VAF to MAF Conversion??

[IMACHU2]

I remember hearing that someone put a V6 Mustang Mass air flow sensor in place of their VAF. Does anybody have an artical / thread or information on this swap??

[wytbishop]

Look on ProbeTalk...they do all kinds of stuff like that. If it's been done, it was done by one of them.

[Nd4SpdSe]

Hmm, that's an interesting idea, but you'd have to get the resistance values. Besides, the ECU is designed to calculate based on volume, not mass (this being the difference between VAF and MAF). Even just changing between a DE VAF and the K8 VAF changes the amount of fuel delivered to the ECU. I know the Mazda VAF looks to be quite restrictive, and I don't blame then, but I think the only solution that works would be Megasquit and the GM IAT

[se7en]

second. That's what I am running....

no vaf, maf, or anything in the intake tract

[wytbishop]

Besides, the ECU is designed to calculate based on volume, not mass (this being the difference between VAF and MAF).

Actually Mike that's not correct. The air entering the engine must be measure by mass or else a barometric pressure sensor must be used to compensate for altitude and temperature (Ideal Gas Law). Our cars do not have any sort of pressure sensing device. The cone in the VAF is acted on my a calibrated spring to hold it closed. The mass of the air entering the engine opens the VAF. Two cars running at the same RPM but at different altitudes would have different air/fuel ratios because the denser air at lower altitude has greater mass for the same volume and imparts a greater force on the cone of the VAF.

Unless I'm wrong and the MX-3 has a baro sensor that I don't know about. That would make me sound pretty stupid.

[Daninski]

I thought VAF and MAF were the same thing just different names. Ya I run the KL02 on my 96 ze.

[wytbishop]

I must correct myself on one point. When I said that two cars at different altitudes would have different a/f ratios that was incorrect. I meant to say that they would be burning different amounts of a/f mixture.

Car A running at the same RPM as car B but at sea level would burn more fuel because the VAF would open wider due to the greater density if the air...so more fuel would be delivered to maintain the a/f ratio.

[Nd4SpdSe]

I still stand by what I said I'd get into it more but I've been and am busy

Btw, I'll be by this week to get those links. I had planned to last week but things really didn't work out.

And on to my defence:
http://www.autoshop101.com/forms/h34.pdf" onclick="window.open(this.href);return false;

[Ryan]

I think I actually agree with wytbishop... Mark the calendar.

does the MAF actually use hotwires? I don't have on of these....

[wytbishop]

Mike...

They are using the word "Volume" in a generic way and it is incorrect. They say volume when talking about the Mass Air Flow Sensors as well as the Vane Air Flow sensors. They are generalizing to speak to people in laymans terms.

In Fluid Mechanics the steady state flow equations always work with mass. Whether talking about a gas or a liquid or a mixture, mass in used because it doesn't change with respect to temperature or pressure. If you look at the VAF example they show with the trap door style measuring plate acted on by a coil return spring, you can imagine the force of the air on the plate and the force of the spring resisting it. If you imagine the air which is in contact with the cross-section of the door you can see that at any given instant there is a certain mass of air pushing on it. At higher ambient pressure or lower temperature, there are more air molecules in that cross-section and so the door would be pushed harder. That would result in a larger opening and while there would be a larger volume of air flowing through the VAF, that is not what is being measured.

Volume is used to describe the function of the VAF (and MAF) because most people do not understand the relationship between volume and density.

[Nd4SpdSe]

You know, i actually see that now...

However, my mind came up with something, as it tends to do...but that would only be 100% correct if the air was pushing against the VAF, but the engine creates a suction effect, so I'm wondering if it's not the force of the air pushing the vaf but the suction of the motor pulling it back...that's where the ECU would only see the volume of the air and not know it's actual mass. The MAF system would probably compensate for that and require the air to push against it. Also a VAF type system wouldn't be able to see the differences and measure accordingly when the VAF is maxed out and take any additional measurements beyond that.

[wytbishop]

You're right that the negative pressure behind the VAF cone (or door) does assist in opening the VAF. Even considering that the negative pressure behind the cone of the VAF is, in part, what creates the force which opens it, in terms of physics that suction is a function of the mass of the fluid in the stream not the volume.

In reality the mechanical means of opening the air flow measurement device, whatever it is, is not really important at all. The key point is that the ECU has to take what ever data is collected and use it to calculate the mass of air coming into the engine in order to accurately calculate the correct mass of fuel to inject. All the ECU sees is the value returned by a potentiometer in the VAF and compares it to a table (the fuel map) which says for (X)volts open the injectors for (Y)milliseconds.

Measuring the volume and calculating the mass based on the pressure and temperature of the flow would actually be more accurate and easier to modulate. More expensive machines do that (and most motorcycles). It requires at least one more sensor (2 more to do it properly) and adds additional cost which is why it's not common on low end cars.

[OROutdoors]

OLD THREAD RESURRECTION ALERT!

I just happened to read this thread because I too was wondering about the mod of replacing a VAF with a Mustang MAS. When I happened to read this thread, I noticed something wrong with what someone said regarding the physics of a gas (air), and want to correct it. Vaccuum is a matter of perception; you can't pull air. You can only push air. Air flows from high pressure to low pressure areas.

When the piston moves down during an intake stroke, it creates a low pressure area. Since the intake valve is open, the higher pressure air in the intake manifold pushes air into the cylinder. The piston doesn't draw the air into cylinder, the air gets pushed into the cylinder by the higher pressure in the intake manifold.

Higher pressure pushes air in all directions, not just toward the low pressure area. In the case of an intake tube (like on a CAI), the walls are solid, so despite the air pushing against the walls, the only place the air can flow is toward the low pressure end of the tube -- toward the intake manifold. So, in relation to the discussion about MAF and VAF, the only way those devices can open is because there is a higher pressure on the filter side of the sensor, that higher pressure pushes the air toward the low pressure areas. In doing so, the air is pushed against the cone of the VAF or the flap of the MAF. That air pressure is what causes the sensor to move -- it is not being sucked from behind -- it is being pushed from in front.

This concept of flow from high pressure to low pressure is critical in understanding Natural Aspiration. Using NA, the pressure in the intake manifold will always be slightly lower than the pressure just outside the filter -- the friction the filter, tubes, MAF or VAF and throttle body apply to the air passing through them has the net effect of reducing the pressure in the intake manifold. The better those restrictions are reduced, the higher pressure in the intake manifold. The greatest pressure in the intake manifold will be achieved when the engine is off. The the pressure will normalize to approx. 1 atmosphere. While the engine is running, on a NA car in proper operating order, the pressure in the intake manifold will be slightly less than 1 atmopshere.

[Note: the "bowl" on the manifold side of the intake valve can have greater pressure than the intake manifold for a very short moment every cycle. As the air has mass and is moving toward the intake valve when it is open, then when the intake valve shuts suddenly, the momentum of the air continues for a moment. Since there is no where for air to move, it hits the end of the tube and stops, building pressure until it the pressure pushes the air backup to tube. This all happens very fast and wouldn't be noticeable to a normal person. People who blueprint engines use this effect to help push air into the engine at high RPM.]

Eric
If this topic has already been covered in a different thread, I apologize for spamming you all.

[wytbishop]

I have actually thought about this thread more than once in the years since we had this discussion. I wondered when it woul dcome up again.

You are absolutley right Eric and reading it again I recognise that I was a bit too loose with the term vacuum. the VAF isn't being sucked open from the negative pressure in the manifold, it's being pushed open by the air on the other side which is moving towards that low pressure area to equalize it.

The point I was trying to make then, is that the "V" in VAF doesn't stand for "Volumetric" and it doesn't measure volume. Then I just got going. I have tried to curb that tendancy since then.

[OROutdoors]

Then I just got going. I have tried to curb that tendancy since then.

I hear you brother! Me too - LOL!

BTW, I did appreciate what you wrote (other than the vacuum part). You seem to have an excelent grasp of Newtonian Physics. I recently read in the back of a SpeedPro Series book about distributor type ignition systems about other SpeedPro Books. They have a book called "How to Build, Modify & Power Tune Cylinder Heads". According to the teaser info, the book has info on how to build a flow bench. That sounds really cool! I think I'll buy the book and see how expensive (or not) it is to build such a beast, and then run tests... It would be interesting to see how varous tasks have a net-effect on flow.