SRF Oil System

updated: Aug 11, 1999 

This page contains all of  the information that I have been able to gather on this topic since the infamous oil pressure shim fiasco at the Topeka pro race.

On the Ford 1.9L engine, the oil pressure relief valve is located just below the oil filter:

When you unscrew the relief valve cap with an allen wrench, the following relief valve parts can be removed from the engine:

The Ford specifications for the oil pressure relief spring are a bit difficult to measure. The spring length is designed to be 1.11" (28.1mm) with a pressure of 9.5 to 10.3 lbs applied to the spring. The free length is not specified, but seems to typically be about 1.995 to 2.000 inches with no compression of the spring.

The oil pressure relief valve sets the maximum oil pressure that the oil pump will produce. If the oil pressure in the engine is too low, there may be two undesired problems: First, low oil pressure may cause insufficient lubrication and that may cause severe mechanical damage. Secondly, low oil pressure may cause the lifters to be only partially filled with oil, causing the engine horsepower to drop abruptly.

The Ford specification for oil pressure in the 1.9L engine is 36 to 65 psi at 2000 rpm, measured at the oil pressure sender port on the side of the engine.

While it is possible to put a shim at either end of the spring, the relief valve piston has an inner bore of about 0.325 inches, so it is easy to install the shim inside the relief valve piston. The following illustration shows such a shim ready to be put inside the piston:

There are three easy methods to adjust the oil pressure at which the relief valve will open:
                 1.) Use metal shims... note that SAE washers are relatively thick, while AN washers are available thinner and allow for better fine tuning,
                 2.) Physically stretch the spring... take care not to overstress the wire,
                 3.) Use a different spring.... by using a thicker wire diameter, you can keep free length the same, so it is more difficult to detect.

Take care not to damage the oil relief valve components since they are not available from Ford as individual parts. They are supplied only with a complete oil pump assembly.

Note:  Under current SCCA rules, any type of modification to the oil relief valve or to the spring is illegal.

For those who want more information about the oiling system, the following diagram shows the basic design of oiling system in the Ford 1.9L engine:

The two most significant variables determining the oil pressure are the mechanical clearances inside the engine and the oil viscosity.

For a given oil viscosity, as the mechanical clearances inside the engine increase due to wear, the oil pressure inside the engine will decrease even though the oil pump is still generating the same volume and pressure into the oil filter. Also, for a given set of mechanical clearances inside the engine, the oil pressure will decrease as the viscosity of the oil decreases.

So, there are two primary reasons that you will see low oil pressure while the oil pump is operating properly:
                 1.)  excessive mechanical clearances inside in the engine,
                 2.)   insufficient oil viscosity.

There are several ways to deal with the low oil pressure problem:
                1.)  correct the mechanical clearances... but that requires an engine rebuild,
                2.)   increase oil viscosity... but thicker oil reduces engine horsepower,
                3.)   increase the oil pump volume... but that is not SCCA legal and requires replacing the oil pump,
                4.)   increase the oil pump pressure... but excessive pressure may explode the oil filter, damage seals, blow out oil passage plugs or even damage bearings.

The Melling web site has quite a bit of interesting information about oil pumps. An oil pump will inherently deliver a certain volume of oil every revolution. So, the faster the engine turns, the higher the pumped volume. As the volume of oil pumped into the engine increases, the oil pressure will increase. The oil pressure relief valve is intended to limit this increase in oil pressure to a safe value.

When the oil is cold, the thick oil is difficult to pump into the oil passages, causing the oil pressure relief valve to open up in order to prevent excessive oil pressure, even at idle speeds. With hot oil, the oil pressure relief valve will generally have no effect until somewhere above 2000 rpm.

At idle rpm when the oil is hot, the oil flows very easily through the engine and the oil pressure is inherently lower. Adjusting the oil pressure relief valve will probably have no effect on the oil pressure at idle rpm when the oil is hot.

On the Red Line Oil web page, they say that you will get a 1% to 2% increase in horsepower for each reduction in oil weight. So, going from the Ford recommended 30 weight oil down to the thinner 20 weight oil should gain 1 to 2 horsepower in our SRF engines. However, the thinner oil will result in a reduction in oil pressure. As a last resort, when better solutions are not practical, the oil pressure relief valve can be adjusted to reestablish reasonable oil pressure while gaining a bit of horsepower by using the very thin oil.

In some racing, such as NASCAR, it is apparently common to use a qualifying engine which puts out maximum possible horsepower and in which longevity is willingly sacrificed for power. One of their techniques is to use extremely light weight oil. Note that Red Line produces a 5 weight oil for such applications. For a specified oil pressure, the losses in the oil pump are probably a bit less for the lighter oil, and the losses in the rest of the engine are greatly reduced with the lighter oil. Reduced power losses in the engine result in more power output.

On the down side, an engine designed for 30 weight oil, such as our 1.9L Ford Escort engine, will not live very long with such thin oil. With zero weight or 10 weight oil in the Escort engine,  the oil pressure will probably be insufficient toward the rear of the engine and you're likely to see cylinder #4 suffer the most since it is at the end of the oil distribution. In all likelihood, you'll soon see the connecting rod sticking out of the side of the engine... but it'll put out a couple of extra horsepower for a little while.

If you want a low risk, SCCA legal solution for low oil pressure, it seems that the safest and easiest solution is to simply use slightly thicker oil or an oil viscosity improver like STP.