There are a few important factors that you must take into account when modifying an electronic fuel injection engine. These are: the pulse duration of the injectors and the duty cycle.
The injector pulse duration is the amount of time that the injector is held open so that it can inject fuel into the combustion chamber. The pulse duration is controlled by the engine control unit (ECU) and is dependent on various sensors in the electronic fuel injection (EFI) system. The longer the pulse duration, the more fuel is added to the air/fuel mixture. The amount of fuel required at any one time varies by the amount of air flow, the air density, the engine load, and the engine temperature. Therefore the pulse duration will vary. However, there is only a limited amount of time that the injector can be held open at each revolution of the engine. This amount of time is reduced as engine speed increases. For example, at 600 RPM the available time is 0.1 seconds (60 seconds in a minute divided by 600 revolutions) but at 6,000 RPM it is only 0.01 seconds. The pulse duration relative to the available time at the engine red line is called the duty cycle and is expressed as a percentage. Thus a duty cycle of 80% means that at the engine red line the pulse duration (the amount of time the injector is help open) is 80% of the available time.
Increasing the Duty Cycle
Some engine tuners will tell you that if your car has a duty cycle of 80%, you have a possible gain of 20%. However, the injector is an electronic solenoid and cannot be held open for too long or it will overheat and fail. In practice most Nippon Denso and Rochester injectors will remain reliable at up to an 80% duty cycle; most Bosch injectors will remain reliable at up to an 85% duty cycle; and most Lucas injectors will remain reliable at up to a 92% duty cycle. Though even at these duty cycles it is still advisable to test the injectors. Test them specifically for their spray pattern and their flow volumes at the maximum duty cycle you require.
On a race engine I wouldn't exceed a maximum duty cycle of 80% as dyno-testing on various have shown that a duty cycle in the region of 60% to 70% produces the best power. This is because a shorter duty cycle does not allow for the proper atomization of the fuel, and proper atomization is important for the proper burning of the air/fuel mixture. I usually aim for a duty cycle at the point where maximum power is reached so as to ensure the longevity and reliability of the injectors. So, how do you adjust the duty cycle? By adjusting the fuel pressure and the injector nozzle size. On an OEM EFI system there are certain limitations on increasing the fuel pressure and the injector nozzle size. I'll discuss increasing the injector size in a while, but you can read more about increasing fuel pressure here.
Increasing the Injector Nozzle Size
Increasing the injector nozzle size will result in increased fuel delivery all the time. As we mentioned in engine basics, a rich fuel mixture results in power loss. Therefore, increasing the nozzle size could have a negative effect on performance and economy. The oxygen Sensor (O2S) will correct the fuel mixture for an injector that is about 20% larger than stock. However, on a pre-1996 EFI system, the ECU will ignore the O2S sensor under full throttle conditions. Furthermore, the OEM ECU will not be able to handle an injector that is more than 20% larger than stock and will suspect that one or more of its sensors are faulty and will revert to its programmed settings, which means that the fuel air mixture will not be optimal and will probably be rich as this is the failsafe setting on the ECU when it suspects that its sensors are faulty.
Increase the Number of Injectors
Another option is to increase the number of injectors. You can do this by adding auxiliary injectors that are controlled by a separate ECU, or by adding a second ECU to control both the existing injectors as well as the additional injectors, leaving the OEM ECU to control the other engine management functions. The latter is called "staged" injection and requires an extra injector for each engine cylinder. With auxiliary injectors you don't need an extra injector per cylinder; instead you can place one or two extra injectors upstream in the intake path. To achieve the best performance, you should install the extra injectors ahead of the throttle body in the intake path as this allows for better fuel distribution and aids fuel vaporization. However, with positive-displacement superchargers, such as Roots, Eaton, or Lysholm superchargers this is not possible because the throttle body must be placed ahead of the supercharger.
While the additional ECU in both systems are fully programmable, and while staged injection is more expressive and a bit more complicated than auxiliary injectors, it does give you far more tuning control and better fuel distribution as you have full programming control over both sets of injectors. Usually, the new injectors are larger than the stock injectors and become the primary injectors with the stock injectors becoming the secondary injectors. The secondary injectors only come online at high engine loads, when extra fuel in required. This setup allows for greater tolerance of minor variations in the air/fuel mixture, and a smoother transition when the secondary injectors are in use. Whenever I need to run larger injectors, and when my budget allows it, I'd go for staged injection every time.