Roots Blower Performance

Discussion in 'PSI Superchargers Tech Questions' started by Mike Canter, Jan 20, 2012.

  1. Mike Canter

    Mike Canter Top Dragster
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    This is to answer Sune's question on Standard Roots blower performance.

    First let me say that anything that I know on screw and Roots blowers has been learned from Patrick Hale and Norm Drazy. All that I am going to say below about Standard Twist Roots comes from Patrick Hale and I hope I do him justice and I hope I don't mess it up. Also, I will simplify it a little for my benefit. If you want more data then contact Patrick or buy his tuning software.

    Sune, you stated that top fuel turned higher than 9000 rpm. I don't believe that is so. From what I know a top fuel car's peak rpm is in the 8500-8800 rpm range and their OD's are not that high. Take a look at the size of the upper and lower blower pulleys.

    I stated in the other post that the upper rpm limit for the maximum Volumetric Efficiency (VE%) for a standard twist Roots was 9000 rpm on the blower. This 9000 rpm is called "Critical Blower RPM". Again that limit is based on the point that Volumetric Efficiency of the blower will be at its highest. Not saying that the blower cannot turned higher because you can and if you turn it higher it will require a lot more horsepower to do so and the VE% will fall off at around 2% per 1000 rpm above 9000. So at 10,000 the VE% is down 2% and at 11,000 it is down 4%. Now keep in mind that boost will continue to climb and you maybe impressed by your boost figures but that is just not the whole story with a Roots blower. What is really bad about a standard twist Roots is the large opening on the bottom. The faster you turn it and the more boost you make in the manifold and the more pressure applied upwards through that large opening against those spinning rotors. This means more horsepower required to spin them. Changes in injector hat design and shoes have helped in improving the "inlet flow Path" and has increased this Volumetric Efficiency a lot and when everything is designed correctly works great. So with better inlet flow some of that loss in VE% can be made up but the loss is still there in the blower.

    Patrick, I hope that was correct and I didn't abbreviate it too much.
     
    #1
    Mike Canter, Jan 20, 2012
  2. sean70ss

    sean70ss Member

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    Roots

    Mike,

    As for as ET I have no idea being the class rules changed and before the screw blower cars were not allowed. I was told by a few guys that it would be really close with the screw cars and roots based off the % of OD and also who has the latest and greatest as well. Sean
     
    #2
    sean70ss, Jan 20, 2012
  3. sune

    sune Member

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    Ahh now i get it, thank you for clearing that up.

    I misunderstood you, I took it as there was no power to be gained above 9000 blower rpm for the standard helix.

    Ok then here is the next silly question: How come standard helix blowers don't have the little pie hole opening in the bottom instead of the large opening, If it is down to the castings not being meant for it, would it not be possible to just put 3/8 aluminum plate between the intake and blower and cut a pie hole in that?
     
    #3
    Last edited: Jan 20, 2012
    sune, Jan 20, 2012
  4. bandit496

    bandit496 Member

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    So, what you are saying is on a standard helix blower is that you really only want to have the rotors spinning at the 9000 RPM for best efficiency, which would essentially only be about 20 % overdrive, given 7500-8000 engine RPM. Anything over the 20% causes loss in efficiency as well as a loss in net horsepower because of the increased demand to drive the blower, right? So, trying to get bigger boost numbers from the blower itself--given the limiting factor of runner size, length, volume, etc. as a constant--by speeding the blower up is a lost cause (that is my real question)? Or is there a marginal increase in net horsepower (in other words I gain 50 horse but lose 35 driving the blower giving a marginal increase of 15)? At what point do you stop beating the blower and buy a more efficient one (keeping in mind bidgets and all)? I like this site!! JW
     
    #4
    bandit496, Jan 20, 2012
  5. Mike Canter

    Mike Canter Top Dragster
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    Yes it takes more power to make power and at some point you are not going anywhere faster. When the VE% drops then the fuel requirement also changes and you have to account for that so there is a big bend in the fuel curve. So you see the boost go up and think it needs more fuel but maybe not so. These days we have found out that we can help bring that VE% we lost back up a little by improving the inlet air flow. This is why the shoes, airflows, the spacers and the carbon fiber hats have all come about. With the best of everything all melded together you will see VE% change but that may or may not be reflected as boost increase.
     
    #5
    Mike Canter, Jan 20, 2012
  6. 3-D

    3-D Member

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    what about the question about filling up the back opening of a standard discharge blower and only having the front opening? does that help?
     
    #6
    3-D, Jan 24, 2012
  7. Mike Canter

    Mike Canter Top Dragster
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    Don't know the answer to that but it it helped it would have already been down by the top fuel guys.
     
    #7
    Mike Canter, Jan 24, 2012
  8. WIZBANG

    WIZBANG Member

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    On the bottom, I think filling the back of the blower case isn't as important as filling it with the correct shape , like a vane , to allow the rotors more scraping surface , maybe ?
     
    #8
    WIZBANG, Jan 24, 2012
  9. 3-D

    3-D Member

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    I don't know about the fuel guys, but they have the blower set back a long way. it would seem that the front discharge on the blowers makes more boost, why would not filling that void at the rear of a standard blower, and not allowing air to disrupt the roters at higher boost be better? I am no expert. if you closed that off and set the blower back 7-8" for discharge in the center of the manifold, would that not be better?
     
    #9
    3-D, Jan 24, 2012
  10. Money Shot

    Money Shot Member

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    I would agree with this statement. The more contact the rotors have with the case the more boost the blower should make. However, It will take more power to turn because of friction..... Yeah....What he said. :D
     
    #10
    Money Shot, Jan 26, 2012
  11. Mike Canter

    Mike Canter Top Dragster
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    Can't visualize that with a standard twist rotor because it is not pulling as much air to the front on the bottom like a high helix does. Remember the air does not go down in between the rotors it goes on the outside of the rotors. Like I said the Top Fuel guys would have done it if it gave them anything. All they would have to do is put a standard twist rotor into a HH case and they we still be legal.
     
    #11
    Mike Canter, Jan 26, 2012
  12. 3-D

    3-D Member

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    do you think it is detrimental to do this?
     
    #12
    3-D, Jan 26, 2012
  13. Mike Canter

    Mike Canter Top Dragster
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    Yes I do but that is a guess because I have never tried it but like I said I bet somebody has. If it worked you can't keep secrets long
     
    #13
    Mike Canter, Jan 26, 2012
  14. nitrowannabe

    nitrowannabe Member

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    Nostalgia Fuel and Funny Car run 6-71 standard rotor in a retro case, less than 20% overdrive but 10,000 rpm. Seems to work for them.
     
    #14
    nitrowannabe, Jan 26, 2012
  15. craig moss

    craig moss Member

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    I had a Kuhl 14 std billet rotors with a small delta it worked ok but still went to the hh retro then it was a diff ball game
     
    #15
    craig moss, Jan 26, 2012
  16. SoDak

    SoDak Active Member

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    I have pics of outlets of top fuel blowers taken at races and at least this one is a delta opening. In my mind if done correctly (to match the std twist) it will work. But I've never done and have no reason too, we can run HH.
     
    #16
    SoDak, Jan 27, 2012
  17. bob szabo

    bob szabo FC / altered

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    some whys

    TOPSIDE TORNADO
    Fuel flings off the rotors. With a forward bottom opening, much of it flings down. In the rear where there is no opening, it flings up. Filling at a rear opening is interfered by this fuel fling. So moving the top opening forward helps inlet flow by a reduction of interference from the fuel.

    BOTTOM OPENING
    Regarding the bottom opening, the delta opening restricts the bottom opening so that compression occurs in the blower before the contents are released into a manifold at boost. The ideal target would be a delta opening small enough to cause blower compression to match manifold boost. in that event, there would be no reversion of manifold boost back into the blower cavity that would need to be pumped out. In fact, Whipple told me that is what he did with his screw blower development for top alcohol was change the size of the bottom opening to adjust the blower boost to match what the racers were doing. However, more recently SSI determined that delta openings restrict mass flow out the blower at high blower speed/mass flow, and square bottom openings generate more mass flow on their blower dyno.

    GOOD OR BAD
    Changing the fuel split from the manifold to the hat can change the fuel fling and blower inlet condensation. Both of which can make one upper opening work better for one setup and worse for another. Likewise they can make one bottom opening work better in one setup and worse in another.

    Bob Szabo
    www.racecarbook.com
    www.airdensityonline.com
    016 419 6649
     
    #17
    bob szabo, Feb 3, 2012

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