Hello everybody,
I started this thread here, where I ended up in developing my own digital ECU for my US 1982 930/16 engine based car, providing the needed duty cycle to the frequency valve for achieving the proper 14,7:1 mixture.
http://forums.pelicanparts.com/porsche-911-technical-forum/1001258-thoughts-replacing-911-sc-ecu-raspberry-solution.html
It uses also a removable LCD unit where all actual values are shown while driving:
Sensor voltage
AFR
Duty Cycle
Fuel Control pressure
Engine (Oil)Temp
RPM
CO
Unit runs flawlessly – works with the original Narrowband sensor but even far better with an external wideband lambda controller and Bosch LSU 49.9 sensor. In my case an Innovate LC-2 controller but would work with all other AFR controllers out there as well.
When setting the CO to
0.5 % factory specs (with sensor disconnected and CO measuring before Cat) than this results in an approx.
45% duty cycle at 950 RpM idle – everything normal so far.
But I recognized (wenn sensor reconnected and engine is warm) when driving the engine at about 2000 RpM and higher the duty cycle decreases significantly down to 15% to achieve the 14,7:1 stoichiometric value based mixture.
I checked my pressures where the system pressure is 4.7 bar and the control pressure is 3.5 bar, so everything perfectly matching the factory specs.
2000 rpm —> ca. 30% duty cycle
3000 rpm and above —> ca. 15% duty cycle!
These are values when crusing very cautious, so no 30% throttle state (which would end up in 65% duty cycle anyway). Spark plug readings also showed an optimal color/state/face, means perfect combustion/AFR.
To me that means, that without an ECU control, the car would run very rich at higher rpm’s, even if not needed.
No further enrichments occur like in case of Euro WLRs do here occur.
Means if 15% duty cycle at higher rpm’s provides the needed 14,7:1, what would happen if you hit the gas for WOT, … here at WOT a „static“ 65% duty cycle mode in my case ends up in 11,0:1 AFR!, which is far too rich.
So my question is, … is there anybody running an AFR gauge/display with a proper idle CO setup of 0.5% who can confirm such low AFR results with a very rich resuting mixture at high rpm’s or at least tell me what AFR value is resulting at WOT or passing the 30% throttle position?
It would be interesting cause after this test, now in my ECU the optimal needed AFR of 12.5:1 when accelerating will be calculated based on the actual running duty cycle when not accelerating.
Means if 15% is the duty cycle when i.E. running the engine at 3000 rpm and 120 km/h then – when accelerating – no static 65% dc will be forced but a calculated increase of the dc from 15% to for example 25% dc will result in the perfect 12.5:1 stoichiometric value based mixture. And will be kept at 12,5:1 by reading the Wideband sensor Bosch LSU 4.9.