From 1967 through 1972, General Motors produced a massive 637 CID gasoline V8—not for racing, of course, but for use in big trucks and buses. Here’s the story on this cast-iron bruiser.

When GM decided to replace its largest gasoline road engine, the giant 702 CID V12 offered by the company between 1960 and 1966 (read the Mac’s Motor City Garage feature story here), the automaker took a more conservative approach for 1967, adopting a conventional V8 configuration. Like the V12 Twin Six, this new V8 was also based on the GMC division’s heavy-duty V6 truck engine series, and with its eight cylinders, it displaced a whopping 637 cubic inches.

The 637 V8 employed the same 5.125-inch bore and 3.86-in. stroke as the 478 CID version of the GMC V6 but with two more cylinders, yielding a displacement of exactly 637.02 cubic inches, or 10.4 liters. (The GMC V6 was also offered in 305, 351, 379, 401, and 423 CID sizes over its production life.) The relatively large bore and short stroke of the 478/637 were selected to reduce piston speed and minimize frictional losses.

Since the 637 shared the V6 architecture’s 60-degree vee angle, a less than optimal arrangement for a V8, a single balance shaft was used to make the engine smooth enough for truck use. The V8 also departed from its V6 sibling in its combustion chamber design, employing a flat deck and a pocketed piston crown, similar to diesel practice and a little like the Chevrolet 348/409 passenger car engine. Compression ratio was 7.5:1, while the air-fuel mixture was fed by a single Bendix-Stromberg WW carburetor.

Though demand was falling by the late 1960s, there was still a significant market for large-displacement gasoline engines in truck and bus applications. Diesels had not yet taken over completely, especially in fire departments and mixed car/truck fleets. Though the V8 gave up 65 cubic inches to the V12 it replaced, the new unit was rated at an identical 275 hp, while torque was a respectable 560 lb-ft. The V8 was also more than a foot shorter than the Twin Six, and thus far more practical in the packaging department. And while the 637 was hardly dainty at 1,219 lbs, it was considerably lighter than the previous unit, which tipped the scales at nearly 1,500 lbs fully dressed.

The GMC V6 engine family also featured a parallel four-stroke diesel line built on the same basic architecture. Known as the Toro-Flow series, these diesels were offered in 351 CID and 478 CID V6 models and, logically enough, there was a diesel based on the 637 CID V8 as well. Utilizing an American Bosch distributor-type fuel-injection system, the D637 was rated at 195 hp and was offered at times in both Chevrolet and GMC heavy-duty trucks. Though it was noticeably less powerful than the gasoline version, the D637 was also less thirsty.

The ultimate version of the 637 CID V8, if you will, was the DH637 twin-turbocharged diesel, which used a pair of turbochargers (each labeled no. 9 in the illustration above) feeding a common plenum chamber (no. 4). Output was a rated 220 gross hp at the governor-controlled maximum speed of 2800 rpm, with 458 lb-ft of torque available at 2,000 rpm.

GMC and Chevrolet truck buyers had a wealth of engine options available in the 1960s, from the venerable Chevy 6 and V8 to the famed 53 and 71 series Detroit Diesels. Among these far more popular engines, neither the gasoline nor the diesel versions of the 637 were ever very common, and they’re even more rare today. The gasoline-burning version of the big V8 is most often found in retired fire equipment. When the Chevrolet Mark IV big-block V8, a more modern and efficient package, became available in heavy-duty truck versions of 366 CID and 427 CID, that was the end of the line for the 637 gasoline V8, and the 1,200 lb brute was discontinued for good in 1972.