The injector styles changed during TPI production. For '85 and '86 production, GM used Bosch injectors in the Corvette. During all of '87 and part of '88, Lucas injectors were installed. In '89, GM went to Multec injectors and used them up through the end of C4 production. GM switched to the Multec injector partly because they were a ball design instead of the pintle like the Bosch-style injector. This means that the ball constantly rotates, which helps eliminate buildup and gives the injector a different seating surface each time, reducing wear on the seats. Also, the Multec injector is higher in the intake, which prevents fouling the tip of the injector. The Multec injector is designed to allow the fuel to flow over the coils of the injector, which helps cool the coils. This can pose a problem if chemicals are used to clean the injectors. Since the fuel flows over the injector coil, so does the injector cleaner. Harsh chemicals in the injector cleaner can deteriorate the protective coating on the coils of the injectors and destroy the injector.
An approved injector cleaner is available from GM; but most of the time, if the injector is not working properly it will need to be replaced. Using good quality fuel is one of the best ways to ensure long life for your fuel injectors. This eliminates the need for cleaners and helps prevent water in the fuel system. When changing injectors, be sure to install the injectors that have the same resistance as the originals. The Multec injectors have around 12.8 ohms of resistance. You can change injectors within a couple of ohms difference, but if you start going too far away from the originals, you'll have problems. Even performance injectors should have around the same ohms of resistance as the originals.
In the pursuit of more performance, GM turned away from the long, tuned tubes of the TPI system and went back to a shorter intake runner. The TPI intake runners were 21 inches long, which provided excellent midrange power; but the engine stopped making power at 4,000 rpm. For the next generation of fuel injection, GM shortened the intake-runner length dramatically to about 3 inches. The LT1 was introduced in the '92 production year. Once again, a base Corvette engine was breaking the 300hp barrier with its short runners and speed density system. The LT1 has a much flatter torque curve than the TPI system, giving up some torque around 3,000 rpm to the TPI, but picking it back up again above 3,500-3,600 rpm. The LT1 engines started production using a MAP sensor for load detection but, after '93, LT1 engines used a MAF sensor with a MAP sensor as a backup system. The '96 LT4 engines also used this MAP/MAF system for fuel mixing.
The implementation of Sequential Fuel Injection was another change for the '94 production year. Instead of all of the injectors on one side of the engine firing at the same time whenever one of the cylinders on that bank required fuel, the SFI fuel system fired the injector for each cylinder independently and only when that cylinder required fuel from the injector. This helped reduce emissions, increased fuel economy, provided less variation between cylinders, produced a smoother idle, and allowed for rpm limitation because of its ability to cut fuel to one cylinder at a time. The implementation of SFI was a great step toward a more controllable fuel-delivery system.
This probably won't be a surprise to most of you, but the ZR-1 didn't play by any of the rules when it came to the exclusive fuel-injection system for the LT5. It always used a MAP sensor, but never used a MAF sensor like the other '94 or '95 Corvette engines. Also, when GM was making the transition between long and short intake runners, the LT5, unbiased, used both. The longer runners were used in everyday driving situations, and the shorter runners were opened when the secondary air inlet control was activated.
Another set of injectors was used and activated by the ECM when the Power Key on the console was turned on to include the proper amount of fuel for the additional air introduced when the secondary air inlets were activated. Also, all ZR-1s ('90-'93 included) used Sequential Fuel Injection instead of Multiport Fuel Injection. This improved the ZR-1's driveability and fuel mileage.
The '97 and '98 Corvettes had return lines with an external fuel-pressure regulator to keep the proper fuel pressure. '99 and later Corvettes use a semi-returnless fuel system. The fuel pressure regulator on a semi-returnless system is located with the fuel filter in the tank. You can see the difference by looking for the fuel lines going through the engine covers on the driver side. If there is one line going through the cover, it's a '99 or later style; if there are two lines (one pressure, the other a return line to the tank), it's the earlier style.