Best way to lower boost without loosing speed
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Best way to lower boost without loosing speed

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    Default Best way to lower boost without loosing speed

    Hi, I am running a SBC and 671 in front of a Berkeley. At full noise it will pull about 5500rpm and close to 15lbs boost. I dont like to take it over 10lbs boost and about 5200rpm at which revs its doin around 115kph. What is the best way for me to lower the boost? I know I could gear the blower down but this will have a negative effect on performance. Could I use the torque of the engine and run a coarser impeller to knock 3-400rpm off but at the same time not loose any top end speed? Its running a 70mm nozzle insert,would going down to a 65 or 60mm insert reduce revs and boost but still keep top end? I dont know what cut the stainless impeller is that I am running.Thanks

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    Senior Member 74Bonneville's Avatar
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    Where are you with the kilometers & millimeters? I feel like I'm back in school.

    Going 1 cut larger on the impeller will reduce your rpm's, maybe the 300-400 your looking for. I'm guessing you'll need a different pulley to lower the boost, others will have more details on how much boost will be affected by lowering rpm's.


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    Senior Member ICECREAMAN's Avatar
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    You can change the rpm by changing the impeller. You can change the boost by changing the blower pulley's. But what you can't do is lower the rpm, lower the boost and keep the same HP output of that motor. It becomes a math equation, your roots blower is going to produce a certain amount of boost at a certain rpm. The only way I can see you get your desired result's is to add cubic inches.
    At least that's how I see it.

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    Senior Member SoldHondaBoughtHondo's Avatar
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    Won't more cam lift/duration..intake/heads that flow better bleed off boost pressure?....depending on what cam he has now it might even make more power.

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    Description Here are my cam specs




    Return to Store Item
    Part Number: 12-404-4 Grind Number:268AH-14
    Lifter Type: Hydraulic Lifter
    Engine Family: Chevrolet 262-400 c.i. 8 cyl. 1955-1998
    Description: Hydraulic-Best cam for 350 TPI with modified computer.
    Cam Family: Computer Controlled Hydraulic Flat Tappet Camshafts
    Specifications Intake Exhaust
    RPM Range: 1800-5800 Valve Lash:
    Valve Timing: 0.006 Duration: 268 276
    Lobe Separation: 114° Duration @ .050" Lift: 222 226
    Intake Centerline: 110° Valve Lift: 0.464 0.464
    Lobe Lift: 0.31 0.309
    Valve Timing @ 0.006 Lift:
    Exhaust
    Closes 20
    ATDC
    Opens 76
    BBDC
    110
    Intake
    24 Opens
    BTDC
    64 Closes
    ABDC
    I was advised to use this cam for my application, I am running 76cc cast heads that have had some clean up work around the bowls. So if I run a coarser blade and drop revs will I go slower?
    Last edited by Huffa; 02-01-2014 at 12:44 PM.

  8. #6
    Ain't Right Racin piston in the wind's Avatar
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    Intercooler.
    QE 2050 W/ a Junk ass screw blower

    90% of the game is half mental... The pursuit of the unknown is priceless & gratifying . Sometimes it just takes good old fashion balls. No amount of reasoning or evaluation can deem it right or wrong. At the end of the day stick with whatever gives the person in question morning wood LOL

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    No intercooler, its bit of a budget built motor,not even forged but its 7.8-1 comp and after over a year of running it and a couple of reasonably long river races placing 1st and 2nd in my class(one race was 160km or 100miles long) its going well. All the same I dont expect it to last for ever but at this stage its not using any oil and is going like the day it fired up.

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    Senior Member Hass828's Avatar
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    Go from that old blower to an efficient turbocharged setup and you will you will achieve those goals.
    "if we keep doing it the same way we always do..we will always get the same results"
    H8-2-W8
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    mo balls than $cents$ IMPATIENT 1's Avatar
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    Quote Originally Posted by piston in the wind View Post
    Intercooler.
    X2 a straub cam wld help a ton as well.looks way small to me and im a true believer in small cams big power.mine is 545 lift

    Dare to be different, if it turns out great you can claim you planned it that way.

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  12. #10
    "BOATLESS" blender over's Avatar
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    Quote Originally Posted by Hass828 View Post
    Go from that old blower to an efficient turbocharged setup and you will you will achieve those goals.
    Hass is that your answer for everything when someone post a blower Or nitrous question?lol.
    Are you going to run the 660 deal they have set up?

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    Senior Member Hass828's Avatar
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    Quote Originally Posted by blender over View Post
    Hass is that your answer for everything when someone post a blower Or nitrous question?lol.
    Are you going to run the 660 deal they have set up?
    Of course not. But the OP had a question about "lowering boost without loosing speed" and going to and efficient turbocharged setup from that old roots deal will free up a lot of HP and allow him to make the same power he is now but do it at a lower boost #. Much more than intercoolers & cams, which can make a small difference.

    I'm not towing all the way to Bakersfield. Don't believe I have anything for those guys anyway. I built my boat to be a very fast lake boat, and went to great lengths to keep that goal in sight. Didn't want to loose the ability to cruise for miles. If I were building a race rig to go out there & hammer those guys, there would be ALOT different about my entire setup.
    I witnessed Jack's deal in action, if those guys can contend with him then they certainly have me beat. I have my rig ready to hit some races this yr though. Will probably run a couple SDBA local races .
    Last edited by Hass828; 02-02-2014 at 07:28 AM.
    "if we keep doing it the same way we always do..we will always get the same results"
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  14. #12
    LP-25.com Infomaniac's Avatar
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    I would have suggested turbos before "budget" was mentioned. Isn't it common knowledge turbos cost 6x what a roots cost?

    But it is definitely a solution to the objective.
    If For Some Reason I Do Something Worthy Of Recognition. God Provided The Ability And Deserves The Credit.


    QE 439 Twin Turbo

  15. #13
    "BOATLESS" blender over's Avatar
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    Quote Originally Posted by Hass828 View Post
    Of course not. But the OP had a question about "lowering boost without loosing speed" and going to and efficient turbocharged setup from that old roots deal will free up a lot of HP and allow him to make the same power he is now but do it at a lower boost #. Much more than intercoolers & cams, which can make a small difference.

    I'm not towing all the way to Bakersfield. Don't believe I have anything for those guys anyway. I built my boat to be a very fast lake boat, and went to great lengths to keep that goal in sight. Didn't want to loose the ability to cruise for miles. If I were building a race rig to go out there & hammer those guys, there would be ALOT different about my entire setup.
    I witnessed Jack's deal in action, if those guys can contend with him then they certainly have me beat. I have my rig ready to hit some races this yr though. Will probably run a couple SDBA local races .
    What type of fuel do u run, just curious.

  16. #14
    Senior Member Hass828's Avatar
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    Quote Originally Posted by blender over View Post
    What type of fuel do u run, just curious.
    Dual fuel system, pump gas for cruising and boost up to 12psi, then switch to another tank with C-16 for racing and boost up to 30psi.
    But in all honesty, the thing makes all the power anyone should really need on 12psi with the pump gas. The boat will run over 120mph this way. But it accelerates way harder when you pour the boost to it.
    Last edited by Hass828; 02-02-2014 at 11:06 AM.
    "if we keep doing it the same way we always do..we will always get the same results"
    H8-2-W8
    [email protected] first pass

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