A good .21 is definitely enough for the SC8.

If it were me though, Alpha .23 with 0801 pipe and an 18T CB.

As mentioned above, the SC8 is basically a buggy with a truggy chassis, but it has quite a bit of extra weight to it - bumpers, cage, huge body, etc.. So engine-wise, I generally think more along the lines of what you'd run in truggy as opposed to buggy.. For ex., probably a B5 instead of a B6 if you get a Werks..

I've run .21's, 26's, and now a .30 in my SC8. Honestly, so far my favorite for all-out performance would probably be a Mach 427 with 2047 pipe, or the Argus A52 with 0801. The Mach I think was faster, but the Argus - more of a real race engine, was crisper.. - better idle, response, etc.. The Mach though is perfect for an SC8 power-wise.. The Argus could have used a bit more top-end..

An Alpha A852 would also be worth a look.. Meant to be pretty brutal for a .21.. Lots of choices.. - Werks, Nova, Alpha, OS - most meaty 21's, a .23, a .25, even a .28 - all good.. Racing though you want run-time. Which is why I say Alpha .23 - runtime of a .21, with more grunt.. Meant to wind out up top really nicely too..

Alpha 352 or 852,, 0801 pipe, and some nice clutch of your choice would all be doable for well under $400 I think.. Check out rcrenew for prices.. I think he's the main Alpha distributor guy.. Another thing about Alphas, in addition to the big power, good runtime, and ease of tuning, is they last a long time..