I've gone through 1 and 1/2 motor mounts so far. Not bad for 100k+ miles I guess..
Anyway I read that if you put poly motor mounts on a cherokee you would get some vibes, actually a lot of vibes from the motor to the unibody.
Is this true? I'd like to have better motor mounts, but the dental work could get expensive.
Poly mounts on
anything are going to give you vibrations, simply because the poly is stiffer than the elastomer (it's not actually "rubber" - it's also synthetic) that is used on OEM engine mounts.
Recall that OEM parts are a compromise, and much of what is done is driven by NVH (Noise, Vibration, Harshness) because people are turning into wusses (I tend to think.) While that's okeh for a Caddy or a Lincoln - I'd expect one of those to be smooth and quiet - it's silly for a truck.
The softer you make a mount, the more torque you'll lose in compressing the thing.
Enter polyurethane. Like using poly suspension bushings, you'll have less "give" in the poly, because it is stiffer than the elastomer used in OEM and OEMR mounts (probably Viton, come to think on it. Or possibly Buna-N.) Since there's a stiffer coupling between the engine and the chassis, there's going to be more vibration transmitted through the mount. Not as much as if you were to, say, machine one out of solid mild steel (I've seen that done,) but more than if you were to make one out of sponges.
The OEM elastomer has to be able to hold up the weight of the engine, handle vibration with minimal transmission, respond to torquing of the engine, and not deteriorate in the presence of oil, grease, fuel, heat, and general crud. Poly does all of those rather better than the OEM elastomer - except for "handle vibration with minimal transmission" - stiffer means stiffer. It's really that simple - there's only so much you can trade off.