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Do 2 turbos at the same PSI, in the same setup, make the same power?

Posted by cornboy, Jan 15, 2021

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  1. cornboy

    cornboy Proven Member

    43
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    Joined Feb 28, 2020
    Manhattan, Kansas
    To give background to the question in the title, I have a 1996 GSX with a stock T25 turbo, and a wacky as hell intercooler system.

    I'm pretty sure the turbo is going out, so I figured I'd replace it with a 14B. I ended up getting a VRSF front mount intercooler to make it all play nicely.

    However, my end goal is to eventually go with a Evo 3 16G and make 300-350 crank horsepower. I figured I'd save a little bit of money and just skip the 14B and go straight to the 16G, and just have it limited to about 12 psi, which should be around stock, that way I don't have to necessarily retune my car right away.

    I'm now second-guessing myself, and think that even with only 12 psi, I'd be making a substantial amount more power, which would require new fuel injectors, maybe a fuel pump, and consequently, a new tune.

    Since the two turbos will be flowing through the same throttle body, would there be the same amount of air flowing through to the engine?
    And more specifically to my situation, can I get away with not tuning it immediately, or should I just go the safe route?
     

    Street Build 264  3

    1996 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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  2. 95REGF150

    95REGF150 Proven Member

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    172
    Joined Mar 7, 2012
    Elk Grove, California
    In short No. Different compressors make different power for the same boost.

    That is because different centrifugal compressors can operate at the same pressure at different efficiencies. And that is just the compressor side. With a turbocharger we are also driving that compressor with the exhaust which creates back pressure and robs power. The two have to be matched or else whatever gain you make with a better flowing compressor is not just being robbed right away by choking the motor driving a small turbine.

    So bottom line you will make more power at 12PSI on a E316G than a T25 because the more efficient compressor and larger exhaust housing can flow more air at a lower temperature.

    Now it won't be a huge gain because at 12 psi you are not that far out of the good efficiency range on a t25. The big gains for this principle come when you try to run the turbo far outside of its efficiency. Say you tried to run a T25 at 20psi. It will be choking the motor back and feeding it such hot air that it's not going to make good power. If you then went to an E316G and ran the same 20PSI the gains would be huge. Because you are in the efficiency range of that turbo.

    Hopefully that makes sense. Everybody loves to talk about boost and PSI but it's only one part of the equation when your trying to make power and its definitely not as simple as this boost number = this HP number.
     

    Street Build 2K  0

    1995 Mitsubishi Eclipse GSX
    manual · 2G DSM
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  3. cornboy

    cornboy Proven Member

    43
    4
    Joined Feb 28, 2020
    Manhattan, Kansas
    That actually clears up a lot. Thank you so much.
     

    Street Build 264  3

    1996 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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  4. JusMX141

    JusMX141 Moderator

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    Joined Dec 13, 2005
    Greensburg, Pennsylvania
    To solve this simply and independently, make two modifications:

    Turbo = pump
    Air = water

    Suddenly when the air charge becomes a liquid things change a bit...regardless of the pressure a given pump is generating we know it's not possible for a small pump to flow the same volume of water as a larger one. This also takes into consideration the distance the air must travel in addition to the size of the piping, throttle body, and intake manifold plenum volume. A smaller turbo (pump) must work harder to fill a greater length of piping, larger intercooler surface, or larger intake manifold plenum with pressurized air (water) and keep it full at various throttle points as an increase in engine speed will also increase airflow consumption.


    So jumping straight to the Evo III 16G may be a fine financial move if you plan on upgrading the rest of your mods in the future to support the additional airflow...otherwise the car may potentially "feel" slower than it would with the smaller turbo as there's easily a 1000rpm difference in spool and response between the TB2566 and Evo III 16G.

    Note the first thing you'll need to upgrade, if you haven't already, is the fuel pump. The factory pump is pathetic. Even something like a simple Walbro 190 with the rewire can pretty much support a 16G at any boost level as long as you're not on ethanol.
     

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    manual · 2G DSM

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    1997 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM

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    1992 Eagle Talon TSi
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  5. cornboy

    cornboy Proven Member

    43
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    Joined Feb 28, 2020
    Manhattan, Kansas
    I am indeed planning on doing to supporting mods at a future date. The problem is that I need to do the other necessary maintenance in order to ensure everything is in good shape before making those mods and getting the car tuned.

    This is disingenuous I believe. If we talk about fluid flow through a pipe, then we can define flow rate as Q, where

    Q = (cross sectional area of pipe)*(squareroot(2*Pressure*fluid_density))

    Because the throttle body is the same, and we're using water, whose density is pretty easily defined as 1g/m^3. Because the area and fluid density are the same, the same pressure WOULD result in the same flow rate, regardless of how hard the pump is working.

    This becomes more complicated when dealing with air. Water's density does change with changes in temperature (https://en.wikipedia.org/wiki/Density#Water), its fairly insignificant until we reach a point of boiling the water, which shouldn't be happening if our intercooler is doing its job correctly. However, air is a lot more tricky, since it has a higher change in density due to temperature and is compressible.

    Overall, fluid dynamics is a very tricky subject and while I appreciate the response, oversimplification really can lead to misinformation, which is already incredibly prevalent in the area of turbos.
     

    Street Build 264  3

    1996 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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