A/F ratio?
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A/F ratio?

  1. #1
    Senior Member Cat Sass's Avatar
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    Default A/F ratio?

    It looks like I am almost the last guy to get an oxygen sensor on my blower motor. What do you guys that already have them shoot for on the A/F ratio. This is a 509 with 8-9 lbs intercooled on pump gas.

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    Senior Member STV_Keith's Avatar
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    In a car, 11.5-11.8. Might be able to go a touch leaner due to you're running an open cooling system and not seeing the temps you would see in a car, but I wouldn't go over 12:1.
    Keith-


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    Senior Member Cat Sass's Avatar
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    thanks kieth, do you think it would be safe to run a little leaner at cruise rpm? I know the car EFI systems are set up to run as lean as 15-16:1 at light load cruising conditions, and 14.7 under normal conditions. I'm not sure if a boat ever sees a "light load" condition. I have noticed that while cruising at 65 MPH around 3600 rpm, my vacum guage reads 0. Seems like this would be about the same as a N/A motor at wide open throttle.??

    any thoughts guys?
    Last edited by Cat Sass; 03-18-2008 at 11:08 AM.

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    Senior Member STV_Keith's Avatar
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    The car scenarios are basically out the window with the kinds of RPM's a boat motor sees to "cruise" across the lake. Given that you're at right near atmospheric while "cruising", I'd shoot for around 13:1, maybe a little richer. Most cars will cruise at 14.7:1 (stoich), but I wouldn't do that in a boat due to the load and RPM.

    BUT, I've never tuned a 4-stroke, supercharged motor in a boat, so I can't give you any info based on past experience...just what I'd probably start at. YMMV.

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    Default Air/fuel Ratio

    11.5-11.8 should be good, however it really can vary depending on your monitoring system equipt.
    Quote Originally Posted by Cat Sass View Post
    thanks kieth, do you think it would be safe to run a little leaner at cruise rpm? I know the car EFI systems are set up to run as lean as 15-16:1 at light load cruising conditions, and 14.7 under normal conditions. I'm not sure if a boat ever sees a "light load" condition. I have noticed that while cruising at 65 MPH around 3600 rpm, my vacum guage reads 0. Seems like this would be about the same as a N/A motor at wide open throttle.??

    any thoughts guys?

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    Cantard 71hallett's Avatar
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    A/F means alot but I would tune with an egt. What we use anyways and lots safer since it would be very hard to say how much load you are really under. P.S. put it on #7
    Paul
    Last edited by 71hallett; 03-22-2008 at 05:47 AM.

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    Senior Member Cat Sass's Avatar
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    Default air fuel results

    well the first round of testing showed:
    14.7 at idle in neutral
    16 at idle in gear
    10.9 at cruise
    10.4 at wot

    Got some jet changes to try, You can really see when the powervalves open on the meter. Neat-o.

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    Senior Member STV_Keith's Avatar
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    Decent amount of power and economy still on the table.

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    It's what we do BDMarine's Avatar
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    Here's what we do.
    Idle 13.5 to 13.8
    Accelerate 12.2 - 12.5
    Cruise no load no boost 13.0 - 13.3
    Cruise with small amount of boost and load 12.5 - 12.7
    WOT 7 - 8 lbs boost 11.8 - 12.0

    These are good general numbers. ALL these numbers will be adjusted depending on what the engine likes. We still look at plugs also.
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    Quote Originally Posted by 71hallett View Post
    A/F means alot but I would tune with an egt. What we use anyways and lots safer since it would be very hard to say how much load you are really under. P.S. put it on #7
    Paul

    Why would you rather tune with an EGT?

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    Member JAY4SPEED's Avatar
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    Quote Originally Posted by BDMarine View Post
    Here's what we do.
    Idle 13.5 to 13.8
    Accelerate 12.2 - 12.5
    Cruise no load no boost 13.0 - 13.3
    Cruise with small amount of boost and load 12.5 - 12.7
    WOT 7 - 8 lbs boost 11.8 - 12.0

    These are good general numbers. ALL these numbers will be adjusted depending on what the engine likes. We still look at plugs also.
    Great info, thanks!

    What are your AFR figures with an unboosted engine at load and WOT?

    Jay
    "You just can't fix stupid!"

  14. #12
    It's what we do BDMarine's Avatar
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    Quote Originally Posted by JAY4SPEED View Post
    Great info, thanks!

    What are your AFR figures with an unboosted engine at load and WOT?

    Jay
    Normally aspirated engines are a lot more finicky in their tuning. There are so many things that determine what they like. Cast iron or aluminum heads, compression, lobe seperation, etc. Lets say it's a 540 BBC with alum heads and 9.75:1 compression with 91 octane. I would start at 12.7 - 12.8 and adjust from there.
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    Cantard 71hallett's Avatar
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    Quote Originally Posted by Warp Speed View Post
    Why would you rather tune with an EGT?
    Sorry, wanted to see if anyone else would reply. We use an a/f but it will tell you nothing about timming or the quality of the fuel you just bought. Egt will. You could be rattling away and your a/f is still good.

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    Quote Originally Posted by 71hallett View Post
    You could be rattling away and your a/f is still good.
    True, but you could be "rattling away" and still not see it with the egt data either.
    Can't remember the last time we used them. Now that doesn't mean they are totaly useless, but I think they are just not as useful as we once assumed.
    Kinda like jetting with BSFC, too many variables effecting the reading to rely on them for tuning.

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