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BackPressure Myth Explained

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  • The Shaolin
    replied
    Nice.

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  • Minivan Tuner
    replied
    Just thought that I would toss this up here since exhaust diameter has entered into the conversation. Granted, this is a general guideline, but a good one to go by none-the-less if you do not want to work the math out for yourself.

    As shown in Maximum Boost

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  • Ricer Ryan
    replied
    Re:

    Originally posted by Green Monstah
    most I've seen that "pop" have an exhaust leak.

    It's the fresh air that does it.
    I have the worlds worst exhaust. Bought the car in november, with an sohc. exhaust will completely be redone when I do the DOHC swap, so go ahead and laugh while you can. so, any way.... I bought the car with 3-5 diff. size exhaust pipes running from the stock manifold to the back. The muffler had a huge rust hole in it, so I cut that fucker off and put a fart can on. thats where you get to laugh at me. to loud. so I put a cherry bomb/thrush straight through glass pack on where the cat should have been (was gone when I bought the car). Now it leaks like an SOB. The glass pack was to big, so I cut it and just clamped it down. HUGE leak, no pop.

    I am just going to put a peice of test pipe in, and put a half ass, muffler on there. BTW, any ideas on a cheap ass muffler that will sound half way decent? I just have to lose that fart can and quiet it down a bit. That will probably be this weekend, because I am buying the rear bumper from an SL2, and that fart can will not fit under the bumper. My current one is cut out at least 4". And there is no way in hell I am going to cut out the one I am going to buy.

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  • Hydrokracker
    replied
    Re:

    Originally posted by Ansalon
    The case is actually that any bigger than that will actually increase backpressure due to turbulent flow in the exhaust piping, aka the engine doesn't push enough "stuff" through the exhaust for a smooth flow. Hence this is why big displacement engines can run giant pipes and it isn't beneficial for 4 cylinders to do the same.
    I'm glad you brought the exhaust gas velocity into this thread. I love seeing civic douche bags with 3" Thermal R&D on their 160hp 2.0L engine. because they have no idea that 3" piping is actually going to allow the exhaust to cool which results in slower gas velocities and therefore it backs up into the exhaust and at higher rpm you lose power (which is the area where they were hoping to improve with that fart pipe).

    Since Turbo back pressure hasn't been discussed I think I'll go ahead and open up this can of worms. Since the turbo is going to create back pressure (a lot of it at high rpm) you want to reduce the pressure behind the turbine to help relieve that pressure on the front side of the wheel, which in turn spins the turbo faster. you want to have a large downpipe to get the exhaust away, but there is a point in which you have diminishing return and eventually a negative return. But on turbocharged engines more heat is involved you run a larger exhaust due to the greater heat generated by the engine itself. So on an N/A motor making 200hp the 2-2.25" exhaust with cat and mufflers will suffice. But on a turbocharged engine a 200hp engine 2.25-2.5" with no cat and mandrel bends is required for minimum back pressure on the turbine. This is why you will see a 2.5" exhaust on a 350hp NA car while there are turbo cars out there with 500hp and a 4" (or the occasional 5" DP on Supras!!).

    Something I have noticed that I don't think people are understanding the concept behind, I have seen a lot of builds in which there is some type of expansion on the piping which is aimed at reducing back pressure. if there is a bottleneck at some point the only way to reduce the back pressure of that restriction is to remove the bottleneck. putting an expansion on the downstream side of the restriction is not going to relieve the pressure, in fact it may actually harm the flow by allowing cooling of the gas charge. an example of what I am talking about is if your turbo has a 2.5" turbine outlet and you stick a 3" DP on it. It's not going to help all that much, especially if that 3" DP goes back into a 2.25" pipe to the rear of the car.

    The Popping that he is referring to could be reversion I agree. But it could be just a raspy exhaust. Hondas are known for terrible sounding exhausts. but at the same time their heads flow so well that there is a lot more air flowing through them as other cars. I read earlier in another thread that the LL0 heads flow 116cfm (I couldn't believe it either) a k-series flows 400cfm at stock lift and 15inHg. That much volume inside of the exhaust and the exhaust pulses slapping against the mass of air could contribute to this rasp. A simple resonator will always remove that rasp (as long as you find a real resonator and not a muffler that is labeled as a resonator).

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  • slowion2
    replied
    Re: BackPressure Myth Explained

    Originally posted by DigitalDC89
    my '97 SL2a pops on decel, but i thinks its because i have an auto, and when i coast down a hill it stays in the highest gear and lets gravity do the work, resulting in an extremely lean condition, usually past my gauges ability (10:1-18:1). if i shift it into neutral, i get a nice stoiched AFR, and no popping.
    well part of that, and part like said earlier is reversion from a sudden low pressure wave following the higher pressure area when you let off the throttle. the same can be said when you are on decel and you lightly stab the throttle, causing the same thing to happen, and that will force a bit of reversion right there. I used to do that to get ricer's attention stopping at a light lol, or to scare them when the downpipe was open [img]{SMILIES_PATH}/icon_twisted.gif[/img]

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  • DigitalDC89
    replied
    Re: BackPressure Myth Explained

    my '97 SL2a pops on decel, but i thinks its because i have an auto, and when i coast down a hill it stays in the highest gear and lets gravity do the work, resulting in an extremely lean condition, usually past my gauges ability (10:1-18:1). if i shift it into neutral, i get a nice stoiched AFR, and no popping.

    Leave a comment:


  • XL1200c
    replied
    Re: BackPressure Myth Explained

    All of my tuning experience is with motorcycles, mainly my HD Sposrtster.

    When the bikes are pooping on deceleration it usually an exhaust leak or lean mixture.

    Leave a comment:


  • TheMU47
    replied
    Re: BackPressure Myth Explained

    I'm sure less pressure makes the car run good.
    The stock system is setup to have 7psi at WOT on a properly functioning system. Any reduction will result in better airflow - increase in power output.

    As for the popping...MOST I've seen had an exhaust leak of some sort....could also be a fresh air reversion from a free flowing system.

    BTW - hollow cat creates turbulence. better off with a straight pipe, or a high flow cat than a gutted one.

    Leave a comment:


  • taylorjh
    replied
    Re: BackPressure Myth Explained

    well i run a stock header a hollow catalic converter to a cherry bomb glass pack which is no back pressure and my sturn seems to run just fine and loud

    Leave a comment:


  • Zerofactor
    replied
    Re:

    Originally posted by The Shaolin
    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.
    that's not true, but what is, is that running open headers, after you shut a car off, causes cold air to rush up the header, and if it's open, the cold air can reach a valve and crack it. happened to my camaro [img]{SMILIES_PATH}/sad.gif[/img]


    and popping exhaust means their's a exhaust leak? interesting.. is that always the case? all the car's in my family with aftermarket exhaust's pop.

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  • bwtucker
    replied
    Originally posted by Green Monstah
    most I've seen that "pop" have an exhaust leak.

    It's the fresh air that does it.
    I thought that too, but I checked mone over and there is no leak. I just had the whole thing wleded up not that long ago. who knows!

    Leave a comment:


  • TheMU47
    replied
    most I've seen that "pop" have an exhaust leak.

    It's the fresh air that does it.

    Leave a comment:


  • bwtucker
    replied
    yea, mine pops too and i have a high-flow cat.

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  • RMR North
    replied
    I just wanted to add somthing. My car is a 97 SC2 i have a stock exhaust maniflold a Vibrent high flow cat, 2 1/4in piping, a Thrush glass pack, and an HKS high Power muffler with the 170mm shell. The piping also goes under the rear suspension. Now i think it sound really good, as do most of my friends. BUT, i does pop when i am slowing down. So it is not just something that cat-less cars do. It even poped when it had a stock cat instaled. So IMO it just has to do with your AFR and how you have laided out your exhaust. Also, i agree that pipe size still plays a big part and that bigger is not better in all cases.

    Thats just my to cents.

    Leave a comment:


  • NGKTECH
    replied
    this is excellenet info thanks guys!

    Leave a comment:

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