Today?s electronic engine management systems can process millions of instruction s per second to continuously adjust spark and fuel for optimum performance. The computer regulates the electronic fuel-injector pulse width (the time that the f uel injector is open) and ignition lead with input from various sensors. One of the key things the computer needs to know is how much air the engine consumes un der a given set of conditions. Three different measurement strategies have evolv ed to supply the computer with this basic information; in order of sophisticatio n they are: N Alpha, Speed Density, and Mass Flow metering. N Alpha A relatively simple design, N Alpha systems use only engine speed and throttle a ngle to calculate the amount of fuel needed by the engine. This system doesn?t m easure airflow directly; instead, engine load is assumed based on throttle-angle versus engine rpm. The various load-rpm points make up the computer?s lookup ta ble, with the amount of fuel needed at each point manually programmed by the tun er. N Alpha systems work well on engines that operate primarily at wide-open thr ottle?such as race cars?but are much less accurate at part-throttle than more so phisticated systems because of their relatively simple fuel map. They generally do not have a closed-loop mode for air/fuel correction, resulting in part-thrott le calibration that is crude at best when compared to other systems. This also m akes them incompatible with modern catalytic converters. Any significant engine change requires remapping. Speed Density Speed Density systems accept input from sensors that measure engine speed (in rp m) and load (manifold vacuum in kPa), then the computer calculates airflow requi rements by referring to a much larger (in comparison to an N Alpha system) prepr ogrammed lookup table, a map of thousands of values that equates to the engine?s volumetric efficiency (VE) under varying conditions of throttle position and en gine speed. Engine rpm is provided via a tach signal, while vacuum is transmitte d via an intake manifold-mounted Manifold Air Pressure (MAP) sensor. Since air d ensity changes with air temperature, an intake manifold-mounted sensor is also u sed. Production-based Speed Density computers also utilize an oxygen (O2) sensor moun ted in the exhaust tract. The computer looks at the air/fuel ratio from the O2 s ensor and corrects the fuel delivery for any errors. This helps compensate for w ear and tear and production variables. Other sensors on a typical Speed Density system usually include an idle-air control motor to help regulate idle speed, a throttle-position sensor that transmits the percentage of throttle opening, a co olant-temperature sensor, and a knock sensor as a final fail-safe that hears det onation so the computer can retard timing as needed. GM?s Tuned Port Injection (TPI) set-ups used Speed Density metering from ?90-?92 , as did ?91-?93 LT1 engines. All ?86-?87 and ?88 non-California Ford 5.0L-HO en gines used Speed Density metering. Most Mopar fuel- injection systems have used Speed Density too. Because a Speed Density system still has no sensors that directly measure engine airflow, all the fuel mapping points must be preprogrammed, so any significant change to the engine that alters its VE requires reprogramming the computer. Mass Flow By contrast, Mass Air Flow (MAF) systems use a sensor mounted in front of the th rottle body that directly measures the amount of air inducted into the engine. T he most common type of mass-flow sensor is the hot wire design: Air flows past a heated wire that?s part of a circuit that measures electrical current. Current flowing through the wire heats it to a temperature that is always held above the inlet air temperature by a fixed amount. Air flowing across the wire draws away some of the heat, so an increase in current flow is required to maintain its fi xed temperature. The amount of current needed to heat the wire is proportional t

o the mass of air flowing across the wire. The mass-air meter also includes a te mperature sensor that provides a correction for intake air temperature so the ou tput signal is not affected by it. The MAF sensor?s circuitry converts the current reading into a voltage signal fo r the computer, which in turn equates the voltage value to mass flow. Typical MA F systems also use additional sensors similar to those found in Speed Density sy stems. Once the electronic control module (ECM) knows the amount of air entering the engine, it looks at these other sensors to determine the engine?s current s tate of operation (idle, acceleration, cruise, deceleration, operating temperatu re, and so on), then refers to an electronic map to find the appropriate air/fue l ratio and select the fuel-injector pulse width required to match the input sig nals. GM used MAF sensors on the turbo Buick V-6 Grand National, ?85-?89 TPI, ?94-?98 LT1, ?96 LT4, and all LS1 engines. Ford has used MAF metering on ?88 California 5.0L engines and all ?89-and-later V-8 engines. MAF systems are much more flexible in their ability to compensate for engine cha nges since they actually measure airflow instead of computing it based on prepro grammed assumptions. They are self-compensating for most reasonable upgrades, as well as extremely accurate under low-speed, part-throttle operation. On the oth er hand, the MAF meter, mounted as it is ahead of the throttle-body, can become an airflow restriction on high-horsepower engines. On nonstock engine retrofits or EFI conversions on engines never produced with fuel injection, it may be hard to package an MAF meter within the confines of the engine bay and available int ake manifolding. Which Is Best? In a perfect world, virtually all street-performance engines would use Mass Air, due to its superior accuracy and greater tolerance for engine changes. In the p ast there was a problem on high-horsepower engines because larger-capacity MAF s ensors were scarce and prohibitively expensive. Nowadays, oversize MAF sensors a re available from Pro-M, Granatelli Racing, and other sources that are compatibl e with Ford engines and computers. Custom MAF calibration keyed to the specific vehicle, engine, and injector size is also available. With a correctly calibrate d oversize meter, reflashing the Ford computer usually isn?t required. (However, before you run out for a larger Ford MAF meter, Fast Track Performance points o ut that the first limiting factors are the puny Ford 19 lb/hr injectors, which c an only support about 320 hp.) Some oversize MAF meters have also become available for the second-generation (? 94-and-later) GM MAF systems, but the selection isn?t as broad as for the Ford g uys. The GM MAF engine management computer isn?t as adaptable as Ford?s. Althoug h it will accept larger MAFs, you can?t go up and down more than one injector si ze with reflashing the computer. Bigger MAF meters are not readily available for old GM TPI systems, but Granatel li says it is possible to adapt Ford meters to them via a conversion wiring harn ess. Custom calibration is required, so Granatelli prefers to do the changeover in-house. For radical engines or engines never produced with fuel injection, an aftermarke t user-programmable computer system is usually preferred. Unfortunately, the mor e-or-less affordable aftermarket systems?including ACCEL/ DFI, Speed-Pro, and Ho lley?are Speed Density?based and don?t support Mass Air metering. Those systems that do?including Electromotive, Motec, and Pectel units?are more costly, someti mes considerably so. However, Westech Performance reports that it is possible to use Pro-M?s adjustable Optimizer MAF meter and a stock Ford Mustang computer wi

th Ford?s EPEC piggyback programmable unit to run Mass Air on any engine. If it is not practical to use MAF on your engine due to packaging or hardware co nstraints, the programmable Speed Density systems are the next best choice becau se production-based Speed Density systems won?t tolerate major engine changes wi thout computer reprogramming, which usually requires the services of an outside specialist; if the reprogrammer isn?t specifically familiar with your combo, the end results may be less than satisfactory. On radical engines (those with cam duration over 240 degrees at 0.050 or less th an 10 inches of idle vacuum), even user-programmable Speed Density systems have difficulty due to an erratic or insufficient manifold vacuum signal. If the appl ication is for a race car operated primarily under full throttle, N Alpha is the solution. If you intend to drive on the street, a system that blends N Alpha wi th Speed Density?varying which is in control per specific operating point and co nditions?may be the answer. The higher-end aftermarket systems, including Electr omotive?s, support this option. As electronic engine-management system usage becomes more widespread in the car crafting community, prices and ease of use should become more user-friendly. Alr eady, the latest Gen VII ACCEL/DFI system has the ability to construct its own b aseline fuel curve, and the new user programming interface is a full-fledged, Wi ndows-compatible program. Can voice command be far behind? Beam me up, Scotty.