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Dumb Blower Question

Discussion in 'The Hokey Ass Message Board' started by 345 DeSoto, Mar 29, 2010.

  1. Isn't the pressure you see on the boost gauge evidence of the compression that is going on in the intake? In most cases it's low comression but it's still compression!
  2. Deuces
    Joined: Nov 3, 2009
    Posts: 22,034

    from Michigan

    You gotta get a boost/vacuum gage. I have one in my car just for checking the vacuum readings for idle and cruising speeds. :)
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 51,241


    Haven't had a boost gage on mine since I pulled the motor out of my truck and put it in my car 14 years ago. Put a lot of miles on without destroying the engine, and it runs ok at the strip (as if it's putting out about 750 hp at the flywheel), so I guess neither one of your points is valid?

    A roots blower with a toothed belt drive can only make so much boost, and it's determined by the relative displacements of the engine and blower, and the drive ratio.
  4. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    The first thing you need to do is know what boost is.

    Boost is unused air. Your engine makes boost when the air in the manifold piles up and becomes more than the engine can process.

    What determines this?

    Several things. How big of a blower do you have? (6-71 in your case)How good are your heads? What are your cam specs? How good is your manifold and what are you using for induction? How much is the blower overdriven?

    To measure your boost you want to be at full throttle above say 6K. Before that there really isn't much if any boost.

    So on a street car that's cruising at part throttle most of the time you rarely see boost at all.

    On a competition engine most cars leave above 4k so the blower works almost instantly.
    (Yes if you are at idle and stomp it boost will climb starting just above 3K) I guesss that would be the simple answer, but I went a bit further.

    If you have the luxury the best thing you can do for yourself is dyno your engine. After you get it "tuned" build a boost profile.

    To do this you need several pulley combinations. You want a good cruise set up and something you can run at your local track.

    While it's common to run 5 to 10% under on the street swaping the pulleys to 5 or 10 over isn't always the best so trying different ratios will give you your best results.

    For example on a 355 sbc if your trying to be conservative and go from 10 under to 5 under you'll see a noticeable differece, but 5% under to 1:1 can drop your peak HP.
    Last edited: Mar 29, 2010
  5. So I guess I wasted all that money, on a blower for my engine...:rolleyes:
  6. Rob Kozak
    Joined: Aug 18, 2005
    Posts: 442

    Rob Kozak

    No you did it to be cool and being cool is neither cheap nor easy.
    There is no better power adder on a hot rod than a blower, you just need to learn about it a little more.
    I said Hot Rod. The best power adder in my oppinion is a turbo.
  7. krooser
    Joined: Jul 25, 2004
    Posts: 4,584


    If you overdrive the blower you'll have boost way down low...makes a car fun to drive in traffic with lots of torque. I don't have my hemi running but mine is overdriven about 10% as it's set-up right now... ya gotta start somewhere. Even running it 1:1 it will give you a kick in the pants as soon as you open the throttle...that's the key to boost. The throttle has to open before the blower gets enough air to operate.. once open the power in lineal... just keeps going without any lag.

    Boost is boost regardless of it coming from a turbo, blower or your ex-girlfriends sweet lips...

    Jimmy blowers, 6-71's, came in two different displacements...large bore and small bore. I'll get that cfm info asap...
  8. madman customs
    Joined: Feb 24, 2009
    Posts: 26

    madman customs
    from odanak

    i have 4 blower cars and love than for any question on blower call big al's toybox he is the blower guy 1 860 354 3237
  9. The boost that you see on the gauge is the air being compressed in the manifold because more air is coming in than the motor can use.The air coming out of the blower into the manifold isn't compressed, it is just being forced into the manifold faster than the motor can use it, that is why you see boost. It is also fairly linear with a positive displacement blower. Once you get into boost, again usually between 2500 and 3000 rpm on most street set ups you will be pretty close to full boost from then on, only building slightly more as the revs go up.
    Centrifugal blower and turbos actually compress the air and they continue to build more boost as they build RPM.
  10. BigChief
    Joined: Jan 14, 2003
    Posts: 2,084


    yeah....instantly facing the traffic behind you will definitely get your attention.
  11. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,811


    Thank heaven somebody else finally said this. In order to move air, the blower first has to have access to air.

    Let's not forget the twin screw supercharger, which also compresses air like the centrifugal superchargers and turbochargers. These three types also tend to create less heat than a roots supercharger.
  12. Does the power band of a blower not depend on the specs of the cam (blower cam, duration, rpm range, etc?) The over/under ratio of the blower is relative to the crank, yet the valves still must open and close while the blower forces more air/fuel mixture creating an instant changes in compression and power? Straighten me out, this is a good read!!

    I road in my dad's 671 blown coupe several times, and whenever he stabs the loud pedel, it was an instant and violent ride like nothing else I've experienced.

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