I know that my .18cvr doesnt really have all that long of run times, compared to some othere engine's. And the extra power has come to so beefing up of the drivetrain, not a whole lot but some where needed. And when you drive these on the pavement it's not all that good for them as there where designed for driving on dirt, or at least my xxx-nt is. If you want a .21 in a car get a 1/8th scale buggy or on road car. As said it would be cool for a littlw while but after about 2 sets of tires in a days worth of bashing because all you do is spin the tires. Well it would kinda get old