Figure 5 details what the transfer function for a stock Ford 55mm MAF sensor looks like. As you can see when airflow increases, the amount of voltage also increases. If you were to install 30# injectors into a 5.0 Mustang and not re-calibrate the air meter or the EEC, it would run way to rich and pump out lots of black smoke. The reason for this is because the EEC still thinks it has 19lb injectors installed, which flow much less fuel at any given pulsewidth than the 30lb injectors. So when the EEC goes through its calculations to figure out how much to pulse the injectors, way too much fuel will be injected. Here's the trick the aftermarket MAF people do (not including Kenne Bell which is calibrated in the same fashion as the stock Ford MAF is). Since the EEC looks at the MAF sensor voltage to determine airflow, what if we were to fool the EEC into thinking it had less air coming in, therefore it would calculate a smaller pulsewidth? Bingo! That's exactly what they do. There are a couple different ways to accomplish this. One way is how Pro-M and Ford Motorsport do it. They open up the electronics on the MAF sensor and modify the circuit to lower the output voltage of the MAF sensor. The shape of the voltage airflow curve remains the same (hopefully), but it is shifted down by a ratio of old injector over new injector. This means the output voltage curve of the MAF electronics is scaled by the ratio of the two injectors. In our case we had 19#'ers and switched to 30#'ers (19 / 30 = .63) which means the new curve is only 63% of the old curve. Fig 6 shows this graphically. Now when the EEC looks at the voltage, it now thinks it's getting less air, and less air means less fuel needed, so it will calculate a smaller pulsewidth which is hopefully close enough to deliver the right amount of fuel.
Another method of fooling the EEC is the way C&L/Vortec do it. By changing the ratio of the main bore of the MAF sensor to the sampling tube, you can make the MAF look like it's getting less airflow too. These MAF's use the stock MAF electronics and vary the output voltage curve mechanically. The last way I have seen to fool the EEC is the way Auto Specialties does it. Their method is similar to C&L/Vortec, but they also use a screw positioned in the sampling tube in order to fine tune the bore to sampling tube ratio. By moving the screw in and out, you change the ratio. Each of these methods looks like it should work well, in theory. There is a problem with fooling the EEC in this way and it's called Load. Remember Load is calculated by the ratio of incoming air to how much the engine can hold. Well, now the incoming air information is all wrong so the Load calculation is all wrong also. Since Load is used to determine what A/F ratio and what spark advance to run at any given RPM, you can probably guess that with a re-calibrated air meter you no longer run correct fuel and spark - and you'd be right! By looking back at fig 4, you'll see the RPM Vs Load spark table and if you notice as you go up in Load, the amount of spark advance goes down. With the 30# re-calibrated air meter installed, Load is going to be roughly 35% less than what it actually is. What you end up with is at some RPM / Load points your running more spark advance which is like bumping up the base timing and makes the car a bit quicker. At other RPM / Load points the spark is so over advanced you can get surging, detonation at part throttle or just plain slow down. Look at fig 4 with the spark table for a 93 5.0L Mustang. Go to 1500 RPM and let's say the engine is operating at a true .60 Load, but the re-calibrated air meter is tricking the EEC into thinking it's only running at .40 Load (roughly 35% lower than actual). Notice that the base spark advance is a whopping 17 degrees over advanced! That's just like setting your base distributor timing at 27 degrees!!! At Wide Open Throttle on a 93 5.0L Mustang, the over advanced situation goes away since the EEC only uses RPM to figure out spark advance, but the SN-95's use Load ALL THE TIME! Now look at fig 3 to see how it can affect fuel on a 93 5.0L Mustang. Let's say we are running 180 degrees engine coolant temp and .70% load. Normally we would want to run somewhere around a 13.04:1 A/F ratio, but since Load is goofed up we actually run near 14.64:1 which is quite a bit leaner than you'd want to be. Figs 5 and 6 show a 95 5.0L Mustang fuel and spark table. Ever wonder why the SN-95 cars seem to be a bit more tricky to get running good? Now you know.
The larger the injector the MAF is calibrated for, the worse everything gets since the error in the Load calculation gets bigger and bigger. Now these problems don't happen to everyone and hopefully I didn't scare anyone by writing this. But if you are experiencing derivability problems and you have a re-calibrated MAF, now you know the reason why it runs like it does. Now there is a benifit of having a MAF sensor re-calibrated and it's sort of a side effect of the process. If you run a stock air meter on an engine that can really pull a lot of air, such as those with a supercharger, you can peg the MAF sensor's electronics. Depending on what year Mustang you have, the 'peg' voltage is somewhere around 4.85 or so volts. The EEC will look at this voltage and think there is something wrong. The check engine light will pop on and depending on how your engine is set up, you could blown a head gasket or worse. What happens is the EEC will think the MAF sensor is bad and use a default air charge table to get it's airflow values based on throttle position and RPM. Normally this table is calibrated so the engine will run richer than normal which doesn't do any harm. But if your pushing lots of boost, it might not be enough fuel. Since the re-calibrated MAF sensor's voltage curve is now lower than the stock one, it takes an awful lot of airflow to peg the meter. That's the good side effect from this type of MAF re-calibration!
Hopefully, this clears up some of the misconceptions about how the Ford MAF system works. If you have any further questions, feel free to e-mail me by click my name above and I'll be glad to answer any further questions you might have.
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