This is an article intended for the archives - could people comment on it and I'll make any required changes before posting. Thanks
NB. This article is written from knowledge of 20XE, C20XE and C20LET engines. Although this information is specifically about these engines, most of what is written will be the same for other engines.
This article also assumes that you have some way of determining your oil pressure. A guide for digital dash users is contained towards the end of the article.
Oil pressure is effectively a pressurised flow of oil through the engine created by the oil pump. This pressure causes the bearings inside the engine to “float” which protects surfaces from coming into contact with each other. The oil protects all moving surfaces including the valve guides, tappets, cam bearing and much more. The oil eventually drains back down to the sump. On its journey, the oil is heated partly due to friction and combustion heat. The oil has to be able to deal with this heat without breaking down.
The hotter oil is, the thinner it becomes. Because of this, when engine oil is very hot, oil pressure drops as it is more difficult to pressurise a fast moving thin liquid. The use of a cooler can reduce oil temperature and help maintain oil pressure at high operating temperatures.
Maintaining oil pressure in an engine is extremely important. A lack of good oil pressure can cause the surface tension of the oil to allow the metal surfaces inside the engine to come into contact thus causing damage. An engine running with too little pressure will not last very long, especially if used at high revs.
At the heart of the oil system is the oil pump. This has an inlet on the bottom. The pumping forced creates low pressure up flow of the pump, which causes the oil to be sucked in from the sump via the oil pick-up pipe. The oil is then delivered through the oil filter, then the pump and then up through the engine. The pump works by using two gears, which are run off the cam belt. These gear effectively squeeze the oil in order to pump it.
Usually the standard pumps are well up to the job of handling big power - especially on the 20XE and C20LET engines. There is no need to buy a more efficient pump for these engines. For high revs, it can be a good idea to replace the gears inside the pump with machined steel items, as these reduce the risk of them breaking up, but again, the standard items will be fine to around 7750-8000rpm.
If you ever need to replace your oil pump, only do so with a genuine GM item. Non genuine items can sometimes be worse than a used 200,000 mile GM item – you have been warned.
Oil Pressure Release Valve.
If you imagine the pump run of the cam belt you soon realise that the higher the rpm, the faster the pump with spin - hence the higher the oil pressure.
Optimum oil pressure is usually gained at about 2500-3000rpm. To stop the oil pressure rising higher at revs greater than this, the pressure release valve opens to allow oil to re-circulate and thus dropping the pressure.
The valve works quite simply using a spring-loaded valve. The valve is fully closed at low revs, but as they rise, the pressure pushes against the valve and the spring compresses with respect to the amount of pressure behind it.
On this principle, any rpm above around 3000rpm will have the same pressure, as the spring will constantly change the position of the valve to do this.
Shimming the spring with stainless washers came increase oil pressure due to the extra force required to move the valve. This is useful if you use extremely high revs or if the oil pump is old and needs a little boost. Washers can be added without removal of the pump, although can be a bit tricky, especially on PAS cars.
SBD do a nylon relief valve kit which is designed to help to overcome particles which can jam between the aluminium housing and the standard steel relief valve. The problem you get with a standard relief valve is as soon as any particles come between the two, the relief valve just sticks (and usually permanently). The nylon relief valve is designed to ride over the top of any particles that become bedded into the aluminium housing. This does promote wear in the nylon relief valve and wherever particles have become embedded this will sometimes cause the relief valve to react slowly. The nylon relief valve also being more compliant than the steel valve does have a tendency to be slightly more erratic on maintaining a consistent oil pressure. You may find a more consistent pressure with the steel valve, but on that one occasion that it does stick, it could be its last and that may go for the engine as well. SBD suggest that if the oil pressure is more erratic than normal and if you had seen higher or more stable oil pressure in the past, check the relief valve carefully and the oil pump for excessive wear.
Sandwich Plate and Oil Coolers.
On higher performance cars, you may find this between the oil pump and the oil filter. It simply directs oil to a cooler on the front of the car. Some are thermostatically controlled and only use the cooler when the oil reaches a specific temperature. If you have an oil pressure gauge and a thermostatically controlled oil cooler, you may notice in traffic that the oil pressure suddenly drops. Many people say that this is due to the oil losing its viscosity at higher temperatures - not so, it is in fact due to the sandwich plate redirecting to the oil cooler which cause a pressure loss.
If you run a racing car, it could be a good idea to run the oil cooler all the time. This can be done by removing the thermostat from its housing, or by fitting a different sandwich plate.
Signs of Problems.
If your oil pressure read maximum even before starting the engine, make sure the sensor is plugged in, as this is impossible in reality.
Check you oil pressure when starting you engine, if it takes a while for oil pressure to build up (more than 8s) it is likely that your oil pump is starting to wear.
Constant low oil pressure could be down to two main reasons. If your oil pressure is very low at low revs, it is most likely that the pump is worn. If pressure is okay at lower revs but just doesn't pick up very much, it could be down to a weak oil pressure valve spring. In either case, try fitting a stainless washer behind the spring to increase the pressure required to open the valve. If this does not work, consider fitting a new oil pump.
What you Should Aim For.
Oil pressure should be at least 55-65psi at around 3000rpm. If it is any lower than this, you should take action. Usually at idle, pressure is at around 30psi - if this drops below 25psi you should check the pump and valve.
It is not recommended to run oil pressure greater than 90psi as this can cause seals to fail. Typically for a high revving C20XE (around 8000rpm), 70-80psi should be fine.
For reference, on an Astra GTE digital dash, each yellow blip represents 10psi - so full shows that oil pressure is over 50psi. If this drops by one yellow bar, I would not use high revs until it is fixed. If pressure is any lower than this, it would be wise to not use the car.
By Michael Warner © MIG Performance Vauxhalls 2002