Blower setup help
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Blower setup help

  1. #1
    Senior Member Scapegoat's Avatar
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    Default Blower setup help

    I need pro opinions on my blower set up. I want to keep it on 91 octane and reliable as possible.
    502 8.75 to 1 originall heads were iron. (Mercruiser 502HP)
    Now have aluminum 115 chamber heads (procomp 320)
    Blower, B&M 420, Lee cooler,
    Pulleys 5.8top &5.8bottom running 1-1
    I have 6.3 top and 5.25 bottom to swap out(17% underdrive.
    Cam is unknown
    Carbs are 750double pumpers
    I bought the boat out of NM where it ran at 4pounds boost. Approximately 4000' I plan on running at lake Havasu and would like to make the changes needed.
    I'm not sure if these new pulleys will put me at a safe level for pump gas.
    Any guidance would be appreciated.

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  3. #2
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by Scapegoat View Post
    I need pro opinions on my blower set up. I want to keep it on 91 octane and reliable as possible.
    502 8.75 to 1 originall heads were iron. (Mercruiser 502HP)
    Now have aluminum 115 chamber heads (procomp 320)
    Blower, B&M 420, Lee cooler,
    Pulleys 5.8top &5.8bottom running 1-1
    I have 6.3 top and 5.25 bottom to swap out(17% underdrive.
    Cam is unknown
    Carbs are 750double pumpers
    I bought the boat out of NM where it ran at 4pounds boost. Approximately 4000' I plan on running at lake Havasu and would like to make the changes needed.
    I'm not sure if these new pulleys will put me at a safe level for pump gas.
    Any guidance would be appreciated.
    Air density at Havasu will vary with temperature and humidity. I assume there is some way you can check that through the airport, or perhaps some other website. Normally speaking, you go 1 jet size for 2,000 ft of altitude change. New Mexico may have been pretty warm as well. If you have to make an error, a little fatter is probably safer than too lean. Once again without doing your homework for you, I don't know Havasu altitude to compare.

    If it was running 4 psi with the one to one ratio, not sure why you would want to underdrive it?

    Absolute best way I know to tune in the boat is if the dry part of the exhaust has a bung for an O2 meter.

    Got any idea of ignition timing? Did you cc the heads? I know some of the imports advertise 115 and are actually 119 cc, but only way to know for sure is to check volume yourself or pay someone.

    JE/SRP sells replacement pistons for the stock 502, so you should be able to look up what the stock dome volume is.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by obnoxious001 View Post
    Air density at Havasu will vary with temperature and humidity. I assume there is some way you can check that through the airport, or perhaps some other website. Normally speaking, you go 1 jet size for 2,000 ft of altitude change. New Mexico may have been pretty warm as well. If you have to make an error, a little fatter is probably safer than too lean. Once again without doing your homework for you, I don't know Havasu altitude to compare.

    If it was running 4 psi with the one to one ratio, not sure why you would want to underdrive it?

    Absolute best way I know to tune in the boat is if the dry part of the exhaust has a bung for an O2 meter.

    Got any idea of ignition timing? Did you cc the heads? I know some of the imports advertise 115 and are actually 119 cc, but only way to know for sure is to check volume yourself or pay someone.

    JE/SRP sells replacement pistons for the stock 502, so you should be able to look up what the stock dome volume is.
    Good advice right there. I would fatten it up to be safe. Most carbs are jetted for N.A. application out of the box. If you add a blower you are increasing air induction and should increase feul to match if you don't have a sensor to check oxy readings raise jet sizes atleast 1-2 sizes and run it and then get a "proper" plug reading at a minimum.

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    Senior Member Scapegoat's Avatar
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    Havasu is about 750' give or take.
    I tested at Elsinore and was hitting 10 pounds on the boost gauge. Didn't cc the heads. I'm going off factory specs. I'll dig up the je piston specs.
    Will check timing and and set it at 28* to start.
    I'll get the Jets situated along with a fresh set of sparkplugs..
    New Jets 82primary 86 secondary
    10.5 pwr. valve front. Plug rears
    R5671A ngk plugs

    What rpm range at idle should be safe for a bravo one drive?
    Last edited by Scapegoat; 07-24-2019 at 08:22 PM.

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    Senior Member Scapegoat's Avatar
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    Definitely great advice, what final compression ratio is safe for 91 octane running this type of setup?

  8. #6
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by Scapegoat View Post
    Havasu is about 750' give or take.
    I tested at Elsinore and was hitting 10 pounds on the boost gauge. Didn't cc the heads. I'm going off factory specs. I'll dig up the je piston specs.
    Will check timing and and set it at 28* to start.
    I'll get the Jets situated along with a fresh set of sparkplugs..
    New Jets 82primary 86 secondary
    10.5 pwr. valve front. Plug rears
    R5671A ngk plugs

    What rpm range at idle should be safe for a bravo one drive?
    I built one of those years ago, stock 502 bottom end, blueprinted clearances. May still be running today in Washington, forget it if ran 6 or 7 lbs boost.

    As I said, Havasu heat may be enough to offset the altitude, but that homework is up to you.

    Try 8 range on the spark plugs.

    Why are you saying 10.5 power valve? Are the carbs boost referenced? If you plug power valves, rule of thumb is add 6-8 jet sizes for that side of the carb.

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    Senior Member Scapegoat's Avatar
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    Teague suggested I start with this setup.
    Last edited by Scapegoat; 07-25-2019 at 09:33 PM.

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