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  1. #1
    Senior Member eliminator's Avatar
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    How much difference does each pound of boost make ?
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    Senior Member Hass828's Avatar
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    Quote Originally Posted by eliminator View Post
    How much difference does each pound of boost make ?
    Open ended question, every setup is different, a set of small choked off turbos making twenty five pounds of boost on a BBC wont make anywhere near the power a large efficient set of turbos at the same boost would.
    And you need to keep in mind that boost is unused air backing up into the intake tract. Would much rather have an engine capable of 1200hp with 5psi of boost than one that makes 1200hp but requires 25psi to get there.
    Last edited by Hass828; 07-26-2010 at 06:43 PM.
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    Senior Member TurboNova's Avatar
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    On a well designed system 14psi will make double what the engine orginally did. 7-8psi about 50% what the engine did without the blower or turbo. Like Hass said it really depends on the parts you are using.

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    Yea boost is a measure of restriction. Not necesarilly an amount of power.


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    the padawan
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    Its about 7-10hp per psi on a efficent system believing the system is not encountering heat soak...


    but like said it has alot to do with set up

    a efi intercooled setup will make more power per psi then a carbed induction.

    turbos ussually see more hp per psi then a blower in SMALL applications

    6/71's and higher roots blowers can make impressive numbers.


    like stated 14.7 is atmospheric pressure, in a perfect world, 14 psi will double the power made before. BUT THIS IS NOT ALWAYS TRUE

    you could make 10psi with a application that is being spun beyond its efficiency and you wont make the power.

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    Quote Originally Posted by TurboNova View Post
    On a well designed system 14psi will make double what the engine orginally did. 7-8psi about 50% what the engine did without the blower or turbo. Like Hass said it really depends on the parts you are using.
    That's the basics I use too... or ~7% more power per PSI added.
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    JUst for comparison i have a 540 with a pair of 47-88's,@ 9 lbs of boost it's making 900 hp.For every lb added it makes about another 100 hp.

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    Quote Originally Posted by Bangingears View Post
    Its about 7-10hp per psi on a efficent system believing the system is not encountering heat soak...


    but like said it has alot to do with set up

    a efi intercooled setup will make more power per psi then a carbed induction.

    turbos ussually see more hp per psi then a blower in SMALL applications

    6/71's and higher roots blowers can make impressive numbers.


    like stated 14.7 is atmospheric pressure, in a perfect world, 14 psi will double the power made before. BUT THIS IS NOT ALWAYS TRUE

    you could make 10psi with a application that is being spun beyond its efficiency and you wont make the power.
    can you explain this?
    Last edited by twntrbo; 07-28-2010 at 10:20 PM.

  11. #9
    the padawan
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    Quote Originally Posted by twntrbo View Post
    can you explain this?

    i can see how this can be interpreted wrong when I ment small I was not talking CID, Iam talking volume. a 8/71 blower will make tremendous power in the right hands and do it instantly, the last roots blower engine I built made 80% of is peak torque at 2500 rpms, try doing that with twin turbos. the technology is coming around with the turbos. but positive displacement blowers will never be beat as a form of induction in my personal opinion.

    so when i mean small i mean not big ass baller status, there are alot of turbo cars, boats etc's that make incredible power but i believe when you cross over the 1000hp thresh hold a blower starts to become more of a beneficial form of induction.

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    The amount of power made by supercharging is related to the compression ratio you end up with per lbs of boost. Static compression ratio is what you see doing a cranking compression test... effective compression ratio is what you see running and determined by many things. Using 8.5:1 for example, you can see that each 2 lbs of boost increases your effective compression ratio by a taste over 1 point of effective compression ratio. This is where you have to match your components carefully when selecting how much over/under driven your supercharger runs. Compression ratio increase will yeild the most hp gain, and essentially what a blower does. As far as power gains are concerned there are many variables that determine that, such as the center line the cam is cut on (108 vs 114), overlap, and your tune. A cam with a 114 lobe center will build more cylinder pressure and better suited for a blower motor (effective compression ratio) than a 108 lobe center. Also having too much overlap (ie 88 degrees or 108 lc) will allow the boost to be blown out the exhaust due to the late closing of the exhaust valves, where having 18 degrees of overlap allows the blower to build more pressure in the cylinder, more effective compression ratio, and more power. Blower motors and the matching of components in your engine are critical if you want to have a fast motor, run it hard and have it live. Make no mistake, if it isn't a part recommended for supercharged engines, your engine will be nothing more than a time bomb. As you can see, the amount of static compression also determines how much effective compression ratio you gain per lbs of boost. This is where component selection gets tricky, and key to making power and having it live.

    Pounds of boost @ 8.5:1 compression ratio
    2..............................9.7:1
    4............................10.8:1
    6............................12.0:1
    8............................13.1:1
    10..........................14.3:1
    12..........................15.4:1
    14..........................16.6:1


    Pounds of boost @ 10.5:1 compression ratio
    2............................ 11.9:1
    4............................13.4:1
    6............................14.8:1
    8............................16.2:1
    10..........................17.6:1
    12..........................19.1:1
    14..........................20.5:1

    Here are the different CID values for an 671 & 871, and not really that much of an increase
    Approximate CID of Air Per Revolution
    A Weiand 671 for example makes 411 CID of Air Per Revolution
    and a Weiand 871 makes 436 CID of Air Per Revolution
    Last edited by Sanger Flatbottom; 07-29-2010 at 01:43 AM. Reason: add info
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    Quote Originally Posted by TurboNova View Post
    On a well designed system 14psi will make double what the engine orginally did. 7-8psi about 50% what the engine did without the blower or turbo. Like Hass said it really depends on the parts you are using.
    No way in hell. There isn't a motor in this world that will double the horsepower at 14-15lbs boost.



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  14. #12
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    Quote Originally Posted by Bangingears View Post
    Its about 7-10hp per psi on a efficent system believing the system is not encountering heat soak...


    but like said it has alot to do with set up

    a efi intercooled setup will make more power per psi then a carbed induction.

    turbos ussually see more hp per psi then a blower in SMALL applications

    6/71's and higher roots blowers can make impressive numbers.


    like stated 14.7 is atmospheric pressure, in a perfect world, 14 psi will double the power made before. BUT THIS IS NOT ALWAYS TRUE

    you could make 10psi with a application that is being spun beyond its efficiency and you wont make the power.
    If all I could muster from my roots blower was 7-10 HP per psi of boost, I'd throw the thing in the garbage and never use it again!
    Where do you guys come up with this stuff
    EFI will not make more power per PSI than a carb. Both inner cooled or notThe motor has no way of knowing what the air passed thru before it got to the blower or the motor. Does somebody tell it?
    Turbos see more power per lb of boost in all applications. Not just small ones.
    And there is not, and never will be a motor that can double its HP at 14-15 lbs boost. The world just is not that perfect and never will be.

    Sanger Flatbottom has it pretty well summerized.



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    Last edited by gn7; 07-30-2010 at 10:21 PM.

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    Quote Originally Posted by gn7 View Post
    If all I could muster from my roots blower was 7-10 HP per psi of boost, I'd throw the thing in the garbage and never use it again!
    Where do you guys come up with this stuff
    EFI will not make more power per PSI than a carb. Both inner cooled or notThe motor has no way of knowing what the air passed thru before it got to the blower or the motor. Does somebody tell it?
    Turbos see more power per lb of boost in all applications. Not just small ones.
    And there is not, and never will be a motor that can double its HP at 14-15 lbs boost. The world just is not that perfect and never will be.

    Sanger Flatbottom has it pretty well summerized.
    Bob I agree on the efi-carb part of this but disagree on the other.

    On an engine that is boosted and makes lets say 1200hp on gas, in a N/A state it may be way over cammed, way low on compression, way to large of heads and probably do well to muster 600hp, then with 15psi of boost it makes the whole combination come together so I would say certainly that 15psi of boost "could" double the hp, esp on an intercooled turbocharged application.
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    Senior Member TurboNova's Avatar
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    Quote Originally Posted by gn7 View Post
    No way in hell. There isn't a motor in this world that will double the horsepower at 14-15lbs boost.
    Sorry, but true. Most of the engine turbo systems I do make double the power at 14-15 psi. My personal engine makes over double at 14psi

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