I just checked the TCS rules and at least for this year the Silver Can is specified for Mini, so I'll guess they're what'll be used at Nats.
Hey Scottrik,
Here's the deal on motors in Minis per the rules for TCS.
You are required to run a sliver can motor, per the TCS web site, for all of the Regional Events.
At the Nationals, which I am leaving for shortly, they have "Hand Out" black can motors and a spec, Orion 3300 V-Max, battery that you must run. The motors are numbered teched and you only get them when you are at the track. No taking them home and working magic on them.
The idea is to make the racing fair for everyone, if you win and go to Japan you will be racing a black can out there.
The black cans are a little faster, 18-20,000 RPM verses 12-13,000 RPM and have a little higher amp draw, 1.7 to 1.9 verse 1.0 to 1.3 on my Indigy motor checker. BUT they don't last any were near as long, like Tip Toe said when it's hot they just seem to go flat.