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Resolved Pre turbo intake pipe size vs velocity

Posted by ec17pse, Feb 25, 2020

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  1. ec17pse

    ec17pse Freelancer

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    Im about to make my pre turbo pipe and ditch my injen intake. I understand the taper helps flow but not sure if i need to add a taper in my new one or not apart from the silicone coulper.

    So my current idea is to use 2.5" tube and then use a 2.5" reducer coupler for the cover.

    Is there a need to go bigger like 3" for the pre turbo pipe? I just have no idea on flow or velocity and its effects.

    Has anyone got data on this that they would like to share so i can build this right

    Thanks
     

    Road Race Build 10K  10  55

    1997 Mitsubishi Eclipse GST
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  2. llxkevinxll

    llxkevinxll Proven Member

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    I don't think it's about flow pre-turbo. But more about a lack of restriction.

    A turbo compressor map y-axis is "pressure ratio". Which is the ratio of discharge pressure over inlet pressure. You want the inlet pressure to be as high as possible. The higher the intake pressure the less the turbo has to work to make your desired manifold pressure. You can't raise the pressure of the atmosphere of course, but you can reduce the amount you are lowering that pressure guiding and filtering the air to your turbo air inlet. Things like an air filter being restrictive or improperly sized and too small of an intake tube will cause pressure drops below atmospheric in the air entering the turbo. I don't think flow is necessarily the concern but more avoiding bends or obstructions that will cause pressure drops. In summary, best case is the biggest, smoothest pipe you can accommodate and as large and unrestrictive of an air filter as you can. Flow per unit area of an air filter goes down as the filter surface area increases so the safest bet is go as big as you can to avoid restriction.

    Gale Banks has covered this pretty extensively. I'll link some videos below.



     

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  3. ec17pse

    ec17pse Freelancer

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    I read about just opening it up and then found conflicting info that it slows the air down if its not tapered or channeled into the compressor properly and creates turbulance which is bad for the wheel.

    Those videos help to see the idea of opening the intake up but thats for the head right? Im not firmikier with cummings setup and whats where.
     

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  4. ishnish

    ishnish Proven Member

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    I think that makes most sense ec17pse. It's basically Bernoulli's principle in that once the piping tapers gradually down, you go from an area of high pressure (larger diameter pipe) to low pressure (smaller diameter pipe) in which the air speed is higher in the low pressure part of the pipe (smaller diameter).

    And this is just me, but I'd think that if you had a reducer coupler that greatly decreased the diameter without a smooth transition, the area of which that wall is where that drastic transition takes place would result in high pressure, more turbulent air. That smooth, fast flow would decrease by a lot I'd assume.
     

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  5. ishnish

    ishnish Proven Member

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    Here's an exaggerated illustration of what I tried to explain earlier described as vena contracta. Notice the turbulent areas and the potential loss of flow from a very large diameter down to a very small. Much more exaggerated but you get the point.
    Flow-separation-and-vena-contracta-in-sharp-edged-entrance-to-tube-31.png
     

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  6. bastarddsm

    bastarddsm Proven Member

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    3-7* taper from inlet to comp housing would be best.
     

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  7. Mech Addict

    Mech Addict Proven Member

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    A consideration also is under what circumstances you want best performance. It should go without saying that all the other restrictions in the intake tract are meaningless if you don't have the throttle wide open, which is generally a small fraction of the operation for most cars, unless they are pure racing, particularly drag cars. Large cross-section (diameter) short path is often good for high-rpm, but Chrysler and others made some very impressive gains in usable torque in other parts of the power band by making longer intake tracts (non-turbo). Look up the long-ram intake of the 60's. If you want to experiment and have a way to test the results, then a sort of informed trial and error is one approach (expensive or time consuming or both, though). Otherwise, it might be best to use a setup that has already proven successful by many users. I'm always amazed at the power levels that folks are claiming. Not saying they're wrong, but wow; 400, 500hp? More?!!! Really (from 2 liters)? And the car stays together? And is drivable (starts? idles? part-throttle response OK?). Cramming more air into the cylinder and compressing without detonation it is the most witchcraft part of output, though. Other elements are more straightforward to understand, though not necessarily easy to obtain, like fuel flow or mechanical strength. An engine is essentially an air pump. Generally smoother transitions in diameter or bends are less restrictive (more flow, same thing).
     

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  8. bastarddsm

    bastarddsm Proven Member

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    thats mostly correct, but in our case the intake pipe it is so far upstream it is unlikely that you will see much for gains from this. Since the turbo inducer is the choke point on a smaller setup, best performance will be gained by presenting the turbo with the highest pressure, straighest, coldest flow. velocity will be a secondary effect, and usually works against pressure.

    And yes, my car for example makes 7-800, and I drive it regularly. Really the only pain is the dogbox. if it had a synchro trans in it, it would mostly drive like any normal car.
     

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  9. ec17pse

    ec17pse Freelancer

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    I think my current coupler for this project is 2.5" to 2.25" as its a direct fitment for the evo compressor cover. I shall look into the degree of it and it might mean i can go 3" to 2.5" then coupler, i will see what i can do on the taper side to help the pressures.

    Good info so far. I know for my current situation its likely not a huge factor but its something i would like to try do well and then i know for the next version
     

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  10. ishnish

    ishnish Proven Member

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    If the inducer is a choke point, wouldn't providing it with high pressure, dense air be the exact opposite of what you want to do? I really am just wondering. Still learning here LOL
     

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  11. Mech Addict

    Mech Addict Proven Member

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    Not too many cars out there making 700+hp. Must be something to romp on it! Of course it is actually torque that breaks stuff; maybe you're turning 15k rpm?;)
     

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  12. ec17pse

    ec17pse Freelancer

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    Well just checking back in as i finally did it and i opted to do smaller shallow reducers along the length of the pipe, so i started with 3" and tapered it down to 2.5" then i used a reducing 45° coupler thats 2.5 to 2.25" for the 16g so its all shallow and hopefully works well.

    IMG_20200325_183627.jpg
     

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