they stiffen the mounting points of the beam where it "fish mouths" to go over the diff and the back of the trans by better distributing the clamping force of the bolts. they make a noticeable difference, more so on stick cars. I have put 3 sets on my zr1s, and even the drm car with the roll bar, I noticed the car puts the power down better and the tendancy for the car to step out under acceleration is reduced.
they make mounting the beam much easier, and reduce the tendancy for the bolts to losen up. another benefit of these is that the bolts tend to dig into the alum beam as it twists which causes them to lose torque. the plates keep this from happening!!!
bill b. did lots of testing with these and I help him market them.
I am a believer!!! but if you don't really push the car, you might not notice much and the improvement is subtle.
fyi,
on my LPE 368 ZR1, the rear mounting on the beam was so lose the engine was rocking. you will notice the shifter moving up and down under torque which is a clear cut sign that the beam bolts are lose. sometimes you can just tighten them up, but if they have started to dig in, they will quickly get lose again. previously, you would have to weld and fix the beam, or get a new one.
now you can just file the ridge off the mounting surface, and put on the plates and the problem is fixed.
best of all, they are money back guaranteed. if for any reason you aren't happy, your $$ is refunded.
I'm also going to put a set on my wife's LT1 automatic just to prevent the bolts from digging into the beam. she drives the car very ez, but I get my hands on it every now and then so the prevention is better than fixing it later.
see
www.zfdoc.com and
www.lt5registry.net (STORE) and the parts for sale thread for info. the beams are pretty slick IMHO!!!