it's the size of the engine and I think any thing over .21 is a big block. I not sure but the revo 3.3 is like a .20 but not a big block
. I think the os tz .18 turbo will run better then most .21 engines in the same price range. I don't have one myself but saw one in a revo and was like OMG that hauls butt. only point I'm trying to make is that it's not the size only that makes a good engine but a lot of other factors as well and don't just look at the number . you can swap to a bigger engine but might have to make some changes. revo has a big block install kit but you don't need it to be honest there are some great .18 engines out. I have a cen fun factor and run a .18 os cv-r and that is the only small block engine in a rc truck I have all the others are big blocks and the best engine I have is the lrp spec 3 .28. big block is just a term for a engine with a bigger stroke.