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Blower Math

Discussion in 'The Hokey Ass Message Board' started by 39Deluxe, Aug 8, 2013.

  1. Blown35
    Joined:
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    Location:
    Connecticut

    Blown35 Member

    I have a S.C.o.T Blower installed on my 284 C.i. 59A Flathead. With the stock dual crank pulley the boost at 3000 RPM is only .5 lbs and I need to get to 7lbs - so I need to calculate an overdrive pulley dimension.

    Is there a chart that will show me what diameter blower pulley I need if I know the drive pulley diameter?:confused:
  2. mechanic58
    Joined:
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    Texas Titty Twister

    mechanic58 Member

    That ain't much boost...do you know for 100% certain that the blower is good? Half a pound at 3k with ANY pulley arrangement just doesn't sound good to me.

    Edit: if the clearances in the blower aren't just right - it won't make much boost at any speed.
  3. Phil1934
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    Ellijay, GA

    Phil1934 Member

    Thr SCoT blower for the flathead is 4500 cc (274 CI) so 1:1 should put you there. Something else is problem.
  4. solidaxle
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    solidaxle Member

    Get Joe Abbin's book "Blown Flathead"
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  5. Rusty O'Toole
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    Rusty O'Toole Member

    If the displacement of the blower is 274 cu in and the engine is 284 cu in they are about equal. No wonder there is little or no pressure.

    To get 7 pounds pressure you will have to increase the output of the blower 50%. Taking atmospheric pressure as 14 PSI you are after 1 1/2 times that.

    This suggests you need to overdrive the blower 50%. In other words the drive pulley needs to be 50% larger in diameter than the driven pulley.

    These are just rough, back of the envelope calculations. I know nothing about your engine or the SCoT blower. It would be better if someone more experienced chimes in. But it sounds like you need to drive it a lot faster.
  6. metalshapes
    Joined:
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    Location:
    Tucson, AZ USA

    metalshapes Tech Editor

    With a 1:1 pulley ratio the blower would pump 548 Cu In , if it would be running at 100% efficiency.
    ( which no blower does, specially not a roots blower )

    Because a 4 stroke engine needs two complete revolutions to complete a cycle.


    You have a baseline to work off, because the boost is known with the pulleys you have.

    Use those as a starting point, and re-calculate from there.

    If those numbers don't make sense, the clearances on the blower are probably off.
    Last edited: Aug 9, 2013
  7. hoop98
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    hoop98
    ALLIANCE MEMBER

    I doubt the displacement is 274, more likely 1/2 that.

    Here is some info, you need to know the CC's and covert to CID.

    Roger Huntington in his 1950 book Souping the Stock Engine had a reasonable estimating equation:

    R = (A x D)/(K x V) where,

    R is the blower drive ratio (crank pulley dia/blower pulley dia)

    A is the desired manifold pressure in atmospheres (14.7 psia = 1 atmosphere)

    D is the engine displacement in cu.in.

    V is the Roots blower displacement in cu.in./revolution

    K is a constant determined by experience. I use 1.8 .

    For example, assume a 276 cu.in. engine, a 142 cu.in./rev blower (e.g. Weiand 142) and 6.6 psi boost,

    A = (14.7+6.6)/14.7 = 1.45

    R = (1.45 x 276)/(1.8 x 142) = 1.56
  8. Kiwi 4d
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    Kiwi 4d
    ALLIANCE MEMBER

    Hmmm could be onto something here as we had a SCOT on a tame 239 flat motor and went sort of ok. The beefed the motor up to 276 and it would run worth a darn the blower seemd to get in the way , changed nothing only put twin manifold back on and it runs real strong.
    So looks like we need to check drive ratios.
    I think the SCOT is around 172 cu.in/rev according to joe abbin. So to achieve a decent boost we might be looking at nearly 40% overdrive .
  9. hoop98
    Joined:
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    hoop98
    ALLIANCE MEMBER

    Looks like a good guess;

    [​IMG]

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