Why have a plenum?
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  1. #1
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    mistareno's Avatar
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    Default Why have a plenum?

    Ok techeads,

    I have noticed lately that several new MPFI 4 cylinders have little or no Plenum Chamber.

    I saw an underbonnet picture from a Turbo Focus and it had what looked like an old style carby manifold with a set of injectors (in the normal position) and a TB mounted to the top of it.

    My question is, what function does the Plenum preform?

    I was of the understanding that it was the "bigger the better" as far as plenums went.

    After having a think about what happens on the intake side of things, I really couldn't work out what the purpose of it was.

    As long as the intake runners are of the same length what other benefit is there?

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    I would have thought that the shorter the path from TB to the start of the intake runner, the better. I thought a large traditional style plenum would create a slight throttle lag. I thought the best in this aspect would be the carby style manifold.

    Anyone know what the go is, and whether the Focus Turbo does indeed have no plenum???

  2. #2
    1000+ Posts dino's Avatar
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    I m not sure if this will help but have a read.....ceartinly interesting reading re.
    intake induction....

    hope it helps...

    http://www.grapeaperacing.com/GrapeA...ionsystems.pdf


    it is pdf....but very small and does not take long to download...


    cheers


    dino


  3. #3
    Fellow Frogger! winnie's Avatar
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    Plenums are a link in the reasonably complicated 'tuned' intake. Bigger is not necessarily better, it depends on engine capacity, rpm where you want to make power, forced induction or NA etc...
    Plenums are used to slow the induction air (Bernoulli tells as this increases pressure) and provide a volume of air "at rest" for the final induction runners to draw from. It also reduces the peak intake air velocities up stream of the plenum - same average but lower peaks if you know what i'm trying to say.
    IMHO (with limited practicle experience) putting a plenum between the throttle body and the intake runners is bad because the same characteristic that reduces the velocity peaks will also produce a soft throttle response, however for a single throttle body it may improve peak power slightly. The solution i like is 1 throttle body / cyl drawing from a plenum.
    Lots of factory turbos do not use a clearly defined plenum for whatever reason the factories have, but i know a few drag racers with turbo engines that go to pretty great lengths to get a plenum of 2 x capacity in the engine bay.
    Last edited by winnie; 8th June 2004 at 09:54 AM.

  4. #4
    1000+ Posts Luca's Avatar
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    Good article that one....

  5. #5
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    I have actually read that article before.

    The problem is, most of the examples are for N/A vehicles (negative pressure).

    When in a positive pressure environment (turbo/supercharging, alot of that theory seems to go out the window...

    Perhaps this is why some of the newer turbo cars have reverted back to a freer flowing plenumless design. A turbo engine by it's very nature usually has oodles of torque but poor throttle response.

    By removing the plenum, perhaps they are also slightly reducing throttle lag.

  6. #6
    1000+ Posts Luca's Avatar
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    Turbos rely more on air speed. The Rajab's did demos and found a fully polished and ported vl head had no difference in dyno figures with the stock head.

    Good point about the more torque/throttle response though.... let me consult the crystal ball.....

  7. #7
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    someone correct me if i am wrong, but :

    plenums are used on a NA tune car to increase volumetric efficiency to near 90-105% VE for a great deal of the rev range (due to long runner inertia on the intake valve).

    since turbo cars are under boost for most of the time then a plenum/runners are not really required. like you said, why bother putting more LAG into a turbo system when all you need is a single t/b and some way of distributing the air to all cylinders.

  8. #8
    1000+ Posts dino's Avatar
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    I suppose it depends on the many constraints imposed upon the engineers....
    On the street a lot of the engeneering practices get thrown out the window if all you r chasing is peak power, power across the rev range or just bottom end......Whereas the manufacturers have to comply with various emission controls, economy demands and so on.....It must be a hard compromise and I wonder how many designers take into account future motorsport use, possible factory performance options (ie a crap manifold on stock makes it a lot easier to design a SPORT option...???)
    As for VE...I dont know.....it seems that many try to compesate with inlet manifold for a POOR head design....again not sure...an example of this would be the first EFI commodore....apparently this manifold (VK????) was a great design and i guess it would have had to be considering head was what ? 20 years old???
    I m not sure how many makes do this ie supply one manifold for NA yet another for FORCED induction....surely that should be the case?....Should the plenum/runners vary when using small turbo/fast spin-up versus big turbo/plenty of lag......then there is the sequential issue....
    All very complicated when u think about it......I m guessing they try and keep construction (and r/d) to minimum cost and are letting the sophisticated electronics DEAL with most fueling issues....
    Metal versus plastic....long versus short.....heated versus cold....economy versus performance.......absolutely amazing......
    We can at least take comfort knowing that french vehicles tend to be equiped
    with some great induction setups....think 205gti manifold, mi16,etc etc



    cheers

    dino





  9. #9
    1000+ Posts Luca's Avatar
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    Remember they changed the 205 manifold to plastic by the end of the run Dino.... changing atomisation, length and positioned the oil filler cap in between the 2 runners.

  10. #10
    1000+ Posts dino's Avatar
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    + diff. ecu and metering.....call it progress...call it new emission rules....god knows......us aussies still miss out on poorer quality fuels....so a lot of the quality design ends up being less efficient...especially on those 40 degree days....ie...i m not sure if the aussie spec cars always end up as optimum tuned for the environment...


    cheers


    dino


  11. #11
    1000+ Posts Luca's Avatar
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    U've got a point....

    Anyway after consulting the crystal ball i've come upon the knowledge that the forced induction cars don't calculate runner length as they rely on air speed. And 6 inches of runner length when the air is travelling at 10 meters per second equates to SFA.

  12. #12
    Fellow Frogger! AlsPug504's Avatar
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    The plenum chamber in normally asperated cars effectively acts as a dam. If the air is slowed down before entering the runners. The absolute pressure rises. This is an advantage to the engine, as when a cylinder is in the induction stroke it draws in a certain amount of air from the dam.

    The bigger the (dam) plenium is relative to cylinder size The less pressure drop occurs across it. That leaves a greater head pressure from which other cylinders may draw. It is important that the pressure behind the tunnels be kept as close to ambient as possible or above as it is that pressure that causes the air to flow into the evacuated cylinder.

    This also can also be multipled by the area across the mouth of the tunnel and so by having bell mouth or in the case of a carby a stack the area is effectively increased and so to is the force applied to the air in the runner.

    The other function, is that in pipe tuning applications. The plenium acts as a Helm Holts resonator. This can be used to broaden the rev range through which pipe tunings effect occurs.

    On rotary engines like the 12a the plenuim is small and the pulses reflections of each rotor are used to the other runners advantage and the plenium size would either dampen or enhance this function dependant on its size.

    Als

  13. #13
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    Don't bedeuten Sie "Helmholtz"?

    Der Stu-meister

  14. #14
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    W.t.f ?

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