Oil Pump Selection: Is High Pressure Or High Volume Better?

If you’re building or modifying an engine, it’s a process that requires a lot of planning and forethought. You need to know how the engine will be used. At what RPM do you want your powerband to be prevalent? What camshaft should you order, and do you need a high-pressure or high-volume oil pump?

That last consideration is a question I get asked quite often, and the oil pump is not a piece of the project that should cause confusion. To help clear the air, I reached out to Mike Osterhaus VP of aftermarket products for Melling Engine Parts and Certified Lubrication Specialist, Lake Speed Jr., at Driven Racing Oils to get some concise input.

However, before we hear from the professionals, we need to get a few things straight. First, oil pumps produce flow, and the flow restrictions within the engine are what build the pressure. However, a pressure-relief valve in the pump actually regulates the allowable pressure. If your engine is experiencing low oil pressure, it is usually caused by a leak or excessive wear in the engine (think oversized bearing clearance or worn bearings as an example of a leak). When this occurs, many enthusiasts believe adding a high-volume pump will help.

“A high-volume pump will increase the flow to the engine, which will provide an increase in pressure. The amount of the improvement depends on the amount of wear in the engine or the size of the leak,” states Osterhaus. But, you still have the leak or worn condition. High-volume pumps are designed to be used when increased demands on the oil system — such as high RPM usage, racing, remote filters, and/or coolers — are added.

Pressure Versus Flow

Let’s consider pressure versus flow inside an engine. The size of a given oil pump is designed to supply the correct amount of oil to meet engine requirements. It produces a specific amount of flow at a given RPM. The resistance to the oil’s flow is from the various clearances within the engine (i.e., bearing and lifter bore clearances).

I’m comfortable with 20 psi at a hot-idle. That is sufficient pressure when the engine is not under load. – Lake Speed, Driven Racing Oil

When an engine is freshly built, the clearances are tight, and the oil pressure is good. As the bearings and lifter bores wear, clearances increase. When this occurs, resistance to flow decreases, and oil pressure starts to drop. “It is normal to see a reduction in the operating oil pressure as the engine wears over time,” affirms Osterhaus. When the pressure drops, we get our first signal that something is wrong in the engine. With the increased clearances/decreased resistance, the flow from the oil pump is at its maximum.

oil pump

Traditional thinking has bearing clearance for street and most performance applications at roughly 0.001-inch of clearance for every 1-inch of crankshaft journal diameter. Another thing to keep in mind is bearing clearance establishes the viscosity of the oil to use within a given oil-temperature range. Generally, the tighter the bearing clearance, the lower the oil viscosity the engine can safely handle.

If bearing clearance is excessive in any location other than rod bearings, the oil pressure will be uniformly low throughout the entire RPM range. If the rod bearing clearances are excessive, the oil pressure will be low at idle and then get lower as engine RPM increases. This is because rod bearings move in a circle rather than on an axis. This means they are subject to centrifugal forces trying to remove the oil away from the bearing.

“Bearing clearances in the engine will have the largest effect on oil demand,” states Osterhaus. “As the clearances increase, so will the engine’s demand for oil. If an engine is built with stock bearing clearances, then a stock-volume oil pump will work best. We need to look at increasing oil flow from the pump if the engine has larger bearing clearances, higher flow-rate lifters, or the addition of piston and/or valve spring oilers, etc. Any modifications that will increase the engine’s demand for oil will require a high-volume pump.”

From an oil supplier’s viewpoint, Speed recognizes how bearing clearances affect the decision, but also explains how the lubricant itself plays a vital role.

“Let’s begin with the definition of proper lubrication — having the right oil (correct viscosity and additives for the application), in the right places, at the right time, and in the right amount,” says Speed. “Now, also consider oil pressure is a result of restrictions to oil flow throughout the oiling system. It is possible to have very-high oil pressure with very-little oil flow. Just a side note, viscosity is also resistance to flow, so the viscosity of the oil itself obviously plays a role in oil pressure.”

oil pump

As the engine spins, the meshed gears within the oil pump also spin, creating flow. The pressure of this flow is regulated by a spring-controlled valve in the oil pump.

“Consequently, it is critical to have proper oil volume moving through the system. However, oil pressure is equally important, especially in hydraulic lifters and camshaft-position phasers. Without proper oil pressure, these devices can’t function correctly. Bottom line? They are both important. It is a both/and relationship instead of an either/or.”

Stock (OE replacement) oil pumps are typically sufficient for use in most stock applications. But, leave the world of stock, and some are not up to the task. When building an engine to deliver increased horsepower, engine RPM, or has the stock bearing clearances altered to be more compatible with a thicker – or thinner – viscosity oil, choosing the correct oil pump is crucial.

“The application always dictates the hardware needs,” states Speed. “Higher engine RPM will increase the pump speed, which can lead to pump cavitation. OE pumps are not typically designed for engine speeds beyond the OE red line. As such, engines modified to turn higher RPM need to be equipped with pumps capable of that RPM.”

The flow and pressure of engine oil created by the oil pump need to fit the parameters of the application. There is an old rule of thumb that an engine needs about 10 psi of oil pressure for every 1,000 rpm. That guideline is still a good rule to follow. However, there are exceptions to the rule. “I’m comfortable with 20 psi at a hot-idle,” says Speed. “That is sufficient pressure when the engine is not under load.” Osterhaus agrees with the 20-psi rule.

We’ve heard that many NASCAR teams actually run as little as 5 psi for every 1,000 rpm. Of course, NASCAR engines don’t spend much time idling. Consequently, the pump’s output volume and pressure can be minimized to deliver just enough oil to keep the engine lubed without wasting excessive power to drive the pump.

oil pump

A Geroter pump works when the fluid passing through the pump enters the suction port between the rotor (inner gear) and idler (outer gear). The close tolerance between the gears acts as a seal between the suction and discharge ports. This seal forces the liquid out of the discharge port.

By comparison, an engine built for street use spends a lot of time idling and running at low RPM. Because of that, the oil pump has to deliver good flow and pressure at all engine speeds. Parasitic horsepower draw is less of a concern when discussing a street engine, and that’s why many street-use oil pumps are set to over-deliver oil to the engine. Many enthusiasts see this as a way to provide insurance against a surprise drop in oil pressure which might damage the engine.

High-Volume Pumps

Have you ever heard of people installing high-volume pumps in high-mileage engines to “fix” low-idle oil pressure and/or valvetrain noise? A high-volume pump can certainly be used for such purposes. But, if an engine has low-idle oil pressure problems and/or valvetrain noise, there are other issues that need to be addressed. The concern could be excessive bearing clearances, pressure losses within the oiling system itself, or possibly oil-flow restrictions to the upper valvetrain.

Installing a high-volume oil pump has improved the operation of many of our customer’s high-mileage engines.” – Mike Osterhaus, Melling Engine Parts

“A high-pressure pump is an oil pump with a higher pressure relief valve setting,” states Speed. “There are three ways to increase oil pressure: 1) Increase the viscosity of the oil (higher viscosity means greater resistance to flow). 2) Increase the pump size, which increases oil flow. Consequently, oil pressure (resistance to flow in the oiling system) will increase. And, 3), increase the pressure relief-valve setting, which reduces bleed from the pump itself.”

I asked both men if they would ever recommend a high-volume or high-pressure pump as a band-aid for low oil pressure. “Not necessarily,” Speed states. “It is better to find out the reason for the low oil pressure. Since oil pressure is a result of resistance to the flow of oil through the oiling system, it is vital to understand why there is less resistance than normal. It is also important to consider the viscosity of the oil as a factor.”

“One of the main “leak” points in an engine are the main and rod bearings. Larger clearances provide larger areas to leak, which bleeds off oil pressure. Driven Racing Oils publishes a bearing clearance-to-oil viscosity chart that helps engine builders, racers, and enthusiasts choose the correct viscosity oil for their bearing clearances.”

According to Osterhaus, “the increased internal clearances found in a high-mileage engine create a higher demand for oil. A high-volume pump will provide assistance in increasing the oil flow to fill the larger internal clearances. This results in increasing the oil pressure. Installing a high-volume oil pump has improved the operation of many of our customer’s high-mileage engines.”

High-volume oil pumps have taller gears and typically flow about 15- to 20-percent-more oil than a comparable stock pump. In some applications, a high-flow pump is absolutely essential. Many late-model engines with cylinder deactivation (some LS engines) require more oil flow for the lifters and cam phasers, so these parts can do the job properly.

oil pump

A high-volume pump has a larger gear than a standard-volume pump. The gear on the left is Melling’s new shark tooth gear, while the gear on the right is a traditional straight-cut gear.

A high-volume oil pump is also necessary if you’re building a performance engine with larger bearing clearances (more than .0025-inch at the main and rod bearings if using 20W-50 racing oil). On the other hand, if you are building an engine with tighter main and rod-bearing clearances, the engine won’t need as much oil volume, and a stock-capacity pump should work just fine. The pressure was not addressed, only volume.

A Stock Pump Has its Place, But…

One of the limitations of all oil pumps is a condition called cavitation. At some speed (dependent on oil viscosity), the gears inside the pump will actually be spinning faster than the oil can flow through the pump. When this limit is reached, small bubbles form along the trailing edges of the gears. This is cavitation.

This condition aerates the oil and causes the pump’s fluid-output to stall. For many stock, straight-cut, spur-gear oil pumps, cavitation can occur when the engine exceeds 6,000rpm. With front-mounted or crankcase-mounted gerotor-style oil pumps (think LS), cavitation may occur at a somewhat higher engine RPM.

It’s no surprise that minimizing cavitation is paramount. Oil-pump manufacturers have developed various pump designs that can reduce cavitation at higher engine speeds. One upgrade is to enlarge the oil-pump inlet and use a larger diameter pickup tube. This will allow oil a less-restricted flow into the pump. By doing so, it can keep up with the increasing speed of the pump’s gears. Another design found in some racing pumps will split the inlet stream into two channels and route half to each side of the gears.

oil pump

Not only do you need a quality oil, but the viscosity plays a crucial role in oil pressure. Driven’s high-performance motor oils are formulated using Pennsylvania petroleum combined with high-zinc (ZDDP) content that has been engineered and tested to deliver increased horsepower and better engine-wear protection under high load. Driven oils are available in various viscosities for high-performance engines.

“OE pumps are designed to meet the original statement of requirements for a particular engine while controlling cost and weight,” states Osterhaus. “Melling is not constrained by these requirements and is free to design and produce a pump that will provide better performance across the engine’s operating range. The OE manufacturers have investigated applying certain coatings to the pump’s internals, but have decided against using them, as they increase cost — even though durability is increased significantly.”

According to Speed, an engine’s operating range is the largest consideration to keep in mind. “The application always dictates the hardware needs. Higher engine RPM will increase the pump speed, which can lead to pump cavitation. OE pumps are typically not designed for engine speeds beyond the OE redline. As such, engines modified to turn higher RPM need to be equipped with pumps capable of that RPM.”

Not All Pumps Are Created Equal

Front-mounted oil pumps — like those on Chevy’s LS, Ford’s modular engines, and Chrysler’s Hemi have a few advantages and disadvantages when compared to in-pan-mounted oil pumps. On the plus side, these front-mounted pumps are a gerotor-style pump that offers better oil-flow characteristics than traditional in-pan pumps with a straight-cut spur-gear design.

Front-mounted pumps are also driven by the crankshaft, which means it rotates at engine speed, which is actually twice the speed of an in-pan, crankcase-mounted pump that is driven by a shaft geared to the camshaft or distributor. The front-mount design also means there’s no driveshaft spinning the pump that can flex, twist, bend, or break.

A disadvantage of a front-mounted oil pump is that it is mounted above the actual oil level in the pan. We must keep in mind, oil tends to drain out of the pump rather quickly when the engine stops running (shut off). This leaves the pump “dry” during the next startup. In-pan mounted pumps are continuously submerged in oil, and this helps retain oil inside the pump. This fact makes it much quicker and easier to self-prime and build oil pressure when a cold engine is started.

oil pump

Front-mounted oil pumps do have benefits when compared to in-pan-mounted oil pumps. On the plus side, these front-mounted pumps are a gerotor-style pump that offers better oil-flow characteristics. A downside of a front-mounted oil pump is it is mounted above the oil level in the pan. Since oil tends to drain out of the pump rather quickly when the engine stops running (shut off), priming takes longer during startup.

Another downside is pick-up tube mounting. A front-mounted pump uses a relatively long pick-up tube to connect to the oil in the pan. In theory, this means a front-mounted pump will take more time to self-prime and generate oil pressure during a cold start. While this time is minimal, it could cause issues — especially if using heavy viscosity oils.

New Oil Pump Innovations

Recently, there have been some innovations in oil pump design that addresses many of the fundamental issues that plague traditional oil pumps. Oil pump suppliers like Melling are constantly striving to improve its products to deliver better performance and more reliable operation. One of these evolutions is a design change that reduces cavitation at higher engine speeds.

While a typical in-pan oil pump uses straight-cut gears to pump oil through the engine, Melling released a new gear design that resembles a shark’s tooth. The technical jargon for the new gear is a helical-asymmetrical gear.

Helical gears utilize teeth that are cut at an angle and have a slightly forward rake. While a straight-cut gear immediately meshes at full contact, the angled teeth on a helical gear gradually become engaged with one another. This results in a quieter and smoother operation. The smooth-meshing of the helical-asymmetrical gears reduces cavitation as compared to the straight-cut gears.

Hopefully, this look at volume versus pressure has given you some insight into knowing what pump is best for your application. There are a lot of variables that can affect how much oil volume or pressure a pump can deliver. For that reason, it’s nearly impossible to say a high-volume or high-pressure pump is right for a blanket application.

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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