;501-D10: The initial civil variant, which was proposed in 1955 with of power at a
brake specific fuel consumption (BSFC) of , a two-stage gearbox with a reduction ratio of 12.5:1, a 14-stage
axial flow compressor with a compression ratio over 9:1, a four-stage turbine, and a , three-blade Aeroproducts A6341FN-215 propeller. ;501-D13: (Series I) Commercial version of the T56-A-1 used on the
Lockheed L-188 Electra, but using
kerosene as the primary fuel and
JP4 as the alternate (instead of JP4 as primary and
gasoline as secondary), and with the gearbox reduction ratio increased to 13.54 from 12.5, which lowers the propeller blade tip speed by 8 percent to for the Aeroproducts 606 propeller; power rating at sea level takeoff, 14-stage axial compressor, 6
cannular combustion chambers, and 4-stage turbine; 13,820 rpm shaft and turbine inlet temperature; certified on September 12, 1957. ;501-D13A: (Series I) Similar to the 501-D13 but using a
Hamilton Standard propeller; certified on April 15, 1958. ;501-D13E: (Series I) Similar to the 501-D13 except for the location of the rear mount; certified on December 18, 1959. and the
Convair CV-580. ;501-D22E: Offered in 1979 as the initial engine for Lockheed's proposed L-100-60 (a stretched derivative of the
Lockheed L-100). ;501-D22G: (Series III) Similar to the 501-D22C but with power rating at sea level takeoff, a three-mount system, auto-feathering, and no water-methanol injection; certified on March 23, 1984. ;501-D39: (Series IV) Offered for the
Lockheed L-100 civil aircraft, starting in 1979 for the proposed L-100-60 as the successor engine to the 501-D22E, producing with propellers; ;501-H2: Engine for the proposed Vanguard Model 30
lift fan aircraft that was entered in a 1961
vertical takeoff and landing (VTOL) transport competition; powered two fans within the wings and two propellers; used a modified compressor for handling larger air flows. ;501-M7B: Replaces the T56-A-7 on an experimental
short takeoff and landing (STOL) version of the Lockheed C-130E (internally designated as the GL298-7) targeted in 1963 for the
U.S. Army; power increased by 20% over the T56-A-7 due to lowering of the gear reduction ratio from 13.54 to 12.49, propeller blade changes to take advantage of the higher resulting propeller rotational speed, and a new turbine with air-cooled first and second-stage vanes and first-stage blades, so the turbine inlet temperature can be increased from for the T56-A-7 to ; a rate engine that is restricted to and about of static thrust on the STOL C-130E, but is capable of thrust at full power and with a larger, propeller. ;501-M22: Internal designation for the T56-A-18; submitted for FAA certification under a new type certificate. ;501-M24: A demonstrator engine started in 1964 that was later used to derive the 501-M62B engine developed for the XCH-62 helicopter. ;501-M25: A four-stage fixed turbine engine similar to the T56-A-15, but with a increase from the T56-A-15's maximum turbine inlet temperature rating of , and a variable geometry compressor for the inlet vane and the first five
stator vanes; investigated in 1965 to power helicopters with a
maximum takeoff weight (MTOW). ;501-M26: A similar to the 501-M25 but with a
free turbine instead of a fixed turbine, and a two-stage gas producer turbine; ;501-M34: A turboshaft engine targeted for a 60-70 seat commuter helicopter proposal from Lockheed-California in 1966. ;501-M56: Engine candidate for the turboprop version of the Air Force
A-X close air support aircraft, requiring of engine power. ;501-M62B: An internal designation for the engine that became the T701-AD-700 turboshaft, which weighed and was intended to power the
Boeing Vertol XCH-62 heavy-lift helicopter; 15 engines built, 700 hours of component testing, and almost 2,500 hours of engine development testing completed before the helicopter project's cancellation. ;501-M69: Engine proposed for transport-type offensive anti-air (TOAA) aircraft versions of the P-3 Orion (stretched derivative) and C-130 Hercules; rated power of , equivalent installed
thrust-specific fuel consumption at cruise of . ;501-M71: A derivative of the T56-A-14 evaluated by NAVAIR in 1982 to achieve 10% lower fuel consumption, 24% more horsepower, smokeless exhaust, and greater reliability. ;501-M71K: (Series IV) A engine using a larger propeller to power the
Lockheed L-100-20 (L382E-44K-20) High Technology Test Bed (HTTB) for
short takeoff and landing (STOL) starting in 1989, but was destroyed when the HTTB became airborne during a ground test on February 3, 1993. ;501-M78: A demonstrator engine for
NASA's Propfan Test Assessment (PTA) program. It had a modified
reduction gearbox that reversed the direction of rotation and increased the output speed from 1,020 rpm to 1,698 rpm. The engine was attached to an eight-bladed, , single-rotation Hamilton Standard SR-7L propeller. Shown as an engine at the 1983
Dayton Air Show, the 501-M78 was flight-tested on a
Gulfstream II aircraft beginning in May 1987. Various flight and ground testing programs were carried out on the engine testbed through June 1989. ;501-M80C: Also known as the
T406-AD-400, a turboshaft engine. primarily based on the T56-A-427, but with a
free-turbine turboshaft added to the single-spool engine; used on the
V-22 Osprey tiltrotor assault transport. ;PW–Allison 501-M80E: A
contra-rotating geared
propfan engine derived from the 501-M80C/
T406 turboshaft engine and intended for use on a 92-seat version of the proposed
MPC 75 regional aircraft; developed jointly with
Pratt & Whitney. ;501-M80R3: A turboprop engine offered as an equal partnership between Allison and Pratt & Whitney to power Lockheed's proposed successor to the P-3 Orion, which was developed for the U.S. Navy's long-range air antisubmarine warfare (ASW) capable aircraft (LRAACA) program. ;501-M80R33: A propfan engine studied for the
MPC 75 that was based on the
T406 core and rated at . ==Industrial and marine variants (501-K)==