Tech Talk #84 – Dry Sumps Save Lives


By David Reher, Reher-Morrison Racing Engines

“In a wet-sump engine, the rotating assembly whips the oil like a milkshake in a blender.”

It’s great to be back in the pages of National Dragster after taking a break from writing columns. I appreciate the many comments from racers and readers, and I’ll attempt to come up with interesting topics now that I’m back on the keyboard.

Perhaps I should have a soapbox, too. I have strong opinions about many aspects of engine building, and regular readers know that I’m not shy about expressing them. My philosophy is to spend money where it provides a long-term benefit. I’ve written previously about the improvements in both performance and reliability offered by titanium valves. In the same vein, I’m an advocate of dry-sump oiling systems for serious racing engines.

I realize that not every racer needs or can afford a dry-sump oil system. However, as interest in big-inch engines and go-fast sportsman classes soars, it’s essential that the equipment keep pace with the performance level. A wet-sump is fine for an entry-level engine – but a powerplant that’s capable of running 200mph really needs a dry-sump oil system. Yes, a dry-sump is more expensive initially, but it’s an investment that pays huge dividends down the road.

When the big-block Chevrolet V-8 was introduced in 1965, it had a 3.76-inch crankshaft stroke. Today it’s common for big-block race engines to have cranks with 5.00-inch and longer strokes. While the crankcase dimensions haven’t changed in nearly 50 years, the size and mass of the rotating assemblies have grown dramatically.

Consequently there is a lot more hardware spinning in the crankcase at thousands of revolutions per minute. The counterweights, crank pins, connecting rods, and pistons create a hurricane that whips the oil in a wet-sump engine into an aerated froth like a milkshake in a blender. As the rotating assembly spins in this dense fog of oil and air, the resulting windage losses sap horsepower.

This maelstrom in the crankcase can also blow the oil away from the oil pump pickup in a wet-sump engine. Even if the oil pressure gauge has a steady reading, the reality is that the pump isn’t supplying solid oil. It’s delivering intermittent oiling as pockets of air are pumped through the system; the gauge only registers the average pressure of this aerated mixture. Engine bearings require a hydrodynamic wedge of oil molecules to prevent destructive metal-to-metal contact. When the oil is aerated, it cannot maintain this crucial cushion between highly loaded parts.

Now consider what happens inside an engine with a dry-sump oil system. Multiple scavenge stages suck the oil out of the crankcase and deliver it to a remote tank. When this oil enters the tank tangentially, the air and oil separate, leaving a solid column of liquid to feed the pressure stage. The improvement in the quality of the oil that reaches the bearings with a dry-sump system is usually apparent when you overhaul the engine. In most instances, the bearings can be reused, while the bearings in a wet-sump motor frequently show the ill effects of intermittent oiling.

Burnouts and deceleration at the end of a run are especially hard on a wet-sump engine. It’s virtually impossible to baffle the oil pan to keep the pickup submerged under 1g deceleration. With many sportsman cars now running above 200 mph, the potential for damage in the shutdown area is real. Even if the engine isn’t running under load, it still suffers incremental damage every time the oil supply is interrupted.

Racers know that crankcase vacuum is beneficial in a competition engine, and some use an auxiliary pump to produce crankcase vacuum in a wet-sump engine. Unfortunately, this vacuum fights the oil pump; you’d really like positive pressure in the crankcase to force the oil into the pickup.

We performed an experiment with transparent oil return lines from the cylinder heads to see the effects of crankcase vacuum in a wet-sump engine. When vacuum was applied, the oil in the return lines scarcely moved; when the vacuum was removed, the oil drained quickly. If the vacuum is strong enough, it simply stalls the oil.

For this reason I recommend scavenge stages pulling only from the oil pan in a dry-sump engine. Current dry-sump pumps will generate 20 to 23 inches of vacuum, so an auxiliary vacuum pump simply isn’t needed. Several manufacturers now offer excellent pumps with twisted Roots rotors at affordable prices. These pumps are much more efficient than the spur gears that were used in the past.

A dry-sump system significantly increases horsepower by reducing parasitic windage losses. With less oil in the crankcase, ring tension can also be reduced. Rings are the chief source of friction in an engine, so reducing drag with low-tension rings is a path to “free” horsepower. In an extreme example, we saw a 70-horsepower gain in an engine with a 5.500-inch stroke after switching from wet-sump to dry-sump oiling. In a 600+ cubic-inch sportsman motor, a dry-sump system is typically 30 horsepower better than a wet-sump.

In pre-LS1 Chevy V-8 engines, the distributor gear drives the oil pump. It takes considerable torque to turn an oil pump under load, and the cam and distributor gears are prone to excessive wear and failure. With a belt-driven external pump, the distributor gear only turns the rotor, so wear is virtually eliminated.

Priming an engine with a dry-sump pump is quick and easy. Pressurizing the oil system with a pulley driven by an electric drill motor ensures that the engine is completely lubricated before starting it. Adjusting the belt is simple: When you can twist the belt 90 degrees, the tension is right.

I’m about out of space, so I’ll climb off my soapbox now. I want to leave you with a final thought: Dry sumps save engine lives! Please remember that.