Shimming the Toyota 4A-G Oil Pump for Higher Pressure Operation
The 4A-GE oil pump is a fairly standard gerotor type oil pump. It consists of two eccentric gears. The inner gear is spun directly by the crank. The outer gear has one more tooth than the inner gear and so spins slightly slower and rotates inside the housing of the pump. Because the inner gear is smaller than the outer gear, a gap exists between the two. As the pump spins, oil is pulled into this gap and as the gearset rotates further, the gap closes, pushing the oil into the engine.
As the oil flows through the engine, it moves through passages, the oil filter, and through the clearance gaps in the bearings. Friction and viscosity resist that flow, so squeezing the oil through these gaps takes pressure. That pressure is related to the flow rate of the oil and to the size of the passage. Small passages and high flow rates mean high pressures. If you try blowing through a small straw, like one of those coffee stirrers, it takes a lot of effort, but if you use a larger straw, it is much easier, because you need less pressure. If you blow gently through the same straw (that is, a low flow rate), it is easy, but if you want to push a lot of air through it, it takes more effort because you need more pressure to get more flow.
As with the straws, moving more oil through the same passages takes more pressure. The pump moves the same amount of oil for each revolution, so as the engine spins faster, the pump moves more oil, and the pressure rises. As the engine speed increases, the pressure will continue to rise and could get to a point where it damages seals and causes leaks. The pump uses a spring and piston mechanism to bleed off oil flow, which in turn reduces pressure and prevents over-pressurization of the oil system. The pressure exerts a force on the piston which is countered by the force from the spring. At a certain pressure, the force on top of the piston exceeds the force from the spring and the piston moves down, allowing oil to bleed back into the pan and preventing the pressure from rising any further.
For engines where more oil flow is desired, one of two things has to happen - either the restriction has to decrease, or the pressure has to increase. The restriction can be decreased by smoothing out the passages - some people refer to "porting the oil pump" or polishing the journals. It can also be done by increasing the clearance gaps on the bearings, but this can affect reliability if taken to excess. When more pressure is required, the oil pump may not be able to deliver the necessary oil pressure due to the relief valve bypassing flow. This is where shimming the oil pump comes into play. By adding a shim to the bottom of the relief spring, the preload on the spring, and therefore the force exerted by it, is increased. The increased force from the spring increases the pressure required to open the bleed valve and in turn increases the system's operating pressure. This allows more of the flow from the pump to make it to the engine, aiding lubrication and cooling. When shimming the pump, the shim should go between the spring seat and the retaining clip. This is because the spring seat has a raised portion that keeps the spring centered.
The science governing this interaction is fairly simple. The force exerted on the piston by the oil pressure is equal to the pressure multiplied by the area of the piston, or F=PA (lbf/in2 x in2 = lbf). The force exerted by the spring is predicted by Hooke's law, which states that the force is equal to the preload distance (x) multiplied by the spring rate (k), or F=kx. At the maximum operating pressure, the forces are equalized so PA=kx.
For this case, the area of the piston and the spring rate are constants, so if you know either the preload or the operating pressure, you can solve for the remaining variable (x=PA/k or P=kx/A).
The full operating specs of the Toyota 4A-GE pump have to my knowledge never been released to the public. While lots of conjecture exists, no actual results were ever documented. I sought to remedy this situation. My experimental rig consists of a modified late model Toyota 4A-G oil pump (p/n 16011-19036 - from the Smallport, Silvertop, and later -GZEs), a modified hydraulic bottle jack, and an AutoMeter oil pressure gauge.
I used the hydraulic jack to create the oil pressure applied to the oil pump relief spring and measured the oil pressure required to open the relief valve at different spring preloads. Because Hooke's law states that the spring force (and therefore the behavior of the relief valve) is linear with respect to the spring preload, I only needed two data points to construct a fairly accurate relationship to predict the oil pressure with a given shim. I took a third to verify the results. The results are below:
Run 1: no shims (stock preload is 0.222in), 64psi
Run 2: one shim, 0.047in (+ 0.222in), 75psi
Run 3: two shims, 0.110 (+ 0.222in), 86psi
One important thing to consider is that this experiment was conducted in a static scenario. In actual operation, the oil flowing through the passages will have a pressure drop due to friction. For engines with long oil runs, such as those with full-flow oil coolers, the pressure drop is greater. You should expect your actual operating pressure to be 2-5psi lower than the static results predict.
Using this chart and the equation of the trendline, you can predict the amount of shims necessary for a desired operating pressure. As an example, if the desired pressure level is approximately 80psi, using the equation from above, the calculated shim thickness is 0.09 inches. It's worth noting that all 4A- engines use the same relief spring setup, so regardless of the generation, these results should be accurate.
If you don't feel like doing math, I created a table with shim thicknesses based on the desired operating pressure: