Tech Talk #99 – More About Oil: A Slippery Subject


By David Reher, Reher-Morrison Racing Engines

“Controlling the supply of oil in the shutdown area should be a top priority.”

More drag racing engines are damaged in the shutdown area than during a race. This is a bold statement, but

after examining thousands of engines in the last 40 years, I’ve come to the conclusion that motoring an engine down at the end of a run can be more destructive than a full-throttle pass.

Drag racing oil systems are designed to produce failsafe lubrication under acceleration. In an engine with a wet-sump system, the g-forces produced by wheels-up launches and all-out passes propel the oil to the back of the motor. Oil pump pickups, pans, baffles, and trays are designed to ensure an uninterrupted oil supply when the engine is under full power. So what happens when the driver closes the throttle and decelerates after the finish line? In too many instances, the engine runs out of oil.

The consequences of intermittent oil starvation are cumulative. Over the span of several races, the damage builds. Eventually the final result can be catastrophic failure.

At this point, some readers may be thinking, “Reher, you’re an idiot! An engine isn’t under load during deceleration, so there are no forces that can damage a crankshaft bearing.” But that’s not true.

When an engine is under power, its pistons and rods are turning the crankshaft. In the shutdown area, the rear wheels are driving the pistons and rods through the transmission, unless the driver has shifted into neutral or disengaged the clutch. Using the engine as a brake subjects the bearings, rods, and pistons to high loads – at a time when the oil system is least able to lubricate these components.

I’ve observed that the vast majority of sportsman racers decelerate with the transmission in gear after a run. I certainly understand their reluctance to shift into neutral. However, the harsh reality is that even a momentary loss of oil pressure as the engine motors down can nick a bearing – and eventually lead to a blackened crankshaft and a broken engine.

A crankshaft’s oil feed holes for the rod journals are typically drilled between 20 and 40 degrees before Top Dead Center. Introducing pressurized oil at this point creates a hydrodynamic wedge that prevents metal-to-metal contact between the bearing and the crankshaft under the intense pressure of the power stroke – the rod bearing effectively rides on a cushion of oil molecules. Now consider what happens when the rear wheels are driving the crankshaft during deceleration. Instead of the pistons and rods pushing the crankshaft down, the crank is now pushing and pulling the pistons against cylinder pressure – and the oil feed is 180 degrees out of phase. Of course there is still residual oil pressure and a weakened hydrodynamic wedge between the bearing and journal, but lubrication is no longer being delivered at the point of maximum load. In fact, most of the damaged crankshafts I have seen show the most wear and distress on the “unloaded” side of the journal opposite the oil feed hole.

How does a racer know if the engine is momentarily running out of oil? Usually he doesn’t. That’s why I strongly recommend that every sportsman racer invest in a basic data logging system. A simple six-channel system can record everything a racer really needs to know: engine speed, driveshaft rpm, acceleration, battery voltage – and most important, oil pressure. These systems are affordable, and they are great insurance. The driver may not notice a drop in oil pressure on a gauge, or he may ignore an oil pressure warning light that flickers during deceleration. However, a data logger will capture even a momentary loss of oil pressure. That’s a wake-up call to improve the oil system or to change the shutdown procedure.

Many steps that racers and engine builders take to increase horsepower with a wet-sump oil system have negative effects during shutdown – a case of unintended consequences. For example, a “big box” oil pan with a flat bottom is great for power, but it’s a disaster for controlling oil in the shutdown area. A vacuum pump that’s plumbed to the valve covers can improve horsepower by creating negative crankcase pressure, but it can also impede oil return to the pan, as I described in my previous column. Consequently when the remaining oil stacks up at the front of the pan during deceleration, the oil pump is sucking air.

I recommend a wet-sump oil pan with a well-defined sump, effective baffles and dams to keep the oil around the pickup, and a windage tray to separate the oil from the rotating assembly. Sportsman motors are bigger now – a “big” big-block Chevy used to be 454 cubic inches, but now our most popular sportsman engines are 565s and 632s. While the crankshaft strokes have grown longer, the crankcases aren’t any bigger, so there is more turbulence to whip up and aerate the oil in the pan.

All of my foregoing comments apply chiefly to wet-sump oil systems. A dry-sump system solves these shortcomings by ensuring a continuous supply of oil from a remote-mounted reservoir. A dry sump tank deaerates and cools the oil, delivering liquid lubricant to the pump instead of the frothy foam of bubbles and oil that sloshes around inside a wet-sump pan. A good dry-sump oil pump can pull 20 inches of crankcase vacuum, producing a horsepower boost without an auxiliary vacuum pump while significantly reducing windage losses. Yes, a dry-sump system costs more, weighs more, and is more complex than a wet-sump system – but for my money, the improvement in engine reliability, longevity, and performance is worth it.

Controlling the supply of oil in the shutdown area should be a top priority for every racer. Running an engine without lubrication is the root cause of many reliability problems. You may not kill a motor the first time that the oil pressure drops, but you’ve already taken the first step on the slippery slope to a broken engine.