84-87 DME problem

[David W]

I have a question concerning the DME system in my 84 Carrera. I have an extra DME box from an 87 911. I know these cars had about 10 plus HP more than the 84’s mainly because of the DME box changes. Well, I plugged the 87 box in the 84 and it ran fine (I could not feel and great difference (but can you feel 10 HP?) except for one problem: When the car is cold, it idles and runs fine. When hot, when you come to a stoplight, the engine will hunt (sounds like it has a radical cam) around the 900 RMP idle position. Eventually, it will settle down and idle fine. If you blip it, it starts all over again. Putting back in the 84 DME restores normal function.

Thanks for the help, David

[Steve W]

There should not be a power difference between a 84 and 87 DME box because the fuel and ignition maps from 84 to 89 are essentially all the same. The differences you read about in the 84s having 207 hp and 87s having 217 hp are just the way Porsche rated its motors. I had posted chip maps of my 84 DME and Chris Bennett's 89 DME somewhere on this board which show the differences. If you are interested, you can search out some of my past posts to find out more. Early on Tom Polzin found out the difference in the later DME boxes is that the later ones disable the onboard 8051 microprocessor and uses additional coding placed in the larger eprom to do the microprocessing instead. He also found out how to modify earlier boxes to take the later 28 pin eproms of the later Carreras, though he is not aware of any power or performance advantages to doing it.

[ChrisBennet]

Hi,
AFIK the "10hp difference" really isn't there. They make the same horsepower on the dyno according to Bruce Anderson.
I'm no DME expert but thinking out loud...
Idle is slightly different between the two, something like 800rpm for the old ones (pre-87) and 880rpm for the later ones. When the idle is set (by a mechanic), the idle stabilizer is disabled, and the idle is adjusted such that the motor idles naturally at 800. In normal operation the DME adjusts the idle to keep it there. If your idle was set to 800 and the new DME expects 880rpm it would have to "pull" the idle up to 880rpm. I think that is where the problem lies. I would readjust the idle to 880.
-Chris

[David W]

Thanks guys. This is a suprise. I thought the later cars had more HP as the DME was improved and that Porsche felt the G50 box could handle more HP. It does make sense (no extra hp) as the CR etc, is the same. Not so for the Euro cars of course.

Any thought on chips? I had a Euro products chip several years ago in my 85 that detonated even with the best premium.

Thanks!

[Tim Polzin]

I'll chime in here. I was the one that converted the earlier box to a later chip. I also found that when I used the converted box, the idle hunted, while with the original box the idle was rock solid. Sounds like Chris's explaination is right on the money.

Tim

[David W]

WOW! This is a great! Is there any advange to the 87 box? If not I will stay with my 84. FYI, Performance Products cat. shows a 84 vs 87 HP and the effert the welmister chip makes. In most areas the 87 is more powerful, both stock and with the chip. Bogus?

Anyone want to buy a DME system with a good box?

[Tim Polzin]

I'm actually not sure there is any advantage to the new chip. The reason I swapped chips was because a blown 5 volt regulator took out all the 5 volt parts on the digital board. The only part I could not directly replace was the CPU because it had internal code. The later 87-88 had all the code in the E-prom. By using a generic 8051 and the later eprom, I was able to revive the blown DME box. I also found no difference in performance between the two. Now at least I have a spare DME.

Tim

[ChrisBennet]

David,
The '87 box has a socketed EPROM and I don't *think* the '84 does. For future upgrades the '87 might be preferable.
Hey Tim, I still haven't gotten a chance to turn that 951 DME into a 911 one yet. I wonder if the injector driver circuits are the same? The 951(944T) drives 2 injectors and the 911 drives 3. Thoughts?
-Chris

[Tim Polzin]

The injectors are fired in banks. On the 4, only two are fired at once, on the six, three go at once. (so much for the idea we had sequential injection). I will be very interested in hearing how that project works out Chris.

Tim

[ChrisBennet]

Originally posted by Tim Polzin
The injectors are fired in banks. On the 4, only two are fired at once, on the six, three go at once. (so much for the idea we had sequential injection). I will be very interested in hearing how that project works out Chris.

Tim

Yes, I figured out that part, I just wondered if the driver output circuit was different. I've got to finish installing my "spare" engine, rebuild my 3.4, and do an engine reseal/car restore before I can play with that DME. So many projects...
-Chris

[Tim Polzin]

Yes, I know the feeling. Too many projects and too little time. If we ever get any warm weather, I hope to be out on the golf course. Have you opened up the 951 box and is there any physical differences between the injector drivers on the 951 box and the 911 box?

[ChrisBennet]

Tim,
I haven't looked at the "analog board" yet. The digital boards look very similar if not identical.
Hmm, I think we hijacked David's topic. Sorry David.
-Chris

[David W]

Thanks guys. I Think Chris's idea to incerase the idle sounds correct. I forgot to mention that when the erratic idle occoured, I could ddisconnect the idle stabilizer and the engine would idle fine.

Question: how do I increase the idle speed?

[ChrisBennet]

The idle screw is on the front of the throttle body, probably under a yellow cap. I think it takes a 7mm socket. You should disable the ISV by jumping sockets B & C in the test socket first.
-Chris

[David W]

Thanks to Chis and everyone! I wil try it next week and report back. (I am driving to the Smokies this weekend (apex city). top off and 911 music!