By David Reher, Reher-Morrison Racing Engines
“Reliability is more important than horsepower when you have to pay your own engine repair bills.”
You don’t have to be a cardiologist like my fellow back-page columnist Dr. Torstveit to appreciate the importance of blood in the human body. I’ve watched enough episodes of “ER” and “Chicago Hope” to know that even a momentary interruption in the flow of blood to the brain can cause permanent damage.
At the risk of repeating a cliché, I’ll state categorically that oil is the lifeblood of an engine. The consequences of erratic oil delivery in a racing engine can be just as traumatic as the medical emergencies on television shows – but instead of comas and strokes, the likely results are broken connecting rods and catastrophic engine failures.
I estimate that nine out of ten connecting rod failures are not the fault of the connecting rod. Almost invariably, a broken rod is the result of a spun bearing -and the bearing failure is usually the result of an oil system problem. If an autopsy of a blown engine reveals even the hint of a blackened crankshaft rod journal, that’s strong evidence that a spun bearing was responsible, not the connecting rod.
When drag racers think about oiling systems, they usually look for ways to increase power by reducing windage and parasitic losses. That approach may be valid if you’re contending for the Pro Stock championship, but it’s risky business for weekend warriors, bracket racers, and Super eliminator competitors. Reher’s first rule of racing is that reliability is more important than horsepower when you have to pay your own engine repair bills.
I should point out the important distinction between wet-sump and dry-sump oil systems. With a wet-sump system, an engine’s entire oil supply is contained within the oil pan; with a dry-sump system, the oil is stored in an external tank. This simple difference in how the two systems manage their respective oil reservoirs has huge implications for racers.
A wet-sump system has the advantages of simplicity, low cost, and light weight. That’s why virtually every production engine in the world uses wet-sump lubrication. A dry-sump system is more complicated, more expensive, and heavier than a wet-sump – but it offers the important benefit of continuous lubrication under all conditions and the promise of increased power through reduced windage.
When you consider cost and complexity, a wet-sump oil system is the logical choice for most sportsman racers. The biggest mistake a sportsman racer can make, however, is to try to duplicate the performance characteristics of a dry-sump using a wet-sump system. This simply can’t be done without sacrificing reliability.
Many sportsman racers look to Pro Stock for inspiration. In some instances, the technology that is used in Pro Stock is applicable to other classes – but lubrication system design is not one of them.
Pro Stock engines are universally equipped with dry-sump oiling systems. The luxury of an external oil storage tank allows a Pro Stock engine builder to use an oil pan that is the size of a small refrigerator and to install a vacuum pump that sucks the air out of the crankcase like a tornado. Pro Stock racers use lightweight lubricants and restrict the oil flow to the absolute minimum. I couldn’t recommend these tactics in good conscience to any bracket or Super eliminator racer.
Using a large-volume Pro Stock-style oil pan with a full-length sump is an invitation to disaster with a wet-sump oil system. I insist on using an oil pan with a rear sump on every Super Series bracket racing big-block we build at Reher-Morrison Racing Engine – even if the engine will be installed in a dragster with plenty of room between the frame rails for a full-length sump.
Here’s why: When a car accelerates at one “g,” the oil stands up in the rear of the pan at a 45-degree angle. At two “g’s” acceleration, the oil is plastered to the back of the pan at a 66-degree angle. The same thing happens when the car decelerates, except the oil piles up at the front of the pan, and the oil pump pickup sucks air. With a full-length sump, there is little hope of keeping the oil pump pickup covered.
The low-viscosity oil that is commonly used in drag racing engines today has the consistency of kerosene when the engine is at operating temperature. You can see for yourself what happens to the oil in a full-length wet-sump pan by filling it with four quarts of water and rocking the pan forward and backward. Tilt the pan at a 66-degree angle to simulate what happens during a two “g” launch and deceleration. Even baffles and trap doors can’t keep the oil pump pickup submerged under hard acceleration and deceleration.
I strongly advocate using a solid windage tray to shield the oil pump in any wet-sump engine. You may sacrifice a little power with a partial solid tray compared to a full-length screen-type tray, but the oil pressure will be much more consistent if you shelter the oil around the pump pickup. The turbulence inside an engine’s crankcase is unimaginable when its crankshaft assembly is spinning at 7,000 rpm and its eight pistons are pumping up and down in their cylinders 56,000 times per minute. A full or partial solid windage tray isolates the oil surrounding the oil pump pickup from this whirlwind in the crankcase.
I have seen the difference in oil pressure that a solid windage tray mounted above the oil pump can make in a wet-sump racing engine. With some full-length screen-type windage trays we’ve tested on the dyno, the oil pressure fluctuates as the engine accelerates. With a solid windage tray mounted above the oil pump to isolate the sump, the needle on the oil pressure gauge hardly moves.
For similar reasons, I am not a fan of crankcase vacuum pumps on wet-sump engines. Lowering the pressure inside the crankcase with a vacuum pump can increase power because the crankshaft has less drag – just as an airplane can fly faster in the thin air at high altitude than it can in the denser air at low altitude. But from the standpoint of the oil pump, a little positive crankcase pressure is actually desirable because it forces oil into the pickup. Positive crankcase pressure primes the pump and helps to ensure a steady flow of oil into the pickup. When the crankcase pressure is artificially lowered with a vacuum pump, there is less pressure differential to push the oil through the pickup tube.
One of the best investments a sportsman racer can make for a wet-sump engine is an oil accumulator such as an “Accusump.” An oil accumulator is a sealed cylinder that contains a floating piston. Pressurized air on one side of the piston forces a reserve supply of oil into the engine if the flow from the internal oil pump is momentarily interrupted. An accumulator works automatically and requires virtually no maintenance except regular cleaning. Unfortunately, accumulators are seldom seen in drag racing. That’s a pity, because they really work.
You may have an oil system problem and not even know it. Most drivers are too busy during a run to watch the oil pressure gauge. If you’ve plumbed the pressure gauge with small-diameter tubing that dampens the gauge’s response, you might not even see dips in the oil pressure. Try connecting the gauge with 1/4-inch I.D. tubing, and mount the gauge where you can see it throughout the run. You might be shocked to learn what’s really happening inside your engine!
The effects of oil pressure fluctuations are cumulative. If the pressure drops to zero when you stop after a burnout, decelerate after a run, or make the turnout at the end of the track, you have abused the bearings. The oil pressure may return to normal when you restart the engine, but you’ve already inflicted some damage. If you repeat the injury enough times, eventually a bearing is going to fail. And when it does, the damage will be very, very expensive to repair.
The oiling system is not the place for a sportsman racer to look for power. The potential rewards are small – perhaps five or ten horsepower under the best circumstances. On the other hand, the risks are enormous. If a trick-of-the-week oil pan starves the engine for oil, you are going to pay a heavy price. A reliable oil system is the best way to avoid a medical emergency for your motor!