Ford Mustang EFI System Tuning - Inside The Black Box Part 2 - Tech

Welcome back for our second look inside the mysterious black box that we call the Ford EEC system. Last month we covered the basics of EFI, with an emphasis on the Ford systems. This month we'll delve into the specifics of the Ford EEC.

But before we start changing around ones and zeros in the programming, we need to cover some of the hardware upgrades and the theories needed/required for performance applications. These can include properly sizing injectors, MAF sensors and fuel pumps, big cams, big-port heads/intakes, superchargers, low restriction cold-air intakes, free-flowing exhaust, and more. These are all common mods when seeking additional horsepower.

What they all have in common is the ability to stuff more air into the engine. The fuel side is certainly less glamorous than adding a supercharger, for example, but you can't ignore the fuel side if you want safe, reliable power gains. With more air, more fuel is needed. It's that simple.

To understand the fuel requirements, you need to first understand Brake Specific Fuel Consumption (BSFC). BSFC is a measure of your engine's efficiency, and it's simply the ratio of fuel flow (in lb/hr) divided by the brake horsepower output. Really efficient naturally aspirated gasoline engines run in the low 0.4 lb/hr-hp range, while richer-running supercharged engines can be upwards of 0.6 lb/hr-hp.

To size your fuel system, start with a realistic estimate of your BSFC (higher numbers are safer to go with if you're not sure), and your estimated horsepower output (again, try to be realistic here). Let's say we're expecting 500 flywheel horsepower, and we'll be running a supercharger. We're expecting a BSFC of 0.6. To find the fuel-flow requirements, multiply the horsepower by the BSFC to get 500 x 0.6 = 300 lb/hr. To convert to gallons/hr, divide by 6.2 to get 48 gal/hr. To get liters/hr multiply by 4.78 to get 183 L/hr.

Now this doesn't seem like much, but it's the minimum fuel flow you need to see at the fuel rails to support 500 hp at maximum fuel pressure. Fuel flow drops off dramatically when you crank up the pressure the pump has to push against (keep this in mind if you're running boost). When looking at fuel-pump flow rating, check carefully how much it actually flows at the fuel pressure you need. Also, keep in mind it's tougher for the pump to push the fuel forward from the tank to the engine under acceleration, so you always want to size your pump a bit bigger. In the above example, we'd go with a 255 L/hr unit and we should be good.

Regardless of how much math you do ahead of time, it's always good to either log fuel pressure during dyno runs (newer EEC V), or install a fuel pressure gauge in the car (older EEC IV). If your fuel pressure starts dropping as rpm/hp increases, it's telling you your pump is undersized, or you have some other restriction in your fuel system. Fix it before it costs you an expensive engine.

For injector sizing, we can use the same math as above to get the fuel flow rate in lb/hr. In our previous 500hp example, we calculated 300 lb/hr. If we have eight injectors, then each needs to flow 300/8 = 37.5 lb/hr. Again, this is the absolute minimum injector size to flow the required fuel at 100 percent duty cycle (DC). We want a bit of a safety factor, and we'd rather not have to run our injectors at anything more than say 85 percent DC. So, taking 37.5/.85 = 44 lb/hr, we could use a 42-lb/hr injector here, but we'd end up pushing it to an 89 percent DC at peak horsepower.

Another solution to run the 42-lb/hr injectors at no more than 85 percent DC would be to run them at a slightly higher fuel-pressure drop. Since our calculated injector size was 44 lb/hr (at 85 percent DC), we need to increase fuel pressure by (44/42)2 = 1.098, or roughly 10 percent. Taking our 39-psi fuel-pressure drop and multiplying by 1.098 gives us about 43 psi. Running a fuel-pressure drop of 43 psi would therefore allow us to run the 42-lb/hr injectors safely.

Lastly on the list to ensure adequate fuel delivery is the MAF sensor. Why discuss a MAF sensor when talking about fuel-system capacity? Well, the ECU can accept only 5 volts as a maximum output from a MAF sensor. If you start flowing enough air through your MAF that it outputs more than 5 volts, the ECU will not "see" that additional airflow and hence not add any more fuel to the mix (i.e., your MAF is "pegged"). This is now dangerous territory, since more air without more fuel equals a lean condition, usually followed by a big BANG!

For our 500hp example, we can calculate the expected airflow at peak horsepower. If we're targeting a safely rich A/F ratio of 11.5:1 (for our supercharged engine), 300 lb/hr of fuel flow will correspond to 300 x 11.5 = 3450 lb/hr of airflow. Dividing by 2.2 gives us 1,566 kg/hr. On a 5.0 H.O. engine, the stock MAF sensor hits 5 volts at only 835 kg/hr. You can now see why running the stock MAF calibration would be a problem on a 500hp engine.