Bumpstop mix n' match? 54 and 46mm
 

So I'm installing the FM rear mounts with my FM springs and Illuminas. I happen to have some FatCat 46mm bumpstops as well as the FM 54mm pieces (yeah, I know they're all really SuperPro or whateves). Anyhow, I was thinking of putting the 46s on the rear in hopes of just staying off them as much as possible. I was also considering the longer ones in the front because I had some fender liner rubbing when I had the 46s up there before.

Thoughts from those with experience? I emailed Shaikh but haven't heard back yet. May give him a call today.

I'd do 46 in front and 54 in the rear. Supposedly the 46's compress a bit less than the 54. The 54 engage a bit softer.

You'll want the shorter stiffer ones in the front, for the same reason that front spring rates are higher. The front motion ratio is higher in front. If you run same rate bumpstops, or softer fronts, the car will tend towards more oversteer as grip/ body roll increases.

You also want to be a bit closer to the bumpstops in front than the rear, at rest.

The FM rear mounts should allow an extra 1" of travel. Shaikh usually suggest the 58mm softs for the rear with 46mm for the front (36mm if you're using the FCM mounts). So you may want to add a 3mm shock spacer (in FCM's Koni kit page) in the rear so that all the bump stops engage around the same time. With the FM rear mounts you shouldn't worry about staying off the bump stops since they add a lot more travel.