Certain silicon type things have different hardness scales, I've taken these quotes from various websites. Note that the first shore A scale numbers are for Devcon Flexane which has a 60, 80 and 94 hardness. The Sikaflex hardness numbers are at the end of post.
There are a number of firmness or "durometer" levels offered. These are:
["60A - For the Daily Driver - This is the softest formula offered that will still make a noticeable difference in vehicle performance. This softer formula is best for tuners that still want to increase performance without trading off as much personal comfort.
80A - Even trade off between performance and vibration when it comes to motor mount inserts. This formula is the most popular for it's ability to significantly increase vehicle performance while still allowing some small amount of flex in the mounts to keep vibration and noise to reasonable levels
94A - Race Quality - Not for the daily driver. This formula is the hardest of the 3 available. Providing the best reduction in motor movement - this version will also allow a rather significant amount of engine vibration to be transmitted into the vehicle.
http://www.energysus...unt-Inserts.asp
Window weld has a rated 55A hardness. Commercial mounts have a hardness rating of 60A for street vehicles. So the vibes shouldnt be too bad. The 75A are recommend for street and track vehicles that make more than average horsepower. This rating of bushing is also for those that would like a stiffer feel and less engine flex. The 85A Grey bushings designed specifically for race cars or drivers that seek a solid engine mount kit.
The thread originated here on ClubRSX."]
Sikaflex is probably the most readily available polyurethane for us to find and is probably the most cost effective
Sikaflex; 291 = 45-50 shore A
227 = 40 shore A
252 = 55 shore A
255 = 60 shore A
It comes down to personal preference, DIY gives someone the options to get the right hardness for what they want AND it's cheaper than OEM or aftermarket
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