set style pumps.”
Osterhaus of Melling says cavitation is a function of the inlet design of an oil pump. The vapor pressure of the oil versus the pressure drop that occurs from the oil pan pickup to the pump is what creates the conditions that lead to cavitation. If the pressure drop is greater than about 4 psi, vapor bubbles will form inside the pump. The bubbles will collapse on the high pressure output side of the pump. But when they do, the force generated by the implosion of the bubbles can erode the metal surfaces in the pump.
The point at which cavitation starts to occur depends on pump speed and the size of the pickup tube, says Osterhaus. “Up until the early 1990s, GM used a 5/8˝ OD pickup tube on its V8 engines. In 1993, GM changed to a larger 3/4˝ tube that flowed better and produced less cavitation. We still make the M55 oil pump for the older applications, but recommend the newer M155 pump with the larger inlet as a retrofit for better oil flow.
Osterhaus says Melling has also improved the inlet design of its pumps to improve flow and reduce cavitation up to 8,000 rpm. This allows the pump’s output to continue to climb without flatlining above 5,000 rpm like a stock pump.
“You don’t really need a high volume pump if you are running stock bearing clearances. This will save power compared to a high volume pump. But if you are running a lot of rpms or are building an engine with looser clearances, then a high volume pump would be the best choice.”
Melling also makes billet pumps for performance applications, which Osterhaus says are an affordable alternative to dry sump systems, and are recommended for engines that rev above 5,000 rpm. A unique feature of these pumps is that the gear shaft extends through the gear into the pump cover to reduce deflections that can rob power from the engine.
Moroso of Gilford, CT, a company well-known for its performance oil pans, says many stock and so-called “race” pumps are not equipped to handle higher oil flow at race speeds. As oil velocity increases, many pumps cavitate and chatter because they just can’t handle the extra demand.
Moroso Racing Pumps are specifically engineered to accommodate the oil flow required by consistently higher engine speeds. Special anti-cavitation slots, feeder grooves and an enlarged bypass area are machined into the housing to bleed oil back to the inlet side of the pump. These features combine to offer a smooth and consistent flow of oil, which improves the pump’s overall performance, extends engine life, and places an even loading on the distributor shaft to eliminate spark scatter.
“Assuming you are looking at an internal wet sump pump, you would look for these characteristics in a modern oil pump,” explain’s Moroso’s Kevin Brown: “Enough pump displacement to yield adequate oil pressure at a hot idle and all the way through the useable rpm range. You want to see a nice clean linear pressure trace until the regulation point of the pressure relief valve. At the point of pressure regulation it should maintain near constant pressure level. And, you should look for a heavy duty casting to withstand the rigors of the racing environment.”
Moroso Racing Pumps include Professional Billet Aluminum Gerotor pump, Sportsman Billet Aluminum Spur Gear Pump, and the cast iron Blueprinted Racing Oil Pump that is CNC machined to ensure the tightest tolerances. The Blueprinted pump has anti-cavitation slots and enlarged feeder grooves to provide a steady flow of oil to each side of the gears for high rpm lubrication, cavitation relief and reduced spark scatter.
Moroso also makes an External Wet Sump Oil Pump that mounts outside the engine (similar to a dry sump setup). This allows the use of a full-length windage tray inside the oil pan for less windage and more power. Moroso says their External Oil Pump also eliminates torsional stress on the camshaft, as well as spark scatter and resultant horsepower loss caused by an internal wet sump pump.
Brown, a Moroso sales engineer, says a properly designed inlet circuit is key to limiting cavitation. “You need to have the correct diameter inlet with minimal lifts and bends to mitigate piping losses and oil velocity limitations, as well as a properly designed inlet strainer (pick up) to minimize pressure loss at the inlet,” says Brown. “Then, you must set the correct pick-up height off the floor of the oil pan to minimize vortexing at the pick up. A well-designed windage tray will help protect the oil from aeration, and of course you must be sure there is suitable oil volume in the oiling system to eliminate any drainback and oil turnover issues.”
Vern Schumann of Schumann Sales & Service, Blue Grass IA, feels a solution for cavitation in the Chevy SB/BB oil pump is his dual feed design that flows 60 percent of the oil into the pump housing through the main inlet, and 40 percent into the housing through a second inlet. There are also cavitation relief channels cut into the pump cover (which are machined with a unique water jet process).
Pickup Restrictions
There is also a risk of starving the engine for oil at higher speeds with heavier viscosity oils. The screens or perforated metal covers on most oil pump pickup tubes are fairly restrictive. And the heavier the oil, the less easily it flows through the pores in the screen or perforations.
Wet sump oil pumps don’t generate a lot of suction, so any restrictions on the inlet side of the pump can starve the pump and reduce oil pressure. Long pickup tubes with restrictive inlets, therefore, are bad news if you are trying to maintain good oil pressure in a performance engine – or even a stock engine.
Schumann says he has researched this issue extensively after discovering his new dual feed high volume small block Chevy oil pump was not realizing its maximum flow potential with commonly available aftermarket pickups.
“Our pump is fully capable of delivering up to 12 gallons of oil flow per minute. But it can’t achieve those flow numbers with any of the aftermarket pickup tubes I’ve tested. The pickups are too restrictive because of the size of the screens or perforations they use.”
Schumann said he has flow tested a variety of aftermarket pickups and found that all are overly restrictive and inhibit oil flow, especially with higher viscosity motor oils. For every 10 point increase in the viscosity of the oil, our tests show about a 5 percent drop in oil flow through the pickup screen.
An OEM style oil pickup with a screen style inlet cover has a theoretical open area of 60 to 70 percent between the wire mesh. Yet it only flows about 30 percent of what it should flow due to the restrictive drag created by the screen. Aftermarket pickups with perforated metal inlets are no better. In some cases, says Schumann, their flow characteristics are even worse!
The size, shape and spacing of the inlet holes in the aftermarket pickups vary from 29 percent to 49 percent (which is actually a lot less than the typical OEM screen style pickup) according to Schumann’s analysis. The flow rates range from a low of 20 percent in the poorest design up to a maximum of 37 percent in the best design – which is far less than what you would expect from an aftermarket “performance” pickup.
Schumann says the industry needs to rethink the validity of current pickup designs because existing designs create a bottleneck that restricts oil flow to the engine regardless of what brand or style of oil pump is used. “The screen or perforated holes at the pickup inlet don’t really protect the oil pump. They just keep big chunks of debris out of the pump. The tolerances inside an oil pump are typically .002" to .005", so any particles larger than that but smaller than the openings in the pickup inlet will be sucked into the pump anyway and damage the pump.”
Schumann said he worked with a dozen different automotive, aircraft and industrial filter manufacturers to develop a cartridge filter or other filter that could be installed in the oil pump inlet or pickup inlet to protect the pump. But they all concluded it couldn’t be done. Any such filter would be too restrictive due to the low suction qualities of wet sump oil pumps.
So Schumann came up with a different approach. Since the pickup screen or perforations don’t really do much to protect the oil pump anyway, and only create a restriction, why not open up the pickup inlet to improve its flow characteristics? Schumann’s solution is to replace the existing screen or perforated metal inlet cover with a 1/4" hex honeycomb cover that has an open area of 92 percent and flows up to 78 percent of its theoretical maximum. There is still some protection to prevent large chunks of debris from entering the pickup, yet much less restriction to impede the flow of oil at higher engine rpms.
Schumann said he is working with several aftermarket pickup manufacturers to produce the new low restriction pickups, and hopes the entire industry rethinks their current designs and adopt his new approach.
“Another modification that can improve flow to the oil pump is to cut the inlet end of the pickup tube at an oblique angle rather than a straight cut. This increases the effective area of the opening and allows better flow into the pickup tube.”
Schumann also cautioned that pickups with welded perforated metal covers may contain weld splatter inside that can flake loose and enter the oil pump. “If you pound the pickup against a pine board several times, you’ll often shake loose several weld BBs that are hidden inside the pickup. At least one pickup manufacturer I know has revised its welding procedure to reduce the risk of weld splatter ending up inside the pickup. But others have not, so you need to check the pickup tubes to make sure there’s nothing lurking inside that could cause problems later on.”
Oil Leaks
One often overlooked area where oil pressure and flow can sometimes be lost in a rebuilt engine (or even a new engine, for that matter) is the area where a bottom mounted oil pump mates to the block. The OEM mounting is simply a flush surface with no gasket or O-ring on most engines. Consequently, if both surfaces are not perfectly flat, gaps may allow high pressure oil to leak back into the crankcase. This can cause a drop in oil pressure at idle.
Schumann’s fix for this is a thin copper gasket that fits between the pump housing and block. The gaskets are currently available for small block and big block Chevy, and Windsor Ford V8s.
Rebuilt Oil Pumps
Osterhaus said hot idle oil pressure is critical in any engine, whether it is stock or performance. An engine builder can resurface the cover on a worn pump to restore end clearances, but it won’t restore the clearances between the gears or the gears and the sides of the oil pump housing. Because of this, a worn pump won’t deliver the same pressure as a new pump.
“We’ve supplied oil pump rebuilding kits to the aftermarket, but the better approach is to go with a new pump because the housing is often worn,” Osterhaus says.
If you’re worried about maintaining the oil pressure in your newly built engine, that’s a good thing. Not being sure of the oil flow demands will be a real problem for your customers.
