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Maintaining adequate oil pressure and volume is absolutely essential for engine durability. The problem is the output of many stock oil pumps, particularly small block Chevy oil pumps, levels off around 5,000 rpm. At that speed, the straight cut gear teeth are skimming over the pump inlet port so quickly there isn’t time to completely fill the voids between the gear teeth with oil.
By Larry Carley
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It’s 6,000 rpm do you know what your oil pressure is? That’s a question every engine builder should know the answer to if they’re building an engine that’s going to survive.
Maintaining adequate oil pressure and volume is absolutely essential for engine durability. The problem is the output of many stock oil pumps, particularly small block Chevy oil pumps, levels off around 5,000 rpm. At that speed, the straight cut gear teeth are skimming over the pump inlet port so quickly there isn’t time to completely fill the voids between the gear teeth with oil. Cavitation forms air bubbles that reduce the volume of oil being pushed through the pump. That, in turn, causes the pump’s output to level off. Oil volume and pressure level off or may even drop.
In a stock motor that doesn’t rev beyond 5,500 rpm, that’s not a concern. But in a performance engine that’s capable of being revved to 8,500 rpm, 9,000 rpm or even higher, a drop in oil volume and pressure could be disastrous.
When a performance engine throws a connecting rod, the underlying cause of the failure often turns out to be a lubrication problem. If the oil pump can’t maintain enough oil flow to the rod bearings, the oil film between the bearings and crank will go away causing the bearing to seize, spin and break the rod. A blackened or blued crankshaft journal is usually proof that the bearing spun and caused the rod failure.
The old school rule of engine building says that most engines need about 10 psi of oil pressure for every 1,000 rpm. That’s still good advice for the average Saturday night racer. But many ProStock drag racers and NASCAR racers are now running significantly less pressure (up to 40 percent less) to reduce the horsepower loss needed to drive the oil pump which is okay provided bearing clearances are tight and you have enough oil volume to keep everything adequately lubricated.
Many top racers today are also running extremely low viscosity 0W-20 synthetic oil. The lower viscosity of the oil means less resistance to flow, so it takes less horsepower to pump the oil. Zero weight oils also shed more quickly from moving parts, which reduces drag and allows faster acceleration. But extremely thin oils require oil pumps and bearings with close tolerances.
Any form of racing that subjects the oil inside the engine to high G-forces also creates problems for the oil pump and lubrication system. If the engine has an internal wet sump oil system, oil can slosh away from the oil pump pickup tube as the vehicle dynamics change. In a circle track or road race engine, the oil wants to slosh sideways away from the pickup every time the car enters a curve. In a drag race motor, the oil wants to climb up the back of the pan and crankcase when the car launches off the line and accelerates down the strip. In an off-road car or truck, the oil can go just about anywhere inside the crankcase when the vehicle becomes airborne. And the thinner the oil, the more it can slosh around inside an oil pan.
No oil pump can maintain oil pressure and a constant supply of oil to the engine if the pickup tube is sucking air. Consequently, wet sump systems in performance engines require baffles inside the oil pan to control the movement of the oil. A well-designed oil pan can do a great job of keeping oil where it belongs so the pump has a steady supply of oil to draw upon.
DRY SUMP SYSTEMS
The other approach to handling high G-force applications is to convert the engine to a dry sump oiling system. These are used by most ProStock drag racers as well as NASCAR and circle track racers.
With a dry sump setup, there is no oil in the pan. One or more scavenge pumps suck the oil out of the pan (and lifter valley in some cases) as fast as the oil flows through the bearings and lifters. The scavenge pump(s) route the oil to an external storage tank, which may hold anywhere from one to five or more gallons of oil. The external tank serves as a reservoir and allows air to separate from the oil. A vent allows air to escape.
Dry sump systems are usually described by the number of stages (pumps) they have, such as 3-stage, 4-stage, 5-stage, etc. Typically, the external pump assembly will include 2 to 5 scavenge pumps mounted in a row, with the pressure pump at the back end of the stack.
In a very simple 2-stage dry sump system, two pumps are stacked together: a scavenge pump to suck oil out of the oil pan, and a pressure pump to feed oil back into the engine from the reservoir. When multiple scavenge pumps are stacked together (usually 3 or more), there’s usually enough scavenging action to suck not only the oil out of the engine but also air from the crankcase. Pulling a vacuum in the crankcase reduces windage and drag for more horsepower. In a high revving engine, pulling vacuum may increase the engine’s power output up to 10 horsepower or more.
The oil pan usually has two to four suction ports that are connected to the scavenge pumps with hoses. The oil pan is also quite shallow since it doesn’t have to hold any oil. This increases ground clearance under the engine and allows the engine to be mounted lower in the chassis for a better center of gravity. The oil pan may be cast aluminum, or welded steel or aluminum.
Gary Armstrong of Armstrong Race Engineering says his company makes a wide variety of cast aluminum dry sump oil pans. “Our most popular pan right now is for the LS1-LS7 Chevy applications. These are all bolt-on pans that use the stock gaskets.”
To push the oil back into the engine, a dry sump system uses an external pressure pump, which is usually mounted in the same stack as the scavenge pumps (all share a common driveshaft that runs down the middle of the assembly). The pump’s output is routed directly to the engine’s crankshaft bearings with hose fittings and an adapter that fits over the old spin-oil filter mount, but some of the oil can also be routed through a valve block or manifold to provide lubrication direct to the valvetrain and/or turbocharger.
The dry sump pumps are usually mounted on the side of the engine and driven with a cogged belt off the crank pulley. On some applications (sprint cars, for example), the pump assembly may be mounted on the front timing cover and are driven off the cam.
Because most 3-stage dry sump systems do not have enough suction to pull vacuum in an engine, racers who want to minimize windage and drag usually go with a 4-stage, 5-stage or 6-stage setup. Tim Mangus of Mangus Precision Pumps says “Bigger motors need more scavenging if you want to pull vacuum in the crankcase.
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