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Blower Math

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

  1. Blown35
    Joined: May 20, 2008
    Posts: 166


    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: Mar 21, 2010
    Posts: 682


    That ain't much 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
    Joined: Jun 24, 2001
    Posts: 2,612


    Thr SCoT blower for the flathead is 4500 cc (274 CI) so 1:1 should put you there. Something else is problem.
  4. solidaxle
    Joined: Jan 6, 2011
    Posts: 251

    from Upstate,NY

    Get Joe Abbin's book "Blown Flathead"
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  5. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 7,865

    Rusty O'Toole

    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: Nov 18, 2002
    Posts: 10,632

    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
    Joined: Jan 23, 2013
    Posts: 1,362

    from Texas

    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

    V is the Roots blower displacement in

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

    For example, assume a 276 engine, a 142 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
    Joined: Sep 16, 2006
    Posts: 1,861

    Kiwi 4d

    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 according to joe abbin. So to achieve a decent boost we might be looking at nearly 40% overdrive .
  9. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    from Texas

    Looks like a good guess;


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