I've had my 1980 928 for a few months now, and this is one of the issues that bugs me the most. The car runs fine, until i hit 4000 rpm and then i feel like i hit a flat spot all the way until redline. I've read that some people have had similar problems, but didnt really find the answer. Any suggestions would be appreciated.

I found this https://rennlist.com/forums/928-forum/36155-poor-performance-above-4000-rpm.html , but i didnt know if the same thing would work due to the age difference.

Thanks for the help

 

No, the link refers to S4 87, and that's a different fuel/electronic system. I asume that you have the mechanical injektors, and no electronics to control it. I think you'll have to check if it's getting enough fuel, check Co2 value's. Any misfire's ? I had the same problem some years ago - mine was missing fuel...!

 

Might be a clogged cat or muffler. Put a gauge to manifold vacuum(you may have a plug on the front of the plenum you can use for this, if you have a 5 spd) and run the engine to 4500 RPM in Park/Neutral. As soon as the engine stabilizes at 4500 RPM, note the reading on the vacuum gauge. Hold the RPM and see if the vac reading drops over 20-30 seconds. If it does, you have some restriction in the exhaust. Most likely the problem would be the catalytic converter, but it could be the muffler.

You can narrow it down further by disconnecting the exhaust just aft of the cat and repeating the test. If the vac reading drops during the test, the cat is most likely the problem. If it doesn't, the muffler is most likely the problem. Note that this part of the test should be run with all your friends over, preferably after plenty of and between 2am and daybreak. Be sure to have the shark blocked into the driveway so that when the cops come you can show you had no intention of driving it with either you or the car in that condition...

The above are not absolutely positive tests, but should be 99%+ accurate. The uncertainty lies in the possibility that the cat and muffler both contribute to the problem and only exhibit symptoms when they are both connected.

I suggest checking this first as it is a very simple test and I doubt fuel delivery or ignition is the issue or I think you would have noticed rough running in addition to lack of power. Another very easy test is to run the car with the hood open in a very dark place and look for arcs along the plug wires.

Note that you DO have some electronics contolling fuel delivery -- your car has an O2 loop unless it's a euro, in which case it may still have a Lambda loop tacked onto the CIS injection system. Please give more details about your car if the above doesn't indicate an exhaust restriction.

The plug in the plenum that I refer to is between the top of this pic and the crossbar, a bit left of center on the plenum from this perspective:

 

Dave,
Thanks for the detailed reply. I'm positive that my exhaust system isnt clogged, not too long ago i had a new cat installed as well as a cat-back exhaust sytstem.

I dont have any misfires, either. I guess this leaves me with fuel delivery?

What other details can i provide to help diagnose the problem?

 

When you say you had a new cat installed... what was the source for this part? If it was not a brand new OEM replacement I'd say do the test anyway. Some cheaper replacements out there do not flow as well as original.

Also, check the secondary ignition wiring as I mentioned, make sure your coil wire is seated properly, look for arc tracks inside the distributor.

 

On the throttle body, there are two microswitches activated by the cam connecting the throttle cable to the butterfly shaft. The upper one must 'click' closed on ~30% opening .... which is responsible for the WOT enrichment signal from the ECU that gives the pleasant kick in the butt ( and bypasses the O2 sensor control).
Check out this switch and wiring.