Now would be a good time to read the site Disclaimer, since there is a strong bias among Z owners against pressure washing the engine bay. I have pressure washed mine at least twenty times without any problems.
I have pressure washed every engine on every car I've owned (at least twenty used cars by now). I won't debate whether it's necessary or not. I would rather work on a clean engine than a dirty one. And it looks nicer. Those are my reasons. Back in the early 90's, I had a problem ONCE after pressure cleaning an engine in my old Datsun 210. I didn't leave the engine run while cleaning. Afterwards, the car would turn over but not start. Two minutes later and a squirt of WD-40 under the distributor cap and the problem was solved. When I first got my Z she was in great shape except the prior female owner had never touched anything under the hood. The engine was black with a ten-year build up of grime.
I developed a routine where I would stop at the same self-service car wash on my way to my GF's house in another town once a week and pressure clean the engine. Sometimes I would go back the next day and repeat. I cleaned my Z engine this way more than TWENTY times. I used GUNK every time, and one time I even used EASY-OFF Oven Cleaner (I don't recommend the latter, removes paint, lol). The car wash also had an "engine cleaner" setting for the pressure wash that I used.
HOWEVER, I was not cavalier about it. I never forced water into any crannies or electrical parts. The instructions on the can of GUNK state to not use on a running engine, but I always kept my engine running. Hint, if the car starts missing or chugging during the pressure cleaning, don't spray water on that spot anymore! After each wash, I sprayed WD-40 into all the obvious connectors to displace the water. This took an extra five minutes at most. Plus, I kept all my connectors packed with dielectric silicone grease.
Notice "Waterproofs" and "Prevents Corrosion" on the label.
I am not suggesting anyone use a commercial extreme high-pressure washer, the kind that can take off paint, or remove flesh from bone. I used the overhead wand pressure washers typically found at a self-service car wash. Since these are readily available to all idiots at large, I would guesstimate they are generating less than 1000 psi at the nozzle tip. For comparison, Sears' most powerful pressure washer with a Briggs and Stratton gas engine will make 3400 psi.
Cleaning the engine bay by hand wasn't practical when I first got the car. The engine was just too dirty. Once I got the engine looking the way I wanted, I stopped pressure cleaning it. Last time I pressure cleaned my engine was in 2004. There were no long term problems.
These engines are not that delicate. A little common sense when cleaning goes a long way. If a person doesn't want to pressure clean their engine then don't. Get out your toothbrush and have at it. But it can be done safely.
I talked to a Nissan technician about this once. He told me that Z's would get towed into the dealer because of failing to start after a pressure wash. He said this fix worked on the last ten Z's in a row he saw. Remove the coil packs. Suck all the water out of the spark plug wells with a Shop-Vac. Leave car in sun with hood open until dry. Replace coil packs. Start car.