Classic Engine: Jeep's Tornado Straight-Six

The two faces of Tornado. The engine is shown here is from the J-Series Wagoneer or truck configuration with the standard paper air filter and a generator fitted.

Story by Jim Allen

Jeep's overhead-cam six, famously called the Tornado, was one of the first mass-produced overhead-cam engines built in the United States after World War II. As whiz-bang as that sounds, the engine actually has very humble roots.

Willys Motor engineer A.C. Sampietro worked out a simple plan: In 1960 he took the existing lower-end architecture from the Kaiser Supersonic/Continental Red Seal 226-cubic-inch L-head (or flathead) six and began designing a new overhead-cam engine around it.

The 226 engine had been incorporated into the Jeep lineup as the Super Hurricane in 1954, and it had an enviable reliability record. It made 105 horsepower, and when it was introduced it was a welcome addition to the horsepower-starved Jeep engine lineup. Like all flatheads, it had breathing and cooling difficulties, but the Continental-sourced engine became almost legendary for durability in Jeeps and the many other applications in which it served until 1973.

Sampietro's engine came to life in February 1961, when the prototypes were tested. By May, the engine had passed its 100-hour full-power certification tests, and the first production engines came off the line in April 1962. The 230-cubic-inch engine was dubbed Tornado, and in a strange twist of fate, the first Jeeps to have them were not the new Gladiator J-Series pickups or Wagoneer SUVs but the old Willys wagons and pickups they were designed to replace.

Although the basic architecture of the Tornado came from the 226, the blocks were considerably different, not the least of which was the lack of cast-in ports and valves used on a flathead engine and the lack of a camshaft. It was also given full-flow oil filtration, which the L226 did not have. The Tornado had the same stroke as the Super Hurricane and only a slightly larger bore. The crankshaft was more or less the same one used in the 226, but it was strengthened by Tufftriding. This was one of the first times the hardening process was used by an OEM engine builder.

Here is the power and torque graph for the engine as designed. Note the torque line. Jeep rated the engine at 210 pounds-feet at 1,750 rpm. This curve shows it achieving 210 pounds-feet before 1,000 rpm and holding above that to about 3,700. Now that's a torque curve!

The overhead-cam system was unique because one cam lobe opened the intake and exhaust valves on each cylinder. This made for a unique camshaft profile, but it allowed the intake and exhaust valves to be precisely aligned in the spherical (don't call it a Hemi) combustion chamber for optimal breathing in the crossflow head. The valves were large for the displacement — 1.88-inch intakes and 1.62-inch exhausts. A good number of parts on the engine were aluminum, including the front cover, water pump, valve cover and intake manifold. The engine weighed 575 pounds, some 40 pounds lighter than the 226 flathead.

Despite the Tornado's undersquare configuration — a 3.34-inch bore and 4.48-inch stroke – it was a bit of a revver at heart. In fact, the engine's 140 hp at 4,000 rpm and 210 pounds-feet of torque at 1,750 rpm was underrated. When Sampietro wrote about the Tornado in November 1962, dyno testing of the two-barrel engine was yielding 155 hp and 230 pounds-feet of torque (gross) as well as BSFC (brake specific fuel consumption) below 0.45 pounds/hp/hour from 1,200 rpm all the way to 4,000 rpm. Sampietro also designed a 153-cubic-inch four-cylinder version of the Tornado that was built overseas.

On the ugly side were the problems with oil consumption and leaking issues seen mostly in the engine's early days. They were relatively easy to fix, both under warranty and in production. Because of the oil consumption and owners not checking the oil until it was too late, a relatively high number of engine failures were reported. Word spread, and even though the issues were largely solved by the next model year, the bad rap followed, and it has stuck to this day. The engine's relative complexity compared with the standard American engine of the day was also an issue for the less well-trained mechanics.

The Tornado didn't die when it was dropped from the lineup in 1965. Its design and tooling was transferred to Industries Kaiser Argentina (or IKA), which also had ties with AMC and Renault. The engine was used in the Torino, a Rambler American clone. With only a few upgrades, the engine was sold at a standard 155-hp level, and an Interceptor version with three side-draft DCOE Weber two-barrels made 176 hp. An economy 181-cubic-inch version (3.34-inch bore and 3.44-inch stroke) was built as well. In 1973, the lower end was upgraded to seven-main configuration, and by the time the engine was discontinued in 1982, there were 215-hp versions floating around. It was used extensively in racing, and it even garnered a few international victories.

The Tornado also appeared in uniform. The low-compression one-barrel version was fitted to the 1967-69 Gladiator- based M-715 military tactical truck. It was mostly the same, except for the waterproofed components and the 24-volt shielded electricals. Another difference was the elimination of the front engine mounting plate, one of the major sources for oil leaks on the civilian engine. The block had bosses, and the motor mounts were situated there to suit the locations used for the civilian 232ci AMC six.

The lower end of the Tornado was unremarkable. Four-main configuration, long stroke, undersquare. The upper end was remarkable for the day, with a crossflow head and overhead cam. The high-compression (8.5:1) engines had a domed piston while the low-compression engines used flat-tops. The spherical combustion chamber allowed for good airflow. If this head were fitted to an oversquare seven-main engine, it would have been a real barn burner. In its long-stroke undersquare configuration, it had good torque with a hint of "peakyness."

The 226ci L-head, whose basic lower end dimensions became the core DNA for the Tornado. It could be argued that a better engine could have been chosen for this adaptation. Engineer A.C. Sampietro's primary mandate was to develop the engine on a tight budget. This engine was originally designed by Continental Motors, a subsidiary of Kaiser Industries.