Motronic fuel quality switch??

[Leland Pate]

Do the 993 models still have this fuel quality switch on DME?

Got this from Steve Wong:

The Motronic fuel quality switch is an eight position rotary switch which adapts fuel mixture and ignition timing for the quality of gasoline in the country of operation. As climates vary throughout different regions of the world, so does the quality of the fuel. The switch is designed to counter engine running problems and does not yield any appreciable performance benefits. In fact, Bosch service documentation warns "Unnecessary adjustment to the switch though, leads to poor drivability, and particularly to "search" during overrun or to increased fuel consumption, in some cases even to engine damage." Porsche option M 240 from the factory (version for countries with inferior fuel) sets this switch to position 4.


That said, it is located along the side of the DME under a plastic plug on the DME's cover. With the DME in your car, the FQS can be accessed from under the driver's seat from the rear. A small three sided tool is needed to fit into the switch and rotate it. A small properly sized flathead jeweler's screwdriver or a trimmed down golf tee works well. Adjustments are possible via the switch to assist in combating issues such as predetonation under extreme conditions. When performing any adjustments, do it with your ignition switch full off. Changing the FQS with the engine running produces no effect or change until the next boot up of the DME unit. Full counterclockwise represents the stock position. If your car suffers from predetonation such as from extreme heat conditions (i.e. 110+ degrees F with your A.C. on) trying position 4 or 5 can help alleviate the condition. If your car suffers from what appears to be a rich running condition, experimenting with position 3 may help. However if you are at extreme altitudes and experience a rich running condition, you may want to check the operation of your altitude sensor located alongside the DME. A simple on/off switch, it should only read closed at altitudes above 1000 meters (3250 feet).

It was an interesting read none the less.

[AFJuvat]

I don't believe they do. I think the last car to have that was the 968. (prod ended 1995)

AFJuvat

[kang]

Sometimes chip manufacturers make chips with two maps, one for street gas and one for race gas (100 octane). They wire a switch to choose which map to use. I don’t know of anyone who does this for the 993, though.

[ischmitz]

Nope. 964 and 993 do not have that switch anymore. There are several maps stored in a 964/993 chip and the DME harness has two coding plugs that decide which maps gets used.

I have made a dual-chip for the 964. It is essentially two chips with a selector switch. Flipping the switch is equivalent to swapping one chip with another. This is very convenient when running street gas and race gas. See here for more details: http://sswl.di.com/~ingo/Chipping.htm

Cheers,
Ingo