DME Motronic Switch Adjustments

[DANNOV]

It is my understanding that the Motronic DME control unit in my 1985 911 Carrera has an internal 8 position rotary switch that can be adjusted "to adapt fuel mixture and ignition timing for the quality of gasoline in the country of operation." Switch position 0, is the default position and provides no correction to fuel mixture or ignition. Positions 1 and 2, give a +3.1% and +6.3% fuel correction with no ignition correction, the other positions give various + and - corrections to fuel and ignition. I got this info from various sources (Rennlist and others) and by means of a search on Google. Is there any meaningful performance to be gained from adjustments to this switch? I have searched this BBS and found no references on this topic.

I would think that a slightly richer mixture should help performance. But because the DME system is a closed loop and the mixture is controlled by the oxygen sensor constantly sending signals to the DME, perhaps that offsets any tinkering with the DME itself and automatically keeps the air fuel mixture at the cleanest setting rather than the most powerful setting. I understand this switch can be accessed through a hole in the side of the DME unit. I would like to look but my 911 is getting new torsion bars and is away for the week. Does anyone have any experience in experimenting with this switch?

[nhromyak]

I too have an 85. I have found no changes become of moving the dial inside the DME box. I was looking for some early ignition timing, using my dial-back to zero timing light. I have checked both at idle and at 5K rpm.

I seem to have a slight miss when decelerating with no load (i.e. blipping the throttle off idle). I was hoping it was a lean mixture. I already checked my intake ports for tightness. I found no difference in acceleration or blipping the throttle. Of course I do not have a CO meter for certainties sake.

Just my .02

[ChrisBennet]

I don't believe there is a performance increase to be gained by using this switch. The DME corrects the mixture under part throttle conditions but at full throttle the system goes open-loop and doesn't use the O2 sensor to correct the mixture.
-Chris

[nhromyak]

That's interesting, perhaps I should try the seat-o-pants with dial and full throttle....

Nick