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How much HP will 3-5 psi of boost make?

Discussion in 'RD's Lounge' started by skav, Aug 30, 2011.

  1. skav

    skav Well-Known Member

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    I'm curious how much HP is created by a pound of boost in a typical Whipple system.

    Say you have a 550ci motor w 8:1 compression and you add a Whipple 5.0 efi system. How much HP is created by each pound of boost? Is it linear? Ie 1psi=50hp, 2psi=100hp...

    What about changing from 4psi to 8 or 9psi?


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  2. Racey

    Racey Maxwell Smart-Ass

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    15 psi would double the potential hp of the motor, as atmospheric air pressure is ~15psi, so adding an additional 15psi of boost is actually 30psi of Absolute pressure, twice the air. Engines make hp based on how much air and fuel they can run through them, and how much energy can be extracted from each pound of air that runs through the engine.


    So 5psi would be about 33% increase in power, 10psi would be 66% increase, 15psi would be 100%. This is very basic rule of thumb for estimation, but it comes very close to predicting power.

    Bottom line, X psi =/= X hp
    X psi = +% of base HP
     
    Last edited: Aug 30, 2011
  3. Boozer

    Boozer Well-Known Member

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    In a perfect world that would be the case but there are a lot of additional things that have to be taken in to account to come to a real world number. For starters adding any type of engine driven forced induction system will rob power from the motor to power the system. Increased intake temparatures will reduce the amount of power the engine is able to produce. The other thing to keep in mind is that no 2 engines are the same. You can take two factory 525's slap the same exact mods on them with the same exact tune and produce very different dyno numbers between the 2 engines.
     
  4. DaveH

    DaveH Well-Known Member

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    This is exactly correct. The only thing i would add is that using this math, you are not taking into account any loss from turning the supercharger, or in the case of a turbo, back-pressure created. the point is, no system is 100% efficient. so even at 15 PSI where you would "double" horsepower, in reality it will be less than that due to the energy required to run the supercharger.

    We see this all the time in REAL WORLD application on the dyno. displacement, manifold pressure, RPM and volumetric efficiency are simple rules of physics that dictate horsepower.
     
  5. Ziggy

    Ziggy SlumLord

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    :smackhead
    I guess you skipped right over the part where Racey said "its a very basic rule of thumb for estimation".
     
  6. Tom Brown

    Tom Brown Epsilon contributor

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    Racey presented some basic math as a rule of thumb, not absolute engineering. In that regard, I think it answers the original question brilliantly.

    Are you guys trying to provide information or make yourselves look smart?
     
  7. Fury25

    Fury25 Well-Known Member

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    Tom,

    how is anyone to take your post serious when your avatar keeps finger f-ing your other hand? I?m just saying I is very distracting? :D

    But on that note I think Racey answered the question perfect with the information given. :bowdown:
     
  8. skav

    skav Well-Known Member

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    How are the chargers rated? Whipple 2.3, 3.3,...=liters per minute?

    8:71, 10:71...= I have no idea?

    If you have a 550ci motor, is it optimal to match the volume of the charger to the ci of the motor? IE 550ci w a 8.0l Whipple?


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  9. Racey

    Racey Maxwell Smart-Ass

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    Don't Quote me on it, but i believe the whipple superchargers are rated in Liter's per revolution, remember your engine has to make 2 complete revolutions of the crank to displace the air from all the cylinders (one revolution for intake/comp, the next for power/exhaust)

    There is a table for roots blower sizing somewhere, I don't know it off the top of my head. I believe for example that a 10:71 is two inches longer than an 8:71, and a 12:71 is two inches longer than a 10:71, 14:71 is two inches longer than a 12:71, I don't know how much additional volume those two inches of root account for, but the information is available somewhere online if you dig around. the 71 refers to the original Detroit Diesel engines they came off of and the 6, 8, 10, or 12 refer to the number of cylidners. The 71 refers to the displacement in inches per cylinder. a 6-71 was for a 6 cylinder detroit with a 71 inch/cylinder displacement, 8 for an 8 cylinder, 10 for a 10 etc...... I don't build or design motors, i leave that to the experts like Brummett, so don't quote me on this stuff, someone may have to come in here and show that i'm full of shit :p
     
  10. SLOPOKE

    SLOPOKE Well-Known Member

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  11. 500bbc

    500bbc Well-Known Member

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    Call Whipple they might have a little input.:D
     

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