How Much Oil Pressure?
Vehicle manufacturers have traditionally recommend a minimum of 10 psi of oil pressure for every 1,000 rpm of engine speed. Using these numbers, most stock engines don't need any more than 50 to 60 psi of oil pressure. With tighter bearing clearances, pressure goes up requiring less flow from the pump and less parasitic horsepower loss to drive the oil pump. In racing applications, the old school of thought was more oil pressure was needed to keep the engine lubed. That's true if bearing clearances are loosened up. But most engine builders today tighten clearances so less oil flow is needed to maintain adequate oil pressure. This approach increases the horsepower output because less power is needed to drive the pump at high rpm.
A stock oil pump is usually more than adequate for most street performance engines, and even many racing engines. NASCAR engines typically get by just fine with no more than 50 psi of oil pressure at 9,000 rpm! Some top fuel dragster and funny car engines are set up so the oil pump will dump excessive oil pressure at high rpm so more power will be routed to the rear wheels.
Some racing engines use a "dry sump" oiling system. With this type of lubrication system, an external oil pump is used to suck all the oil out of the crankcase to reduce "windage" (oil drag) on the crankshaft. The oil is routed to an external tank and an oil cooler before it is recirculated back into the engine.
High-Volume & High-Pressure Oil Pumps
In applications where more oil flow is desired either to increase oil flow or pressure for better bearing lubrication and cooling, an oil pump with longer or larger gears may be installed. The physically larger surface area of the gears pushes more oil through the pump at the same rpm than a stock pump. A high-volume oil pump typically flows 20 to 25% more oil than a stock pump. The increase in oil flow produces an increase in oil pressure at idle, which helps compensate for increased bearing clearances. Consequently, some people may install a high-volume pump in a high-mileage engine in an attempt to restore normal oil pressure. But oil isn't metal, and the only real cure for low oil pressure is to replace worn bearings and restore normal clearances.
High-pressure oil pumps are another option. A high-pressure pump contains a stiffer relief valve spring that does not open until a higher pressure is reached (75 psi or higher). The actual flow rate of a high-pressure pump may be no different than a stock pump, or it may be higher if longer gears are used. Either way, the pump will increase the system oil pressure reading at high rpm when the pump is working hard, but it won't have any affect on idle pressure when the pump is turning slowly.
A high-volume or high-pressure oil pump may be recommended in engines where bearing clearances are looser than normal, in engines where an auxiliary external oil cooler has been added to improve oil cooling and in racing engines where a oil accumulator has been installed.
Friday, January 23rd, 2009 AT 6:05 PM