converting DMEs
 

What is involved in converting from a early DME to a late DME?

Can it even be done? I'm not so sure . . .

with a 86 or 87 DME - the AFM and Idle Stableizer Valve should also be changed. because the 88 & 89 had different comp ratio, I think using that DME/chip would not help you pass a smog test unless you changed the pistons too.

Yes you can.

You wont have to add an idle stabilizer valve but you'll need a later AFM (different voltages and flow curves). It's literally plug and play.

The plug-ins are the same for both the DME and AFM.

I've read the AFM must be the same year as the DME used as there were changes through time but that info was debunked by Frwilk.

I changed out mine with an 87, 944 and didn't need the stabilizer.

Cool, does the new DME take care of the early shudder?

You can't use the later idle control vavle (stabilizer) with out replacing the DME to engine wire harness. You can use the DME with the later AFS.

"Cool, does the new DME take care of the early shudder?"

Yes, the code in the later DME removes the shutter. The reason you have to match the DME and AFM is that the early AFM is 0-8v and works only with the early DME. Conversely, the late AFM is 0-5v and works only with the late DME.

Many 944-spec racers swap to the late DME/AFM combo to make use of the plug-in chip socket.

There is an extra sensor that the late DME used that can cause an irratic idle on the early engine since the early cars didn't have it from what I've read.

Have you used a late AFM and DME on an early car without modifying the wiring harness and had it work okay?

Which extra sensor is that?

The DME/AFS combo is swaped out all the time.

hall sensor on the distributor?

that's only on the 16v

And it's on the back side of the cam gear.

Also there are two knock sensors in the 16 valves. And their idle control valve works in a different way than the others.

What does AFM stand for?

Porsche lists the device as an AFS; Air Flow Sensor. Others have taken to calling it an Air Flow Meter. As it does not intentionally restrict the air flow into the throttle body it is a sensor and not a meter. This unit also has a temp sensor for the incoming air.

the Idle Control Valve is what I was thinking of. Is that only on the 16V cars?

All of the 944's have an idle bypass for cold start. The early 944's are electrically heated to cause a bi-metalic spring to close of the opeing.

The later 944's have an active valve where the DME controls the opening. There are two kinds of these; one is bi directional the other (16 valves) is single direction with a spring to return it to open.