High Pressure Fuel Pump
The high pressure fuel pump on the FK8 is a major component that is also a major headache for tuned FK8s. The largest single limitation to making more power on an FK8 is the high pressure fuel pump. Even on the factory tune, the factory pump is near its limits.
Thank you to Derek Stevens from Hondata for providing much of the technical information in this article.
Operation
The fuel pump has a piston that is driven by the exhaust camshaft with a 4 lobe cam, providing one pump cycle per cylinder cycle. There is a non-return valve on the fuel outlet and an ECU controlled solenoid to block the fuel inlet. The ECU varies the volume of fuel delivered by the pump during each cycle via the solenoid.
There are two stages to a fuel pump cycle:
- Intake: the solenoid is opened, the piston lowers, and fuel fills the fuel pump cylinder from the low pressure pump.
- Outlet: the piston rises, pushing the fuel back out of the inlet until the desired amount of fuel is in the pump cylinder. the ECU energizes the solenoid and the remaining fuel is forced past the non return valve into the fuel rail via the high pressure fuel line.
If the ECU energizes the solenoid at bottom dead center, then the maximum volume of fuel will be pumped out to the fuel rail. As the ECU delays the solenoid, progressively less fuel is pumped out.
The fuel pressure is a result of the ECU matching the pump output volume with the injector demand in order to reach a fuel pressure target.
Duty cycle
Hondata allows you to monitor and datalog the actual fuel pump duty cycle. It is the percentage of the fuel pump stroke that is used to deliver fuel as measured by the camshaft angle. If the solenoid is energized at BDC, fuel pump duty will be 100%. The closer to TDC the solenoid is energized, the lower the fuel pump duty.
The fuel pump cam lobe is not triangular but more of a sinusoidal shape, causing actual fuel delivery to not be linear. This is more apparent as the fuel pump approaches 100% duty. For example, at 90% duty cycle the pump is delivering roughly 96% of the maximum amount of fuel.
