ORIGINAL: sexyfingaz
in my opinon .21's have more bottom end power for fast take off's, most 1/8 tracks are usally reasonably tight so bottom end power is needed more than top end.
big block's (.28,.30 etc) are good for open tracks with fast straights but lack bottom end power and are also illegal except for sportsman and unlimited classes.
That depends all on porting and timing. The .28s pretty much just give you more torque because of the displacement. I don't think you can run the same kind of RPMs with a .28 then a .21 because of the size of your internals either. If you have a buggy go with the .21... it doesn't need the torque of a .28 and you'll benifit from the RPMs and fuel consumption. If you have a truggy go with the .28 because you'll need the torque.