Oxygen sensor on mpi boats?

[Black86comp]

Is it just programmed in a little rich, or do the boats with mpi have oxygen sensors in the exhaust manifold or heatriser.

Thanks
Shane

[DKJBama92Mariah]

Since the exhaust is a "wet" water cooled exhaust, an o2 sensor would not work. The ECM has computes the correct air/fuel ratio from readings from the mass airflow meter, intake air temp sensor, and coolant temp sensor. It probably uses some other sensors too, thats just the three that popped into my head.

Cheers,
DKJ

[Black86comp]

I was just curious, I was wondering how they did, for the reason that you said, since it is a wet exhaust.

It must be set from the factory, I wonder how it changes with elvation if it can't read the A/F ratio. Hmm. I need to do some digging on this, reason being, I ski at 5000+ in Utah, and my 351 can't quite keep enough speed for footing. Sea-level is no problem, but I lose somewhere in the range of 10-20%, and thats with reset carb jets power valve and timing.

[DKJBama92Mariah]

Originally posted by Black86comp
I wonder how it changes with elvation if it can't read the A/F ratio.


It uses the mass airflow meter. It detects the mass of the air going through it, not the volume. Remeber physics? Mass = density x volume.

Basically, all the meter is is a wire running through the intake air stream. The ECM passes a current through this wire. As the air flows through the meter, it cools the wire. As a conductor gets colder, its resistance to the flow of electricity goes down.

The denser air at lower elevations wil pull more heat from the wire than the less dense air at 5k feet. Thus, the computer sees that the engine is getting more oxygen and increase the amount of fuel. It also uses the reading from the intake air temperature sensor. With these two it can calculate the mass and density of the air entering the engine, and from that it derives the air pressure per the formula above. Therefore, the engine knows nearly the exact amount (mass) of air entering the engine and will tell the injectors to squirt the appropriate amount of fuel to create the ideal A/F ratio. The fuel maps in the compuer tell it what the ideal ratio will be for that particular point in time. It chooses the fuel map based on readings from the throttle position sensor and coolant temp sensor.

This is an EXTREMELY simplified example, but it serves the purpose. An o2 sensor isnt needed to create a good A/F ratio, it just does a little better job than this method.

Cheers,
DKJ

[DKJBama92Mariah]

Shane,

I apologize. I gave erroneous information. Looking at the indmar PDF owners manual, i learned that the Indmar motors do not use a Mass Airflow sensor nor do they use an intake air temperature sensor. They use a MAP (Manifold Absolute Pressure) sensor. I'm not too familiar with how this gadget works. But its about the same principle. It was used on earlier EFI systems in cars. It serves about the same purpose though. Here is a GREAT explanation from a auto website



Manifold Absolute Pressure (MAP)
The Manifold Absolute Pressure sensor measures changes in the intake manifold pressure resulting from engine load and speed changes. The computer sends a 5-volt reference signal to the MAP sensor. As pressure changes in the intake manifold occur, the electrical resistance of the MAP sensor also changes. By monitoring the sensor output voltage, the computer can determine the manifold absolute pressure. The higher the MAP voltage output the lower the engine vacuum, which requires more fuel. The lower the MAP voltage output the higher the engine vacuum, which requires less fuel. Under certain conditions, the MAP sensor is also used to measure barometric pressure. This allows the computer to automatically adjust for different altitudes. The computer uses the MAP sensor to control fuel delivery and ignition timing.


I think that just about covers it. I KNOW i couldnt explain it any better.

Cheers,
DKJ

[Black86comp]

Thank you very much, I need to do some work on my boat in the next year or two, and I am considering a swap to mpi, for fuel saving purposes but especially for performance. But that write up was great. I really appreciate it.

Shane

[84 Rider]

The above information is correct.

An O2 sensor in the exhaust provides feedback to the ECM on the A/F ratio that the ECM can use to fine tune the A/F ratio on a continual basis. This is called "closed loop" operation. "Open loop" operation is most commonly used in marine applications and functions quite well.

The main and driving factor for the use of an O2 sensor in automotive applications is the catalytic converter. Modern catalytic converters operate most efficiently when the A/F ratio is "dithered" between rich and lean. Dithering means the ECM cyclically adjusts the A/F ratio to slightly enrichen or enlean the A/F ratio. This strategy is employed so that the catalytic converter can operate at peak efficiency and the manufacturer can comply with emission regulations. The strategy is not driven by improving performance.

I think much of the mis-information on the effects of O2 sensors is propagated by the O2 makers and the auto parts stores trying to sell an item that is relatively easy to change to the average car owneer on the promise of improved performance and mileage.

[84 Rider]

If you have a 351 I have a complete set of upper and lower manifolds, throttle body, injectors, sensors, and wiring harness that I would be willing to sell.

This setup is off a '96 ford van. It is not a GT 40 intake setup.

This setup was planned for my correct craft with 351, but now that is for sale with the Holley setup while I focus on the 454 equipped Supra.