Classic cars are much more delicate and susceptible to damage from a normal engine bay cleaning service. They were not designed to be soaked in water or to have very heavy cleaners used on them. Most of the classic cars we see are not daily driven anyway so there isn’t as much road grime buildup under the hood area but general driving will cause a build-up of dust, dirt and a small amount of debris that can easily be cleaned by hand or steam to avoid any water damage occurring during cleaning. The air intake or carburetors aren’t sealed as tightly as more modern cars so heavy water cleaning is always avoided. The rubber seals, hoses and belts can also be very fragile especially if it’s an all-original vehicle so you must avoid very harsh cleaners. The wiring also tends to be more exposed than modern vehicles so in order to avoid any electrical issues cleaning by hand or with small amounts of steam is the safest method. The chrome or metal finishes on valve covers, exhaust manifolds or air cleaner housings are also sensitive and must be gently cleaned to preserve and protect them.