Results presented down below are from modified M106 engine:
- 3.0 bar / 43.5 psi fuel pressure
- Adjustable fuel pressure regulator
- 0.6 bar / 9 psi boost pressure
- L-jetronic 200N wastegate
- Three-inch stainless exhaust
- Modified M30 cylinder head
- 93.00 mm forged pistons
- Fuel cut delete chip
With NTK meter following afr numbers were tracked:
- Idle 13.5 to 14
- Cruise 15.5. to 16
- Full throttle 12 to 12.5
For better understanding operation of 745i engine, installing wideband air/fuel-ratio meter is best thing to do. Even on non-modified car it gives very usable information. There are several manufacturers of meters (NTK, AEM) to pick from. Sensor should be installed away from turbo, according to NTK at least 0,5m down turbine. Before metering basic tune-up items should be covered (injectors, distributor, filters, plugs, valves, idle speed and throttle body). Adjusting basic ratios can be either done via air flow meter or with adjustable fuel pressure regulator (easy way).
Idle is smooth in this setup, leaning idle to 14.5 makes car run rougher (fuel pressure 2.5 bar). Cruise afr numbers are at 100 km/h speed with fourth gear locked. Full power afr needs more reliable testing, ratio of 10 and 11 show up but seem to average above 12. On sport, open throttle from stand-still makes car hesitate bit when shifting up to second gear. Air fuel ratio goes sidelines? as maximum boost is not yet reached on first gear. Rich mixture can make feel of power loss momentarily, in particular due less accurate air flow meter based system.
Acceleration on full throttle with single muffler exhaust and no floor insulation, makes you consider ear protection. Cruising generates exhaust resonation both sides of 2000 rpm, especially on highway speeds. Irritation is forgot after acceleration in a tunnel. Forged pistons play well along, the sound past 3000 rpm is breathtaking. Idle tone is on heavy side but sound is promising. Any stock system flow restrictions are long gone. On motorway exhaust quiets down at 3000 rpm, sport selected, the engine pull is insane.
Numbers listed up above give guidance what one should expect while metering. According to Autospeed article on afr ratios idle can be around 13 to 13.5. Cruise ratio around 15.5 to 16 is fine due light load conditions. High load afr on boosted engine should be 11.6 to 12.3. Most factory boosted cars will run very rich, like afr of 10 being common on high loads. This is safety feature which helps protecting engine by cooling down mixture with excess fuel. It does not go well with best power but can help combat high intake temps or lesser quality fuel.
BMW spec for idle AFR can be converted from Technical data (please see Files/BMW M106 repair files/img114.jpg). BMW list CO target as 1.5 % by volume. This converts to AFR of 13.94, so shoot for 14:1 at hot idle. For AFR conversion table, just Google “co% to afr table”. If you don’t have a wideband on hand, to get basic information how your idle AFR is, you could use annual inspection data of emission test for this. For example the 1986 745i this site is all about, got reading of 1.49 CO% in the test year 2022. That is spot on for air/fuel-ratio.
On highway full boost is seen above 2000 rpm on fourth gear locked (economy) and before 3000 rpm on locked third (sport). From stand still full boost comes on second gear. Off boost engine is easy going and old eh-transmission makes smooth job on economy. Part throttle acceleration response is great, traction is lost quickly on wet surface. Up shifts are quick on sport mode and speed builds up fast. On sport transmission down shifts aggressively when throttle is just opened and can surprise. This is highlighted above stock M106 boost levels.
Following meters while driving can be difficult. For reliable results data logging is needed. There are special logging devices that accept analog signals from engine and can supply voltage for standalone sensor. Devices save data to memory card or directly to computer via USB. AFR meters usually provide analog output, beside this boost pressure is needed for rudimentary logging. Boost can be measured by MAP sensor, it needs own voltage supply. Australian company eLabtronics sells USBP Module for 5-channel logging with single supply voltage.