Roots max rpm

Discussion in 'PSI Superchargers Tech Questions' started by racecar1517, Aug 7, 2017.

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  1. racecar1517

    racecar1517 Member

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    What is the recommended max speed for a hh1471 on methanol?
    Thanks ITA
     
    #1
    racecar1517, Aug 7, 2017
  2. Mike Canter

    Mike Canter Top Dragster
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    The Volumetric Efficiency of a High Helix roots type supercharger drops off sharply at 14,625 rpm. So if your engine high Point was 10,000 rpm you would have to have a 46% overdrive to reach it. It is at 9000 rpm for a standard helix roots type supercharger
     
    #2
    Mike Canter, Aug 7, 2017
  3. Bjs344

    Bjs344 Member

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    I always thought 13 or maybe 13,200 was the magic number?
     
    #3
    Bjs344, Aug 8, 2017
  4. Mike Canter

    Mike Canter Top Dragster
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    Nope, it is 14,625 rpm
     
    #4
    Mike Canter, Aug 8, 2017
  5. racecar1517

    racecar1517 Member

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    Were always on the conservative side. We have been under 12500.
    Thanks Mike
     
    #5
    racecar1517, Aug 8, 2017
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  6. Mike Canter

    Mike Canter Top Dragster
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    You still have the problem of the faster you turn a roots blower the more it heats the incoming air charge and the more horsepower it takes to turn it. Meaning that you may actually be losing power by turning it too fast below that 14,625 rpm.
     
    #6
    Mike Canter, Aug 8, 2017
  7. racecar1517

    racecar1517 Member

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    Ok, here's the issue behind the question. I've been working on increasing the fuel to the motor. Thinking if I feed it more it will go faster. I have gotten it from 10.2 gpm to 12 gpm at 8500rpm, but the et is the same! So next was to add more air.
     
    #7
    racecar1517, Aug 8, 2017
  8. Mike Canter

    Mike Canter Top Dragster
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    A biggest percent of ET is traction off the line to 330 ft. Increasing power to the track is the fastest way to improve ET. So how boost are you making at 8000 rpm and how much fuel is going into the motor at 8000 rpm
     
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    Mike Canter, Aug 8, 2017
  9. racecar1517

    racecar1517 Member

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    Boost at 8000 32.68. Flow 11.10.
     
    #9
    racecar1517, Aug 8, 2017
  10. Mike Canter

    Mike Canter Top Dragster
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    That is pretty well what I would flow
     
    #10
    Mike Canter, Aug 9, 2017
  11. jay70cuda

    jay70cuda Well-Known Member

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    2.94 boost/fuel ratio. I'd say so!!!
     
    #11
    jay70cuda, Aug 9, 2017
  12. bob szabo

    bob szabo FC / altered

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    Just an FYI: Methanol is not a monopropellant fuel. It burns at the stoichiometric air to fuel ratio of 6.45 to 1. Any amount of methanol beyond that ratio (ie smaller number) does not combust. Some of it forms aldehydes. That is what burns your eyes. As far as I know, the formation of aldehydes from excess methanol do not contribute to power. The remainder goes out the pipes as raw fuel. It may be vapor or droplets.

    If you are adding fuel in the high end and the car is running faster, you may be too lean from ram air. But once you are delivering enough fuel to burn at the stoichiometric air to fuel ratio, plus what ever extra is needed to cool the combustion chamber to keep it below the self ignition temperature before the spark plug ignites the mixture, any extra remains there throughout the combustion and exhaust cycle. In those cycles, the extra inhibits power by absorbing heat and occupying space that increases pumping losses. Our book, 5000 HP on Methanol has about 6 chapters that expand on that explanation.

    Bob Szabo
    bob@racebook.com
    https://racecarbook.com/store/
    look up 5000 HP on Methanol for more info for this thread.
    https://airdensityonline.com for free 3 day AD forecast and ProCalc videos
     
    #12
    bob szabo, Aug 11, 2017
  13. Mike Canter

    Mike Canter Top Dragster
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    Bob, just to make sure everyone understands this that if you run a blown alcohol car at an AFR of 6.45:1 you will probably have to go and pick up your crankshaft off the track.
     
    #13
    Mike Canter, Aug 11, 2017
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  14. racecar1517

    racecar1517 Member

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    Wow, I feel more lost now!
    I have been lead to believe a a/f ratio should be between 3-1rich and 4-1 lean. What does the boost to fuel flow ratio have to do with anything?
     
    #14
    racecar1517, Aug 11, 2017
  15. Mike Canter

    Mike Canter Top Dragster
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    Stay with those AFR numbers and you will be good.
     
    #15
    Mike Canter, Aug 11, 2017
  16. jay70cuda

    jay70cuda Well-Known Member

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    Boost divided by fuel. Every 1000 rpms. But only on a clean pass. 2.6 is real rich
    2.7 is nice and happy
    2.8 is starting to go lean
    2.9 is lean.
    3.0 is picking up parts.
     
    #16
    jay70cuda, Aug 11, 2017
  17. Bjs344

    Bjs344 Member

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    I would be skeptical about using boost/fuel. I swapped out my 584 short block for a 493 a couple years ago. Boost went up by and I had to take a couple numbers out of the main to make it run.
     
    #17
    Bjs344, Aug 13, 2017
  18. TOL

    TOL Active Member

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    I think there are 2 things going on here at the same time, which might be causing confusion???

    Some people use boost/fuel ratio as an "approximation" of the resultant in-cylinder air/fuel ratio. Nothing wrong with that, but you can't universally use the same boost/fuel rules of thumbs for all engines and all supercharger/compressor types or even all fuel delivery types or blends of fuel.

    Some people use means to actually measure, or predict, the true air/fuel ratio within the chamber. What is the desired ratio? It depends upon the engine, and supercharger/compressor type, and of course the fuel delivery type and fuel blend.

    Both work given sufficient experience with a certain combination. The end goal is really to know the later, with repeatability.
     
    #18
    TOL, Aug 13, 2017
  19. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    TOL beat me to the punch.

    There is no universal boost to fuel ratio. While we can say with a fair degree of accuracy what fuel FLOW is due to calibrated flow meters, what we do not have an accurate measurement of air FLOW because we are using manifold pressure aka boost instead. You may have 1000 cfm air flow (nice easy number) but a number of factors such as CID, cylinder heads, and camshaft grind, where it's installed, along with air temp at a given boost all affect what the boost will read with that same 1000 cfm of flow coming out of the supercharger.

    It is certainly a good frame of reference, but it's not gospel.

    I have tuned cars hauling ass in the 2.6 zone, and I have tuned cars that hauled ass in the upper 3's. Both were not hard on parts either.
     
    #19
    Will Hanna, Aug 17, 2017
  20. Mike Canter

    Mike Canter Top Dragster
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    Will, that all depends on how you are computing AFR. There are some fairly accurate ways of doing it and some inaccurate ways of doing it.
     
    #20
    Mike Canter, Aug 17, 2017
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