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GMC logo.png
V6 engine
1960 GMC V12 702cid.JPG
GMC "Twin Six" heavy duty engine 702 cu in (11.5 L)
ManufacturerGeneral Motors
Configuration60° V6, V8 and V12
  • 305 cu in (5.0 L)
  • 351 cu in (5.8 L)
  • 379 cu in (6.2 L)
  • 401 cu in (6.6 L)
  • 432 cu in (7.1 L)
  • 478 cu in (7.8 L)
  • 637 cu in (10.4 L)
  • 702 cu in (11.5 L)
Cylinder bore
  • 4.25 in (108 mm)
  • 4.56 in (115.8 mm)
  • 4.87 in (123.7 mm)
  • 4.875 in (123.8 mm)
  • 5.125 in (130.2 mm)
Piston stroke
  • 3.58 in (90.9 mm)
  • 3.86 in (98 mm)
Block materialCast iron
Head materialCast iron
ValvetrainOHV 2 valves x cyl.
Fuel systemCarburetor
Fuel typeGasoline and Diesel
Cooling systemWater-cooled
Power output150–254 hp (112–189 kW)
Torque output260–585 lb⋅ft (353–793 N⋅m)

GMC Truck produced a unique 60 degree V6 engine family from 1959 through 1974, in gasoline and Diesel versions. V8 and V12 derivatives of the basic design were also produced. Examples of this engine family were found in pickup trucks, Suburbans, heavier trucks and motor coaches.

V6 engines were produced in 305, 351, 401 and 478 cubic-inch (5.0, 5.8, 6.6, and 7.8 respectively liter) displacements, with considerable parts commonality. During the latter years of production, 379-and-432-cubic-inch (6.2 and 7.1 L) versions with enlarged crankshaft journals were manufactured as well.

GMC produced a 637-cubic-inch (10.4 L) 60° V8 with a single cam shaft using the same general layout (bore and stroke) as the 478 V6. The 637 V8 was the largest displacement production gasoline V8 ever made for highway trucks.

The largest engine derived from the series was a 702-cubic-inch (11.5 L) "Twin Six" V12, which had a unique block and crankshaft, but shared many exterior parts with the 351.

Diesel versions of the 351, 478 and 637, advertised as the ToroFlow, were also manufactured. These engines had no relationship to the well-known Detroit Diesel two-stroke engines produced by General Motors during the same time period.

All versions of the GMC V6 used a six-throw crankshaft, which when combined with the 60 degree included cylinder angle, produced a smooth running engine without any need for a balance shaft. Spark plugs were located on the inboard side of the cylinder heads and were accessed from the top of the engine. This position allowed for shorter spark plug wires and kept the spark plugs away from the hot exhaust manifolds, something which was emphasized in sales literature. It was also perceived as easier to access them for maintenance. These GMC V6 engines were noted for durability, ease of maintenance and strong low RPM torque.

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The 304.6-cubic-inch (5.0 L) 305 had a 4.25 in × 3.58 in (108 mm × 91 mm) bore and stroke. The 305 was GMC's standard pickup truck and Suburban engine from 1960 to 1974, and was one of the first V6 engines produced by an American company. The 305A was equipped with a single barrel carburetor and produced 150 hp (112 kW) gross at 3600 rpm and 260 lb⋅ft (353 N⋅m) gross at 1600 rpm (measured without air cleaner or accessories in an ideal environment). GMC also made a B, C, D and E version of the 305, with the E version producing 165 hp (123 kW) gross at 3600 rpm and 280 lb⋅ft (380 N⋅m) gross torque at 1600 rpm.


The 351-cubic-inch (5.8 L) 351 had a 4.56 in × 3.58 in (116 mm × 91 mm) bore and stroke. The 351 was available as an E series (351E) and Magnum series (351M). The Magnum featured a larger 2 barrel carburetor and an open port intake, bigger intake and exhaust ports, larger diameter valves, and larger exhaust manifolds. The 351E shared the same parts as the 305 with the exception of a larger bore. In 1973, the 351 was replaced by the 379.


The 378.6-cubic-inch (6.2 L) 379 had a 4.56 in × 3.86 in (116 mm × 98 mm) bore and stroke. The 379 was a 351 with a 478 crankshaft. Net power was 170 hp (127 kW) at 3600 RPM and 266 lb⋅ft (361 N⋅m) torque at 1600.


[1] The engine was further enlarged for the 400.7-cubic-inch (6.6 L) 401. It has a 4.87 in × 3.58 in (123.7 mm × 90.9 mm) bore and stroke, and was produced from 1960 through 1972. The 401 engine produced 205 hp (153 kW) gross at 3200 rpm and 377 lb⋅ft (511 N⋅m) gross torque at 1400 rpm while the Magnum version produced 237 hp (177 kW) gross at 4000 RPM and 372 lb⋅ft (504 N⋅m) gross torque at 1600 RPM. This engine was used in the 4000 series and larger trucks.


In 1973 & 1974 there was also a 432.2-cubic-inch (7.1 L) version with enlarged crankshaft journals (4.875 in × 3.86 in (123.8 mm × 98.0 mm) bore and stroke). The 432 was a 401 with a 478 crankshaft. It produced 190 hp (142 kW) net at 3200 RPM and 336 lb⋅ft (456 N⋅m) net torque at 2000 RPM. The 432 was a Magnum engine though it was never designated as such.


The 477.7-cubic-inch (7.8 L) 478 was one of the largest V6 engines ever built. Gross output was 254 hp (189 kW) at 3700 rpm and 442 lb⋅ft (599 N⋅m) at 1400 rpm. Bore and stroke was 5.125 in × 3.86 in (130.2 mm × 98.0 mm). It was introduced in 1962 for the 6500 series trucks.


The 702-cubic-inch (11.5 L) V12 "Twin Six" was offered in United States GMC commercial trucks, and as a special order option in Canada. It was mistaken as two V6 engines welded together, but it is its own separate engine design based on a single casting.[2] It used four separate exhaust manifolds, two separate carburetors and intake manifolds, two separate distributor caps driven by a single distributor drive,[2] and other parts from the 351 V6. 56 major parts are interchangeable between the Twin-Six and the other GMC V6 engines to provide greater parts availability and standardization. It produced 250 hp (186 kW) net SAE horsepower. Torque was 585 lb⋅ft (793 N⋅m).

See also


  1. ^ gmc truck parts & illustration manual 1955-1964,1965-7
  2. ^ a b "GMC Twin-Six V12 Myths". 6066 GMC Trucks. June 1, 2007. Retrieved 2008-10-05.
This page was last edited on 19 August 2019, at 22:13
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