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Jaguar V12 Motor Mount Upgrade

Jaguar V12

Motor Mount Upgrade

The typical front engine, rear wheel drive car has the engine/transmission assembly mounted on three rubber mounts; one on either side of the engine, and one at the transmission.  Under most conditions, these mounts are usually in compression; the weight of the engine/transmission is sitting on the rubber.  However, with the transmission in low gear and the throttle floored, enough torque may be generated at the drive shaft to cause the engine to react the other way, which is to try to lift up on the left side.  On the typical automobile -- the Jaguar XJ-S and XJ12 included -- the only thing that resists this lifting is the fact that the rubber elastomers in the motor mounts are glued to the metal brackets they are mounted with.  When the mounts are a little old, the rubber rips away from the bracket.  Then the engine is free to lift up until it hits something, which may be the underside of the hood, where it'll leave a nice dent showing on the top.  Often a belt-driven fan will rub against a shroud, making a deafening screech.

Such a motor mount is a terrible design, and in this engineer's opinion inexcusable; rubber should never be loaded in tension on such a critical application, and it'd be almost too easy to design a mount to fit in the same location that can safely be loaded in either tension or compression without ripping apart.  The attachment on the top of a shock absorber is a good example; if the motor mount consisted of a similar arrangement of a threaded post with two rubber doughnuts, the only result of a shot piece of rubber would be some rattling, whereupon the owner could just drop by the mechanic and have the doughnuts replaced for a few bucks.  Having the body shop bang out the lump in the hood would never be necessary.

In the case of the Jaguar XJ-S, the problem can be even more serious.  There is a freon hose across the top left of the engine, and the throttle linkage consists of a bellcrank and two pullrods just below that.  When the motor mount breaks, the owner who floors the throttle may have the engine lift up until the left pullrod jams behind the freon hose.  Now the throttle will be staying floored, regardless of operator intentions.  Anyone who lives to tell about it is advised to file a lawsuit; I'll be happy to testify about the stupidity of the design.

People who race cars with these type mounts never leave them stock; they simply won't hold up under racing conditions, and a failure can be dangerous.  In some cases, they simply replace the rubber mounts with a metal attachment, but this is unacceptable in any situation other than racing because it transmits too much vibration to the chassis.  Another option is to add a fourth engine support, sometimes called a dogbone or torque link.  This is a link between the engine and chassis that is put under tension when the left side of the engine tries to lift.  It may contain rubber doughnuts so that vibration isolation is unaffected, but they are loaded in compression rather than tension, and will accept all the torque load the engine can dish out before the OEM mount is stretched far enough to threaten its integrity.  This is a complete solution.

The problem:  There's basically noplace to put such a link under the hood of an XJ-S!  So, Greg Meboe came up with a different solution: he modified the OEM mount on the left side by installing a pin through it that limits how far it can be stretched.  Some pictures of his XJ12:

The pin is welded to the top plate and passes through the mount itself out the bottom plate, through the small tan bumper, and into a self-locking nut.  In compression, this mount is free to move as before.  When tension is applied, however, the mount can only stretch so far before that tan bumper hits the bottom support, positively preventing it from stretching any farther.

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