Umm, locking differentials are for going off-road and rock crawling. You are probably thinking of an Limited Slip Differential (LSD) which will engage both drive wheels rather than creating a one wheel burn out. LSDs are available through several manufactures. As far as the injector question goes, if you are non-turbo then factory should be quite sufficient. If you are not sure, you could always visit RC Engineering website (www.rceng.com) and use their formula to figure out the optimal injector size for your applicaton.