What does "stiff" mean for engine mounts?
 

I was reading a super old thread on engine mounts about Cusco being stiffer than Perrin mounts, and that Perrin is more recommended for daily driving. What exactly does "stiff" mean in terms of engine mounts? NVH I can understand, but "stiffness" ..?

Harder to accelerate? Harder to turn? Harder to steer? Feel more bumps in the road? Decreased lifespan on parts that need to flex? :iono:

Mostly more NVH (more engine/tranny noise & vibrations transferred/heard in cabin). Would expect longer lifespan and better clearance for some parts (recalling issue with Ace headers clearance/rubbing if on worn engine stock mounts). As for steering/feeling bumps .. engine mounts are not exactly suspension parts, are they? :)

What does stiff generally mean? Apply it to the material of the mount and there is your answer.

Basically stiffer mounts = less engine movement.

All it's means is the material is harder and you generally get more noise, vibration, harshness.

These Poly's are measured with a Durometer. An instrument that measures the hardness of rubbers and plastics.

https://en.wikipedia.org/wiki/Shore_durometer

Some may be Shore A = 90 Less flexible

Some may be Shore A = 80 More flexible

OEM rubbers would be far less, I would imagine. AND, more compliant but not great for that "direct feeling"

That's all. It has noting to do will turning or anything else.

FWIW, I will be replacing all of my suspension/mount bushings with poly. Doing my homework still on the ones that are offered. Again, some are harder/softer than others.

Stiff = Less Compliance.

What you'll notice, in addition to the NVH is a potentially more direct feeling of what the engine is doing (actually, this is simply another manifestation of the same thing). Thinking back to sophomore physics - Newton's third law. For every action there's an equal and opposite reaction. This applies to engine torque. When the engine/ transmission twists the drive shaft, the opposing force twists the engine in the opposite direction - much like winding a spring. The softer the mounts, the more the engine "winds up." Stiffer mounts reduce the springiness yielding a more direct connection between throttle inputs and car performance. Similar things happen on the rear of the car with the diff mounts.

For my purposes, stiffer engine mounts don't make sense. Stiffer steering rack bushings and shift linkages do. But, in the end, everything is connected to everything.