I’ve placed this inquiry in the 2.6, 2.8, 2.9, 4.0 and SOHC 4.0 V6V6 Tech section because I am referring especially to the 2.9 Liter Engine non-California Emissions model. I’ve been trying to understand the basis of the speed density method. I understand that by it mass air flow is determined by utilization of a formula. To complete the formula the MAP sensor, Engine Speed sensor and Air Temperature sensor is used and the mass air flow rate is then calculated by the EEC-IV. My question is this; dose anyone know the actual formula and how values determined by each sensor are used in it?

 

I dont know the actual formula, but I can tell you exactly how they work. I have a programmable speed-density system on my 390. They're really not that hard to understand.

There is a base fuel map, with RPM on one axis, and the MAP sensor value on the other. The computer is then programmed with modifiers for such things as air temperature, coolant temperature, rate of change of throttle position, etc. It then determines the needed injector opening amount.

To get the actual formula you would need to hack the computer. In most cases, it's not necessary to change the formula itself, just change the values in the computer. That is what you need to be concerned with.

 

I don't know specifically how it's programmed into the comptuer, but the basic equation that governs the speed density system is the ideal gas law, which you can study in any intro to chemistry book. P(V/n)=RT P=absolute pressure, V=volume, n=moles of gas, R=universal gas constant, T=absolute temperature. V/n is the inverse of the density.

Given a nominal 2.9L displacement, that means the engine moves ~2.9 L of air every 2 rotations (4 stroke engine). V=(dis/2)*(rotations/min)=V/min. Put this into the ideal gas law, and you get n/min of air. Ideal fuel mixture is like 1 part fuel/14.7 parts air, so we can now calculate how much fuel to deliver. Knowing the properties of our fuel injectors, the computer calculates how long to leave the injectors open. Then, using the feedback loop governed by the o2 sensor, this value is fine-tuned to give the optimum fuel mixture. The computer then used the other sensors to further optimize for the specific operating conditions.

As I understand it, that's the theoretical basis.

 

Thank you, for your quick responses. The information is quite informative. I've always preferred speed density due to its easy maintanance and the map sensor’s greater reliability verses a mass air flow sensor, but I’ve never quite understand how it worked.

 

Just from reading the Factory Service Manual, one comes upon Ford's claim that the EEC-IV utilizing the speed density method is adaptable to changes. Outside of installing an aggressive cam I wonder just how adaptable it really is. Based on the ideal gas law and its applicability in the speed density method am I correct in saying that’s as the pressure sensed by the MAP sensor decrease the mass of air increases?

 

About as far as the O2 sensor compensation limit.

You are correct, in that when the MAP sees higher absolute pressure there is more air flow, but the computer does not know this. The actual airflow is inferred based on displacement, cam timing, MAP value, and RPM. The tables in the computer are calibrated for a specific engine efficiency at specific loads and RPM's.

When you port the heads and add headers, for instance, it changes the airflow efficiency through the engine. So it becomes more efficient at certain RPM's and less so at others. The computer doesn't know this and runs it lean or rich. WOT, with airflow increased, it would run lean. At low RPM's, velocity has probably changed and may cause it to run rich. Now like I said, in closed loop (low engine loads) the O2 sensor would probably be enough to compensate, but at WOT you would absolutely be running lean.

 

Would this only prove true with an aggressive cam? What effect would a mild cam have on the computer, would it not then been able to compensate even at WOT? <?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>

 

The computer does NO compensation at WOT! Speed density simply has no direct means of measuring airflow. It simply reads the RPM, MAP (obviously at 100%) and injects the factory predetermined amount of fuel. Any time you increase airflow, by any means, it will cause the engine to run lean.

Consider this. What happens to intake manifold vacuum, at WOT, when you increase the flow capacity of the cylinder heads and exhaust, either through headers, porting, or cam? The intake manifold vacuum increases! So the computer goes to a leaner setting, precisely when it should be going to a richer setting. Even if the intake has plenty of flow capacity and vacuum does not increase, there is more air flowing through the engine and more fuel should be injected. Speed density simply will not compensate for this.

It is possible that a mild cam close to the stock specs would work ok. But if you're not increasing the airflow, where is the advantage of switching out the cam? I'm sure there are "speed density approved" cams, as well as numerous stories of people putting headers, etc on their speed-density equipped engines. It's just a matter of amount. Were it may have been running 13:1 fuel to air ratio before, maybe it's running 14:1 after! You'd never know it. To do it right you'd need to add more fuel for the extra air. Speed density simply doesn't do it in open-loop operation.

Are you starting to see the problem?

 

As long as you dont change the vacuum signal to the map sensor, usually the engine can be tuned with timing and fuel pressure. Thats why you can get away with just about anything but a radical cam change on the SD computer.

 

Or bigger injectors. But you better be sure you have a wideband O2 sensor to make sure you get it right!

 

I hear a lot of talk about wideband O2 sensors, what exactly are they and how are they different to the stock O2 sensor?

 

Specifically what would you change with the timing?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

 

Wide band O2 sensors will tell you the actual air/fuel ratio, whereas narrow band O2 sensors, like the computer uses, are only accurate near stoichiometric. Normally people put the wide band O2 sensor somewhere in the exhaust and have a readout on the dash to aid in tuning.