External Oil Filter + Accusump for the Porsche® 356/912

An illustrated discussion by Eric Nichols

Why an external oil filter?

The stock oil filter in the 356/912 engine is a "bypass" design, which only filters a small portion of the oil during its circuit through the engine. The drawbacks of this design are discussed in Harry Pellow’s books, and Duane Spencer’s book, "Porsche 356 Performance Handbook".

Why an Accusump?

The 356/912 engine is a "wet sump" design, which relies on gravity to direct the oil back to the oil pump intake in the bottom of the crankcase. While this may have worked well for 1950’s-era tire technology, today a well-developed car can generate in excess of 1 g of cornering force. If this force is sustained for more than 10 to 20 seconds (such as during a long sweeping turn, or in a series of linked turns all in the same direction), most of the oil in the engine will accumulate in the outboard cylinder head. When this happens, the oil pump intake tube sucks AIR rather than OIL. This results in a sharp drop in oil pressure, right when you need it most. While this will rarely happen in a street-driving situation, it can happen often enough during racing to require preventative measures.

There are several preventative measures available. Some could be called "partial solutions". The partial solutions include:

  • Windage tray. This is a piece of sheet metal placed in the sump to slow oil slosh. It is described in Duane Spencer’s book.
  • The Super 90 centrifugal oil pickup. This clever design uses a sliding weight to shift the oil pickup point to the side of the crankcase that is on the outside of the turn. This has two disadvantages: it isn’t sufficient to overcome extreme oil slosh, and if it should ever stick in one direction, then your engine will be suitable only for NASCAR style oval tracks.
  • Don’t drive so fast. Not an option!

Currently there are three alternative "complete solutions":

  • Dry sump conversion. Leo Droughton described this awesome project in a ‘70’s-era Panorama article, which is repeated in "Up-Fixin der Porsche," Volume V. It involves retrofitting an oil-scavenge pump and an external oil tank. A nice idea, but very difficult to engineer. A similar system is used in the 911, and many other modern sports car engines.
  • Deep sump. This is the favorite solution of many 356 racers. The major drawback is ground clearance. I used this solution when I was running 195/60-15 race tires. With the lowered the ride height, this resulted in marginal ground clearance… speed bumps were impossible, and the sump would occasionally scrape the crown in the road. Scary! With 205/50-15 tires, the ground clearance is too small even for track-only usage. This prompted the following solution:
  • The Accusump. The Accusump is a "hydraulic accumulator". It is a large cylinder containing a free-sliding piston. On one side of the piston, the cylinder is closed, with air filling the enclosed space. The other side of the piston is full of oil, pressurized by the engine’s oil system. As pressure rises, the air compresses, the piston slides toward the closed end, and the Accusump accumulates more oil. As pressure drops, the air expands and forces oil back into the engine. This has the effect of damping out sharp changes in oil pressure. Should the oil pump suck air, the Accusump provides 10-30 seconds of oil pressure, which is usually sufficient to finish the turn without harm.
The Project

Here is a photo of the project from the upper engine bay:

The Details:

Besides filtering the oil and improving engine reliability, the modification needed to satisfy two additional specifications:

  • It had to be "reversible," and require the fewest number of alterations to the car and original components.
  • It had to be aesthetically pleasing, at least to me


Rather than alter the original third piece of the engine case, I acquired a spare third piece at a swap meet, and performed all permanent modifications on it. The original third piece is hanging on the wall of the garage, just in case my car is inducted into the factory museum.

Besides the modifications to the case, six holes needed to be drilled: four in the body to mount the Accusump, and two larger holes in the rear pulley shroud to route the new oil lines to/from the filter. A spare pulley shroud was donated to this cause.


I wanted a tidy installation, and as near a "factory" appearance as possible. This meant that the oil filter should be placed in the same location as the stock filter, and should have similar (but larger) "plumbing". The Canton/Mecca oil filter (made by the same folks as the Accusump) fit the bill. Fortunately, 912s are blessed with a large open space immediately behind the engine. This is a tidy spot for the Accusump, and it allows the plumbing to be simple and short.

System Schematic:

All oil lines were plumbed with 10AN stainless steel braided hose.

Modifications to the case and oil pump:

The existing oil pump outlet passage was tapped and plugged with a ½" NPT plug. For the oil return line, the boss on the left side of the third piece was drilled and tapped for a 3/8" NPT fitting. A 3/8" NPT to 10AN male swivel 90° elbow was fitted. Fortunately, the position of the fourth main bearing was concentric with the original piece, and required no adjustments when the new third piece was mated to the rest of the case.

The oil pump cover was drilled and tapped for a ½" NPT fitting. A ½" NPT to 10AN female swivel 90° elbow was fitted to the cover. WARNING! The forged oil pump cover is thin, and this results in very little thread engagement. The pump cover can be thickened by welding between the webs, but this can distort the cover, rendering it useless. Don’t ask me how I know! I consider this threaded connection the weakest part of the current system. The swivel fitting may provide some strain relief. I think Vic Skirmants now sells a custom pump cover that does not suffer from this problem. Although the system has been leak-free for 8 years, I plan to upgrade the pump cover connection at the next opportunity.

Since the oil filter has no anti-drainback provision, the pump discharge was connected to a 10AN low-cracking-pressure check valve. This is a mil-spec unit -- quite expensive, but very high quality. It also serves to prevent backflow during startup, as the Accusump acts as a pre-oiler.

The details of the third piece, the check valve, and the passages through the engine skirt are shown in the following figure:

The resulting clearance between the check valve and a Bursch exhaust system is TIGHT, as shown on the following figure. Note that the elusive 912 heat shield is made even more necessary.

The following photo shows the lower oil lines and fittings tucked in behind the 912 heat shield and a Bursch exhaust. 912 owners may also note the factory-issue brackets to support the rear engine tray, fitted to the exhaust pipes.

Modifications in the upper engine bay:

The filter uses a stock mounting bracket with a new, smaller-diameter stainless steel strap. The outlet connects to a 10AN male branch tee with a ½" NPT on the run. One branch returns oil to the third piece of the case, the other leads to the Accusump. Folks that are interested in just a full flow filter without the Accusump could replace this tee with a simple 90° elbow adapter.

A photo detail of the filter is shown below:

The Accusump includes an electric solenoid valve, keyed to the ignition, with a defeat switch on the dash. The defeat switch allows the ignition to be left on without the motor running, while preventing the Accusump from emptying itself back into the engine. This is useful, for example, during static timing, or for listening to the radio while parked.

Stuff I would do differently next time:
  • I would use Vic Skirmants’ improved oil pump cover. Vic can be contacted at (810) 575-9544 or [email protected].
  • I would do all the plumbing fittings and hose ends in a consistent color, preferably silver, or better yet, gold. Gold would match the Cad II plated fasteners even better. Anyone know a source for gold anodized fittings? Shiny silver-finish fittings are available from Russell Performance (http://www.russellperformance.com).
Parts List:

A list of necessary parts can be found here.

Most of the plumbing pieces, including the filter, check valve, and Accusump are available from Pegasus Auto Racing Supplies, 1-800-688-6946, or Baker Precision at http://www.bakerprecision.com.

Some of the adapter fittings are not common, but be obtained from the Earl's catalog through either of the vendors listed above.

Most of the prices shown in the parts list are from the most recent (late 1999) Pegasus catalog.

If you found this information useful, or have any comments or questions, please email me at [email protected].

Keep the 356/912 Faith!