There's Power In The Pump

Historically, oil systems used in race cars worked with a greater volume of oil, so special (and expensive) oil pans were needed. The additional oil volume was needed to make up for the fact that more oil would be slow to return to the pan because it was trapped in and on the rotating assemblies or was pooling in the heads or the lifter valley. This was done to preclude running the pump dry and starving the engine of oil. With a dry-sump system, you still need to be concerned with windage issues, and you will still be using scrapers to reduce the total amount of oil that clings to the rotating assembly. But you can be assured of a steady supply of oil to the pump because you are feeding the pump from a tank. The tank is replenished by scavenging and pumping the oil back into it from multiple places on the engine. In this way, you can run enough oil in the system to make sure that you always have a steady supply.

There is a good bit of science involved in a quality dry-sump tank. The tank is part of the system and isn't just designed to fit in an obscure area of the car. It is more than just a vessel to hold oil. It becomes a tool that assists in making sure the oil is free from air in suspension by filtering the oil through various strainers and foam. The tank can also be used to prepare the oil prior to a race. It is not uncommon to see heaters in the oil tank that ensure the oil is up to temperature before the start of a race. In a well-designed system, the dry-sump tank is a part that adds value to the whole as it helps develop horsepower.

Let's spend a bit of time talking about the pump itself. Dry-sump pumps are constructed in stages. From the simplest perspective, a dry-sump pump is just several oil pumps run on a common driveshaft that is driven off of the engine. Just think of multiple oil pumps stacked end to end. There is the pressure pump, which draws oil from a remote tank. Then there are a number of other pumps, usually at least three, that pull the oil out of the engine and return the oil to the tank. The term used for this process is scavenging. This scavenging action is the heart and soul of making more power. The scavenge pumps are not always moving only oil. They also evacuate air from the crankcase.

This is where the story really gets interesting. As the crank spins and the rods are moving the pistons up and down, there is a whirlwind of activity in the crankcase. The scavenge pumps move a good bit of air out of the crankcase-enough to pull a vacuum inside the crankcase. The vacuum can be as much as 22 inches of Mercury. This vacuum allows the piston rings to seal better. Rings? Aren't we talking about oil pumps? Contrary to popular opinion, rings seal in a multitude of ways. The popular idea is that rings seal just on the face, OD (outside diameter) of the ring against the cylinder.

While true, that is only one of the three ways rings seal. The other two are the sides (top and bottom) of the ring against the ring land in the piston. This aligns with the three potential leak paths that compression gasses can follow: past the face of the ring, by the ring end gap, and around the ring past the ring lands. One of the oracles of piston ring technology, Keith Jones of Total Seal, had this to say: "The rings are responsible for the greatest level of parasitic drag in the engine. When you are using a dry-sump oil system and you are pulling a good vacuum, you have the ability to adjust the ring package to take full advantage of the lower pressure in the crankcase."

For a ring to seal against the outer face and the cylinder wall, it has to exert a load against the cylinder wall. This is accomplished by the ring tension against the cylinder wall. The greater the force against the cylinder wall, the greater the seal-up to a point. The problem is that the force to move the piston up and down within the cylinder goes up. Add three rings to each piston and then multiply that force by the number of cylinders in the engine. "The oil control ring is the biggest offender," states Jones. "The drag number can be quite high." Remember that we are only producing power on every other stroke. The piston is going on a free ride for three of those four strokes. While the ride is free for the piston, the ride will cost your engine power.

If we were to place a set of rings that were optimized for a dry-sump system into an engine with a wet-sump oiling system, we would create an oil burning machine. The rings would not hold compression and/or power stoke pressures. Also, the oil ring would allow an excessive amount of oil into the combustion chamber. None of these things are good.

The increase in differential pressure created between the top of the piston and by the lowering of the pressure in the crankcase is contributing to making the rings seal better without additional tension. Remember the three areas of sealing. This differential pressure forces the rings to seat harder to the top or the bottom of the ring land, depending on the direction the piston is traveling. This mP (Delta Pressure) created by the pan scavenging is one of the keys to develop better ring seal with lower parasitic losses due to ring drag. We are able to reduce the ring tension and lower the parasitic drag on the piston(s). Power levels increase. It is a win-win scenario. Just how big of a win is it? Jones says that the number is 19 to 22 hp on the engine in their house Sprint Car (a 360ci injected small-block), and that seems to be the norm. So, depending on the engine and the state of tune, it seems that a 3 to 5 percent gain in horsepower is completely possible. Those numbers were gathered using 15 inches of vacuum. So, using typical racer logic, if a little is a good thing, more should be even better. Why not add several more stages to the pump and pull a greater vacuum, or mount a separate vacuum pump to generate even higher levels of vacuum? While a possibility, the power required to run an additional pump may negate any gains, especially at high engine speeds. The consensus among the engine builders we spoke with was that a level of vacuum greater than 15 inches causes more problems than power. Many were concerned with wristpin bushing failures, due to the oil mist being evacuated from the crankcase. It's just like your mom always said: everything in moderation. The reason they were concerned is that many have tried to run the vacuum levels above the 15-inch level and have experienced failures. They were not postulating on this point-they had an empirical data set from which to draw. The lesson here is to learn from the knowledge of others. These engine builders paid to learn these lessons so that you wouldn't have to.