Acceptable range for fuel rail pressure

[Z4m40i]

I’ve been trying to do more research on this topic about the stock Bosch HDP6 pump found on b58 TU, and was wondering if people can provide insight.

I know that the target operating pressure is 350 bar (5000 PSI). I’m under the impression that the HPFP will crash at ~4300 PSI (-700 PSI based on kern417’s video), and a tuner video stated they try to avoid going below -400PSI

I don’t hear much about pressure spikes though. Ive seen some logs, including mine, spike 300-400PSI+ above 5k PSI target both during WOT and coming off throttle. Are there any risks to this? I’ve only seen one source that there is a pressure relief valve built into the HPFP that operates at 5800 PSI (400bar).

1) what would you consider an acceptable range for rail pressure? It seems like up to +/-400 PSI is tolerable.

2) while there are risks to the rail pressure dropping, what are the risks to excessive pressure spikes?

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[razorlab]

The spikes are normal and part of the PID system of the HPFP pump. Another datapoint you need to look at is the HPFP angle. It maxes at 126*. Think of the angle as the demand.

Here is an example of the HPFP angle and the fuel pressure "spike" you are talking about:

 

[Z4m40i]

Thank you for the reply

What is the association between HPFP angle, rail pressure, and rpm? I’ve seen rail pressures drop while the HPFP angle isn’t maxed, and I also see rail pressures increase past target and HPFP angle is several degrees below the max.

I’ve been under the impression that if fuel rail pressure dips, the HPFP would be maxed and if the pressure is too high, I’d expect the angle to decrease to move towards target.

 

[razorlab]

That first screenshot is telling me you are very close to maxing out your oem HPFP. Be careful, or use less ethanol.

The HPFP will normally work "harder" at lower rpm as it's cam driven, so you will see more angle lower in the RPM and less angle higher in the RPM. The angle will usually also follow the boost. So if you have any fluctuation in boost, you will see it in the angle as it is responding to the demand. If it's a big enough fluctuation, you will see it in the rail pressure. Like my example I showed above on spool with a slight boost spike.

Example of HPFP angle following boost:

As far as the actual rail pressure fluctuating, that is normal because of the PID system. If it sees a call for more demand quickly, it will increase angle to meet that demand, slightly overshoot, then drop and recover. Think of a garden hose when you quickly turn the water supply up, the water will flow out fast, then drop a little, and recover to a steady state.

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