Roots boost

Discussion in 'PSI Superchargers Tech Questions' started by mightymouse, Sep 22, 2014.

  1. mightymouse

    mightymouse Member

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    Is a roots blower linear? Finally put on a reliable data logger on car. The boost stays linear with rpm tell around 8500. From 8500 to 9500 it goes from 39lbs to 49lbs. I know boost is a measurement of back pressure so did the blower really pick up at that speed or did the motor become less efficient? Sbc with 15* heads, 10/71 HH 36%
     
    #1
    mightymouse, Sep 22, 2014
  2. SoDak

    SoDak Active Member

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    What kind of overdrive are you running?

    I think as a general statement you could say modern blowers are pretty linear, but when you get to the nitty gritty details there not exact, thus one reason supercharger dynos are important.

    My guess is you surpassed a rotor speed threshold and your air is now really hot creating the extra psi.
     
    #2
    SoDak, Sep 22, 2014
  3. mightymouse

    mightymouse Member

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    Blower speed

    36% over shifting at 9500 is like 12900 blower speed. We do chill are fuel. Going to start monitoring intake temp next time out. It's a kobelco 10/71 superman, be leave it has the k9 rotor So it's not
    The newest out there.
     
    #3
    mightymouse, Sep 22, 2014
  4. turbo69camaro

    turbo69camaro Member

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    That's the nice thing about a SBC boost is almost liner i had a kobeco 14-71 on a 387 ci motor and @ 15% over it had 32 psi @ 7200... 36 @ 8200 42 @ 9200
     
    #4
    turbo69camaro, Sep 22, 2014
  5. mightymouse

    mightymouse Member

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    boost curve

    well, then I am not far off from your boost numbers then, 33lbs@7200, 39lbs@8200, 46lbs@9200.
    Had the quckdata, that rarely worked right, now have a rpm with a lot better data to work off of. Normally don't spin it that hard ,but the higher I spin it the faster it runs. Put a new cam in and learning how it acts too.
     
    #5
    mightymouse, Sep 22, 2014
  6. Mark Leigh

    Mark Leigh Member

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    Are all 8 cylinders firing or are you dropping a cylinder ???
     
    #6
    Mark Leigh, Sep 22, 2014
  7. mightymouse

    mightymouse Member

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    all 8

    all cylinders look good, mater fact the plugs where burnt down 2 threads when I ran it up to 9500 and shifted. Put 10 in the main the next pass to keep it safe, was fighting a slipping converter last weekend at the same time.
     
    #7
    mightymouse, Sep 22, 2014
  8. Mark Leigh

    Mark Leigh Member

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    Your new cam profile might be an issue ---- what happens if you tighten the exhaust lash ?
     
    #8
    Mark Leigh, Sep 23, 2014
  9. Mike Canter

    Mike Canter Top Dragster
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    The answer is NO the boost output of a Roots blower is not linear. Two reasons: FIRST. The injector hat supplies ram air so the faster you go the more boost you have. Look at your boost at 8000 and at 9000 in first gear and record it now go and look at the boost figures at 8000 and 9000 rpm in high gear. Ask yourself why they are NOT the same if the blower is operating at the same rpm in first gear and high gear.
    Second: The Roots supercharger was never designed to supercharge a drag racing engine to 30-plus psi levels. It can supercharge an engine, but not to the degree required in drag racing without the high cost paid in overall operating efficiency. A Roots maintains fairly good efficiency up to 10 psi, but beyond that, its efficiency drastically drops off.
    With the Roots, all of the rotor surfaces "see" all of the air pressure all of the time. The overall efficiency -- mechanical, volumetric and adiabatic -- of the Roots begins at 40 percent and drops off to 20 percent at the end of a run.
    One of the problems with a Roots is that it compresses air by back flow. Air is taken in at the inlet port at the top of the Roots at 14.5 psi -- atmospheric pressure at sea level. It carries this 14.5-psi air in the spaces between its lobes and along the outer cases until it reaches the outlet port at the bottom, 180 degrees later. The air is deposited in the intake manifold where it piles up under pressure. That is where the problems begin.
    The high-pressure plenum air continually wants to flow back into the low-air-pressure rotor spaces. This force tries to turn the rotors, and crankshaft, backwards. It takes a tremendous amount of power from the engine to turn the rotors against this pressure.
    This back flow causes a tremendous amount of air heating. Moving pressure waves of air between the plenum and the rotor spaces at the rate of six cycles per revolution and produces typhoon-like conditions inside the intake manifold. When the air is moved around to that degree, high temperatures are encountered due to friction.
    As a result, air density is decreased, which makes the supercharger's job all the more difficult. (It was fitted to the engine to increase air density in the first place.) To make up for the rapidly thinning air, the Roots has to be turned faster, which really takes a toll on efficiency.
    Like said: The overall efficiency -- mechanical, volumetric and adiabatic -- of the Roots begins at 40 percent and drops off to 20 percent at the end of a run.
     
    #9
    Mike Canter, Sep 24, 2014

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