The 1977 Delta 88s and other GM B-body cars were considerably downsized from their predecessors in length and wheelbase (116 in (2,900 mm) – the same as the four-door 1973–77 A-body Cutlass Sedan) and nearly 900 lb (410 kg) lighter in weight, with curb weights dropping to between 3,500 and 3,600 lb (1,600 and 1,600 kg) depending on model. Other than a reduction in shoulder room, however, interior room was not adversely affected; in fact, headroom and rear seat legroom increased. Both base Delta 88 and Royale models were now only offered in two pillared body styles; a two-door coupe and a four-door Town Sedan. The 1977–1979 Custom Cruiser was now based on the Delta 88's B-body rather than the Ninety-Eight's C-body (thus also sharing the coil spring suspension, rather than the multi-leaf spring) and came with a two-way tailgate rather than the clamshell of 1971–76 models. A fuel economy gauge was optional.
The standard engine was now a 231 cu in (3.8 L) Buick V6 with a Turbo-Hydramatic 200 automatic transmission. An Oldsmobile 260 was the base V8, followed by either a Chevrolet 350 (option code LM1) or Oldsmobile 350 (option code L34). Initially, the Oldsmobile engine came in California and high-altitude cars, the Chevrolet engine came in cars with Federal emissions equipment. The Olds engine returned later in the model year. Oldsmobile's new 403 was the top engine option and came with a THM350 transmission. During this time, GM used GM or Oldsmobile engines in Oldsmobile cars. This caused a situation known as the "Chevy-mobile" affair. GM settled with some Oldsmobile owners by offering them warranty extensions for the Chevrolet engined Oldsmobiles, or the option of returning those cars in exchange for an Oldsmobile with a genuine Rocket V8. The "return car" option wasn't used much, because the owner had to give GM money based on the mileage of the Chevy-mobile, which could become expensive. This began the era of "corporate" engines, and for many years GM advertisements would include a disclaimer stating '"Oldsmobiles (or other divisions) are equipped with engines manufactured by various GM divisions, subsidiaries and affiliates worldwide."'
As in previous years, base Delta 88 and Royale models differed mainly in exterior and interior trim. Base Deltas had a full bench seat available in cloth-and-vinyl or all-vinyl upholstery, while Royales had a notchback bench seat with armrest or optional 60/40 notchback bench, also available in cloth-and-vinyl or all-vinyl trim. All 88s featured an all-new instrument panel with a horizontal sweep speedometer and heater/air conditioning controls moved to the center of the dash above the radio from the left side of the dash, and continued with the "Message Center" bank of warning lights. The new dash was highlighted with woodgrain trim. The dimmer switch moved from the floor to the turn-signal lever.