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Electronic Fuel Injection Tech - EFI Facts & Fallacies

Jim Allen | Writer
Posted September 1, 2005

How to use injection to maximize engine power

Fine-tuning your EFI system, whether as a stand-alone mod or to enhance other engine mods, is worth some power. A stock 8.1L engine like this gained 57 hp with just a change to a programmable ECM and some serious dyno time to find the ideal power map.

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.

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