Electronic Fuel Injection System - EFI Basics

Each intersection point between manifold pressure and rpm represents one combination of engine load and engine rpm. The number in each of those boxes or cells represents the amount of time the injector will remain open, called the injector pulse width. For a typical speed-density map, the lower righthand quadrant of the map would represent high-rpm, WOT engine operation. Tuning is actually very simple. Changing to a larger number in each box increases the amount of fuel delivered to the engine. A smaller number reduces injector flow time, which leans the air/fuel ratio for only that one cell. Tuning is even easier if you combine your efforts with the feedback air/fuel ratio information from a wide-band oxygen sensor with the data-logging capability offered by most of the more sophisticated aftermarket EFI systems.

The spark table operates and is tuned in very similar fashion to the base fuel map. The horizontal and vertical scales are the same. The boxes inside the spark table represent the actual ignition timing. As you may already know, light throttle and light load allow you to put more timing in the engine (just as with vacuum advance), while at WOT at low engine speed you will want to reduce the total amount of timing you put in the engine; otherwise detonation and engine damage may occur.

Most modern aftermarket EFI systems will also create initial fuel and spark maps that will allow almost any engine to start and run and will give you a base from which to begin tuning. Plus, with the growing popularity of aftermarket EFI, complete maps are now out there that car crafters are willing to share with other entry-level users. There are a couple more screens that you need to deal with in terms of acceleration enrichment (similar to the accelerator-pump circuit on a carburetor), idle-quality adjustments, and cold-start features (similar to a carburetor choke), but besides these additions, there's not much more to the basics of EFI tuning. It really is that simple.

Fuel-Injector Sizing
One area that seems to cause more than a little confusion is fuel-injector sizing for a given engine application. While it may sound difficult, it's actually pretty easy. All electronic fuel injectors are sized based on the maximum amount of fuel they can inject, generally expressed in terms of pounds per hour of fuel (lb/hr). This also depends on the fuel pressure at the injector. Most injectors used in the performance industry are rated at 3-bar (43.5 psi). As an example, let's look at a performance injector rated at 40 lb/hr. Multiply that capacity times eight injectors to get 320 lb/hr of fuel. If we assume a generic brake-specific fuel consumption (BSFC) number of 0.50, this means that to make 1 hp for 1 hour at WOT demands 0.50 pounds of fuel. Given this, to find out how much horsepower this size injector can feed, we multiply 40 (lb/hr) x 8 injectors x 2 (to give us 1 pound of fuel to make the math work out), which equals 640 hp. This isn't quite accurate because it would demand 100 percent open time on the injector-called a 100 percent duty cycle. This is very hard on injectors and burns them out, so the typical industry safety margin is 15 percent. So if we multiply 640 x 0.85, we get 544 hp. This is what you could expect a 40-lb/hr injector to safely deliver.

Now let's add some variables. Let's say you've built a killer small-block that can make 500 normally aspirated horsepower and you need to size the injectors, but you are also considering adding a dry 200hp nitrous system that will require the additional fuel from the injectors. The formula for determining injector size is:

By Jeff Smith

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!