Wet Sump Oil Pumps & Oiling System Upgrades

The Science Behind Wet Sump Oil Pumps & Oiling System Upgrades

All automotive internal combustion engines are equipped with some type of pressurized oiling system. Oil lubrication is essential for reducing friction, preventing wear and cooling vital engine components.

Pumping oil to the main and rod bearings creates a friction-reducing hydrodynamic film between the bearings and crank that allows the crank to spin with minimal friction. The constant flow of oil through the bearings also provides much needed cooling.

Splash lubrication from oil flinging off the spinning crankshaft provides lubrication for the cylinder walls, pistons and rings, as well as cooling for the pistons.

Oil pumped to the cam and lifters (or cam followers in the case of an overhead cam engine) provides friction reduction and wear protection for some of the highest loaded parts inside the engine.

Oil flowing up through the pushrods to the rockers provides lubrication and cooling for the rockers, valve springs and valve stems.

The force behind all of this oil moving around inside the engine is the oil pump. The spur gears or gerotor gears inside the pump pull oil from the oil pan and push it into the oil galleys which route it to where it is needed inside the engine. The oil pump generates flow by pushing oil through the system, but pressure is actually created by the restrictions in the oil filter, oil galleys, bearings and orifices inside the engine. If there was no resistance to flow, there would be no oil pressure reading on the gauge. Consequently, the relationship between oil flow and oil pressure depends on the output capacity of the oil pump and the restrictions in the oiling system.

Open up bearing clearances, install a low resistance oil filter, add piston oilers to improve piston cooling or additional oil lines to the upper valve train for additional lubrication and cooling and you’ll see a corresponding drop in oil pressure readings – unless a larger volume oil pump is installed to compensate for the reduced resistance in the oiling system.

Tighten up bearing clearances or install restrictors to reduce oil flow to the upper valve train area and you’ll see a corresponding increase in oil pressure readings.

The viscosity of the motor oil in the engine also influences oil pressure and flow. Thicker viscosity racing oils (like 20W-50) do not flow as easily as thinner viscosity oils (like 0W-20 or 5W-20), especially when the oil is cold. Thicker oils are used with looser bearing clearances while thinner oils are a must with tighter bearing clearances. If you try to run an engine with relatively loose bearing clearances on a thin oil, the oil will flush through the bearings too quickly resulting in a loss of oil pressure. Likewise, if you try to use a thick oil in a late model engine that has relatively tight bearing clearances, you’ll get higher oil pressure readings but reduced flow through the bearings, which could result in bearing wear or failure.

Recommended rod and main bearing clearances on an “old school” SB/BB Chevy, Ford, Mopar, etc. build might be .002 to .0025˝ for a stock or mild performance build, or as much as .003 to .004˝ for a race application (assuming the engine will use a thicker viscosity racing oil).

For a late model Chevy LS, Ford modular or Chrysler Hemi engine, the recommended bearing clearances would be much tighter, say .0015 to .002˝ with maybe an extra .0005˝ for a performance motor with a lighter viscosity motor oil.

The point is that oil flow and oil pressure depend on a number of variables, and all of these things need to be considered when choosing an oil pump for the application. Stock engines do just fine with stock oil pumps. They don’t really need extra flow or pressure to provide adequate lubrication, wear protection and cooling. The traditional rule of thumb is most engines need about 10 PSI of oil pressure for every 1,000 RPM of engine speed. However, many late model engines and even some performance engines get by with even less, say 5 to 7 PSI of oil pressure per 1,000 RPM.


When deciding how much oil pump you actually need for an engine build, you have to consider (1) the application, (2) how much horsepower the engine will hopefully make, (3) the RPM range of the engine, (4) bearing clearances, (5) intended oil viscosity, and (6) any “add-ons” that may create increased demand for flow such as an external oil cooler, piston oilers, variable valve timing, cylinder deactivation (displacement on demand in some late model production engines), extra upper valvetrain oil lines or added oil lines to a blower or turbocharger.

Most stock oil pumps are adequate for stock or mild performance applications for everyday driving and even some occasional weekend racing. However, as horsepower and RPMs go up, so too do the demands on the oil pump and oiling system.

The output from most stock oil pumps flatlines above 5,500 RPM. The pump’s ability to keep increasing flow plateaus and may even drop off because oil flow up the pickup tube and through the pump can’t keep up with the whirling gears inside the pump. This happens for several reasons. One is that the inside diameter of the pickup tube restricts flow. The cross-sectional area of the pickup tube inlet can also become a restriction at higher RPMs. The passageways inside the pump casting may also inhibit flow above a certain speed. Sharp corners don’t flow as well as rounded corners. The viscosity of the oil can also prevent the oil from moving quickly enough to keep up with the moving voids between the pump gears. This can lead to cavitation and air bubbles in the oil, which will cause oil pressure to drop. Aerated oil is bad news because it can’t maintain proper lubrication and pressure to the bearings.


Original equipment Chevy LS oil pumps are notorious for producing fluctuating oil pressure readings at highway speeds. The problem occurs because of loose manufacturing tolerances in the pump as well as flex in the pump cover and restrictions in the pump inlet tube. Reengineered aftermarket replacement pumps for these applications have better build tolerances and redesigned ports that help maintain steady oil pressure.

Front-mount Chevy LS oil pumps can also be hard to prime when a freshly built engine is first fired up. Start-up pump failures can occur is the pump is not prelubed, or if the gerotor gears are not perfectly centered within the housing.

Chevy LS applications have become very popular thanks to the engine’s power potential and availability. The LS engine family’s basic architecture is the same so the oil pump’s on these engines are interchangeable. The two most common original equipment GM oil pumps used on these engines are a standard volume pump (12586665), found on most of the Gen III engines, and a high volume pump (12612289) that is used on most of the Gen IV engines, many of which have Active Fuel Management (cylinder deactivation) and/or Variable Valve Timing (VVT).

The stock Chevy LS pumps are generally adequate for stock applications, but can be replaced by aftermarket pumps that have better flow characteristics (18% better over the stock replacement, and 33% more flow for a high volume replacement pump). You don’t really need a high volume pump on an older Gen III LS engine because most of the Gen IIIs (with a couple of exceptions) don’t have Active Fuel Management or VVT. However, on the later Gen IV engines that do have these oil-hungry add-ons, the high volume pump is needed to maintain adequate oil flow and pressure to the engine.


All oil pumps have a spring-loaded relief valve to vent excess pressure. The relief valve limits peak oil pressure for the system and serves as a safety valve to prevent dangerous overloads that could damage the filter or oil lines, or possibly blow out a press-fit oil galley plug. When the pump reaches the preset limit, oil pushes the relief valve open and follows the path of least resistance to relieve internal pressure.

Some people like lots of oil pressure because it gives them peace of mind.  But lots of pressure doesn’t necessarily guarantee lots of flow or adequate lubrication. Increasing pressure will increase flow somewhat, but not as much as what you get with a high volume oil pump. The flow rate of any pump depends on the height of the gears, the size of the gears, the number of teeth or lobes per gear, the size of the inlet and outlet ports, the design of the pump housing and pump speed. Taller gears with better flow characteristics though the pump will flow more volume.

A “high pressure” oil pump is one that has a higher relief valve setting than stock. Instead of dumping pressure at say 40 to 60 PSI, a high pressure pump may not open until system pressure hits 70 or 80 PSI. A higher pressure setting just means the pump keeps pushing oil into the system until you hit peak pressure and the relief valve opens.

The relief pressure setting on many pumps can be adjusted by changing the stiffness of the spring behind the relief valve. Just remember that increasing the oil pressure relief setting also increases resistance and the horsepower needed to drive the pump. This, in turn, increases stress and loading on the pump drive shaft, distributor gear and cam drive. Too much oil pressure increases the risk of twisting or breaking a pump drive shaft.

Likewise, installing a high volume pump that pushes more oil to the engine also requires more horsepower.  But in many instances, you need the extra volume for a variety of reasons (to compensate for looser bearing clearances or Variable Valve Timing or piston oilers or an external oil cooler and filter). Some engine builders prefer to install a high volume pump as “insurance” so oil pressure doesn’t drop too low or fluctuate at idle when the engine is hot. Often, this is a bandaid approach to solving an oil pressure problem caused by sloppy tolerances in the engine itself.

The key to building an efficient engine is to choose an oil pump that delivers as much oil as the engine needs but no more, and to keep pressure at a minimum. That’s the winning strategy behind today’s NASCAR engines as well as many late model stock oiling systems. Keep everything lubed but don’t go overboard on pressure or volume.


A new line of oil pumps is on the horizon to be introduced at the PRI show and will feature a unique “external” relief valve setup with dual ball valves. The dual ball valve arrangement allows faster dumping of pressure when engine RPM suddenly drops, as happens twice a lap in a circle track engine. The manufacturer says the pumps will be available in the spring of 2017 for most popular appliations (SB/BB Chevy, Ford, Mopar, Oldsmobile & Pontiac).

When a race car enters a corner, the driver lets up on the gas and the throttle snaps shut. Engine speed drops from 7,000 RPM back to idle, causing oil flow to drop from 10 gallons per minute down to two. But at 7,000 RPM the engine has built up quite a bit of excess oil pressure, so all of this excess pressure has to go somewhere. Some of it will blow out through the oil pump’s internal relief valve. Some of it may rush to the upper valvetrain and flood the valve covers. And some of it may actually reverse flow out of the pump and go backwards down the inlet tube.

Most oil pumps with an internal relief valve can’t vent the excess pressure quickly enough when a sudden and drastic RPM drop occurs. The pressure trapped inside the pump will push back hard against the distributor gear and cam drive, causing spark scatter as well as increased loading and wear on the distributor gear and cam drive. Depending on how much oil backflow down the inlet pipe occurred, it can also cause a momentary loss of oil pressure and/or some oil aeration when the driver hits the gas coming out of the corner. It happens so quickly you may never see a big drop in the pressure reading on an oil pressure gauge, but that is what’s happening inside the pump.

The new oil pump design with external dual ball relief valves can dump up to 140 percent of the pump’s output instantly when engine speed suddenly drops, and the pressure relief settings for each of the two valves can be set separately for a more gradual bleed-off of excess pressure instead of venting the excess pressure all at once.

The manufacturer says a ball relief valve is more reliable than a cup or spool style valve because the latter are prone to sticking if any dirt or debris gets into the pump.


Anything that increases the flow requirements of the oiling system (such as external plumbing, piston oilers, looser bearing clearances and/or higher engine speeds) will require more flow from the oil pump. Add only as much volume as is actually needed, and no more unless you want to waste horsepower.

If you’re running a lot of oil pressure, a replacement pump with a thicker, stronger casting or billet construction would be recommended.

If increased oil flow at higher engine speeds is needed, choose a replacement pump that will accept a larger diameter pickup tube. Also, choose a pickup tube inlet design that offers minimal restriction to flow. Some pickup inlets with drilled holes or mesh screen that is too fine can restrict flow at higher speeds.

Other oiling system add-ons that can improve engine longevity and reliability include aftermarket oil pans with increased oil capacity and baffling, an external oil cooler and external larger capacity oil filter. Increasing oil capacity in the pan reduces the risk of oil aeration or starvation, especially in applications that experience extreme G-forces when cornering or hard acceleration and braking. A well designed pan will keep the oil where it belongs and assure a steady supply of oil to the pickup tube. A larger oil capacity will also help the oil run cooler.

If money is no object and rules permit it, a dry sump oil system is the ultimate setup. With a dry pan, multi-stage vacuum pumps, an external pressure pump and oil reservoir, there’s almost no risk of aerating the oil or starving the pump. The oil will run cooler, and with a shallow pan the engine can be mounted lower in the chassis for better handling. Most dry sump systems will also pull enough vacuum inside the crankcase to significantly reduce windage and drag on the crankshaft. Dry sump systems are the only way to go road racing, and are great for circle tracks if rules permit (most don’t).

A less costly alternative to a full dry sump system is a wet sump with an external oil pump. This type of setup is a good upgrade for late model Chevy, Ford and Chrysler engines with high front-mounted oil pumps if the external pump is mounted low for easier self-priming. For drag racing applications, the pump can be mounted low and in the back of the engine to assure a steady supply of oil from the pan to the pump. ν

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