it depends on how much grease,oil and dirt etc we are talking about.if theres is quite a lot of grease baked on the engine bay definitely one has to use a stronger ratio (4:1) 4 parts water and 1 part super degreaser.As Shafi bhai mentioned agitation will help quicken the job and will result is better cleaning.
so what u have to do is,spray super degreaser liberally .wait for a minute.then start agitating making sure u reach down as far as u can.
then wash away.
use a trim dressing to protect the plastics.I used DC trim Protectant.