Engines Exposed: Dry Sump Lubrication Systems
I remember sitting in high school physics and having the teacher explain to us why centrifugal force is a “fictitious force.” The example she used was a car turning a corner and the passengers feeling like there’s something pushing them against the side of the car. In fact, the passengers are trying to continue in a straight line and the car is turning across their path. When I started studying automotive technology, I started to realize that these forces affect the entire car, not just its passengers. In fact, it’s most pronounced when it comes to large fluid reservoirs in the car (which is pretty much what humans are, come to think of it) – namely the oil pan and the gas tank. In both of these containers, the fluid within sloshes around as the car is subjected to various forces. Given that the pump for both of these is stationary, the fluid can only move so much before the pump becomes starved. Obviously, this is a problem in high-performance applications, so a solution needed to be developed.
For this article, we’re going to be looking at just such a solution, focusing on the oil system. As a primer, the oil pan in a typical engine, called a wet sump system, is mounted at the bottom of the engine block. It’s bolted to the block and catches oil as it returns from lubricating the engine. Many oil pans are nothing more than the name would suggest: simple reservoirs to hold the liquid. In high-performance applications, however, oil pans often have baffles (metal plates with engineered passages and trap doors for oil flow) installed that prevent the fluid from sloshing around as the car accelerates in various directions. This works for most applications, but there’s another option for those who want an oil system that will function under extreme forces while providing additional performance benefits.
Long before people were “slamming” cars by using airbag suspension systems, race car drivers realized that having the weight of the car close to the ground improved the handling by lowering the center of gravity. The center of gravity is the location of the average of the mass of the car. Cars with high centers of gravity roll more in corners, much like how it’s easier to fall over if you’re carrying someone on your shoulders. When the weight is concentrated close to the ground, the car is much more stable. At this point, you’re probably starting to wonder what any of this has to do with oil systems. Read on…
The average passenger car weighs approximately 3,500 pounds, with the engine and transmission accounting for approximately 500 of those pounds. Given that the engine is such a significant portion of a car’s overall weight, there’s a clear advantage to mounting it as low as possible. The oil pan presents a problem, however, as it’s generally 6-10 inches of extra material below the engine. In the 1950s, automakers began to search for solutions to this problem, and began experimenting with a radical, race-bred technology which can still be found in performance cars today.
In a wet sump system, the oil pan houses the oil pump and serves as the reservoir for all the oil. If you pull the pump out of the pan and move the oil reservoir away from the engine, you’re left with a very shallow catch basin where the pan used to be, as you can see on the 2015 Corvette Z06 engine below. Adding a drain to this catch basin allows the oil to be pumped to a reservoir, where it is de-aerated (allowed to settle) and cooled, often using an external oil cooler, before being pumped back into the engine. This is known as a dry-sump system because the oil is pumped out of the sump, the oil pan, quickly, which leaves it “dry.”
There are four primary advantages to a dry sump system. First, the shallow oil pan allows for the engine to be installed closer to the ground, lowering the car’s center of gravity and improving handling. Second, a shallow oil pan means that there isn’t a big reservoir of oil under the engine that’s susceptible to pump starvation from cornering forces. The oil is pumped to a reservoir that’s specifically designed to account for the forces that occur during cornering – usually a tall, cylindrical shape. Third, the reservoir (or sometimes reservoirs) can be mounted anywhere in the car, which can be designed to improve weight distribution and balance. Finally, the oil is able to release any air or crankcase gases that accumulate during extended periods of driving in the upper part of the rev range. This means that the oil that’s returned to the engine will do a better job of lubricating and cooling because it has less air in it.
But there are challenges presented by a dry sump system, and they tend to fall into the realm of practicality. Dry sump systems are significantly more expensive than a wet sumps due to their additional complexity and parts. A dry sump system could cost upwards of $3,000 depending on the design. Furthermore, these systems take more oil and add complexity to every oil change, since you’ll need to fill the reservoir, the lines between it, and the engine. As a result, a car dry sump car like a vintage air-cooled Porsche 911 takes a whopping 12 quarts of oil at each change. Still, if you’re designing a car for high-performance driving and don’t want to risk engine damage due to poor lubrication when you really need it, those few extra quarts of synthetic and a few more dollars in your build is a small price to pay.
Like classics? It’s always Throwback Thursday somewhere.