Why Do the Oil Pump Gears Fail?
Every mechanical device is destined to fail with any amount of use; the time to failure though depends on the rate of usage or abuse. One mechanical device that tends to fail on the Ford Mod Motor is the oil pump gear. Granted, the stock gear is perfectly adequate for normal engine operation; the issue is only common when dealing with hi-performance engines. The question we receive time and time again: “Why do the oil pump gears fail?”
How the Oil Pump Gear Works
The oil pump used on the Ford Mod Motor is a gerotor design with an outer and inner gear setup. When the gears rotate, the intake side separates creating a void between the gears, which creates a vacuum that sucks in the oil. The oil is then moved to the discharge side through the gear motion. On the discharge side, the gears come back together squeezing the oil out. Tight tolerances and rigid design allow it to pump high viscosity liquids (e.g. Oil) at high pressures. In the Mod Motor the oil pump is driven directly by the crankshaft rotation and therefore, subject to the force oscillations of the engine.
The Forces on the Oil Pump Gears
Neither power nor RPM is the culprit as neither equates to the actual forces within the engine while under operation. Force alone only explains part of the issue because torque (force with leverage) aggravates the conditions as the engine torque output increases.
The first component to examine is the engine crankshaft. On the Mod Motor the crankshaft has 4 phases per revolution with peak pulses of force every 90 degrees of crank rotation. Visually this is comparable to the human heart beat with every peak being a pulse (thump, thump, thump, thump). The blower, depending on the model it may have 5 pulses per revolution of peak force. Finally, the oil pump itself has 8 pulses per revolution of peak force. All of these are normally out of phase, but it is possible for them come into phase or close enough to increase the vector components of acceleration and velocity.
Acceleration has a time component and the above pulses are of very short duration (rapid acceleration and declaration). There is also distance (the clearance between gears/crank in this case, which seems small but things are relative) to calculate which, gives velocity and its derivative acceleration. The rapid change in acceleration can be defined as jerk or lurch:
Jerk is an extremely important component in precision mechanical operations, such as the operation of a combustion engine. Ford engineers would have spent considerable time working on reducing the jerk from normal operation (predictable behavior).
Abnormal Operation and Unpredictable Behavior
On a stock engine, under normal operation, the crankshaft has predictable angular forces and thus torque. Adding abnormal operation (i.e. bigger blower, pedaling the car, on-off traction, 2 step, bouncing off the rev limiter, hole-shots, flat shifting etc.) causes excessive jerk (quick acceleration changes during the pulses) allowing forces to exceed the limitations of the engine components. In the case of the Mod Motor, timing components break, oil pump gears fracture, crank snouts break/crack, crank keys fail, etc.; anything with a mass and angular velocity can be compromised.
Ford engineers designed the oil pump gears to survive under normal operation for as long as possible. They did not design the gears for hi-performance or race engine applications. A stock set of Mod Motor oil pump gears are manufactured from powdered metal, which is quite strong by itself, but it is also very brittle and unable to sustain impacts (instantaneous changes in force).
The solution is to use gears that are constructed from a material that is not as brittle (softer) such as 10xx steel or 4340 steel. These materials are more expensive to manufacture and requires very accurate machining and surface grinding (100 millionths) versus simply pouring powder into a mold. In normal use the Ford gear will last forever as it wears well versus 4340 gears which wear much faster when left unhardened.
The Bottom Line
This market is not standard operation; today’s hi-performance applications are pushing the limits of engines more and more. Ford engineers have taken the time to account for what happens during normal operation, but they have no control over owner operation or the conditions created by modifications.
Real World Examples
The photo to the right shows a crank snout that has been completely destroyed by hitting the rev limiter. As the engine blew past the soft limiter in the tune, it proceeded to bounce off the hard limiter. This action caused repeated acceleration and deceleration in short intervals (jerk). Since the crankshaft has angular momentum it was compromised by quick changes in force.