[eschaider]

A ¼ inch air gap will minimally produce heat damage on the adjacent painted surface and will certainly come into physical contact with anything that close just because of normal engine movement in the motor mounts under normal driving conditions. What we euphemistically call spirited driving will produce mechanical damage.

Open the hood on your car and blip the throttle on your engine. Watch how far the engine moves — that's in neutral! Imagine what happens when the tires are hooked up! You need to provide more clearance than ¼ inch, or you will experience physical contact between the pipes and whatever they are closest to.

Solid mounts will provide the least engine movement (not zero, but the least). They will also produce the least desirable driving experience because they will transmit all the engine vibration to the chassis. That vibration will, in turn, be transmitted to everything attached to the chassis or in the car (you?) and, in time, turn your car into a rattle trap. Did I mention it will make the driving experience less attractive and more numbing the farther you drive the car.