MAFs essentially measure the mass flow of air at any given point going into the engine. Since combustion air/fuel ratios are mass based, this is all you need to know about the air entering the engine. MAP is not needed. It doesnt matter what boost of vacuum level you are at.

This is also why our cars require recirc valves....if you measure the mass air flow and then blow it off when going off throttle, you have incorrectly measured the amount of air entering the engine, thus the car will go rich and stumble. Recirc works becasue the air is just put back into the intake, nothing is lost.

 

To reiterate the answer to your question, yes the evo uses a MAP sensor.

dryad001, I haven't heard this but it makes sense. Once you are over one bar you are back in open loop anyway.

 

MAF equipped cars will use a MAP as a BAP (barometric pressure sensor), which takes a reading of the outside air so the ECU can compensate for high altitude recalculation of the load table. In other words, at higher altitude, the same load = less air mass.

If a turbo car uses a MAP that is plumbed into the intake, it can be assumed it is reporting manifold pressure pursuant to electronically controlling boost or otherwise.

The EVO's MAF is designed to pass air such that it causes a small mirror to vibrate at a frequency proportional to the volume of air. A laser bounce off the mirror measures the vibration frequency.

 

Looks like someone has had fluid mechanics

 

What you are saying then, is Karman Vortex MAFs (like our EVOs) need to have a MAP to help them intrepret the outside air density? It seems to make sense, but I'm under the impression that the less air you have the less "eddys" are formed in the Vortex and counted, meaning less air mass is going through the system.

Speaking of the higher altitude idea ... are you saying that if you took an EVO at a low altitude and put a very specific load at a specific rpm on it with only a MAF sensor and count so many "pulses" coming out of the box, and then took it to a higher altitude, and put a very specific load on it also and counted the pulses, the pulses coming out for the MAF would be exactly the same?

The short question is, less air density going through a static area, at the same speed as more air density would produce the same Karman Vortex MAF reading?

Thanks,

jcnel.

 

AFAIK, the K.V. meter measures the volume of air, apparently by measuring velocity through a given area. The ECU converts this to mass (kg/hr) using sensor input for ambient pressure and intake air temp. Even so, I don't know if the answer is 'yes' or 'no', as it may be that pressure fluctuations affect engine operation more so than they affect the accuracy of the meter, and/or the change in response of the meter may not be directly proportional to air pressure (i.e. a function may be used to calculate the estimated difference).

 

Exactly. von Karman meters are supposed to be very insensitive to pressure and temperature so the vortex shedding frequency is linear in velocity over a wide range of conditions. Way to go, Theodore!

Dave

 

Just pondering the whole K.V. concept all over again ... if you have a dense mass of air going through the meter, and a less dense mass of air going through the meter ... both at the same velocity, the denser air mass would effectively produce less eddys in the KV meter. I state this because dense air means moving ultimately more mass, and more mass would be harder to make a nice eddy. Now how much altitude effects this eddy, I don't really know.

Looking at this electronically, the MAP sensor wouldn't have to be a real high speed, well-filtered, type like we find on a lot of cars (Toyotas) ... just something they could even sense once every minute, or sometimes once every time the car is started. Its kinda like checking the daily weather forecast.

Another possibility for having a MAP is low velocity air readings in the KV meter ... like throttle plate closed, at the stop light, idle conditions. KV meters really have a tough time sensing the eddys. There could be an idle crossover point where the system uses a rough idea idle map using the MAP sensor, instead of the KV meter at that point.

I'm rambling at this point, but some good points have been brought up.

Cheers,

jcnel.

 

Where the MAP is used for BAP signal, you are correct that it is typically not a real-time measurement, but something that is sampled only periodically.

As far as K.V. signal, my initial suspicion is that realistically speaking, it isn't much of an issue. Furthermore, the O2 sensors provide continuous mixture correction, so it constantly updates what it learns from the MAF signal. Because of this, if you venture to an area of high altitude, the ECU will automatically compensate.

 

you don't necesssarily need either sensor..

there's at least one mirage that has NO airflow sensor at all. No Flap, no Karmen, no MAP

NOTHING>

 

Somehow, I don't think we'll be removing our sensors anytime soon.

 

You dont NEED any sensors if your car does not use them to calculate. Im not sure of any EMS that would not use one... it would be a huge waste. Carburated vehicles do not need one nor would it help except for tuning slightly. Our MAP sensor is used for limp mode supposedly, thats it. It is on the intake manifold and causes no harm when removed. It is not accurate enough to use for anything purposeful nor does it read high enough boost to be used as any boost guage (Even if it WAS accurate enough to do so). I dont understand why you care so much? You would like to log your boost using a stock sensor or what? You could buy a GM sensor that will work for many different tuning/logging devices.

ps: it would be completely stupid to remove any airflow sensor... they are highly effective for ensuring proper air/fuel mix. Don't ask me why a mirage would not use one unless it is carburated.

 

Would you like an answer to that question?

Your post did come up with some insightful information, thank you.

jcnel.

 

ok guys... so every MAF turbo car has to have a MAP sensor, correct?

 

No. A MAP sensor could replace a MAF sensor... they are both designed to measure the amount of air going into the motor... they do the same thing. Almost every stock ECU does not use any load maps based upon boost because they are not supposed to be changed. It would be like a vehicle using both a magnet speed sensor on a wheel and a tranny speed sensor in the tranny. It is two sensors doing the same thing. What would be the point in a stock ECU reason?