Tech Talk #21 – Dry Sump Oiling: How to Tame the Hurricane in your Engine

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

Economists and stock market analysts look to leading indicators to divine the future, but I have discovered a more accurate way to predict what will happen. I simply listen to my customers. The trend is clearly toward bigger, more powerful engines for all types of sportsman racing.

I can say categorically that interest in bigger engines is expanding (pardon the pun). While a 502 or 522-cubic-inch engine was once considered a big-inch powerplant, I now frequently field calls from racers asking about 615 and 632-cubic-inch motors. With the introduction of a new generation of Pro Stock-style CNC-ported cylinder heads, horsepower levels are climbing faster than the cost of electricity in California.

In this era of more powerful, more sophisticated sportsman engines, I am advising my customers to seriously consider the benefits of a dry-sump oiling system. A dry-sump lubrication system has four major advantages over a wet-sump system: increased power, improved reliability, better cooling, and enhanced safety.

As piston displacement increases, so does the volume of air displaced inside an engine’s crankcase. A piston displaces an equal volume of air both above and below itself with every crankshaft revolution. For example, a 632-cubic-inch big-block running at 8,000 rpm moves more than 2,900 cubic feet of air per minute inside its crankcase. As one piston moves downward in its cylinder, another moves upward, so the total volume inside the crankcase and oil pan remains constant – but this rapid movement of air from side to side and front to back, combined with the rapidly spinning crankshaft assemble, creates a hurricane of incredible intensity inside the engine.

You can imagine the effect this hurricane has on the oil in a wet-sump system. Even with baffles and windage trays, the oil around the pickup is constantly buffeted by the air displaced by the pistons. The gale forces created by the rise and fall of pistons No. 7 and 8 can literally whip the oil in a wet-sump reservoir into a froth. The large counterweights that are required to balance a stroker crank also act as huge fans that churn the oil like a blender.

Thus we have the potentially deadly combination of extreme bearing loads and marginal lubrication. Consider that each piston in a 1,250-horsepower engine produces 156 horsepower. That is more than four times the output per cylinder of a production big-block – yet the crankshaft bearings that must withstand these loads are no larger than the bearings in a low-rpm stock engine. With the high surface speeds and the huge centrifugal forces generated by a long-stroke crank, the rod bearings can be stressed beyond their limit unless they receive continuous lubrication.

Removing the oil from the pan and storing it in an external reservoir solves these problems. First, a properly designed dry sump tank will de-aerate the oil before returning it to the pressure pump. By removing trapped air, a dry sump ensures that the pump supplies only pure, liquid oil with its maximum lubricating qualities intact.

A dry-sump oil system can also use a much larger volume of oil than a wet-sump. Adding a couple of quarts of oil to a wet-sump system can actually create more problems by increasing aeration and windage. With a dry-sump system, increasing oil capacity with a large external reservoir tank simply improves the overall effectiveness of the lubrication system.

An external oil tank also protects the engine against fluctuations in oil pressure. Keeping the oil pump pickup in a wet-sump system constantly submerged is extremely difficult, even with trap doors and baffles. With an external dry-sump reservoir, you won’t see the oil pressure gauge hit zero after a burnout or when the parachute opens.

Removing the oil from the pan also reduces the demands on the piston rings to control the oil on the cylinder walls. Dry-sump engines can use oil rings with less tension than wet-sump motors. These low-tension oil rings cut friction and parasitic losses, leaving more power to accelerate the car.

A dry-sump system also allows you to optimize the oil pan design for maximum power. A wet-sump pan must be designed to collect the oil in a confined space; consequently it cannot have a large interior volume since this would dissipate the oil over a wide area. A dry-sump pan, in contrast, can be as big and wide as the chassis allows, with kick-outs and other windage-reducing features that improve engine output. Simply switching from a restrictive wet-sump system to a dry-sump lubrication system typically produces a 20 to 25 horsepower improvement in a large displacement engine, even without optimizing the piston package with low-tension rings. A dry-sump pan can also be shallower than a wet-sump pan to allow the engine to be mounted lower in the chassis.

An external belt-driven dry-sump oil pump eliminates the load and wear on the distributor gears in engines such as the big-block Chevrolet V8. Turning a high-volume internal oil pump requires considerable torque; this torque must be transmitted through the camshaft and distributor shaft. When we overhaul a wet-sump racing engine, the distributor gear almost always shows high wear. With a dry sump, the distributor gear only turns a plastic rotor, so wear is negligible.

My advice to racers who are considering a dry-sump system is to use a five-stage pump. I recommend three scavenge stages for the oil pan, one for the lifter valley, and a single pressure stage. The difference in cost between a three-stage and a five-stage oil pump is insignificant compared to the total cost of the pump drive, brackets, oil tank, oil pan, and lines – components that are required for any dry-sump system regardless of the number of stages.

Dry-sump lubrication also dramatically improves engine cooling, which is especially important for sportsman racers who have little time between rounds. In an air-cooled motorcycle engine, for example, it is really the oil that does most of the cooling by transferring heat away from the pistons and valves. The same effect takes place in a water-cooled engine – it is primarily the oil that pulls heat out of the piston tops and friction points such as bearings, lifters, pushrods, rocker arms, and valve springs. An aluminum dry-sump oil pan, aluminum oil tank, and the associated lines make a very efficient radiator to cool the oil. Maintaining a near-constant oil temperature can also improve consistency, which wins races in the brackets and heads-up sportsman eliminators.

Finally, a dry-sump system oil system provides an extra margin of safety. Most oval-track classes require dry sumps because they reduce the danger of oil getting on the track after an engine failure or accident. I wouldn’t race a fast drag car without a dry sump for the same reason. I have seen engines with dry-sump pans break connecting rods without putting an appreciable amount of oil on the track. There is a relatively small volume of oil inside a dry-sump engine, and it is widely spread out, so even a catastrophic engine failure seldom presents a hazard to the driver.

Certainly not every drag racer needs a dry-sump oil system. A dry-sump system is more expensive, more complex, and heavier than a wet-sump. A well-designed wet-sump system can provide troublefree service for the vast majority of engines. But with the growing popularity of big-inch motors that are capable of producing Pro Stock power levels, a dry-sump lubrication system becomes an excellent investment.