Engine Mounts Keep Things Quiet, Make For A Longer-Lasting Vehicle

By Gary Goms

Although engine mounts normally last well over 100,000 miles, they should be inspected if unusually high levels of chassis-borne vibration and noise are present.
During the early 1930s, the Dodge Brothers revolutionized automotive design when they introduced the concept of “floating power” in their line of automobiles. Unlike the magnification effect of metal-to-metal engine mounts, the Dodge Brothers’ rubber engine mounts minimized the reciprocating, rotating and crankshaft torsional vibrations produced by a piston engine. The result was a much quieter, smoother-running and longer-lasting automobile.

Engine Mount Failures
Unfortunately, the effects of heat, oil and environmental exposure eventually reduce the ability of any rubber engine mount to dampen vibration. In addition, torsional stress tends to separate the mounting rubber from its metal attaching plates, which allows the engine and transmission to shift position in the vehicle’s chassis.

During the early days of mechanical clutch and cable throttle linkages, the first indication of worn engine mounts was a continual need to adjust both linkages. Broken engine mounts on any conventional rear-wheel drive chassis are likely to cause the engine-mounted fan to contact the radiator shroud or allow the engine’s exhaust pipes to contact the frame.

In contrast, broken engine mounts on modern transverse-mounted engines aren’t as obvious because they are attached to a sub-frame called an engine “cradle” which, in some cases, is attached to the body by its own set of rubber mounts. In addition, a torque strut located between the engine and the body reinforces the conventional engine mounts by absorbing engine torque.

Nevertheless, broken engine mounts or torque struts in transverse engine installations can aggravate a condition known as “torque steer,” in which the vehicle’s steering will pull in one or the other direction as the vehicle accelerates. Last, excessive fore-and-aft movement caused by a broken engine mount or torque strut on a transverse engine installation can cause the exhaust flex pipe located between the engine and catalytic converter to break during acceleration.

Types Of Engine Mounts
Early engine mounts were manufactured by bonding a rubber pad directly to a top and bottom metal plate. With the advent of more powerful engines during the 1960s, a number of cases of unintended acceleration occurred due to engine torque actually holding the mechanical throttle linkage wide-open when the mounts separated. Since the 1960s, most engine mounts are designed as “captured” mounts that mechanically prevent an engine from rotating in the chassis if the mount separates.

Most recently, a few import and domestic auto manufacturers offer active hydraulic engine mounts in their high-end vehicles. Active hydraulic mounts dampen engine vibration by using electrically controlled orifices operating in a closed chamber filled with a viscous fluid to control engine movement. During lower engine torque loads, both large and small orifices remain open to dampen engine vibrations. During high engine torque loads, the large orifice is closed to better control engine torque.

A more leading-edge design contains a magneto-rheological (MR) fluid that increases viscosity when exposed to a magnetic field. As with conventional active engine mounts, the fluid flows through a fixed orifice in a closed chamber. The engine management system then changes the dampening capacity of the engine mount by electronically modifying the magnetic field surrounding the MR fluid.

On a more basic level, some aftermarket manufacturers offer captured polyurethane engine mounts designed for high-performance applications. For safety’s sake, polyurethane engine mounts should be recommended for extreme operating conditions such as drag racing or off-road competition.

Selling Engine Mounts
Although engine mounts normally last well over 100,000 miles, they should be inspected if unusually high levels of chassis-borne vibration and noise are present. Similarly, if the engine exhaust or accessories are rubbing against the frame or sheet metal, suspect a bad engine mount.

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