Electronic Fuel Injection Tech - EFI Facts & FallaciesPosted in How To: Engine on September 1, 2005
Fuel injection is on every 4x4 that's newer than about 15 years, but truck owners still approach EFI modifications with shaky hands. We had a rare opportunity to get deep inside a company at the cutting edge of fuel injection development, Precision Automotive. The Technical Services engineering department of this company operates a test lab where fuel-injection programs are written from scratch. They are hired by various OEM and aftermarket manufacturers to write computer code for fuel-injected engines of all types. On any given day, you can see just about anything on a dyno, from an ordinary four-cylinder econo-box engine, to fire-breathing V-8, a 600hp light-truck diesel, and even a helicopter engine. Our time there yielded a wealth of information that's bound make those hands a little less shaky. We asked Alan Tehan, founder, CEO, and chief gearhead at Tech Services, for some tips on modifying fuel injection.
What are the best and simplest EFI tuning tricks?
Overall, headers and exhaust systems are the most bang for the buck. The ECM can adapt in most cases to offer improvements in performance. If you're talking about tricking the ECM by messing with sensor inputs, think again. Generally, there are very few tricks that won't be negated by the ECM. From the early '90s, the manufacturers have installed algorithms that are "trained" to look for variations that are not synonymous with "normal" operation and override them. The government has mandated this. Your trick will only last long enough for the ECM to figure it out and revert to some predetermined programmed function akin to "limp-home mode." Your gains, if any, are short lived.
OK, what are the worst tuning tricks?
One of the more common tricks is to fool the coolant or air-temp sensor into thinking the engine is running colder than it really is to increase fuel delivery. The engine may run richer, and in certain circumstances and at certain times, that might be beneficial. The problem is that an engine is not a linear animal. It may want more fuel at certain airflow rates but not others. When you richen by temp-sensor fooling, you richen at all speeds and loads. A sprinkling might work as well.
When all is said and done, the oxygen sensor will trim the air/fuel ratio back regardless-unless, of course, it comes up MIA. Can you say bye-bye to fuel efficiency? The same things happen when you increase fuel pressure, plus some other negative effects. Low-temp thermostats are OK and may offer small gains, but never use one that opens lower than 180 degrees. At between 140 and 160 degrees of coolant temp, the system will go into fuel-enrichment mode and you are back to the "sprinkling-can effect."
Many performance enthusiasts think Mass Air Flow (MAF) systems are better than Speed Density (SD) systems. Do you agree?
Everything is a trade-off. MAF systems generally are able to react to component changes (i.e., performance mods.-Ed.) easier because they measure actual airflow instead of "computing" airflow based on engine speed and manifold pressure. They are more susceptible to the vibrations and transient airflow reversions that come with more aggressive camshafts. Additionally, there is some question as to their effectiveness with boosted (i.e., supercharged/turbocharged) engines. Overall, I would agree that performance tuning by owners is easier with a MAF system.
In a similar vein, many people think sequentially actuated fuel injection (SFI) is better than batch (or group) actuated. Do you agree?
There is little power difference using one versus the other-at least on the engine systems we have quantified. SFI is supposed to offer a little better control of fuel overruns and emissions, as well as idle quality. The OEM can program for it because they have the equipment to monitor individual cylinder air/fuel ratios, combustion temps, and combustion pressures at each firing event. Most aftermarket labs cannot do this, so unless these factors can be accounted for in the mapping, there is no great advantage to SFI from an aftermarket perspective. Don't forget that the injector "squirts" before the intake valve opens, so intake runner design, injector targeting, and location have some effect as well. Additionally, oxygen sensor feedback is usually on a bank-by-bank basis, so fuel trimming during normal operation is usually accomplished equally on all cylinders on a bank.
To what degree, if any, will larger throttle bodies on a port-injected engine cause "soggy" bottom-end performance? Do OEM-size throttle bodies limit upper-end performance?
With low-rpm performance, throttle-body size is not critical, since it's not responsible for atomizing the fuel. Intake runner size and design is much more an issue. In the high-rpm ranges, a larger throttle body may offer a performance increase, but only if the fuel rate is increased to match. Injector fuel flow rates and engine airflow rates are closely related. Intake valve size is more of a limiting factor than throttle-body CFM rates on the engines I've tested.
Same question for throttle-body fuel injection?
The answer is largely the same. There are more issues with keeping the fuel atomized when you have more distance for the air and fuel to travel, but the injectors in a TBI system do a better job than a carburetor. Also, intake manifold design, port location, or spray targeting are so poorly engineered in some port-injected systems that their advantage over TBI is largely negated.
What's the best way to compensate for the effects of a "hotter" cam profile?
The best way is to use a stand-alone Electronic Control Module (ECM) and build a fuel and spark map commensurate with the cam profile and power curve. There really isn't another good way to accomplish this. As I mentioned previously, some MAF systems will compensate for small cam changes quite well, but injector sizing becomes a major factor fairly quickly here.
Alright, how do you know when you don't have large enough injectors, and how do you choose new ones?
Generally, if you modify your engine beyond a very low level, you will out-strip the injector performance. Frequently, you can't tell if your injectors are too small, and that can be a major problem. The only chance you have to "feel" the problem is at WOT (Wide Open Throttle). If it feels like you're running out of gas, serious damage is near. Lower-rpm, heavy-load situations are more insidious because, unless you can electronically detect detonation from a lean condition, your first indicator will be when you "toast" the engine. Ideally, you choose injectors based on dyno test and flow-bench evaluations. Outside the lab, there are some formulas that can get you into the ballpark, but it's important not to exceed an 80-percent injector duty cycle below WOT or you can overheat the injector.
The other important consideration is being able to control the injector for low-speed operation. A high-flow injector may supply too much fuel for the engine at low speeds, even at its shortest pulse width. That's why the OEM injectors are sized very close to maximum flow for the base engines because that allows for the closer calibration parameters needed to meet emissions requirements. Again, a stand-alone ECM with careful calibration can go a long way towards controlling large injectors at lower speeds to make a tractable engine.
Can't you increase fuel pressure to compensate for inadequate injector flow rates?
Bad, bad science! There are pressure spikes that occur in most fuel rails due to the opening and closing of injectors. They usually equate to about 40 percent over the rated injector-operating range. That means a 40psi injector might be operating at nearly 80 psi, but it will probably work OK in most cases. If you run your fuel pressure up to 60 psi to increase injector flow, your pressure spike may now reach over 100 psi. Some injectors won't open against that kind of pressure, and you end up running some cylinders lean. Lean mixtures burn valves and cause detonation-bad! Better to keep the designed pressure and install higher-flow injectors.
Engines with knock sensors can adjust to lower-grade fuel. How much power is lost when coming down from, say, 92 octane to 87?
A lot depends on the engine. Many of the new cylinder head designs-the GM Vortec (4.8, 5.3, 6.0, 8.1), the newer Fords (4.6, 5.4), and the DaimlerChrysler engines (3.7, 4.7, 5.7 Hemi) for example-have had combustion optimized to the point that we crank in much less timing advance than we did previously. On these engines, the change is minimal with stock calibration. On older engines, either a factory fuel-injected engine with an "old-tech" combustion chamber, or add-on fuel injection on a previously carbureted engine, the effect might be more pronounced. More aggressive fuel and spark mapping would change all of that in both cases.
Are there any negative effects in day-to-day driveability with going to a larger-bore mass air sensor on a MAF system?
We have not done any quantification in our labs to authoritatively answer this question. I doubt it would make any difference unless the feed to the ECM is altered. If the larger sensor uses the same 5-volt reference and signaling, I would suspect little difference.
What are the common pitfalls to converting carbureted engines to EFI, both aftermarket kit and home-built?
Fuel delivery is the number one problem, particularly the placement of the fuel pump. In my experience, over 80 percent of the problems are fuel-delivery related. In my opinion, only an in-tank pump should be used. Why? "Fuel shear"-that's an engineering term for aeration. Air in the fuel lines in anything but tiny amounts can have a major effect on driveability and performance. Erratic low-speed performance, vapor-locking, and lean conditions under load are just a few possible symptoms. On a typical EFI conversion, an inline pump is installed with a filter between the pump and the tank. The flow through the filter shears the fuel, which can also cause problems with the pump. The pump should be protected by a filter, but it's better placed in the inlet of a submerged pump. If an inline pump must be used, a submerged fuel "sock" is a better idea than an inline filter in front of the pump. We've also found that certain types of fittings will cause fuel shear. Just like optimizing intake or exhaust flow, all bends in the fuel line should be gentle ones, and transitions inside the lines as smooth as possible. After fuel problems, poor electrical connections are the big problem. Wires connected by the "twist and tape" or "Scotch Lock" methods are inadequate.
What is the major key to performance improvement in an EFI engine?
It's simple: Optimum fuel and spark for the particular application. In milder cases, that can come from an aftermarket chip or programmer. Many of these are emissions-legal. If you want serious increases, combine internal and external engine mods with new calibration. If you want calibration that will give you driveability comparable to OEM, in most cases you need a stand-alone programmable ECM, some dyno time, and a laptop with programming software. With time, effort, patience, and thought, it's possible to do that without the dyno. If the calibration is optimized for the engine, you should get optimal results.
Any specific calibration tips for trucks and 4x4s?
Traditionally, with a heavy load, we add fuel and subtract spark. This keeps combustion-chamber temperatures in the safe zone and allows for reliable engine power. For four-wheeling, we might devise a more aggressive acceleration enrichment calibration to allow the throttle response to be more positive.
BSFC: Brake Specific Fuel Consumption. A measure of how efficiently fuel is consumed in the engine, or how many pounds of fuel is being used per horsepower produced in an hour.
Detonation: Also called "pinging" or "spark knock." A too-rapid release of energy that creates excessive combustion-chamber pressure and temperature. The result is engine damage.
ECM (Engine Control Module. a.k.a. "ECU"): The electronic "brain" of the fuel-injection system. Controls fuel delivery and often spark.
Fuel Enrichment Mode: The warmup period when the engine is in open-loop mode and following a predetermined program rather than being controlled by oxygen sensor inputs.
Group Fired Injectors: The injectors are actuated in groups. A V-8 is typically fired in banks of four.
IAC (Idle Air Control): A motor-operated air-bleed valve controlled by the ECM to adjust idle speed.
Injector Duty Cycle: The amount of time the injector is open versus the time it is closed.
Knock Sensor: Essentially a very sensitive "microphone" mounted on the engine that's tuned to the sound frequencies typically found during detonation (pinging). When they are detected, it converts this to a signal that the ECM can read so the timing can be adjusted.
MAF EFI (Mass Air Flow Fuel Injection): The two main reference signals are the MAF sensor, which measures actual airflow, and engine temp. Finer adjustments are influenced by any other sensors that are present.
MAP (Manifold Absolute Pressure) Sensor): A device that reads manifold pressure (vacuum). The ECM uses this signal as a reference to measure airflow.
Oxygen Sensor, a.k.a "Lambda" sensor: A device inserted into the exhaust system that measures oxygen content in the exhaust. The ECM uses this as a reference to adjust fuel mixture
PCM (Powertrain Control Module): Controls and integrates the engine with the transmission and transfer case. May also connect to an Electronic Traction Control system.
Pulse Width: The amount of time, in milliseconds, the injectors are open.
SFI (Sequential Fuel Injection): The injectors are opened sequentially according to the firing order.
Speed Density EFI: The engine has three primary reference sensors, the MAP sensor, engine temp, and engine speed. Airflow is calculated from these inputs and basic fuel delivery is adjusted to match.
TCM (Transmission Control Module): Some rigs use a separate transmission-control module to control the shift on an electronically controlled transmission. It will react to inputs from the engine or ECM.
TPS (Throttle Position Sensor): A device that converts the angle of the throttle butterfly into an electronic signal
VCM (Vehicle Control Module): Essentially a PCM with another name. It may integrate some other non-engine/drivetrain systems into the mix.
Building and Tuning Electronic Fuel Injection
By Ben Strader
(Cartech, 2004; 128 pp.)
Ben Strader has been firmly established as one of the gurus of the performance fuel-injection realm. Up to now, his vast knowledge has only been dispensed in the seminars he does in various parts of the country. Now you can learn those EFI performance secrets at home. Strader's book is not light reading, but the mental effort is worth the time. The book is broken down to cover the basics of electronic useful injection and individual chapters on the setup and tuning tricks for the various popular aftermarket EFI systems. That includes bolt-on systems to convert a carbureted engine, or tunable stand-alone ECMs for modified fuel-injected engines. Some of the systems covered include ACCEL, AEM, Autronic, Edelbrock, EFI Technology, Electromotive, F.A.S.T., Haltech, Holley Commander 950, MoTeC, and SDS. If you have an unanswered EFI performance question or an unfulfilled project, this book is the latest and greatest on the topic.