects the fuel being injected to reduce the likelihood of unspent gas going out your tail pipe. To my knowledge (which is admittedly limited) most of the header systems have you buy a device to bypass this system - aka o2 'simulators'. If I was doing a header set up, I would like to see one with a bosses for the o2 sensors. It may still be an issue since we do have a limited variable timing set up to that would need to be looked at to really dial in a motor.

    Feel free to flame as to my knowledge for this particular set up is limited. But regarding waste gases - I am an expert! ;) :eek:

    Oh yeah, my Thermodynamics and Internal Combustion Engine instructors should be glad he didn't read this answer.

    PS - Back-pressure is not a bad thing - especially on a truck that works!
     
  5. Jul 14, 2010 at 6:49 PM
    #5
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Staff Member

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    http://www.tacomaworld.com/forum/technical-chat/19967-how-air-goes-thru-your-engine-basic.html

    Have you ever noticed that if you make a quick "puff" at a candle from a short distance, that the air, even though you are not blowing anymore, still makes it to the candle and blows it out? That's because air has "mass", and when in motion, it must be acted upon to stop it. Have you ever noticed that as a kid when blowing bubbles, if you try to blow a bubble to keep it aloft, it seemed that sometimes the bubble would be passed by the air, only to wiggle around, and then "follow" the air you just blew? These little things also apply in an engine.

    As the air is going through the intake, and through the throttle body, there is a bit of turbulence created by the throttle plate. As it continues on to the intake runners, the air is straightened out into a nice laminar flow, as the intake valves open, and the piston is rapidly moveing downward, the air rushes in to fill the combustion chamber, flowing down the sides of the cylinder wall, hitting the top of the piston and moving up the other side and thru the middle, and swirling. as the intake valves close very fast, its like a door slamming shut. The air that was following in to the combustion chamber is now suddenly stopped. This sends a "shock wave" backwards through the intake runners, and if timed properly, this "shockwave" or "Pulse" is now helping to "push" more air into another cylinder.

    Meanwhile, as the air in the first cylinder is being ignited, and producing power on the downward stroke, the piston now begins its upward stroke. The exhaust stroke.This is where scavenging comes into play. Scavenging is when the exhaust valves open and the exhaust exits the cylinder leaving a vacuum behind it. At that point the piston moves the exhaust out rapidly in a "puff". This puff, once again has mass. There is a high pressure area leading the way in front of the exhaust pulse, and when the exhaust valve closes, it happens abruptly, creating a low pressure area behind the exhaust pulse. This pulse travels down the exhaust, and past another cylinder. The low pressure of the pulse "pulls" the exhaust gasses from the next cylinder, when doing this, it creates a vacuum in the combustion chamber helping to "pull" more air into the cylinder as the intake valve is starting to open while the exhaust valve is still open, this is called overlap.

    All these pulses need to keep up their velocity to remain efficient. They do this by staying hot, and by the size of the exhaust pipes. Newer vehicles with variable valve timing (VVT) keep these pulses timed throughout the rpm range. There is a "Goldie Locks" effect taking place. Too large of a pipe, and the pulses slow, exhaust cools too fast, and the engine looses efficiency. Too small of a pipe, and the exhaust is sped up beyond how the timming of the valves was designed, the exhaust temps rise, and the engine looses efficiency. Any change in the system, be it a larger exhaust, or whatever, has an effect on it. Sometimes positive, as the auto makers have to abide by emissions standards, but many times negative, as some aftermarket manufactures just want it to "sound cool".

    When installing a Turbo, or Supercharger, the scavenging effect is not as important, as the higher pressure "pushes" everything through the system, in which case, a larger free flowing exhaust would be the most beneficial.

    I have oversimplified this just a bit, as I don't have time to go into EVERY thing, or break down the physics involved, but thought it would be a good read for those thinking about changeing their exhaust system and wondering how the new exhaust may affect performance. [​IMG]
     
  6. Jul 14, 2010 at 6:53 PM
    #6
    RelentlessFab

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  7. Jul 14, 2010 at 7:02 PM
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    Cape Codder

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    Good Reading as Always, Thanks for More Education.
     
  8. Jul 14, 2010 at 7:09 PM
    #8
    HondaGM

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    what about a crossover pipe,and how this affects scavenging?
     
  9. Jul 14, 2010 at 7:13 PM
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    HomerTaco

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    I've learned something.....:D
     
  10. Jul 14, 2010 at 7:25 PM
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    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Staff Member

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    On "V" configured engines, and running a Dual exhaust, the cross over pipe is used to use the low pressure of one exhaust pulse from one bank, to speed up the pulse from the other bank.
     
  11. Jul 14, 2010 at 7:34 PM
    #11
    HondaGM

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    Also... There is yet another option.... The MAC Prochamber.

    It looks like a box, or a muffler where the 2 header leads enter one end and then exit the other in the location of the H or X pipe.

    It is essentially a combination of all three basic designs , incorporating the crossover flow of an X pipe the open buffer of an H pipe and the passive pulse control of a ported baffle channel pipe. MAC is the ONLY maker to have this design.

    Basically it combines the exhaust into a single box, where the 2 inlets extend into the box a few inches to prevent reversion and open dumping exhaust into the box. The outlets are flush with the back of the box and there is a baffle between the sides with ported slots directing the flow of the inlets to cross to the other side. The Box holds backpressure at a steady rate, which eliminates scavenging.

    There are many who believe the Prochamber will give increased performance values. Everyone using them will tell you they make a deeper yet quieter tone to the exhaust note.
     
  12. Jul 14, 2010 at 8:52 PM
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    HBMurphy

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  13. Jul 15, 2010 at 8:18 AM
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    XrunnIT

    XrunnIT Well-Known Member

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    i like talking about exhaust... ;)
     
  14. Jul 16, 2010 at 12:34 AM
    #14
    TacomaMtnMan

    TacomaMtnMan [OP] those who seek fear shall be followed by fear

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    I appreciate it guys! a ton of great information here and now I feel better. :)


    What is everyone's experience with the exhaust you are running on your personal trucks?
     
  15. Jul 16, 2010 at 7:13 AM
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    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Staff Member

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    Im running the TRD, and am quite happy with it. It doesnt give the best gains, but I like the sound. If looking for the best gains, a header, and full exhaust would give the most, but your still only looking at about 10 to 15 hp.
     
  16. Jul 26, 2010 at 8:15 AM
    #16
    OH-MAN

    OH-MAN Well-Known Member

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    Exhaust used to be very restrictive in OEM setups.
    The cars had large engines and the manifolds, mufflers, and piping were cheep and made for quiet.
    Hot rodders started opening up exhaust systems and made huge gains espically with intake improvements as well.

    The OEMs caught on and in efforts to improve performance and milage started putting better systems on over the years.

    They work pretty damm good nowadays so a lot less improvement per dollar spent on exhaust systems is the norm.

    All thing considered sound is what I am after on my Tacoma.
    I don't think my TRD catback adds any power or milage improvement, just my enjoyment.
     
  17. Jul 28, 2010 at 10:13 PM
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    RDA

    RDA New Member

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    Really interesting material guys; thanks for posting. Helps explain why when I had Longo install the TRD CAI, I felt a gain, but 8 months later, when they put the TRD exhaust on, the engine seemed to gain a substantial amount of power particularly at high rpm when merging onto freeways. Yeah, all I have is the butt-dyno, but I've owned enough cars to know when I feel a substantial increase in power. The engine seems to run smoother, too. For those who don't want something too loud, the TRD exhaust is just about perfect. No droning at highway speeds like I've heard with other setups.
     
  18. Aug 22, 2010 at 8:34 PM
    #18
    RugbyTaco

    RugbyTaco Rhum.

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    In regards to headers, is there a difference in gains/loses when in comes to long and short tube headers?
    i've seen some info on this but nothing real concrete
     
  19. Aug 22, 2010 at 8:36 PM
    #19
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Staff Member

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    Shortys generally give gains in the low to mid range, while the long tubes give the gains up top.
     
  20. Aug 22, 2010 at 8:40 PM
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    RugbyTaco

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    Shortys it is then, thanx
     
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