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Fuel Injection Conversions 101 - Fuel Injections
Basic EFI Knowledge Is the Difference Between Your Project Being a Drem or a Nightmare
They don't make cars like they used to, that's for sure, which is actually something we can all be thankful for, particularly if we're talking about the fuel system in your car or truck. Remember the good ol' days of carburetors, needles, jets, floats, choke plates, and vacuum-operated secondaries, plus gum and varnish to stick it all together? Don't forget about stalling, rough idle, percolation, cold starting (an oxymoron at best), hesitation, stumbling, surging, pinging, and fouled plugs. Yes, all that brings back memories, doesn't it?
The '89-'93 5.0 Mustang-based EEC IV fuel-injection setup is one of the most common swaps for carbureted Ford owners looking for an easy EFI setup.
The heart of any EFI system is the computer, also called an engine control module/powertrain control module (ECM or PCM), depending upon the company. A standard EEC IV 60-pin model is shown here (left). The most common is the A9L from five-speed mass air-equipped 5.0 Mustangs.
But things were so much simpler then. You could tune your car with a screwdriver instead of a computer and, as long as you didn't put too stringent a definition on performance, things stayed simple. But who are we kidding? In search of more power, we'd strap on a new intake manifold, drop in a Holley 650, and head to the track. We and the domestic manufacturers explored a far-out range of carburetion possibilities, such as six-packs, dual-quads and triple or quad Webbers. Yeah. Things didn't stay simple for long.
Around the same time, the Europeans started playing around with this business of fuel injection in order to get more power out of their silly little engines.
In North America, there were a few notable experiments, including the '57-'65 Corvettes, which were equipped with Rochester fuel injection. Well, that turned out to be a good idea after all and now, 40 years later, you can't buy a car in North America that isn't equipped with fuel injection.
Open To Improvement
If you're already thinking of converting your small-block motor to fuel injection, you've probably heard of the many good reasons to consider the project. Among these are improved performance, higher horsepower and torque, better gas mileage, easier starting, and the potential for reduced emissions.
You can do the job fairly cheaply using OE parts from donor cars, if you don't mind doing some research and wiring yourself. There are also a number of commercial conversion kits that supply everything you need to get the job done, with a minimum of fuss.
So why does fuel injection work so well, anyway? In order to understand this, we need to look at what you expect from your car's engine and what the engine needs to deliver the goods. When you start up your car, it may be warm or cold, but in either case, the air/fuel ratio (AFR) needs to be somewhat rich. During warm-up, after starting, and during warm-idle conditions, the AFR is usually maintained slightly on the rich side so that the engine doesn't stumble when you want to drive away.
Once the engine is fully up to temperature and you're in a steady cruise mode, it's best to bring the AFR to what is called "stoichiometric" which, for gasoline, is 14.7 parts of air to 1 part of gasoline. This is the most efficient AFR for completely burning the gasoline and having nothing harmful dumping out the exhaust. It's also the hottest-burning AFR, so when you jump on the throttle, things have to change again or you could melt some pistons after a while.
At wide-open throttle (WOT), the AFR must go rich again in order to generate maximum power while lowering combustion temperatures. Finally, on deceleration, you'll want to shut off fuel flow to the engine to help with engine braking, unless rpm is getting below about 1,500. Then you'll want a slightly lean AFR so the engine is ready to go when you hit the pedal again.
So it seems that we expect a lot from the engine, and keeping it happy requires a number of adjustments. Carburetors typically offer a limited number of adjustments that can't really compensate for all possibilities. You may be able to set up the baseline AFR by changing needles or jets and set the idle speed easily. What is far more difficult to handle, for example, is the change from summer to winter gasoline blends, or compensating for altitude changes while on a trip across the mountains. A carburetor is completely unable to adapt to these changing conditions as you drive, and is known as an "open-loop system." At best, your setup is a happy medium, but once you stray too far from that medium, you're not going to stay very happy.
Close The Loop
In order to make the necessary adjustments to keep the engine running as we like it, there has to be some feedback to a control system that can make the adjustments. Now, in the modern context, we're really talking about full electronic control, which includes spark timing as well. The fuel injectors themselves are a key component in the whole system because they can be controlled, and that adjusts the amount of fuel delivered to the engine. Controlling the fuel injectors is an electronic control unit, roughly equivalent to a desktop XT computer from the '80s. Finally, there is a broad array of sensors that are read by the controller. From your perspective, the most important sensor is the throttle position sensor because that is how your go-pedal instructions are read by the ECM.
Other sensors are also connected to the ECM so that it can read information about the engine, such as coolant temperature, rpm, and crankshaft position. A few other sensors are used to determine information about the incoming air and the outgoing exhaust gas. In creating a feedback, or closed-loop, control system for the engine, the oxygen sensors in the exhaust plumbing are the most important.
We previously mentioned air/fuel ratio and the particular ideal or stoichiometric ratio, where gasoline is fully consumed along with the supplied oxygen. In general terms, it's the primary objective of the ECM to adjust the fuel supply so that the oxygen sensor reads zero oxygen content in the exhaust gas. This indicates that the highest efficiency with lowest emissions has been achieved.There is now a closed-loop control system looking after engine operations. When the signal from the oxygen sensor deviates from the target level for a given set of conditions, the ECM can change the amount of fuel delivered, which will affect the amount of oxygen in the exhaust gas. If that change is in the right direction, smaller adjustments are made for fine-tuning. If the change is in the wrong direction, other appropriate changes are made. This kind of feedback was never used with a carburetor.
Over the years, there have been several versions of Ford's Electronic Engine Control (EEC) systems. The original EEC I debuted in 1978 to control ignition timing, exhaust gas recirculation, and secondary air injection by way of the Thermactor Air pump. EEC II arrived the following year, adding oxygen sensor feedback and digital fuel volume control by way of a stepper motor.Central fuel injection (CFI) debuted in 1980, along with EEC III, which was updated to control one or two central fuel injectors located in a housing that sat where the carburetor used to be.
|Max Power By Injector Size|
|Injector||N/A Power||Turbo Power|
|Flow Rate||at 90 percent||at 90 percent|
|(in lb/hr)||Duty Cycle (in hp)||Duty Cycle (in hp)|
Note that in the case of the turbocharged engine, the fuel supply pressure would have to be increased by at least the same psi as the amount of boost being used.
To avoid much of this complexity and planning, install a kit designed for the engine in your Mustang. We'll have a look at this kit in an upcoming issue, so join us then.
In 1984, EEC IV was released and included the first level of standardized onboard diagnostics (OBD-I). A decade later, EEC V arrived, incorporating updated diagnostic and reporting capability, known as OBD-II. Most recently, EEC VI arrived with added capability to handle more electronic sensors and control functions. EEC VI is fully based on what is called flash memory, a kind of permanent but reprogrammable memory.
For V-8 Mustangs, CFI arrived in the mid-'80s and persisted through the '85 model year on AOD trans Mustangs only. In 1986, the 5.0L Mustang engine was converted to sequential electronic fuel injection (SEFI), which provided one fuel injector for each cylinder. With this design, fuel is sprayed directly into the intake valve port. Because of this, there is no chance of fuel "puddling" in the intake runners, so much tighter control on the amount of fuel dispensed is possible.
Beginning with California-bound Mustangs in 1988, Ford began the migration from a speed-density-based control system to a mass airflow (MAF) sensor system. MAF systems have proven to be more adaptable and "mod-friendly" than their predecessors. If you're considering a conversion to electronic fuel injection using parts from a donor car, you should make sure that it's at least an '88 California, or an '89-or-later Mustang. The compromises and performance issues associated with earlier systems are simply not worth the aggravation.
Design Your Own
If it hasn't become plainly evident by now, engineering your own EFI system is a notably complex task. Still, you might be in the situation of having started with OE Ford parts and needing to update certain aspects because of your engine. One of the least understood issues in this scenario is that of finding the appropriate size or capacity of parts to handle an engine of increased power.
If the 302 engine in your classic Mustang has about the same output level as a '90 5.0 Mustang donor car, then all should go fairly smoothly during the conversion. However, if you have increased the output with intake or compression changes, you'll need to get more fuel to the engine to fully realize that increased power.
Dealing with Ford Racing replacement parts, one of the first things you need to know is that the flow rate of fuel injectors assumes a particular pressure difference between the fuel rail and the cylinder, which should be about 39 psi. When you look at the flow rate of any particular injector, it's based on this pressure difference. If your soon-to-be fuel-injected mill is also supercharged, then you're going to have to increase the fuel rail pressure by the same amount as the boost that you're putting into the engine if you want to rely on the published injector flow rates.
Fuel injectors are really nothing more than electronically controlled valves. When they're open, the fuel flows into the cylinder. If you don't supply enough fuel to the engine, it can't make the power that it's otherwise capable of. When sizing a fuel injector, the duty cycle must be considered. This is the ratio of the amount of time the injector is open during a given period. To maintain good control over the fuel delivery, the duty cycle shouldn't exceed 90 percent.
To get to the point where you can figure out what injector size will support what horsepower level, you'll have to either understand or blindly accept the idea of brake-specific fuel consumption (BSFC). This is a number that represents the amount of fuel needed to generate 1 hp at the flywheel over a one-hour period at WOT. The actual number can vary widely, according to the type of engine and any power-adders that are being used. For most production engines, the accepted number for BSFC is 0.50. Extreme examples of normally aspirated, pure-race engines can show numbers as low as 0.40, while supercharged engines are usually in the 0.60 range and turbocharged engines can run as high as 0.65.
With these two ideas firmly under our belts, we can move ahead. Let's say that your engine has been putting out 300 hp at the flywheel in carbureted form. To support that power level using fuel injectors, you need to know what size of fuel injector to use. Start by multiplying the flywheel horsepower by the BSFC. In this case, the engine is naturally aspirated, so we'll use a BSFC of 0.50. Then multiply by 300 hp. We'll need to supply 150 lb/hr of gasoline to the engine (300x0.50). Since this is a 5.0L engine, there are eight injectors, so each one must handle 18.8 lb/hr per cylinder (150/8). We also said that a 90 percent duty cycle is the maximum to use for reliable operation, so we have to add in this extra capacity.
Taking the current flow requirement per injector and dividing it by the duty cycle, we arrive at 21.0 lb/hr (18.8/0.90). The closest actual injector is 24 lb/hr, so we're not far off. You should never use an injector smaller than what you have calculated. Remember, if you short-change yourself on the numbers here, you're just placing an artificial limit on the maximum output your engine can have. Table 1 (p. 44) shows what maximum flywheel output levels can be supported by common injector sizes, both for normally aspirated engines (BSFC=0.50) and booster engines (BSFC=0.65).