MTCFORUM
02-13-2002, 09:33 PM
Everyone wants to get an air meter, I dont' think I've ever explained recently, how the EEC uses the air meter signal.
First off, they are not air meters, they are MASS air meters on these cars. The older cars, like turbocoupe, used a vane air meter than measured the volume of air, not the mass.
The air meter actually measures mass of air that is flowing into the engine. The EEC also knows the mass flow rate of the injectors. The injectors on Marks are 24#/hr. Finally the EEC knows what air/fuel ratio to command. If you are flowing, for example, 20#/min of air into the motor, and want to run 14.64:1 air/fuel ratio (which happens to be the chemically correct a/f raito for heptane, which is basically gasoline). So with 20#/min of air and a desired a/f ratio of 14.64, you'd need 20/14.64, or 1.366#/min of fuel. Just for your minds sake, let's multiply this by 60 to get #/hr of fuel, so we need 1.366 times 60 or 81.97#/hr of fuel through all the injectors. Since these engines have 8 injectors, each injector would have to inject 81.97/8 or 10.25#/hr of fuel. With a 24# injector, this is 10.25/24 or 42.7% of full injector opening.
This part is straight forward. If you change air meter and don't tell the EEC, you will deliver the wrong amount of fuel, period. But what else happens...
The EEC calculates what's called load. Load is nothing more than volumetric efficiency or VE. VE is the efficiency of the engine to pump air. Since this these are 4.6L engines, in two engine revolutions, you can move in and out 4.6L of air. If the engine only moves 2.3L of air, then it's VE is 2.3/4.6 or 50%. Since the EEC knows mass of air, air temp and barometric pressure, it knows the volume of air that is moving through the engine at all times, so it can calculate VE.
The spark and fuel tables are tables for the amount of spark to deliver and the a/f ratio to command if you are NOT open loop. Open loop is when you are richer than 14.64:1 a/f raito and no longer using the O2 sensors for feedback, this could also be called power enrichment, a throw back to the carb days. The spark and fuel tables have RPM on the X axis and load on the Y axis. The output is then spark or a/f ratio. If your air meter transfer function is off, this is the input voltage from the meter to the EEC and what the flow is at that voltage, you will calculate VE wrong and hence grab the wrong spark and fuel values from the table.
This is why the air meter transfer function is so critical to making the engine work right. VE gets an input into almost everything, even what the trans feels like when it is shifting.
Now, someone will say that you can get an air meter calibrated for a given size injector and not change the EEC. This is right and wrong. Let me explain.
Let's say you have 19# injectors and some air meter. You want to put in 38# injectors. You send your air meter off what they do is TRY to make it so at every voltage point your reading out from the air meter is original injector size divided by new injector size, in my example, 19/38 or 1/2 of the original value. Now, the EEC thinks there is very little air flow into the engine. Let's say before at 2 volts it was 12#/min, now with the "new" air meter, to get that same 12#/min you are at 1 volt. So, you now running at 1 volt with 12#/min of air and the EEC thinks you are running much lower flow, thus giving you a lower fuel pulsewidth, in fact, if the air meter was set up right, you will have 1/2 the pulsewidth and the amount of fuel going into the engine will be correct.
This is the part that's right. Now the wrong part. Load is calculated wrong, in this example,it's 1/2 of what it should be. So, you grab the wrong values out of the spark and fuel tables. This typically ends up in knock on some cars, since you are leaner with more spark.
Questions?
jerry
First off, they are not air meters, they are MASS air meters on these cars. The older cars, like turbocoupe, used a vane air meter than measured the volume of air, not the mass.
The air meter actually measures mass of air that is flowing into the engine. The EEC also knows the mass flow rate of the injectors. The injectors on Marks are 24#/hr. Finally the EEC knows what air/fuel ratio to command. If you are flowing, for example, 20#/min of air into the motor, and want to run 14.64:1 air/fuel ratio (which happens to be the chemically correct a/f raito for heptane, which is basically gasoline). So with 20#/min of air and a desired a/f ratio of 14.64, you'd need 20/14.64, or 1.366#/min of fuel. Just for your minds sake, let's multiply this by 60 to get #/hr of fuel, so we need 1.366 times 60 or 81.97#/hr of fuel through all the injectors. Since these engines have 8 injectors, each injector would have to inject 81.97/8 or 10.25#/hr of fuel. With a 24# injector, this is 10.25/24 or 42.7% of full injector opening.
This part is straight forward. If you change air meter and don't tell the EEC, you will deliver the wrong amount of fuel, period. But what else happens...
The EEC calculates what's called load. Load is nothing more than volumetric efficiency or VE. VE is the efficiency of the engine to pump air. Since this these are 4.6L engines, in two engine revolutions, you can move in and out 4.6L of air. If the engine only moves 2.3L of air, then it's VE is 2.3/4.6 or 50%. Since the EEC knows mass of air, air temp and barometric pressure, it knows the volume of air that is moving through the engine at all times, so it can calculate VE.
The spark and fuel tables are tables for the amount of spark to deliver and the a/f ratio to command if you are NOT open loop. Open loop is when you are richer than 14.64:1 a/f raito and no longer using the O2 sensors for feedback, this could also be called power enrichment, a throw back to the carb days. The spark and fuel tables have RPM on the X axis and load on the Y axis. The output is then spark or a/f ratio. If your air meter transfer function is off, this is the input voltage from the meter to the EEC and what the flow is at that voltage, you will calculate VE wrong and hence grab the wrong spark and fuel values from the table.
This is why the air meter transfer function is so critical to making the engine work right. VE gets an input into almost everything, even what the trans feels like when it is shifting.
Now, someone will say that you can get an air meter calibrated for a given size injector and not change the EEC. This is right and wrong. Let me explain.
Let's say you have 19# injectors and some air meter. You want to put in 38# injectors. You send your air meter off what they do is TRY to make it so at every voltage point your reading out from the air meter is original injector size divided by new injector size, in my example, 19/38 or 1/2 of the original value. Now, the EEC thinks there is very little air flow into the engine. Let's say before at 2 volts it was 12#/min, now with the "new" air meter, to get that same 12#/min you are at 1 volt. So, you now running at 1 volt with 12#/min of air and the EEC thinks you are running much lower flow, thus giving you a lower fuel pulsewidth, in fact, if the air meter was set up right, you will have 1/2 the pulsewidth and the amount of fuel going into the engine will be correct.
This is the part that's right. Now the wrong part. Load is calculated wrong, in this example,it's 1/2 of what it should be. So, you grab the wrong values out of the spark and fuel tables. This typically ends up in knock on some cars, since you are leaner with more spark.
Questions?
jerry