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IGNITION, FUEL SYSTEMS AND ELECTRICAL PROPERLY SELECTING ELECTRONIC FUEL INJECTION COMPONENTS (continued...) As mentioned earlier, all injector flow rates published in this catalog have been determined at a pressure of 39.15 psi (270kPa) across the injector, but to what does the phrase “across the injector” refer? To understand this fully, we first need to discuss three different methods of measuring pressure. The first is called absolute pressure. This is defined as the pressure relative to a complete vacuum, such as would be found in outer space. For instance, atmospheric pressure (the air we breathe) is typically around 14.7 psi absolute (29.93inHg) at sea level, depending on temperature and weather conditions. An engine that has a vacuum signal of 12 “inches” simply means that the absolute pressure in the intake manifold is 12inHg less than the atmospheric pressure. When you subtract the 12inHg from the atmospheric pressure of 29.93inHg, you are left with a positive pressure of 17.93inHg, or roughly 9 psi absolute as compared to a complete vacuum. Sometimes you will see absolute pressure in psi written as “psia.” The second is called gauge pressure, which is pressure relative to atmospheric pressure. In general, everyone is most familiar with gauge pressure, because it is what you measure when you check the air in your tires or when you connect a fuel pressure gauge to the fuel rail. An engine which makes 6 psi of boost at sea level is actually equivalent to 20.7 psi absolute, (14.7 + 6 = 20.7). Sometimes you will see gauge pressure in psi written as “psig.” The third is called delta pressure and is very much like gauge pressure, but instead of being relative to atmospheric, it can be relative to any other pressure, such as the pressure in the intake manifold. Sometimes you will see delta pressure in psi written as “psid.” When we quote pressure “across the injector,” what we really mean is the delta pressure (or difference) between the fuel rail and the intake manifold. On CRP systems, the rail gauge pressure is constant while the delta pressure varies depending on manifold pressure. This means if a fuel pressure gauge is connected to the rail, the reading it gives will be constant. On CIP systems, the system controls the delta pressure, either by use of a mechanical regulator referenced to the intake manifold (in a traditional or “return” system), or by the use of the FRPT and the PCM (with ERFS). This means that if you connect a fuel pressure gauge to the fuel rail on one of these systems, you will see fuel pressure vary depending on intake manifold pressure. This is because the gauge is measuring gauge pressure, which is relative to atmospheric, but the EFI system is controlling the fuel rail pressure relative to intake manifold pressure which is changing depending on engine load (your right foot) among other things. On a naturally aspirated engine, the manifold pressure at idle is typically around 10 psia, and the manifold pressure at WOT will be atmospheric, so typically at the fuel rail you will see approximately 30 psig at idle and at least 39.15 psig at WOT, depending on whether or not you have ERFS and whether or not it is boosting pressure for one of the reasons mentioned in the previous section. On a forced-induction engine, the highest manifold pressure that the engine can reach will be atmospheric plus the maximum boost your configuration can obtain. This means that to keep 39.15 psid across the injector, the gauge pressure will have to increase by the same amount as the maximum boost. A couple of examples should make these concepts more clear. First, consider a naturally aspirated conventional return fuel (non-ERFS, non-MRFS) EFI system with a mechanical vacuum referenced regulator set at the stock pressure setting. The system will try to keep the pressure across the injector at 39.15 psid regardless of engine load, so if you have a fuel pressure gauge attached to the fuel rail, you will see a maximum pressure of 39.15 psig at WOT if the system is doing its job properly. Now consider a forced-induction engine making a maximum of 10 psig boost, also with a conventional EFI system and mechanical regulator set to the stock pressure setting. The system will still try to keep the pressure across the injector at 39.15 psi, so this time your fuel pressure gauge attached to the rail should read a maximum of 39.15 + 10 = 49.15 psig. If it never gets to 49.15 psig at WOT, your fuel system is inadequate for your engine. You will need to either increase the capacity of the pump, minimize the voltage loss between the pump and the battery or decrease the pressure loss between the pump and the engine through the use of larger lines, etc., and re-test. Do NOT try to “tune around” this type of fuel delivery problem. It will bite you in the long run, and can result in hard-to-diagnose problems at best all the way to engine failure at worst. Note that during a WOT event, the fuel pump in the forced-induction engine must supply fuel at a higher pressure than in the naturally aspirated engine. As mentioned in the previous section, this means that the fuel pump supplying the forced-induction engine will have a lower maximum flow rate capability than the fuel pump supplying the naturally aspirated engine. This is a critical concept to grasp because it means that in general, for engines with equal brake horsepower, the fuel pump supplying the forced-induction engine will need to have more capacity than the fuel pump supplying the naturally aspirated engine! For important information about the proper usage of performance parts, please see page 14. See pages 286-292 for important safety, emissions and warranty information. www.fordracingparts.com 215


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