[1]: 44 In addition to improving thermodynamic efficiency, the regenerators serve to muffle engine noise and heat, reducing exhaust temperatures.

The resulting system, which Allison branded Power Transfer, gave the GT-309 an engine braking effect and improved fuel economy at partial load.

[3] As external combustion engines, the GM Whirlfire gas turbines were capable of burning a wide variety of fuels; for example turbine engines burning powdered coal were fitted to a Cadillac Eldorado and Oldsmobile Delta 88 in the early 1980s as a response to the 1979 oil crisis.

[4][5] Other potential sources of fuel included methanol, ethanol, liquefied coal, and shale oil.

[15][16] Externally, the Turbo-Cruiser was distinguished from piston-powered buses by "turbocruiser" script lettering on the sides, blanked-out rear windows, and a large central exhaust stack at the roof.

The rearmost seats were replaced by "a complete mobile laboratory with a large [rear-facing] instrumentation panel" for two engineers.

Other planned improvements would target throttle lag, which was caused by accelerating the gasifier turbine to peak speed, and lack of engine braking.

[21] GT-304 (1956) was the first GM gas turbine to include a regenerator, which used exhaust heat to warm intake air, improving fuel consumption to 0.77 lb/hp·h.

[23]: 31–32 The GT-304 also was fitted to the first Turbo-Titan, a heavy-duty Chevrolet Model 10413 truck-tractor with tandem rear axles; Turbo-Titan was tested with various loads, demonstrating superior acceleration and gradeability compared to the Loadmaster V-8 engine that was removed, a 322 cu in (5.3 L) overhead valve V-8 with 195 hp (145 kW) output.

[18]: 346 Firebird III had a two-cylinder auxiliary power unit for accessories and a special grade retarder to simulate engine braking, which Jan Norbye criticized as resulting from "the refusal of the turbine experts to tackle the problems at the base ... these two systems seem of dubious value except in an application where cost is no object".

[18]: 358–359 The engine was redesignated GMT-305 in 1959 and further development for regular production was handed off from GM Research to Allison Transmission.

[27] The first GMT-305 prototypes began shipping in November 1959 for fitment to U.S. military vehicles, including the M56 Scorpion and a 28-foot personnel boat.

[18]: 382 The final evolution of the GM gas turbine engine was the GT-404, which was developed by DDAD and had dropped the Whirlfire branding.

GT-404 output was 325 hp (242 kW) at gasifier / power turbine speeds of 37,103 / 30,830 RPM, with a stall torque of 595 lb⋅ft (807 N⋅m) and BSFC of 0.475 lb/hp·h.

[35] In addition, it served in the electrical power generator set for the MIM-104 Patriot surface-to-air missile fire unit.

[36] The list price of the first versions, at US$9,000 (equivalent to $50,000 in 2023), reportedly was "barely adequate to cover production costs" and not competitive with conventional diesel engines.

[34]: 92  Under the demonstration program, four GT-404-powered RTS-II (T8H-603) transit buses and four MCI MC-8 Americruiser coaches were placed into revenue service for the MTA (in Baltimore, Maryland) and Greyhound Lines, respectively.

The MTA terminated its test in July 1981 after three months in revenue service;[34]: 51  by the time the study concluded in 1983, it had been placed under the auspices of NASA.

[42]: I-5, I-6  Externally, the modified RTS-II turbine buses for MTA required a boxy cover at back of the bus to accommodate the relocated air conditioning condenser, changing the profile from a slanted rear end to a boxier look.