Understanding the BMW N55 and N20 HPFP Capabilities

G'day guys, 

Today I would like to discuss the capabilities of the HPFP (High Pressure Fuel Pump) used on the N55 and N20 engines.

The HPFP better known as the BOSCH HDP 5 is a mechanically driven high pressure fuel pump driven by the camshaft on the N20 and a separate module that is chain operated via the crankshaft on the N55. 

A lot of the N55 community already know how weak the stock HPFP Pump is even on a stock turbo. What a lot of the N20 community isn't aware of is that the HPFP is also a severe limiting factor in making sizeable power on a stock turbo and through to larger turbo setups. Typically you'll see the EWG equivalent Motors max the HPFP earlier than the PWG cars. 

The HDP 5 Revision BMW chose to install on the N55 and N20 is the 200 BAR capable unit. That means the pump has been tested and rated to be able to provide a continuous 200 Bar of fuel pressure to the common rail and to the 6x or 4x HDEV5.2 BOSCH 1050cc (@100BAR) Injectors.

For the N55 this means being just able to supply enough fuel for around 15 - 18 PSI mid range with boost tapering into the high rpm range on 98 RON (93 Octane). This is however also dependant on the associated bolt on intakes, inlets and downpipes. As performance inlets, intakes and downpipes allow a turbo to flow more air and more air requires more fuel.

Where this becomes a problem for the N55 and N20 is commonly in the 3000 - 4500 RPM range, as the cam drive that operates the piston on the HPFP isn't spinning fast enough to force the pump to provide more fuel flow. Combine that with an ethanol mix that's too highly concentrated for your boost level and users will see the fuel rail pressure drop, which causes the injectors to starve and the car starts to misfire. Usually the misfire is enough for users either to have their tune revised to avoid that scenario or they look to upgrade the fuelling system. Luckily the N55 is well looked after in the fuelling upgrades department, with HPFP, PI and CPI upgrades.  

On the N20 its a little more complicated, firstly on 98 RON (93 Octane) the N20 won't see any fuelling related challenges until the turbo flows around the 36 - 42 lbs/min mark or the ~ +/- 10%, 420 Crank HP range - Note this is based on a Dyno Dynamics figure as its the dyno we use as a baseline for any R&D. However what we have also seen is in colder climates this HPFP limit can be lower as the colder air has more oxygen content in it thus requiring more fuel (We have seen in -5 to 5 Deg C on stage 1 turbo's the HPFP dip at just 22psi in the 3000 - 4000 RPM Range!). On ethanol mixes, typically the Stock turbo can be seen to max out the stock HPFP on mixes greater than E40 on the 28i model BMW's or E60 on the 20i model BMW's. To run Full E85 even on a stock turbo N20, you either need to run boost around the 16-18PSI mark (28i models are primarily referenced here), or install fuelling upgrades. Thankfully the N20 finally has some love in this department, with HPFP, PI and CPI upgrades now available. 

If a user continues to push the car without the necessary adjustments in the tune or the fuelling upgrades to avoid HPFP dip that has been discussed, then some of the damage that can occur is:
- Damaged injectors usually in the form of stuck/leaky Injectors (a leaky injector can cause hydro lock if its severe enough).
- Piston ring/Piston crown damage.
- Piston Skirt Damage.
- Spark Plug damage.

What we have seen on some tunes in the community is that they try to limp around the HPFP limitation by asking the HPFP to run at a higher pressure say 248 BAR (24 mPa) instead of 200 BAR, or they try to run as close to max power lean as they can (usually only done on ethanol mixes due to the cooling factor of the fuel). However these "band-aid" fixes don't bowed well for the life of the motor, asking the HPFP to run at a pressure above its certified and designed pressure rating severely reduces the lifespan of the pump. On top of that, increasing the pressure output of the HPFP doesn't actually increase the HPFP volumetric flow, it does however increase the flow rate of the injectors, but the injectors aren't the limiting factor in this situation. The risk to also try and run at max power lean for a given fuel allows for the chance that the combustion chamber gets that little bit too hot for whatever reason (sudden drop in fuel pressure, that little bit of boost spike, or hot IAT's to name a few), and your engine is sent to kingdom come with nothing left but a huge mess and the cost of engine builds/swaps.

Its worth noting too, being these are 4 cylinder motors they are under considerably more stress and load per cylinder than their 6 cylinder counterparts. A N55 running 24PSI is only 4 PSI per cylinder, where as the 4 Cylinder is running 6PSI per cylinder, the increased force within the combustion chamber creates more heat. Besides running "cold air intakes" or "big Intercoolers" or "sitting the turbo within its peak efficiency range" or "Meth/CP Injection" a primary focus on these 4 cylinder motors especially on stock cast pistons is to run as close to "Max power rich" as possible. This helps minimise piston ring damage from excessive heat, or injector damage as DI injectors are easily prone to heat damage. 

If you're facing HPFP limitations or seeing/experiencing some unpleasant results on your car, its important you get it checked out ASAP to avoid potential irreversible damage.

I hope this Blog post has been informative to you (The Reader), and look forward to discussing more BMW Tuning related topics in future with you. 

Max Power Lean = Running the leanest AFR during WOT that makes the most power, it is the most efficient use of fuel but provides minimal protection to combustion heat.

Max Power Rich =  Running the richest AFR during WOT that makes the most power, It isn't the most fuel efficient way to tune, but provides superior heat mitigation to the cylinders over "Max Power Lean". Stock Cast pistons do not like hot environments, it can cause cracked pistons and damaged piston rings etc.

Peace Out and Enjoy! 


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