460 V8. Really nice looking example of the model for someone who wants one. https://chicago.craigslist.org/chc/cto/d/1977-mercury-grand-marquis/6564383482.html
My grandpa had a dark blue Mercury Marquis like this (his was older) my brothers were very little when he had it, used to call it big blue. Grandpa got it fully loaded, salesman said he needed the big 460 to control the power everything... Grandpa drove truck his entire career... Never went past 55mph... Always taking it in to the shop for tune ups because he'd never blow out the carbon... Went from loving it to hating it and traded it in for a Mercury Monarch IIRC.