Porsche Dry Sump Systems
Your engine requires a supply of clean oil of the correct viscosity at the recommended temperature to pressurize and circulate throughout the engine to lubricate and protect all the moving parts. Without this lubrication the reciprocating and rotating parts will suffer catastrophic failure. Additives in the oil also serve to protect engine components from wear and corrosion.
Very early on, vehicles relied on “Oil Splashing” within the engine. Oil was picked up in the sump by the big end of the connecting rods and hurled into the crankcase where a series of galleries collected the oil and through gravity, fed the bearings and gears. Cylinder walls also relied on this form of lubrication. Besides being inefficient, anyone who has been swimming knows the resistance felt passing their hand through water. Imagine the power loss created passing rotating parts through something as viscous as oil.
Clearly a better solution was required.
Improvements began with a combination of the ‘Splash’ system with the addition of an engine driven oil pump to pressure feed the main bearings.
The rest of the engine was forced to rely on ‘Splashed Oil’. This soon found its limits and it was clear a fully pressurized lubrication system was needed to advance engine performance and reliability.
We now see the introduction of forced lubrication systems that no longer rely on random oil splashing nor gravity for lubrication. An engine driven pump now sucks oil through a pickup tube from a pan
(wet sump) located on the bottom of the engine. This oil is then pressurized through oil galleries which feed the oil through a filter and on to feed the working parts of the engine. Gravity then returns the oil to the engine oil pan.
I’d like to discuss the variety of designs we see in use.
These are the Wet Sump, Deep Sump, Accusump and Dry Sump.
In discussing each type, lets first recap the engine requirements.
∙ Clean oil requires efficient filtration during
circulation.
∙ Cool oil, a design that allows the oil to reduce in
temperature before being re-employed as a large
amount of engine cooling is performed by the oil.
∙ A steady uninterrupted supply of oil is required by
the pump, oil starvation causes component failure.
High performance driving creates large centrifugal
forces on fluids. This sloshing during acceleration,
braking and cornering can starve oil and fuel
pickups.
Most early Porsche 4-cylinder engines are wet sumps. These engines were also equipped with ‘cannister’ oil filters. This is a ‘Bypass Oil Filter’ and only filters some of the oil, some of the time. This design could only hold 4 liters of oil, which under hard braking or cornering could create a situation where the oil sloshed up the sides of the sump and the pickup tube could be starved for oil.
Modifications for the 356/912 system include ‘Full Flow’ oil filters, external oil coolers (there is a small oil cooler on the motor), a Deep Sump or an Accusump system.
∙ A full flow oil filter will filter all the oil sent to the
engine, it normally holds an additional ½ litre of oil
in the filter and lines.
∙ An external oil cooler is effective but difficult to
mount on anything other than race cars.
∙ A deep sump bolts onto the bottom of the engine
where the pickup tube and screen is. It increases
capacity but reduces ground clearance by about
2 ½”. Due to the design, it has the added benefit of
acting as a baffle. It is usually finned aluminum for
additional cooling.
∙ An Accusump system uses an oil accumulator tank.
When you start your car, your engine pressurizes
their system which then holds a reserve of oil.
If the engine oil pickup is momentarily starved for
oil, the Accusump reservoir maintains engine oil
pressure until the pickup is once again submerged.
The best solution to engine lubrication is the ‘Dry Sump’. If reliability is the criterion, to me, a Dry Sump is the answer. In 44 years of driving, racing, and servicing 911’s I have yet to see a failure not caused by neglect or incompetency.
*Note – A key component is the need for the oil to
cool surfaces within the engine.
Designers committed to air cooling have four strategies at their disposal.
►The first is a fan to move the air past the cylinders
much faster.
►The second is attaching fins to the cylinders and
heads to increase the surface licked by airflow by a
factor of 10 or more.
►Third is temperature differential; ambient air rarely
tops 100 degrees. The greater the temperature
differential between the engine and the air, the
more heat is transferred away from hot areas.
Air cooling takes advantage of nature’s infinite
heat sink and is a shrewd choice for car, aircraft,
and motorcycle engines.
►Efficiently circulating and cooling oil removes
unwanted heat from the engine.
Dry Sump Lubrication and Porsche
One of the great attributes of owning a Porsche was whatever worked on the racetrack usually found its way into their production cars.
Referring to earlier in this article we established an engine requires an uninterrupted supply of clean, cooled oil. This is addressed by utilizing a Dry Sump system.
Track ready!
Porsche first utilized a dry sump in
this design in 1953
with the introduction of the Type 547 engine designed by Ernst Fuhrmann.
The Mezger Engine
The use of a dry sump carried through into the 911 (901) with an engine designed by the legendary Hans Mezger. When Mezger designed the flat six for Porsche he planned the engine with a true Dry Sump system.
A gear driven auxiliary shaft drives two oil pumps. This means oil that is returned to the bottom of the engine is immediately removed by a second pump, filtered, and stored in a large tank separate from the engine.
Cylinders supplied by Mahle were a “biral” design consisting of a thin iron bore surface surrounded by a liberally finned die-cast aluminum muff.
A gear-driven auxiliary shaft below the crank powered two oil pumps and the two overhead camshafts.
∙ Uninterrupted supply – The 911 Dry Sump system holds its oil not in the bottom of the engine, but in a separate tank usually located in the right rear fender. This tank is tall and narrow and with the oil pickup located at the bottom of the oil tank, there is no chance of starving the pickup of oil during extreme acceleration, braking or cornering. The oil filler cap and dipstick are in the engine compartment.
∙ Clean oil is accomplished through full flow filtration.
In 1972 Porsche debuted
an external oil tank access, located
on the right rear quarter panel.
Owners could now check their oil level and add oil from outside the car.
However, this ‘gas-style flap’ design proved to be a problem.
Due to the propensity of people to fill their oil tank with gasoline, this feature was immediatel discontinued.
Cooling of the oil in the 911 is accomplished in several ways. There is an oil cooler on the rear of the engine which is fed air from the same fan that cools the cylinders. In addition, there is the option of additional oil cooling through a loop leading to the front wheel housing or below in the nose. Some models were fitted with a ‘radiator style’ front oil cooler.
Although we call these Porsche’s “Air-Cooled” a great deal of the engine cooling is provided by the oil. The dry sump system allows the engine to circulate almost a case of oil, much more than could be accommodated by a traditional wet sump engine. The oil can thus be very effectively cooled before being reused to lubricate the engine.
The 911 system provides several warning and metering gauges for the owner which are not intuitive to those with no experience with dry sump engines.
911 Oil Gauges and how to use them
The Oil Pressure (Druck Press) gauge
The oil pumps are driven by a layshaft which instantly responds to engine RPM.
For this reason, it is normal for the pressure to show very low at idle then rise immediately with engine RPM. There is also a low oil pressure warning light.
*Note – I have never seen an oil pressure failure on a 911, but lots of defective switches. Thankfully they are
inexpensive and easy to change.
Oil Temperature gauge
IMPORTANT
It is paramount to have the entire system up to temperature before driving the car aggressively.
It is critical that 911 owners pay attention to the temperature gauge.
When the engine is first started the oil is cold and too thick to provide the lubrication the engine needs to operate at high RPM’s.
911 engines grow about 10mm wider as they heat up, they need to be fully up to temperature before they are driven hard.
Sodium cooled valves also do not function properly until brought up to temperature.
The oiling system has thermostats which open the cooling circuits as the oil heats up. There is a lot of oil held in the front cooling loops which needs to be integrated into the system before the temperature is fully stabilized.
Oil Level gauge
This has to be the most widely misunderstood gauge on the 911.
It is intended that the driver can know the oil level while driving without stopping and physically checking the oil level by pulling the dipstick from the dry sump tank.
*Note – When returning the dipstick to the tank, be sure it is located in the tubular dipstick holder or it
will just disappear into the tank… this is not plan A.
There is a float sensor located in the top of the dry sump tank which measures the top litre of oil, this mirrors the dipstick level and sends the signal to the gauge.
*NOTE – The criterion for reading this gauge is the same as if you physically check your oil level with the
dipstick. When the car is fully up to temperature, on a level surface and idling the gauge will stabilize
and give you a reading.
When you are moving this gauge is meaningless and must be ignored.
Checking oil level in a dry sump system is the exact opposite of a wet sump, the engine must be hot, level and idling.
I can’t tell you how many times garage attendants would point out that the engine should be switched off to check oil. Another Porsche feature is a large fuel capacity.
When filling my 911, at about 70 liters, attendants would often start looking under the front of the car for fuel leaks…great fun.
*Note – The system is designed in such a way that the correct oil level is shown with the engine running
at idle (two pumps operational) The thermostats must be open, so the oil held in the front loops will
be reflected on the dipstick. When the engine is shut off gravity takes over and some oil flows into the
engine sump. If you check your oil level with the engine off, the dipstick will read low. If you now add oil
to the tank, when the engine is restarted, the scavenge pump will return all the oil that has been hiding
and you will have a large mess on your hands.
The engine holds so much oil, my strategy was always to wait until the level gauge was reading fully in
the red zone (when on a level surface, hot and idling). I would then pull the dipstick, check, and top up
as needed to be about halfway up the dipstick. I never liked filling it to the top, as too much oil got into
the intake, through the tank breather.
The opposed engine (aka Boxer) has many advantages including a very low centre of gravity. Without the need for a bulky wet sump oil pan, the engine can be set lower in the chassis. Some Porsche race cars would invert the gearbox which became the limiting factor, to get the engine even lower in the chassis.
The location of the oil tank in a Dry Sump system was also flexible.
On one of my 911’s I built in the 80’s I used a 935-style tank located in the front compartment where the gas heater was normally housed (aka – the smuggling compartment).
This transferred the weight (pic below) from the rear of the car to the centre and held additional oil.
I ran huge Aeroquip hoses right through the centre of the car down the tunnel. There’s nothing like the sound of gurgling oil to augment the exhaust note of a carbureted flat 6.
I will note that my 1988 Carrera which we endurance raced had a totally stock factory oiling system which operated flawlessly throughout several race seasons. A tribute to the Porsche Dry Sump system.
*Note – Porsche now uses what they describe as an “Integrated Dry Sump” on most models
but retained a true dry sump on some GT cars and Turbo’s.
The true dry sump was last featured on all 911’s in 1998.
This tach has nothing to do with Dry Sumps……
I just like looking at it.