Safe A/F ratio..
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Safe A/F ratio..

  1. #1
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    Default Safe A/F ratio..

    .......Finally got my friends boat up and running and in the water and am wondering what I should shoot for as far as SAFE A/F ratios...

    The boat is a 27' Carrera with a bravo drive.. Sooo not exactly light and it gets run pretty hard..


    The engine is a 509 cid with JE "flat top" pistons, cast iron rectangle port heads with Inconnel exhaust valves, and a Weind 256 blower with twin Holley 750's on it.
    Not sure about the cam specs but it has a fair amount of lope at idle..
    Timing is currently set at 25° at idle with 32° total in by 3000 rpm.
    We have an AEM air fuel meter running in just one collector...

    We took it out for a short run just to see where everything is currently....
    (BTW it's running on 93 octane with "up to" 10 % ethenal fuel )

    Running on just the primary circuit It'll pull up to 3500 rpm with the A/F ratio was running right around 11.5:1

    Once the secondaries opened up the A/F ratio was running about 12.5:1

    It made a total of 6 pound of boost at WOT (not sure about rpm, I didn't get to look at the tach)

    Correct me if I'm wrong, but it seems to me I need to get those numbers switched around...
    .. but then again this is the first blower engine I've ever tuned..

    (glad it's my buddies and not mine )

    Thanks,

    Doug
    Last edited by Boatally Insane; 05-06-2011 at 10:40 AM.
    Some call it A.D.D. I call it "multi tasking"

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  3. #2
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    Default Af

    11.5

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    gn7
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    You may consider putting a little jet in the secondaries. You don't really need it that rich on the primaries because your not really making much if any boost while your running on them.
    Yes, if you could flip the numbers, it would be better.
    I would consider locking out the dizzy, or at the very least, speed up the timing curve. Might help with the part throttle AFR.

    The boost is not rpm sensitive, so it is unimportant what rpm you were turning when you read the boost.
    Boost is throttle opening sensitive. You have to read max boost, at max throttle. It will read the same boost from 2000 to 6000 when the throttle is wide open.



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    You may consider putting a little jet in the secondaries. You don't really need it that rich on the primaries because your not really making much if any boost while your running on them.
    Yes, if you could flip the numbers, it would be better.
    I would consider locking out the dizzy, or at the very least, speed up the timing curve. Might help with the part throttle AFR.

    The boost is not rpm sensitive, so it is unimportant what rpm you were turning when you read the boost.
    Boost is throttle opening sensitive. You have to read max boost, at max throttle. It will read the same boost from 2000 to 6000 when the throttle is wide open.

    ARE THESE BLOWERS CARBS WITH MANIFOLD REFERENCED POWER VALVES, OR ARE THEY OFF THE SHELF 750s?



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    The power valves ARE manifold referenced...

    As far as boost goes... He was being easy on the stick as it IS a 5000# + boat with a stock 1989 Bravo drive turning a 5 blade Hydro motive so we didn't want to grenade the drive the first time out at least..... But whoever mounted the drive gave it a very aggressive "X" dimension as it will still "blow out" a 5 blade prop getting on plane if you're not easy on it..


    Shet DID manage to run 78 mph at about 5000 rpm which I thought was pretty respectable for the first time out with little tuning...

    We also noticed that it's running out of fuel pressure at WOT which we DO need to address, It runs 9 psi at idle but only 3 psi at WOT however the A/F ratio DID stay consistent when the secondaries were open.




    Thanks,

    Doug
    Last edited by Boatally Insane; 05-06-2011 at 11:04 PM.
    Some call it A.D.D. I call it "multi tasking"

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    gn7
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    you would be surprised how much fuel is flowing at 3 psi. You need to get it up, and that may require a different pump.

    If you are running ANY boost at the speed you saw 11.5 afr, your on the power valve. They will come in any time the manfold has less than about 4 inches of vac. on most blower carbs.

    Power valves are ALMOST useless on most boats because they are running below 4 inches almost anytime they aren't idling.

    I would check to see if the secondaries have PVs. If not, and the pri and the sec are jetted the same, there is you answer. On the pri, with the PV open=little rich, but on the sec, with no sec PV=little lean.

    Get the fuel pressure up.



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    ....Just a quick update....

    My buddy went through the carbs this past week. Put in 6.5 power valves (there aren't any in the secondaries). Took some jet out of the primaries and put a bit more jet in the secondaries...

    Now it runs 12.5 - 13.0 A/F on the primaries but starts to lean out past 13.0 on the secondaries while still maintaining 6psi fuel pressure, so we'll have to put a bit more jet in the secondaries before we melt something.....

    BTW we learned a VERY important lesson.. Do NOT use the clear plastic float level plugs from Jegs with Ethonal fuels... They lasted about 1/2 hour before they "crystalized" and disintegrated... Could've been really bad if we hadden't still been at the dock tuning....


    Doug
    Some call it A.D.D. I call it "multi tasking"

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    Has he ck'd the elbow at the fuel tank pick up for the anti syphon ck. valve ?
    It is a very small opening and restrictive.
    Northeast Member

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    Quote Originally Posted by Boatally Insane View Post
    ....Just a quick update....

    Now it runs 12.5 - 13.0 A/F on the primaries but starts to lean out past 13.0 on the secondaries while still maintaining 6psi fuel pressure, so we'll have to put a bit more jet in the secondaries before we melt something.....

    Doug
    Since you leaned out the primaries, now there wasn't a bunch of extra fuel when you come in on the secondaries. Sounds like it's about right on the primaries now. Typically ~13.0:1 out of boost, 11.5-11.8 on boost.
    Keith-


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