702ci Thunder V12 GMC\Evansville, IN
And now…something completely different. Back in the early ’60s, GMC built a run of monster, cast-iron V12 engines intended for use in gasoline-fed, over-the-road truck engines. The GMC engine that propelled this craziness—a 351ci V6 intended for early pickups—was a brute in its own right. These were big, 4.56-inch bore and 3.58-inch stroke engines displacing more than 58 ci per cylinder. GMC then decided to cast a huge, one-piece, iron, 12-cylinder block that mounted four separate iron heads and two intake manifolds. The displacement from this FrankenJimmy effort became 702 ci, but the truly massive number is its scale-bending 1,485-pound mass! These engines experienced a limited production run between 1960 and 1965, and Thunder estimates there are less than 200 of these engines still around. But that didn’t prevent owner Bob Walker from scouring the countryside in search of these 60-degree behemoths. He has rebuilt several of these engines, offering both parts and completed engines. As a turnkey engine, it will set you back roughly $18,000, and if you are building a 1/10th-scale replica of the Titanic, this might fit in the engine room. For more, hit ThunderV12.com.
1. Thunder considers 5,000 rpm as the absolute maximum for these engines. As such, they are best defined as low-rpm torque monsters. Thunder rates its engines at 425 hp at 4,200 rpm and a minimum of 630 lb-ft of torque at 1,800 rpm*.
2. The GMC V6 and V12 engines used a unique bellhousing pattern, but Thunder has developed an adapter that allows the user to bolt up a whole series of GM-style transmissions such as a Muncie four-speed, TKO five-speed, TH400, 4L80E, and even the new 6L90E.
3. Once you get past the massive girth of this GMC V12, you might notice the spark plugs are located up by the intake manifold rather than down near the exhaust. Also, note that Thunder has converted these engines to a waste-spark, distributorless ignition. Induction is handled by a pair of 500-cfm, two-barrel Demon carburetors.
4. That’s a one-piece GMC iron-block that contains the forged, 180-pound crankshaft, four-bolt mains, seven main journals measuring 3.124 inches in diameter, and pistons fitted with 1.24-inch- diameter wristpins. The cam is located much higher in the block than in most 90-degree V8 engines, which shortens the pushrods and makes the valvetrain far more stable. The four cylinder heads are held in place with a total of 56 head bolts. Imagine doing three passes to torque the bolts—that’s 168 reps with the wrench!
5. One thing that may not be obvious by looking at these engines is that the power is incredibly smooth. With a 90-degree V8 engine, combustion occurs every 90 degrees (8 x 90 = 720 degrees). While most V8 engines can be very smooth, consider that with a V12, there is a combustion event every 60 degrees. The closer the combustion events occur, the smoother the engine will operate. This is also why inline-four cylinders tend to shake more than a V8.
If the GMC has an Achilles heel, it is the oiling system. Thunder remedies this with a custom, externally mounted Peterson oil pump plumbed with a combination of -10 and -12 hoses that connect to a custom oil pan. According to Thunder, these engines are incredibly durable, with stationary applications running for more than 200,000 hours of service, including rebuilds. That’s the equivalent of almost 23 years of continuous running time.