Exhaust back pressure





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    Default Exhaust back pressure


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    Default Re: Exhaust back pressure

    I am at this stage doing an in depth study of exhaust systems.

    Bigger is not always better.

    Very few people realize it is all about flow velocity. Does not help you enlarge a system but the intake velocity and capacity stays the same.

    Interesting article Shaun.
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    Default Re: Exhaust back pressure

    Good evening gents.

    I came across a very interesting test on that subject on youtube the other day. The guys had a v8 motor on a dyno with 63mm free flow system and got x amount of kw and torque.
    They then changed it to 76mm free flow system. I must say it is carb fed and they played around with the jets on the carb. Cant remember if they went bigger or smaller, but they did get more kw and torque from the motor. If I remember correctly though it was at different rpm than with the 63mm system.

    So my conclusion is trial and error on petrol engines turbo or non turbo. I might be totally wrong.

    What I can add is that I have a 76mm system on my 2.5 diesel drifter with no silencer. It was going like a champ, but my children normally sit on the back in the canopy and it started getting a bit too loud for them so I fitted a small free flow box. It does feel like the power has dropped slightly. Turbo not spooling so easy at low rpm anymore. So DVR in my opinion is spot on, on the turbo diesel with a big straight pipe. Just a pitty it is a bit noisy.

    Cheers
    Fazda

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    Default Re: Exhaust back pressure

    Quote Originally Posted by Fazda View Post
    Good evening gents.

    I came across a very interesting test on that subject on youtube the other day. The guys had a v8 motor on a dyno with 63mm free flow system and got x amount of kw and torque.
    They then changed it to 76mm free flow system. I must say it is carb fed and they played around with the jets on the carb. Cant remember if they went bigger or smaller, but they did get more kw and torque from the motor. If I remember correctly though it was at different rpm than with the 63mm system.

    So my conclusion is trial and error on petrol engines turbo or non turbo. I might be totally wrong.

    What I can add is that I have a 76mm system on my 2.5 diesel drifter with no silencer. It was going like a champ, but my children normally sit on the back in the canopy and it started getting a bit too loud for them so I fitted a small free flow box. It does feel like the power has dropped slightly. Turbo not spooling so easy at low rpm anymore. So DVR in my opinion is spot on, on the turbo diesel with a big straight pipe. Just a pitty it is a bit noisy.

    Cheers
    Fazda
    You cannot compare turbo and normal aspirated. You get to a size even with a turbo engine that going bigger will not give you any gains. The only gain will be the flow of extra money from your account going bigger than needed

    But then I am still an appy trying to understand the workings of an engine
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    Default Re: Exhaust back pressure

    Nice read here too - especially page 2:

    http://www.myturbodiesel.com/wiki/tu...l-turbo-faq-2/

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    Default Re: Exhaust back pressure

    Quote Originally Posted by grips View Post
    You cannot compare turbo and normal aspirated. You get to a size even with a turbo engine that going bigger will not give you any gains. The only gain will be the flow of extra money from your account going bigger than needed

    But then I am still an appy trying to understand the workings of an engine
    exactly and me too
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    Default Re: Exhaust back pressure

    In my time most school lighties with their 2 stroke 50's found out the hard way that a 2 stroke exhaust is tuned to the engine and messing around with it will not help in their quest to increase top end from 83,2 km/h to an all time 50cc record of 84,1 km/h.

    It is basically the same principle that applies to tuned intake manifolds where the pulses are used to enhance the induction process.

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    Default Re: Exhaust back pressure

    Quote Originally Posted by Woestynbrak View Post
    In my time most school lighties with their 2 stroke 50's found out the hard way that a 2 stroke exhaust is tuned to the engine and messing around with it will not help in their quest to increase top end from 83,2 km/h to an all time 50cc record of 84,1 km/h.

    It is basically the same principle that applies to tuned intake manifolds where the pulses are used to enhance the induction process.
    in my time we had a very good grip on the concept of expansion boxes and their tunability in relation to the exhaust port height

    We had a dude in our school that had an MR50 that would happily thrash 125 4-strokes on top end.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by grips View Post
    You cannot compare turbo and normal aspirated. You get to a size even with a turbo engine that going bigger will not give you any gains. The only gain will be the flow of extra money from your account going bigger than needed

    But then I am still an appy trying to understand the workings of an engine
    Exactly, and I can't help feeling that in these modern ultra high tech small motors with huge power and torque and a excellent fuel consumption, the manufacturer has already optimized things to a very large degree.

    Even if you do manage some gains, I suspect they will at the cost of something/s else.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by Fluffy View Post
    Exactly, and I can't help feeling that in these modern ultra high tech small motors with huge power and torque and a excellent fuel consumption, the manufacturer has already optimized things to a very large degree.

    Even if you do manage some gains, I suspect they will at the cost of something/s else.
    exactly, and then I hear of these industrial area guys with their 4x2m workshop and they try and maintain their free-flow/chip/whatwhat will be better than something that has firstly had several YEARS of rendered CAD/CAM R&D as well as YEARS of bench testing.........

    Now maybe in the old days, when block and head casting was done in a cave, but today?
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    Default Re: Exhaust back pressure

    Quote Originally Posted by jelo View Post
    exactly, and then I hear of these industrial area guys with their 4x2m workshop and they try and maintain their free-flow/chip/whatwhat will be better than something that has firstly had several YEARS of rendered CAD/CAM R&D as well as YEARS of bench testing.........

    Now maybe in the old days, when block and head casting was done in a cave, but today?
    Very true most of the modern engines I have worked on there is almost zero room for improvement.
    I have seen very few of the latest head designs where gas flow and porting will make a difference.

    Turbo or supercharging are almost the only way to increase power on new high tech jobbies.

    My little experience with intake runners and ports is that you sometimes get better performance with smaller high velocity ports than big ones. In most cases you only benefit at the very end of the rev range with huge ports. Huge ports can hamper low and medium power ranges with a lag of velocity.
    Last edited by grips; 2016/09/27 at 09:51 AM.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by grips View Post
    Very true most of the modern engines I have worked on there is almost zero room for improvement.
    I have seen very few of the latest head designs where gas flow and porting will make a difference.

    Turbo or supercharging are almost the only way to increase power on new high tech jobbies.

    My little experience with intake runners and ports is that you sometimes get better performance with smaller high velocity ports than big ones. In most cases you only benefit at the very end of the rev range with huge ports. Huge ports can hamper low and medium power ranges with a lag of velocity.
    Exactly why they invented dual stage carburetors,

    First stage, small venturi, good engine response, smooth etc. Add second stage with larger venturi, max power at high revs and WOT.
    Cheers

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    Default Re: Exhaust back pressure

    And all the V6 owners laugh at me and my 57mm pipe on the V6 ha ha ha ha
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    Default Re: Exhaust back pressure

    Quote Originally Posted by Fluffy View Post
    Exactly why they invented dual stage carburetors,

    First stage, small venturi, good engine response, smooth etc. Add second stage with larger venturi, max power at high revs and WOT.
    I always want to build myself a flow bench.

    Without a flow bench you will always be guessing about gains and mods.
    Last edited by grips; 2016/09/27 at 10:01 AM.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by Fluffy View Post
    Exactly why they invented dual stage carburetors,

    First stage, small venturi, good engine response, smooth etc. Add second stage with larger venturi, max power at high revs and WOT.
    dual stage is primitive

    multi-stage multi-port direct fuel injection is better

    add variable valve timing and variable inlet manifold length technology and THEN you have something cool.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by jelo View Post
    dual stage is primitive

    multi-stage multi-port direct fuel injection is better

    add variable valve timing and variable inlet manifold length technology and THEN you have something cool.
    Multi stage is a fancy name for dual port. Have yet to see a modern engine with more than two ports per cylinder.
    So dual is multi and multi is dual.

    The use of variable manifolds, valve timing and compression does not really influence max power output but ensure maximum power across the rev range.

    If you take an old 16 valve Cosworth BDA it is not to far off the power out put of modern engines with the same capacity,
    Only difference is the Cosworth had all its power at the top end of the range.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by grips View Post

    If you take an old 16 valve Cosworth BDA it is not to far off the power out put of modern engines with the same capacity,
    Only difference is the Cosworth had all its power at the top end of the range.
    undriveable power-band

    The point is that modern head, inlet, and injection tech blows the sox off anything built 10 years ago
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    Default Re: Exhaust back pressure

    Quote Originally Posted by grips View Post
    Multi stage is a fancy name for dual port.
    nope

    multi-stage the way I meant it is where the ECU alters the injection profile several times in one injection cycle, pre- and post ignition.

    You cannot POSSIBLY get that level of fuel delivery with a jetted carb.
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    Default Re: Exhaust back pressure

    Quote Originally Posted by jelo View Post
    undriveable power-band

    The point is that modern head, inlet, and injection tech blows the sox off anything built 10 years ago
    Very true.

    One must remember that tech developed by racers and performance divisions have found their way into production engines.

    It is still amazing to see what companies like Cosworth have achieved 40 to 50 years ago.

    The basics of performance have however not changed over the many years. You need to fill the camber as quickly as possible with the largest volume of air/fuel it can take. Then you need to get rid off that as quickly as possible.

    So in the end it does not matter if it is a carburetor or injection it is about the time and volume of the fill.

    Modern tech is just way easier to control and can make split second adjustments.
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    Default Re: Exhaust back pressure

    the big issue with old school fixed cam timing and stuck manifold lengths was you ALWAYS had a power band effect : a torque cam and tuned headers and inlet manifold would give say peak torque @ 1,500rpm. This engine would NEVER rev.

    Similarly, that Cosworth had to have its clutch slipped so that it could pull away. NOTHING under probably 4,500rpm?
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