Yes a tether is mandatory in all classes. They will NOT let you run with out one.
In improved you are allowed 1% overbore, stock head but it can be machined, pipes, reeds, any available clutches, stock suspension and tunnel. Picks, after market wheels, airbox mods etc.
You can google isr rules and you should find some information there.
They will want you to have a rear tunnel guard and a clutch guard that comes down the side. You will need to meet the minimum driver sled combo weight of 695 lbs. That is for a twin and I am going off of memory.
Let us know if we can help you out with anything.
OC
QUOTE(Xc Special @ Aug 20 2008, 11:17 PM)

will a tether switch be required for improved, ? ill be in stonewall ,
ssra ,, isr rules what and where are they on the net,