The engine mounts hold the engine in place securely but they have to stop as much of the engine vibration
from passing into the rest of the car. The 2 side engine mount are the main pair that support the engine.
The front mount controls the twisting that the engine encounters during acceleration.
There is a fine balance between support and vibrational damping. The more (or harder) rubber you use, the more
vibration is passed through to the car. The less (or softer) rubber you use, the strength of the mount is affected.
The standard mount does a good job not passing vibrations through to the car but they don't seem to last. It is very
common for 1 of the 2 supporting rubbers to split all the way through, here are 2 mounts that we removed recently.
It's worse on the CDI engine as it naturally moves more but this is also a common fault on tuned petrol models.
It takes a little longer for the standard petrol models to destroy the mount but this will happen eventually.
It will be a common problem starting from 2014.
When the front engine mount fails, the engine can move more. This decreases efficiency of the engine
as more power is used to twist the engine and this power is taken away from the wheels. It will
decrease the power to the wheels and eventually you will get a loud "clonk" noise when both
rubbers finally break and the inside of the mount strikes the outside of the mount.
Several companies sell replacement performance front engine mounts. They go from £90 + postage for the BMS mount to £180 + postage for the Smarts4You Racing mount. They both do a reasonable job although the amount of vibration passed to the car can be a bit much. They really are designed for race cars.
Well, ideally you'd be best off applying this to the mount you have on the car before it breaks but after is fine. A new mount costs about £45 but there is little point just fitting a new one as it'll go the same way as the last one.