The first few time`s I was ultra paranoid and only used spray bottles, but I saw a youtube video of a reputable detailer soaking down a 6.1 Hemi and figured if he can do it without harm so can I. Over time I started using more and more water.
My last engine bay wash I recently did I went totally banana`s (rinsed, degreased, rinsed, APC`ed, rinsed, degreased agian, rinsed, APC`ed again, rinsed) because I awas applying CarPro Dlux. I would say I`d say I used around 50 gallons of water directly in the engine bay and all I did was wrap up my alternator with two layers of plastic bags (no other prep). My air filter was off as I was cleaning it in preparation for racing (CAI K&N washable cone filter).
I spent about 2 hours applying the coating everywhere and by then it was mostly dry. Started right up without any issue. Did some WOT pulls and timing was spot on (0 short knock even at WOT).