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Where should AFR readings be for MPI 509

Old 05-29-2018, 05:36 PM
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Default Where should AFR readings be for MPI 509

Been seeing some dark soot on my tips and started doing some testing. Picked up MEFI Burn and FAST Dual AFR setup. This is a 509 with around 500 hp at the prop on a 11K boat (real heavy). Saw a carb thread (Smitty's) and after posting but then realized again I should not have said anything. In any case I'm trying to figure out if there is any tuning I might try. Do plan to turn my code 44 flag OFF. Here are my readings

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Old 05-29-2018, 06:24 PM
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Is nt the goal to be around 12-13 for most if not each engines? I thought so at least!
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Old 05-29-2018, 06:31 PM
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You need to know what the load or vacuum is at the different rpms also.
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Old 05-29-2018, 07:20 PM
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This is a single engine setup, so the one motor is really working hard with a B3 drive. I can put a vacuum gauge on but have not seen the vacuum readings before when dealing with AFR's. The O2 sensor is reading how well the fuel is burning to my understanding, so how does knowing the vacuum give more info ? Not saying its not good to know, just don't understand the correlation
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Old 05-29-2018, 08:54 PM
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Originally Posted by AllDodge View Post
Been seeing some dark soot on my tips and started doing some testing. Picked up MEFI Burn and FAST Dual AFR setup. This is a 509 with around 500 hp at the prop on a 11K boat (real heavy). Saw a carb thread (Smitty's) and after posting but then realized again I should not have said anything. In any case I'm trying to figure out if there is any tuning I might try. Do plan to turn my code 44 flag OFF. Here are my readings

You aren't out of line drastically in any regime. I would tweak things a little if so inclined.

Specifically in the 750 rpm band I would lean out to 13.5-13.8.

The 1800-2400 rpm band the boat is transitioning from a displacement hull to a planing hull, hence a fairly high load (MAP). You might pull a little fuel, and I mean a little fuel around 1800 but only if knock is under control.

2400 rpm is rich for my taste. I'd shoot for 12.5 -12.8.
The rest looks fair to me. Splitting hairs if you want to change things.

Don't touch the decel range. They will virtually always show rich unless you really shrink the BPW in the 40kPa or less column(s) which I don't recommend.
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Old 05-30-2018, 08:10 AM
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Appreciate your thoughts
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Old 05-30-2018, 11:27 AM
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I expected a stock Merc tune to be more in the 11s. That’s a better map than I would’ve guessed.
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Old 05-30-2018, 11:38 AM
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It isn't stock, it was tune by Eddie a few years ago. Since then I have stripped the hull and epoxy painted
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Old 05-30-2018, 08:00 PM
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Originally Posted by Trash View Post
You aren't out of line drastically in any regime. I would tweak things a little if so inclined.

Specifically in the 750 rpm band I would lean out to 13.5-13.8.

The 1800-2400 rpm band the boat is transitioning from a displacement hull to a planing hull, hence a fairly high load (MAP). You might pull a little fuel, and I mean a little fuel around 1800 but only if knock is under control.

2400 rpm is rich for my taste. I'd shoot for 12.5 -12.8.
The rest looks fair to me. Splitting hairs if you want to change things.

Don't touch the decel range. They will virtually always show rich unless you really shrink the BPW in the 40kPa or less column(s) which I don't recommend.
I agree. Lean out the idle. I would not mess with anything else. Remember your seeing the average of 4 cylinders. You can still have a lean cylinder and show an average of mid 12s.
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Old 05-30-2018, 09:05 PM
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Originally Posted by AllDodge View Post
This is a single engine setup, so the one motor is really working hard with a B3 drive. I can put a vacuum gauge on but have not seen the vacuum readings before when dealing with AFR's. The O2 sensor is reading how well the fuel is burning to my understanding, so how does knowing the vacuum give more info ? Not saying its not good to know, just don't understand the correlation
It helps as the engine needs more fuel when it's under a heavier load. My friends engine will pull 2-3" of vacuum at 3000 rpms getting in plane but once on plane pull 8-9". Needs more fuel getting in plane than when cruising at that rpm .

If you have mefiburn use the scanner to see how much load you are putting on the engine with the MAP readings. Very valuable tool.

Last edited by onesickpantera; 05-30-2018 at 09:09 PM.
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