High Pressure Fuel Pump

From FK8 Wiki

The high pressure fuel pump on the FK8 is a major component responsible for delivering your fuel in tandem with the injectors and the low pressure fuel pump/lifter pump inside of your gas tank. It 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 pump is near its limits.

Operation

The fuel pump has a cylinder and a piston that is driven by the engine's exhaust camshaft with a 4 lobe cam, providing one pump cycle per cylinder cycle. On the fuel inlet there is an ECU controlled solenoid and on the outlet there is a non-return valve. The ECU varies the volume of fuel delivered by the pump during each cycle by using the solenoid.

There are two stages to a high pressure fuel pump cycle:

  • Suction: as the piston begins to lower, the solenoid is de-energized and opens, and fuel fills the fuel pump cylinder from the low pressure fuel rail.
  • Pressure: as the cams rotate and push the piston back up, fuel is forced back out of the inlet until the desired amount of fuel is in the pump cylinder, at which point the ECU energizes the solenoid and the remaining fuel is instead 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 and is instead returned to the low pressure rail.

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. From factory, the commanded fuel pressure maxes at 200 bars and will target 200 bars during WOT. There is also a physical pressure relief valve at 250 bars, which you will sometimes see in datalogs when starting the engine, letting off the throttle just before redline, or when using Hondata's full throttle shift.

The ECU has a hard-coded limit of 200 bars and the pressure cannot be increased beyond this.

Thank you to Derek Stevens from Hondata for providing most of the technical information in this section.

Monitoring the fuel pump

Pressure

You should be able to read the high pressure fuel rail pressure with any OBD2 scanner as well as Hondata and KTuner. On a normal tune, it will target 200 bars at full load and ideally should stay within 5 bars of the commanded pressure. Drops of up to 10-15 bars from DIFPCMD, while not ideal, are acceptable. Any larger drops risk the ECU ending up in protection mode.

Pump duty

Hondata allows you to monitor and datalog the fuel pump duty itself rather than just the resulting pressure. 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.

This data channel can be invaluable for tuning an FK8, since you can see exactly how far you are from the limit. However, it is a Hondata-exclusive feature.

Tuning implications

The primary thing a Type R tuner is working against is the fuel pump. More tuning info can be found on the Tuning article.

RPM Limits

The OEM high pressure fuel pump prevents a K20C1 from running at more than 7200 RPM. Past this point the fuel pump piston will float, the pump will very possibly damage itself, and ultimately cause damage to the engine itself due to fueling going wrong at redline. The factory calibration and all basemaps leave it at 7000RPM.

Reaching max duty

The fuel pressure will drop and the injectors will extend their duration to compensate. By doing this, flow is slightly increased and the A/F ratio is maintained. However, this compensation mechanism only works for so long and eventually you will lean out.

At around 160 bars from 200 commanded (a 40 bar drop) the ECU will go into protection mode, the engine will start putting out black smoke, and power will be massively reduced. There's a theory that the ECU is doing double injection at this point, but it is not known exactly what it is doing.

Even the best tunes will occasionally hit max duty due to the dynamic nature of our ECU. It's almost impossible to account for every scenario. Touching 100% duty every once in a while is acceptable.

Decreasing fuel pressure

You can increase fuel flow slightly (5-10%) by decreasing fuel pressure, but dyno testing has shown this can result in power loss. Additionally, the compensation mechanism relies on there being some flow overhead with reduced pressure. If you use this increased flow for yourself, you're also reducing how much the ECU can compensate when you hit max duty.

Ethanol is limited

Due to our fuel pump limitations, Ethanol usage is also severely limited. Generally when adding ethanol you greatly increase boost along with timing, but we have to pull boost slightly back due to our fuel pump limits. The ignition advance that ethanol enables makes up for it, but we still can't use it to increase boost.

Duty limiter

Hondata provides the ability to limit the fuel pump duty by reducing target air charge whenever the duty goes past a set percentage. This is especially helpful with flex fuel calibrations - if a low-quality E85 station suddenly got a good batch of E85, you could end up hitting fuel pump limits.

Upgrading

At the moment, the only way to upgrade your fuel pump while retaining the factory ECU is by tuning with Hondata and purchasing their fuel system upgrade, which will provide roughly 24% more flow over the OEM pump.

With a Motec, Syvecs, or any other standalone ECU you can make use of an XDI pump sold by 4 Piston. This pump will soon be compatible with Hondata as well.