[Question] Blower air/fuel and altitude question
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Blower air/fuel and altitude question

  1. #1
    Senior Member Cat Sass's Avatar
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    Default Blower air/fuel and altitude question

    I just had a professional engine builder tell me something that I disagree with, so I was hoping to get some input from the motor gurus on here.

    This is a 510 BBC, 8-71, carbureted, pump gas motor in a 25 eliminator. It runs well at sea level. I am taking it up to lake Tahoe, which is about 4500 ft I think. My plan was to lean the jets in the carbs a few sizes (4)and change the blower from 15 over to 23 over.

    This guy is telling me that speeding up the blower will lean out the air/fuel mixture, and that I shouldn't re-jet.

    I think that a mechanical injected motor would lean out by speeding up the blower, but I don't think it works that way on a carbureted motor.

    Any thoughts?

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    Quote Originally Posted by Cat Sass View Post
    I just had a professional engine builder tell me something that I disagree with, so I was hoping to get some input from the motor gurus on here.

    This is a 510 BBC, 8-71, carbureted, pump gas motor in a 25 eliminator. It runs well at sea level. I am taking it up to lake Tahoe, which is about 4500 ft I think. My plan was to lean the jets in the carbs a few sizes (4)and change the blower from 15 over to 23 over.

    This guy is telling me that speeding up the blower will lean out the air/fuel mixture, and that I shouldn't re-jet.

    I think that a mechanical injected motor would lean out by speeding up the blower, but I don't think it works that way on a carbureted motor.

    Any thoughts?
    Tahoe is at 6225 ft above sea level

    density altitude on a hot day can make the engine think its at 8-9000 ft

    I would keep the blower drive at 15 over

    drop down a couple jet sizes and put a lower pitch prop on

    be sure you get a inspection before putting it in the water

    We used to go to Tahoe, but not any more

    To many fees and not worth the trouble
    If you ain't the lead dog the view is always the same !

  4. #3
    gn7
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    He is full of shit!
    But before you go pulling a bunch of jet out of the thing, remember there is more to A/F than just alt. Yoiu can have air at sea level that is every bit as bad as 4500 ft high lake. And visa versa.
    Chances are it will worse, but owuld speed up the blower first and then look at the plugs and adjust from there.
    the blower speed with carbs does not change the A/F. Its changes the amount of air and fuel inducted by the engine, but the A/F remains pretty much the same. If the air is shit, then you need to lean it down.
    You are correct that a MFI will lean an engine down by speeding up the blower, and may run leaner at alt simply by speeding up the blower. MFI is only aware of RPM and throttle position and oblivious to alt. density, or atmospheric pressures, and air flow thru the engine. They must be tuned for that. Thats why MFI guys live and die by a weather station.
    Carbs work strictly off a pressure DIFFERENTIAL.


    Let the BS rebuffs commence.



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    Last edited by gn7; 07-13-2011 at 04:31 PM.

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    Senior Member Cat Sass's Avatar
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    Quote Originally Posted by BigSteve View Post
    Tahoe is at 6225 ft above sea level

    density altitude on a hot day can make the engine think its at 8-9000 ft

    I would keep the blower drive at 15 over

    drop down a couple jet sizes and put a lower pitch prop on

    be sure you get a inspection before putting it in the water

    We used to go to Tahoe, but not any more

    To many fees and not worth the trouble
    6225 ft . Wow, no wonder it's such a turd up there. I didn't have any luck finding a prop, so I guess I'm stuck playing with jets and pulleys

  7. #5
    gn7
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    Cat Sass, don't listen to the wacker packer. He is still jealous because he can't turn up the boost on his wacker when he is faced with altitude.

    Air+fuel=HP You can just remove fuel to match the air, but the density still sucks. Power drops in relationship to the amount of fuel you pull out and move the sam ecubic feet of air. The idea with the blower is to try and get the manifold density back to where it was at sea level. To do that, you will have to move more cubic feet of bad air than at sea level. The bad air cannot support the fuel per cubic foot that good air can, so you need to pull fuel out, but you still need to pack the manifold to get back to something approaching the sea level density.

    With the right pulleys and jets, you can regain everything you loose from the altitude.

    Being a pilot, Big Steve knows this, and knows thats why they first started messing with superchargers and turbochargers for aircraft at altitude. He just doesn't what to see you regain the power because, he can't. Just the way wacker types are.

    turn up the blower and be glad you have one.



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    Last edited by gn7; 07-13-2011 at 10:00 PM.

  8. #6
    Platinum Member BigSteve's Avatar
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    GN 7 is the blower man to listen too

    But Bob my Verado outboard has a screw blower and boost by pass valve and I run 20# of boost

    However,Tahoe on a 85-90 degree day I have to drop from a 28 pitch to a 24 and wish I had a 22

    Guess what I found on my Verado motor

    A little mosture damaged on the cam and Merc gave me a new motor and updated screw blower

    After 71 months of WOT on a 72 month warantee

    now i get a extra year on the new motor

    Such is life
    If you ain't the lead dog the view is always the same !

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    Senior Member bp298's Avatar
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    Quote Originally Posted by gn7 View Post
    Air+fuel=HP You can just remove fuel to match the air, but the density still sucks. Power drops in relationship to the amount of fuel you pull out and move the sam ecubic feet of air. The idea with the blower is to try and get the manifold density back to where it was at sea level. To do that, you will have to move more cubic feet of bad air than at sea level. The bad air cannot support the fuel per cubic foot that good air can, so you need to pull fuel out, but you still need to pack the manifold to get back to something approaching the sea level density.
    minor correction... it's "oxygen+fuel=HP". there's plenty of air up there, just a lot less of the good stuff in each cf


    Quote Originally Posted by BigSteve View Post
    Guess what I found on my Verado motor

    A little mosture damaged on the cam and Merc gave me a new motor and updated screw blower

    After 71 months of WOT on a 72 month warantee

    now i get a extra year on the new motor

    Such is life
    steve, if you posted this in the prf, the herd would be all over getting your tax rate jacked up...

  10. #8
    gn7
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    Quote Originally Posted by bp298 View Post
    minor correction... it's "oxygen+fuel=HP". there's plenty of air up there, just a lot less of the good stuff in each cf
    I KEEP THIS UP, AND I WILL PUSH MY HAIRLINE BACK TO THE BACK OF MY HEAD!



    Hey, at least when I do it, peoples lives aren't at stake




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