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Some information i came accross and would like to share

Properly selecting electronic fuel injection components.

One of the most commonly misunderstood aspects of electronic fuel enjection is how to selev the correct size fuel injectors, fuel pump and mass air flow (maf) sensor for a particular engine horsepower output. The following information is inteded to offer a very brief tutorial on properly selecting the most common EFI components.

Fuel injectors

First and foremost, adding larger fuel injectors alone WILL NOT create extra horse power. The purchase of larger fuel injectors should only be considered when your engine has exceeded the horsepower capacity of the existing fuel injectors, at which point larger injectors are then required to SUPPORT teh additional horsepower. If you add larger injectros than-stock injectros to an otherwise stock engine, you should not expect any horsepower increase whatsoever.

The nominal injection pressure for most Ford EFI systems is 39.15 psi (270kPa) "across the injector". The term across the injector takes manifold pressure and fuel rail pressure into account, and is usually referred to as "delta pressure". Ford racing's fuel injectors are always rated at 39.15 psi delta, so the fuel injector sizing discussions found bellow will assume a fuel pressure of at least 39.15psi delta.

There are some exceptions to the above mentioned nominal injection pressure. In relatively recent years, emissions regulations have become so stringent that the government is now regulating the emissions output that gasoline vehicles are allowed to produce even when the engine is not running. This is referred to as "evaporative emissions" and results from the unburned hydrocarbons (raw fuel) emitting into the atmosphere from the fuel tank, fuel lines, injector leakage, intake manifold etc. when the engine is shut off. This is the fundamental purpose of the charcoal canister (and hydrocarbon trap in the air box on many vehicles) and is also the reason tht ford switched to the returnless fuel systems (RFS) found in production vehicles today. These systems have only a fuel supply line from the tank to the engine, with no return line. The primary reason for these systems is that evaporative emissions increase as the temperature of the fuel tank increases. On a convetional return system, the fuel is sent to the engine through the supply line, and the excess is returned (via the mechanical fuel pressure regulator) to the tank through the return line. Since the engine is hot, this process heats up the fuel and thus increases evaporative emissions
To combat this, the returnless fuel systems were invented. Currently ford uses 2 primary types of RFS which are called Electronic Returnless Fuel Systems (ERFS) and Mechanical returnless fuel systems (MRFS). the latter is the simpler of the two systems and controls the fuel rail to a constant pressure via a regulator in the tank, which is typically set to around 60psi. The powertrain control modul (PCM) then calculates the pressure across the injector either by inferring or measuring manifold pressure and substracting from what it knows is the rail pressure set point. ERFS, on the other hand, has no mechanical regulator at all, but instead has a Fuel rail pressure transducer (FRPT) mounted on the fuel rail that measures fuel rail pressure relative to manifold pressure and feeds that information back to the PCM. The PCM then controls the fuel pump driver module (FPDM) which in turn varies the voltage to the fuel pump (or pumps) in the tank to supply the correct pressure and flow rate to the injectors. Most of the time this pressure is maintained at 39.15 psi delta, but when the fuel temperature rises, this pressure can be boosted in order to delay the onset of boiling the fuel. Some vehicles also boost the pressure under some conditions in order to get away with using smaller flow rate fuel injectors for various reasons beyond the scope of this tutorial.

Both V6 and V8 mustangs have used ERFS since the 1999 model and continue to do so today. The purpose of going into all this detail is to convey the message that if you choose your fuel injectors based on a pressure of 39.15psi delta (which is the pressure that ford racing specifies flow rate), the injectors will be correctly sized regardless of which fuel system you actually have, and also to show you that fuel pressure on ERFS vehicles can chage based on a number of conditions. These concepts will be important in the rest of this tutorial. If you are trying to compare injectro flow rates and you have a flow data at one delta pressure, you can easily calculate the flow rate at a different delta pressure as follows:

flow rate at new delta pressure = (flow rate at old pressure) x (square root of)(new pressure / old pressure)
example: what is the flow rate of an injector at 43.5 psi if it is rated at 60 lb/hr at 39.15psi?
flow rate at 43.5 psi delta = 60 x (square root of)(43.5/39.15) = 63.2 lb/hr

You can use the following information to properly determine what size injectors are needed for various applications. For this example, we will use a naturally aspirated 5.0 V8 making 300hp. Keep in mind that this is FLYWHEEL (also known as brake) horsepower, not wheel hp.

Engines require a certain fuel flow rate that is generally measured in lb/hr (pounds per hour) and can be calculated via knowledge of their brake specific fuel consumption (BSFC). By definition BSFC represents how much fuel (in lb) is required per hour per each break horsepower the engine produces. Most naturally aspirated production gasoline engines generally operate on a 0.42 to 0.52 lb/hp-hr BSFC at wide open throttle (WOT). High performance gasoline and race engines (12.5:1 cr and higher) which tend to be extremely efficient can sometimes have a BSFC as low as 0.38 to 0.42. More celarly stated, this means that if you have a gasoline engine that makes 300 whp, its total maximum fuel requirement in lb/hr can be calculated as follows:

fuel flow requirement = (brake horsepower) x (BSFC)
example: a 300hp naturally aspirated gasoline powered V8 requires what size fuel injector?

First, assume a BSFC of 0.50 lb/hr and injection pressure of 39.15 psi accross the injector

300hp x 0.50lb/hp-hr = 150 lb/hr maximum total fuel flow requirement

Since this is the total fuel flow requirement to the engine, we must now divide this by the number of injectors being used to determine the flow rate necessary for each injector so that you can select the correct size injector. In this example we have an 8 cyl engine using 1 injectro per cylinder, which gives 150/8injectors = 18.8 lb/hr per cyllinder.

So, technically, the engine needs a 19 lb/hr fuel injector to support 300hp, but this will require that the injectoris at nearly a 100% duty cycle in order to achieve this horsepower level. Duty cycle refers to how long the injector needs to be open (flowing fuel) in order to supply the required amount of fuel. If the injector needs a 100% duty cycle at a particular engine speed and load to inject enough fuel, that means it is open all the time. Under most conditions, fuel is injected when the intake valves are closed, which helps with fuel atomization and efficiency. If the injectors need to be on 100% of the time to supply enough fuel, this means that some fuel is being injected while the intake valves are open. Depending on the overlap of the cam in the engine, some of this unburned fuel can be blown right past the exhaust valve, or be poorly atomized, which makes for a less efficient combustion process. Perhaps more importantly, operating a fuel injector between roughly 85% and 99% duty cycle does not give the injector sufficient time to close before it is commanded to open again. This can cause extreme variability in the amount of fuel actually injected, which can sometimes result in a rich condition. Similar issues exist at the low end of the flow region at extremely low duty cycles, but this is highly dependent on the type and flow rate of each model of injector. In this case the injector does not have enough time to fully open before is commanded to close again, which causes extreme variability which causes a lean condition. For these reasons, we generally recommend selecting an injector with a flow rate sufficiently high that will not be required to exceed an 85% duty cycle. So to figure out what size fuel injector will result in an 85% dc, divide the original result by 0.85: 18.75 lb/hr / 0.85 = 22.1 lb hr requirement.

Since the next popular injector size is 24 lb/hr, this is the correct injector size tat you should choose for this particular application. Keep in mind that this discussion assumes your fuel pump, lines, regulator, etc. are sufficient to be able to manintain at least 39.15 psi accross the injector at all engine speeds and loads ( even under boost, if applicable). Now that yo have selected an injector, the calibration or "tune" in the PCM must either be changed or a different MAF must be used.
This calculation can also be revised to give the maximum safe hp a set of injectors can support, which gives:

make safe hp =[(injector size) x (total # of injectors) x (max duty cycle)] / BSFC

Example: the following guide is a general fuel of thumb for sizing fuel injectors on an 8cyl engine using a BSFC of 0.50. Forced induction engines typically range from a BSFC of 0.55 to 0.65, with the latter value arising from the fuel enrichment necessary to keep exhaust temperatures bellow 1650 deg F and catalyst temperatures below 1700 deg F.

Naturally aspirated: (19lb x 8 x .85)/.50= 258.4 or approx 258hp @ 85% duty cycle
Forced induction: (19lb x 8 x .85)/.55= 234.9 or approx 235hp @ 85% duty cycle
Forced indution @ 0.65 (19lb x 8 x .85)/.65 = 198.8 or approx 199hp @ 85% duty cycle
 
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