What is duty cycle?

Duty cycle is the amount of time that the fuel injectors are switched on. A duty cycle of 0% means that the injector is not on at all, a duty cycle of 100% means the injector is on constantly. Duty cycle at idle is normally only a few percent, rising as revs or load increases.

Honda and duty cycle

Honda usually runs their injectors to 100% duty cycle at redline. They appear to adjust the fuel pressure to achieve this. A good example is the early B16A and B18C engines. Both have 240cc injectors, but the B16A has 38 lbs fuel pressure and the B18C has 45 lbs fuel pressure. The difference in fuel pressure is roughly the difference between engine capacities (remember that fuel delivery from extra fuel pressure is not linear). If the B16A has a touch more fueling it is because it has a higher specific output. I believe that Honda runs their injectors to 100% to get accurate delivery at part load and revs, which in theory will help emissions, economy and drivability. The implication of Honda running their injectors to 100% duty cycle is that there is not much scope to increase the engine output on the stock injectors, since the only other way of increasing fuel delivery is to increase the fuel pressure. With forced induction you fairly soon get into crazy fuel pressures unless you increase the injector size.

We have not seen any problems with engines running rich or lean once the stock injectors get past 80-85%, but it is not recommended to exceed 80% duty cycle with replacement injectors. Otherwise you will run the risk of the injector not being able to open and close quickly enough, which causes the injector to float half open. This will lean the mixture out, which is not going to healthy for an engine at high revs and under full load.

In terms of power change from different sized injectors it appears that there is little difference with the Honda engine changing the injector size if the overall fuel delivery is kept the same by reducing the injector duration. We have dyno tested different sized injectors and did not find any difference in engine output between 240cc and 440cc injectors on a B16A engine (we adjusted the injector duration to give the same lambda readings).