[Benno]

Since this forum is a safe place to discuss vacuum pumps, I thought I would share some information.

Take a look at these before and after shots of the pistons heads of my 1992 GTS (cylinder 5-8 only for now). The before shots are with the vacuum pump system installed (50K -70K miles). The after shots are of the last 3000 miles without the vacuum pump system and using the late GTS breather system. Oil baffle under the OFN in both cases. I was shocked to see so much oil caking on the piston heads in such a short period of time.





Also I am seeing quite a few knocks under load indicating the octane decrease from oil injection. Timing and plug condition verified.


I thought I would share this information because it shows just how much oil is going through the breather system on an early GTS. Also, I noted more oil consumption and more oil collecting in the plenum after removal of the vacuum pump.

Some background: I had removed the vacuum pump system because the Provent 200 air oil seperator was being overloaded and I was getting oil mist over the front of the engine. This was made worse after I converted the Automatic transmission to a 5spd and now have more time at higher RPM's which produced more airflow output from the vacuum pump and more oil mist on the engine face.

I am thinking I should re-install the vacuum pump and try to solve for a high flow filtration system. I would appreciate some suggestions on high flow oil separation systems. Here are the air flow rates for my vacuum pump.


I am looking for a separation system that is free flowing or can handle up to 800 LPM flow rate. I would prefer to keep the system closed loop if possible but would consider something that offers a blow off above a certain airflow.

[worf]

I am completely unsurprised with the pictures. I’ve taken apart and/or scoped several very young GTSs and they all had massive deposits on the piston tops.

What we know:
928s have an “undersized” crankcase breather system.
GTSs have that problem AND no oil drain holes in the piston skirts.
GTS long-throw crank significantly worsens windage thus exacerbating both preexisting conditions.

A crankcase vacuum pump will work especially well on a GTS because the reduction in windage will help keep mucho-excess oil off the cylinder walls.

What to do?

Drill the oil drain holes in the piston skirts.

Too ‘advanced?’

Two Provents?

My “research” suggests that an NA 928 needs the equivalent of 1.0 to 1.5 square inches of breather volume. If such a system will be closed then you’ll need to be creative with the air guide.

There’s a lot of discussion on hernancia’s breather design thread.

[Benno]

Thanks for the response Worf. I'll re-read that thread and follow up. Rebuilding the GTS motor and adding the oil drainback holes is definitively on the 1-2 year plan assuming I can find qualified help to do the engine work. In the near term, I think re-installing the pump is a good idea.

[maddog2020]

drilling the piston drain back holes is the single best thing you can do for GTS engines. I was part owner of a shop back in the late 90's and the local dealership was referring customers to us for GTS's that were out of warranty and had oil burning and pinging issues because my partner had a really good relationship with the local dealership & they let him tear down a "core engine" after a replacement before they sent it back to Porsche. Granted he had worked for AMG before coming to the US, and had heard about the early GTS's were grenading engines on the Autobahn at speed.

[Benno]

Thanks for the feedback. Yes, tearing down this 92 GTS engine is on the future plan, it will need the ring land drilled, as well as, replacing the connecting rods. And of course, hone the piston bores etc. I just need to find a qualified builder that is relatively local to Portland, OR. I don't have the experience or the tools to do the work.