BkoolB3,
The T4 case is alum and that is one plus that it has over the T1 Mag case. There is no set standard as to when to go to a alum case. If you run a turbo or power added that increases cyl PSI to a point where cyl studs atart to pull a alum case is the way to go.
I have had a few Mag cases get damaged from hard driving, if i had used a alum case i think this damage would not have happened. Most of all worn lifter bores, i had my mag case sleeved with alum so they will not wear out again.
You can go either way and have just as many happy miles from each. Some people want the peace of mind and or the cool factor of the alum case.
geoff.