12/10/2025
Adding new wrinkles to my brain today.
GMC Twin Six V12 is one of the most fascinating and formidable gasoline truck engines ever built. Introduced in 1960, this 702 cubic-inch (11.5-liter) monster was created by GMC for heavy-duty commercial use, designed to deliver immense torque and durability without the complexity of a diesel engine. What made it truly special was its construction — essentially two 351-cubic-inch V6 engines combined on a common crankshaft, sharing a massive block and crankcase to form a perfectly balanced 60-degree V12.
This engine produced 275 horsepower at 2,400 rpm and an astonishing 630 lb-ft of torque at just 1,600 rpm — perfect for moving heavy loads with effortless low-end grunt. It featured a single four-barrel carburetor, dual distributors, dual ignition coils, and four exhaust manifolds feeding into two massive exhaust systems. Despite its huge displacement, the Twin Six was smooth-running thanks to its even firing order and excellent internal balance, making it ideal for continuous operation in dump trucks, fire engines, and large commercial haulers.
The engineering behind the GMC Twin Six was impressive for its era. It shared many internal parts with the GMC 351 V6, which simplified maintenance and parts availability. The cast-iron block and heads gave it legendary toughness, and its 12 pistons worked like clockwork to deliver diesel-like pulling power on gasoline fuel. The downside was fuel consumption — under load, it could drink fuel at a staggering rate, which ultimately limited its appeal as diesel engines became more efficient and widespread.
Today, the GMC Twin Six V12 has achieved cult status among engine enthusiasts and collectors. Its sheer size, thunderous exhaust note, and mechanical beauty represent a unique period in American engineering when creativity and brute force ruled the heavy-truck world. Whether displayed in museums or roaring to life in restored trucks, the Twin Six remains a mechanical masterpiece — a bold experiment that showed just how far GMC was willing to go in the pursuit of power.
