Supercharged '87 911 engine surge

[rarall]

Hi, looking for some help on a problem I had this weekend while enjoying a ride in the 911. I've attached a picture of the 'Knoxville supercharger' set up that I have on my car for orientation. While under boost accelerating onto a Florida turnpike, boost was about 2 psi in 5th gear (4000+rpm - 100mph), I had to quickly back off the throttle....I didn't hear a specific noise, but the car began to 'buck' as I decelerated (no backfire or any other noise but surging engine noises). At 60mph and 2300 rpm the car behaved normally even under mild acceleration up to 3200rpm, then 'bucking' again. Below that it would cruise but with occasional 'bucking', no rpm spikes are seen with this surging (oil pressure normal, no abnormal engine indications). Once off the turnpike, all gear shifts were normal and the car drove ok below the 3200 threshold except when throttling down while in gear. Once at home, inspection shows no discrepancies with supercharger plumbing, all other plumbing is intact. Revving the engine staticly finds the rpm becoming erratic about 3200rpm. The exhaust pipe is coated with heavy dry (not oily) carbon residue (normally this is dry and clean) as if the engine is running extremely rich. Exhaust does not smell fuel rich per say. The DME computer is a brand new rebuild (10 hours on it).
Any thoughts?

[Rodsrsr]

Are you using a BEGI rising fuel pressure reg? If so, make sure that you have the vacuum restrictor in the signal line. I was chasing the same symptom for months before I figured this out. With the Paxton the boost comes on very gradually and often times the BEGI will cause a rich condition mid range. The restrictor limits the amount of vacuum which lowers the pressure.

[Tippy]

Are you building boost in no-load conditions? Have a wideband?

I've had hell with Motronics over 6 months of cutting out in light loads. Never figured out and went MegaSquirt.

Anyways, that is very cool to see a supercharged Carrera. Would love to ride in.

[rarall]

I am using a BEGI fuel pressure regulator...I'm investigating your suggestion and integrity of the vacuum and position of the vacuum restrictor signal line. Thanks!! I'll let you know what I find out.

[Rodsrsr]

The restrictor is just a small plastic piece that's inside the vacuum line where it plugs into the regulator. You may already have it there but check first. If you dont have it, you need one, and if its there I would dial back the top threaded adjustment about 1/2 a turn. Its getting too much fuel too soon. I ran that exact set-up for years so I'm very familiar with it. If you dont have the restrictor BEGI can send you one. You need an air/fuel gauge too.

[guru944]

Rarall:

My i suggest a ignition upgrade as well MSD Blaster 2/3 Coil.
This will improve your power and efficiency. by 30% or more over the OEM Bosch Coil.

Later.

[Tippy]

MSD will burn up the Motronic computer from what I've heard.

[guru944]

Tippy:

Negative, only if its improperly wired....
Installed MSD 6A & Blaster 2 Coil(s) with 8910 module(s) on the 89 SE and a 6BTM on the the 87S Club Sport. Both work like Champions. Even the exhaust tone changed. Smother Revs straight through no glitches.... Not sure where you are getting that info but do the research. If you need help PM Me.

Blaster 2 Coil Hi-Performance - 8202

Easy upgrade... best 45$ you can spend....at Pep Boys or Autozone

Later..

[Jim2]

Originally Posted by guru944

Rarall:

MSD Blaster 2/3 Coil. This will improve your power and efficiency. by 30% or more over the OEM Bosch Coil.

Later.

Hehe, yah, power and efficiency by 30%, must have said this right on the shiny packaging.

Sorry Rarall, back on track now trouble shooting your problems.

[guru944]

Rarell:

What are your no load vacuum readings at a startup (open loop), warm (close loop) and moderate driving loads(no boost). Your vacuum should be ~-17hg at idle and -10 ~2 good all while under 0psi. Any Back firing or "Poping" after it gets warm. Consider check your mechanical and electronic timing settings(saturation).

Had a simular situation and the timing was off by 7-9' found it and it cleared up...thank GOD.

later...

[Tippy]

Originally Posted by guru944

Tippy:

Negative, only if its improperly wired....
Installed MSD 6A & Blaster 2 Coil(s) with 8910 module(s) on the 89 SE and a 6BTM on the the 87S Club Sport. Both work like Champions. Even the exhaust tone changed. Smother Revs straight through no glitches.... Not sure where you are getting that info but do the research. If you need help PM Me.

Blaster 2 Coil Hi-Performance - 8202

Easy upgrade... best 45$ you can spend....at Pep Boys or Autozone

Later..

From my understanding, the factory Bosch coil has a resistor in it. Without this, obviously it puts a higher strain on the DME. I was told by many who were EE's that running an MSD box on the DME will too burn up the DME. Not sure which components cannot handle though, just posting what I was told and read here.

I'm pretty sure "lorenfb" stated this too, and he is a Motronic rebuilder. I think he no longer participates here though, he did not like aftermarket DME chips.

If it does not hurt, this would be great for the person running boost on the C3.2 due to simplicity. I personally own one from 15 years ago.