Skill Level: Moderate to advanced
Applicable Model Years: '97 to '04
Cost: Around $500 for a complete set
Tip: Use dielectric grease to coat the electrical terminals when you plug the new injector into the wiring harness. This not only provides a lubricant for the terminals, but it also serves as an oxygen barrier to guard against oxide formation.
Performance Gain: Depends on condition of existing injectors and level of engine modificationComplementary Work: Change the fuel filter if you have a '97 to '03 Corvette
Before we get into the specifics of the injector swap itself, let's take a look at the C5's fuel system. It comprises eight injectors, a fuel rail, a .375-inch fuel-supply line, and an electric pump in the fuel tank. This differs from earlier Corvettes in that the engine uses all of the fuel passing down the fuel rail. Fuel pressure is regulated to 58.02 psi, or 4 bar, by a regulator in the fuel tank. There is always an exception to the rule. In this case, the exception is the '97 model, which uses a traditional return line, and whose fuel pressure is regulated at the fuel rail entrance and referenced by manifold vacuum.
The amount of fuel that an injector delivers is determined by two factors: the length of time it's held open and the pressure of the fuel system. This flow is measured in pounds per hour. The C5 uses a variety of different injectors with flow ratings varying from 26.4 lb/hr to 29.1 lb/hr.
While upping fuel pressure seems like an easy way to increase fuel flow through the injectors, such is not the case with the C5. The reality is that if you increase fuel pressure, the PCM will shorten the amount of time the injector remains open. This is called pulse width, and it's measured in milliseconds. The result is that you'll end up with the same amount of fuel reaching the combustion chamber as was the case at the lower pressure.
The only instance in which you might need to increase fuel pressure is if you run at very high rpm levels for an extended period of time. You may gain a few horsepower, but this improvement will come at the top of the power curve, so you're unlikely to notice it on the street. Also keep in mind that constantly operating an injector at its limit-as you might at an open-track event-will shorten its life span.
As rpm increases, the window for fuel delivery becomes shorter. For example, at 4,000 rpm there are 30 milliseconds to deliver all the fuel that's necessary, but at 6,000 rpm the span shortens to 20 milliseconds. At some point, the injector will simply stay open all the time because it can't cycle fast enough.
Duty cycle refers to the percentage of total time that an injector is open. If an injector has a 50 percent duty cycle at a given rpm, that means the injector is open half the time. The normal rule of thumb is that you don't want to go much above 80 percent to 85 percent duty cycles. Operating above this level will cause the injectors to overheat, and fuel delivery may become erratic.
Fortunately, the stock injectors on your C5 can support 400 to 425 hp, provided they're in good working condition. Adding larger-than-necessary injectors won't do anything to increase power, but it can reduce duty cycles. Switching to a non-standard injector size will necessitate a PCM recalibration-preferably performed on a chassis dynamometer-to compensate for the altered flow characteristics.
If you're determined to wring out every last pony, you might consider purchasing a set of injectors that have been flow tested and matched to a tolerance of more or less 1 percent. Most injectors are built to within a 5 percent variation. The matched set will give you a slight improvement over standard injectors, though the improvement will be so small you may only notice it on a dyno.
Pintle: This is the most common type of injector. It features a small needle, or pintle, that moves in and out of a matching orifice. When the injector solenoid is energized, the needle is pulled back, allowing fuel to spray out. This yields a moderate spray pattern that works in almost every type of engine. Most OEMs use pintle-type injectors.
Disc: Both Bosch and Lucas favor this type. It uses a small disc, which covers a flat plate with a series of holes in it. Actuation is similar to the pintle type. The Lucas version moves the disc into the body of the injector for a quicker response. Disc injectors generally offer a very narrow spray pattern, which seems to work nicely in the LS engines.
Ball: Rochester, a division of GM, makes a ball-and-socket injector. This design provides for better atomization and a wider spray pattern. It's also more expensive.
Changing the injectors in a C5 is a simple, straightforward process; just be sure your engine really needs it. I believe that 99 percent of the injectors ever installed in C5 Corvettes are still functioning properly and are perfectly capable of supporting the needs of the engines they feed. In fact, in 130,000 miles I've replaced one injector on my car.
If you do decide to swap your stock injectors for larger ones, be sure to budget some money for dyno time and a recalibration of the fuel-delivery tables in your PCM. The only thing worse than a nonfunctioning set of injectors is an improperly matched one.