ENO is pronounced Ee-no, ACS is pronounced as three letters, A C S, because it's easier to say "my Ee-no freewheel broke" rather than "My E N O freewheel broke". With ACS, If you said "Acks freewheels are shit" people wouldn't really understand you, so you say "A C S freewheels are shit" instead. I understand what you're saying with GU, the fact it's in capitals means you should say Gee You, however, do you say Zed Oh Oh, or do you say Zoo? (If you search Zoo on tarty, you'll see it's written as ZOO! on every product description). MBK cant be pronounced as a word, so you say Em Bee Kay, rather than Mmmbuk, but GU can pronounced, so people say Goo. However Is also true, in no time you'll start pronouncing Zoo Zee Oh Oh.