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Originally Posted by 42pilot
Here are three trivia questions for everyone on our 35d motors:
1. How many turbos does our TwinTurbo Power engine have?
For the 35d F15 in the US, it uses a single VGT (variable geometry turbo) with a dual plane (see attached image taken from TurboDirect SA) compressor - multi-length vanes - every other vane is shorter than the next. The VGT is more efficient since the ECU controls the turbine's blades (the hot side) to spool up faster or when you call for more power (wide open throttle for example). At cruise, the engine needs much less air so the blades are adjusted to provide less boost even though the engine has higher RPM. Bi-turbos in series are not as efficient as a VGT.
2. How many injector events are there per combustion cycle?
BMW dumped piezo technology in favor of a very fast solenoid injector on a high pressure common rail fuel system releasing 6 separate injections per cycle. This has two advantages: finer spray pattern (high pressure) over 6 injections makes a quieter and more efficient engine (higher mpg), longer dwell time during the actual injection making more torque. This is why I am paranoid about messing with the timing on the chip. The injectors no longer provide one single event during a 360 degree rotation. It might be spread out over 10 degrees and screwing with this too much will really add heat to the engine.
By the way, I just read where BMW has electronically limited the torque in our engines to provider longer life on components, like drivetrain, cam chains, u-joints, etc. That's why the torque curve spikes early then plateau's out and holds it. Unless we re-program the ECU, we will never improve torque past the pre-determined amount set by BMW.
3. Which is more important for power relative to boost: PSI or MAF?
PSI means nothing without volume. Mass Air Flow (temp and volume) is what is important. For example, I can have a can of compressed air to clean my computer that is compressed to 60 psi, but it's being released through a 1 mm hole. Open the hole to 75 mm at 60 psi and now you're talking volume. Cool that volume via a good intercooler, making it denser (packed more air per given volume), and you've got real power opportunities.
I find this stuff interesting.
When I read in an earlier post that new cars have tens of millions of lines of code, I didn't think much of it. Now the deeper I go investigating BMW's tuning strategy by speaking with diesel mechanics and tuning engineers, the more I am amazed at the sophistication in our engines and why. Again, it is why I am cautious of chips simply modifying a signal and essentially fooling the ECU of the prevalent conditions.
Anyway, I am learning that BMW has this engine de-tuned for emissions, driving friendly power (so you don't snap your neck from a dead stop) and economy. I am learning the diesel engine in the F15 is really meant to cruise/tour. It's quiet (thanks to the injectors), great economy (thanks to the VGT, injectors, tuning), has power at cruise (thanks to injectors, tune strategy, VGT) and very comfortable.
My $.02...
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Thanks 42pilot for the great info. I'm currently on F/3 will keep you all posted.