Powerflex Lower Rear Engine Mount Insert PFF1-1120BLKFor C510. 5 and C514 gearboxes. For models with TCT gearbox, use PFF80 1120. This part is inserted into the original bush, to increase stiffness. If fitting this to the Diesel or TwinAir engine vehicles, an increase in noise and vibration is to be expected. This should be considered as a performance upgrade. Road Series insert increases stiffness at 1mm by 113%, at 2mm by 173% and at 3mm by 225%Black Series insert increases stiffness at 1mm by 143%,