Classic cars’ engine longevity and trouble-free driving is dependent on proper lubrication which comes from the engines oil pressure at both an idle as well as under driving load.
Oil pressure is an important factor in the longevity of most internal combustion engines. Engines with a forced lubrication system, oil is picked up by a positive displacement oil pump and forced through oil galleries (passageways) into bearings, such as the main bearings, end bearings and camshaft bearings. Other components such as cam lobes and cylinder walls are lubricated by oil jets.
Sufficient oil pressure ensures that the metal of the rotating shaft (journal) and the bearing shell can never touch, and wear is therefore confined to initial start-up and shutdown. The oil pressure, combined with the rotation of the shaft, also hydrodynamically centers the journal in its shell and cools the bearings. Such a bearing is known as a fluid bearing.
Oil pressure is higher when the engine is cold due to the increased viscosity of the oil and increases with engine speed until the relief valve in the oil pump opens to divert excess flow. Oil pressure is lowest under hot idling conditions, and the minimum pressure allowed by the manufacturer’s tolerances is usually given at this point. Excessive oil pressure may indicate a blocked filter, blocked oil gallery or the wrong grade of oil. Low oil pressure indicates worn bearings or a broken oil pump.
Some vehicle engines have an oil pressure switch that switches a warning light on at low oil pressure. Some vehicles have an oil pressure gauge in the dashboard or instrument cluster.
Oil pressure is created by a fluid flow restriction in the outlet line of the pump, not by the pump itself. Examples of such restrictions include big end and main bearings, as well as camshaft and rocker gear if fitted.
The normal oil pressure in an internal combustion engine varies from one brand to another and one model to another. What’s normal for a Chevrolet engine may be different for a Toyota engine and the normal pressure for a 4.3L Chevrolet may be different than on a 5.7L Chevrolet motor. Still, oil pressure is an important monitor of how well the engine oil is protecting the moving parts of an engine.
Engine Oil Pressure - Mid-Range is Best
Oil pressure will vary in most engines depending on whether the engine has just been started and the oil is cool or if the motor has run long enough to warm to normal operating temperature. The pressure will vary somewhat with the speed the engine is turning. That’s why many vehicles only have gauges that show low, normal, and high ranges. Check the specifications for the exact make and model engine for your vehicle to determine the correct oil pressure operating range in pounds per square inch. Gauges that register in PSI, usually range from 0 to 100 and the normal zone on these is between 20 and 80 PSI.
Considerations
The oil pressure gauge measures the resistance to the oil being pumped through the engine by the motor’s oil pump. The temperature of the oil, the type of oil and its viscosity all affect that resistance. The age of the engine affects oil pressure, as well. New, tight engines will register higher oil pressures than the same engine thousands of miles later after it has been broken-in and has experienced some internal wear.
Low Verses High
Low oil pressure indicates insufficient lubrication is being provided to the motor. A reading less than 20 PSI or under the normal range on the gauge is cause for immediate attention. The problem could be as simple as a low oil level or a signal of serious engine problems. High oil pressure readings are uncommon and may be a faulty gauge or an improperly functioning oil by-pass valve.
Finally
No matter the classic car or engine knowing your engines oil pressure is at the proper level is one of the keys to a trouble-free driving experience as well as avoiding expensive engine repairs.
