Lot's of great points here! I have seen 3 S55's break at the track (1 the day my bearings went) and I know the crank hub fixes seem to work. I had actually considered just selling my 135i and grabbing and M2 Comp, but I'm not in a position to take on that payment increase. (the dealers are offering good deals on 2019's).So regarding dual oil coolers.... I have a large setrab type oil cooler connected to the oem thermostat, and I still have the problem described. If the engine oil pump picks up air, I dont think it makes any difference how much oil is sitting in the oil cooler? The in/outlets are located on the top of my oil cooler, so the air would just push right though.
I also have a second oil cooler, which is driven by an electric pump connected to the front and rear of the oil sump, circulating the oil in the oil pan. I did try turning that pump on and off, and it doesn't make any difference in regards to the oil pressure issue. It just lowers the oil temps a bit. The oil is moved from the rear of the pan, ie out side the baffle, to the front of the pan. It makes sense that it doesn't help with oil pressure, because the issue happens when I trail brake, where the oil would already have moved towards the front of the baffle. In fact it might help to reverse the pump, but then I have the issue where it might run dry when accelerating. The rear of the pan is lower than the front.
And in general I would say, spend all the available cooling space on radiators instead of oil cooling. Three days ago a friend of mine hit +118C with this S55 on track, while his oil temps barely moved past 120C. My M135i friend would also hit power reduction due to coolant rather than oil temps here in the southern chinese humid heat.
If you update rods and bearings, you risk a bad installation which could lead to premature failure. Really I would say the easy answer is to go S55. It also has a pump to pull oil from the turbos under high G forces.
Regarding turbos for tracking you need something with headroom, so they have an easy job, and a turbo that uses the best exhaust wheel material that can handle the elevated egts. Look at the M235i racing. OEM turbos and less than stock power. That is my best advice. But you wont listen to that hahaha - I certainly didn't myself. Also look at the M235iR control arms for great inspiration.
The engine rebuild would be done by a local BMW guy who builds E36/46 race cars for our AER series here in the states. His cars go very well, but the caveat is he is very busy and this would be side-work for him.... but I have time on my hands.
Is there a way to get the M2+ N55 dual pick-up (and pan) for our engines? I've been scouring the ebay, etc and haven't seen anything.
I've been running stock turbo, with JB4, then MHD and MHD was misfiring like crazy... I heard from some people the burble tune on track can cause issues? So, anyhow, I resigned to stock tune and kept improving lap times. But, I did run stickier tires (I normally run RE71 or NT01 and switched to the AR-1.) I chased down a supercharged e92 M3 and that's when the oil temps went crazy and I shut down on track (300F).
Plan for rebuild is a CSF 7046 Radiator as I have a DCT. I hear you about the aeration issues with any oil cooler.
I also run RaceLouvers.com hood extractors which have definitely helped with coolant temps as well as aero.