I remember on my old MS6, I used to monitor knock with the sub-par DTEC I ran for a while, and running around ~14-15psi, I experienced that dreaded “lean spot” around 3500rpms or so (this was back in the day before CP-E released the Standback or the HPFP upgrades). I remember even having numerous conversations with Jordan at CP-E about this issue and trying to come up with a HPFP solution (go figure, I sell my car and they release a HPFP upgrade haha). Either way, depending on a number of conditions, that motor would typically hit around ~12.5:1 AFR under ~14-15psi, and would sometimes go as lean as 13:1 AFR at around 3500rpm. When that happened, if I was logging, I would typically see at least 1 to 3 counts of knock.
In contrast, my BMW makes peak boost at an even earlier RPM, but will hold stoich AFR’s (14.7:1) to about ~4500rpm or so, and even after that, will target between 12.5:1-13:1 AFR’s all the way to redline. I’ve seen numerous logs, with these cars pushing similar boost pressures & timing as the 2.3 DISI motor, running much leaner, without knocking. Granted, the factory timing isn’t as aggressive as some factory turbo cars I’ve seen, and the DME/ECU is much more reactive to possible knock (it helps that the N54 comes with multiple knock sensors), but still, these things run very lean without issue under full load.
For reference, the N54 even runs a higher compression ratio (10.2:1) than the 2.3 DISI motor (9.5:1), so this further adds to the equation, as the higher compression motor should be even more prone to knock, but is not. They’re also both inline’s, so no real variance there (i.e. like a boxer motor that runs hotter/needs to be tuned more conservatively than an inline ect).
Although I think I’ve found the reasons behind why BMW can run their motors so much leaner. From my research, it appears the major difference is between the injector type and the placement of the injectors themselves.
The BMW uses piezo injectors, whereas I believe the MZR motor uses solenoid injectors, and from what I understand, piezo injectors are typically lighter and react faster than solenoid injectors. This could be a large contributing factor, but I think a large part of the reason why the N54 seems so much more detonation resistant is because of the placement of the injectors. The N54 seems to place the injectors right next to the spark plug, almost pointing towards one another, and the theory behind this is that at the point of spark ignition the air:fuel charge is rich, whereas the surrounding area is running considerably leaner, but because the immediate area around the plug is fuel laden, the risk of detonation is considerably diminished.
From what I understand, VW has done something similar in Europe, allowing for AFR’s as lean as ~60:1 (under no/extremely low load of course haha) without detonation. This seems to be in contrast to the methodology that Mazda/Ford are using. They seem to position the injector on the side of the head(s) pointing towards the piston, which in my mind would mean they’re “bouncing” fuel off of the piston rather than spraying it in the same direction as the injector.
This is what I’m talking about, here is the BMW N54, notice the position of the spark plug & injector:
Here’s a cutaway of the 2.3 DISI MZR motor, with what I believe to be the injector circled:
This appears to be the same design Ford is using in their new EcoBoost design (in the Taurus SHO), this is an image more clearly depicting that design:
This is how that design would work in contrast:
From what I can tell, the method used by BMW appears as though fuel would be more equally dispersed, but also that their theory behind spraying fuel directly towards the spark plug really does work in real life application (increased detonation resistance). I’m assuming the higher pressure fuel pump in the N54 (around 200bar or ~3000psi) and more advanced/faster injectors help with injecting fuel with more precise/accurate timing. Now if only those crafty German bastards would make a damn fuel pump that would be reliable haha!!
Either way, since many of the members on this board run DI motors, I figured I would share what I’ve found. Have a good one!
-Brandon
