when is boost a restriction?

Discussion in 'PSI Superchargers Tech Questions' started by jdgmntl1, Dec 6, 2003.

  1. jdgmntl1

    jdgmntl1 New Member

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    Me again guys!
    When does boost become a measure of restriction?
    I know it's not an alcohol car but we traded heads on a buddy's mustang for him and found that even though horse power on the dyno went up measured boost had dropped by 2psi. Also noticed that two alcohol cars with nearly the same combo for power(blower same bottom end the same but one with ven heads that were 2.500 and one with 2.600's the 2.600's ran away at the top end (newer bigger better) the boost gauge was noticeably lower an was verified when I asked if their racepak was the same and and it was. If the heads are slowing the velocity of the air why do so many try and spin the blower harder to bring up boost when in reality it may just be eating up horsepower to make a bigger number. I'm new to this so please don't bite my head off, just trying to learn to go fast. JDD
     
    #1
    jdgmntl1, Dec 6, 2003
  2. andy wilfong

    andy wilfong Active Member

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    I explain this very same things to alot of customers, first of all when we measure boost, we are only measuring what is in the manifold not in the cylinders, which would be the ultimate thing, for example, if you ran a motor on the dyno with onported heads and say it had 25# of boost, than took the heads and ported them, the motor would flow better, so your boost would drop but the power would go up. hope this helps!
     
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    andy wilfong, Dec 6, 2003
  3. Bill Naves

    Bill Naves Member

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    to give you another angle. if you break a rocker and can't fill a cylinder the boost goes WAY up.
     
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    Bill Naves, Dec 6, 2003
  4. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    like andy said, boost is not eating up the power. with a roots, you will reach an inefficiency point where it is taking more hp to turn the blower than the extra overdrive is generating 'extra' boost. the old 'point of deminishing returns' concept.

    with a psi, you don't have that problem, so the harder you can turn them, the more boost the better. this is all things staying the same. when you start comparing apples to oranges, boost may change and hp may appear to be at some degree of negative correlation, but like andy pointed out, the real cause is in other areas.

    in fact, steve boggs, and many others argue nhra went the wrong way when trying to make tafc a cheaper class by lowering the overdrive from 125 over to 92 over. he argued that they should up it to 150. why? one of the big things on the psi is making the air go supersonic. at 125 over, that happened at roughly 9500 rpm. randy goodwin will tell you they tried turning it 9800 and it didn't run any better than when they turned it 9200. slow the blower down to 92 over and now supersonic is up around 11,000. so its whoever can rev it the hardest. needless to say how much cheaper 9200 is than 10,500 on a big stroke motor like a tafc. hence, bogg says at 150 over, supersonic could be reached at 8500.

    another angle on boost and blown alky combos is boost per cid vs cid. many claim, and i am of this belief myself, that it is more important to have more boost per cid opposed to cid. thats why we run a 451 motor in jason's car when we could run up to a 466 at the same weight. the way conway explained it to me is you are putting 2000 cubic inches of air into a 450-460 ci motor.... the bigger the hole you put that into, bigger volume hole with the same volume of air = less pressure, or boost. he also explained to me there are frictional losses associated with bigger cubic inches.

    i would venture to say that boost is probably the single most important factor on a blown alcohol motor.

    hope this helps.
     
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    Will Hanna, Dec 6, 2003
  5. DHE

    DHE New Member

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    Will,I agree that boost is the most important factor in a alchohol motor,but I will guarantee you that a 466 cu. motor is faster than a 451,both having the same overdrive!It is a physical law.The only reason to have a smaller motor is the weight to cubic inch ratio used a few years ago.Now that the minimum weight is set there is no advantage to run a smaller motor.

    Thanks for a GREAT site!
    Dale Hill
    DHE Racing Engines
     
    #5
    DHE, Dec 7, 2003
  6. Serge TAD 182

    Serge TAD 182 New Member

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    Will...

    It will be great to get a answer to my personnal question relate to the HP/TQ of a 451 cu in Dragster engine. I ve sent you a personnal E-mail and did not received any feedback. Do not want to know your personnal combo , just need a approach to the HP/TQ curve between 7000 and 10000 rpm .

    Thanks !!! Serge (Frenchy Canadian)
     
    #6
    Serge TAD 182, Dec 7, 2003
  7. andy wilfong

    andy wilfong Active Member

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    sometimes more boost doesnt equal more power, there comes a point that the manifold temp, will hinder the torque curve of the motor, so spinning the blower faster to make more boost isnt always the answer.
     
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    andy wilfong, Dec 7, 2003
  8. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    mr hill -

    5.26 duane shields 455 ci
    5.27 david wells 451
    5.27 rick santos 451
    5.29 marty thacker 451

    it could be a case where we are both right to an extent... the reason why the 451, which in my opinion is a better motor may be a bore/stroke/rod length relationship that is happier than the 466. i really dont think there is that much difference anyway. you are right, ci was much more important back in the weight per cid days.
     
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    Will Hanna, Dec 7, 2003

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