When picking an oil pump for an engine build, what do you select: standard replacement, high pressure, high volume, or both (high pressure and high volume)?
That’s a question we hear regularly. But before we lay out the differences, let’s review the oil flow path in a common Chevy power plant.
First, oil is pulled in through the pickup screen into the oil pump. From here, the oil pump pressurizes the oil and then pushes it through the oil filter. Next, the oil exits to the main oil gallery where it is delivered to the main bearings along with the cam bearings. Simultaneously, the oil flows through a series of drilled passages in the crank to the connecting rod bearings. Oil from the main oil gallery moves into the lifter galleries, eventually delivering oil to the lifters.
Splash lubrication is oil that is flung off the crankshaft. That oil is used to lubricate the cylinder walls along with the pistons and wrist pins. Meanwhile, oil pumped into the lifters is directed by way of the hollow pushrods to lubricate the pushrod tips, rocker arms, guides and of course, valve stems. Oil spray from the pushrods lubes (and as a consequence, cools) the valve springs.
As you can see, the oil path is extensive (somewhat convoluted) and long. There are a lot of factors that can influence the oil pressure your engine sees (from a given pump), including bearing clearance, oil type, and oil weight. If the bearing clearances are too loose, oil is simply sprayed away from the bearings at a faster than normal rate. Basically, it’s a “leak” and the oil pump might not be able to keep up with the demand. That usually means the oil pressure drops. On a similar note, as oil becomes contaminated (for example with combustion by-products), it dilutes. As the oil becomes warmer (and more contaminated), it tends to be less viscous.It’s also becoming more and more filled with gaseous bubbles. Those bubbles do not contribute to oil viscosity of lubricity and, consequently, oil pressure drops.
In a typical (stock) passenger car engine, the oil pressure is usually between 30 and 40 psi. And that’s sufficient to provide adequate lubrication to the various moving components. But most high performance (or race) engines require 50-60 psi or more for proper lubrication. There’s an old racing rule of thumb that still applies here: Approximately 10 psi of oil pressure is needed for every 1,000 rpm. As a result, a 7,500 rpm engine will mandate approximately 75 psi of oil pressure.
We spoke to Mike Osterhaus, Product Development Manager at Melling Engine Parts, about oil pumps and oil pressure. Mike adds this: “A common misconception is that the oil pump produces the engine’s oil pressure by itself,” said Mike Osterhaus, Product Development Manager at Melling Engine Parts. “In truth, the oil pump creates a flow of oil to the engine, and the restrictions in the oil passages of the engine produce the pressure (restriction of oil flow = oil pressure). The faster the pump turns, the greater the flow of oil to the engine resulting in more pressure.”
Oil pressure is directly related to the amount of flow passing through the bearing clearances, according to Osterhaus.
“Think of placing your thumb over the end of a garden hose,” he said. “The flow of water coming out of the hose is constant but as you press your thumb against the end of the hose the flow is restricted. As you press harder, the pressure against your thumb and the pressure inside the hose goes up. This directly relates to the bearing clearances in an engine. Tight bearing clearances require less oil to fill the space between the bearing and the crankshaft, camshaft, or connecting rod. As the clearances increase, so does the amount of oil needed to fill the space between. Bearing clearances have the greatest effect on oil pressure.”
Most oil pumps for common high performance engines incorporate some form of pressure relief valve. It is set by the manufacturer to go off at a specific pressure. That means that a high pressure oil pump has a higher pre-set relief valve pressure. Spring tension on the pressure relief valve is what determines maximum oil pump pressure. When the pump pressure exceeds the pre-set limit, the relief valve lifts off a seat within the pump housing. When that happens, excess oil is bypassed to the oil pan. The oil pump manufacturer has no control over what the clearances are in the engine or what type and what viscosity of oil is in the engine. Because of this, many high performance oil pump manufacturers include an additional spring to increase pressure (Melling even offers special oil pressure relief spring pack sets–there are 5 different springs in each pack). You can also shim the existing spring in order to increase pressure (some springs and shim packs are held in place by an easy-to-remove roll pin; others such as certain Melling High Performance Pumps incorporate an Allen head plug to retain the spring). Essentially, you set the oil pressure produced by the pump for your engine. Keep in mind that too much oil pressure isn’t good either (remember that old 10 psi per 1,000 rpm rule).
Melling’s Osterhaus offered some valuable insight into setting oil pressure:
“Because oil pumps have the capability to provide enough flow to create pressures in the engine above 150 psi, a pressure regulating valve (PRV) is required. The PRV is typically located within the oil pump assembly, and its purpose is to regulate the oil pressure so it cannot over pressurize the engine. t does this through the use of a valve and a calibrated spring designed to open at a specified pressure allowing some of the output oil flow to be redirected to either the inlet of the oil pump or back to the crankcase. It is important not to change the manner in which the bypassed oil is redirected. Changing the path of the bypassed flow from the inlet of the pump to the crankcase will reduce the performance of the pump resulting in engine damage. Optional springs are available which can increase or decrease the pressure required to open the PRV. Changing the spring will affect the engine oil pressure for engine speeds above the point at which the PRV is open. Melling does not recommend modifying springs by clipping coils off. This will change the pre-load on the valve and may result in a PRV failure, which can cause engine damage. Shimming a spring too much can cause the spring to go solid restricting the amount of oil flow that is allowed to bypass back to the inlet. This will result in higher oil pressures and increasing the risk of spring failure.”
Volume is the other consideration. The amount of oil that flows through the engine is again dependent upon the application, but most pro race engine builders like to see a range of between 8-12 gallons of oil flow per minute (gpm). The trick here is for the oil pump to maintain pressure at this flow rate in order to avoid bearing to journal contact (remember, the engine bearings “live” by operating upon a thin layer of oil). If that thin layer of oil is removed, then the engine fails–plain and simple.
Keep in mind that as you turn the engine over faster (more rpm), the rate of oil loss from the bearings will increase. The pump has to be capable of keeping up with that loss (demand). If it doesn’t then the oil pressure will drop (most often instantly). That’s why there is a need for pump with adequate volume (delivering the target 8-12 gallons of oil per minute gpm).
So when do you switch to a high volume pump?
“Switching to a high volume oil pump is necessary if an engine builder plans to use larger bearing clearances or is expanding the amount of oil contained in the lubrication system (adding remote filters or an oil cooler),” Osterhaus said. “The additional flow from the high volume oil pump will fill the additional spaces and maintain proper oil pressure. The additional flow from the oil pump is created by increasing the displacement of the pumping unit (gear or rotor set). To do this, the physical size of the pump must increase so additional room is required in the engine. This means that clearances between the oil pan and other components must be verified during engine assembly, and some modifications may be necessary to maintain proper [pump and/or pump pickup to pan] clearances.”
As you can see, there are a large number of variables that can have an effect upon the oil volume a pump can ultimately deliver. So what’s the right pump for your application? The short answer is: It depends. It depends on the clearances in the engine. It depends on the intended use of the vehicle. It depends on the oil. It depends on maximum engine speed. When it comes to oil pressure, the only way to really find out is to build the engine and then work with the relief valve to specifically adjust it.