The smallest V-12 in this trio displaced 340 cubic inches, and the largest had a capacity of 544 cid. In between, a 398-cid V-12 showed the greatest promise. Like the other two, the 398 kept the five-inch bore centers that all Franklin engines used after 1922. It also had the same bore and stroke (3.25-4.00 inches) as an earlier-production Franklin six.
Engineer John Rogers, in studying Shoemaker's V-12 designs, soon discovered that when it came to building actual running V-12 prototypes, there was no room inside the 60-degree vee for lifters and pushrods.
The intake manifold and cooling fins took up too much space. So he ended up with two camshafts nestled low in the crankcase and pushrods running up the outsides of the cylinder barrels.
Most vee-type engines of the late 1920s and early 1930s used updraft carburetors. The problem with this system is that, in an air-cooled vee engine, an updraft carburetor would have interfered with the cooling airflow.
Just before Shoemaker left Franklin, several carburetor manufacturers announced downdraft carburetors, so he chose a Stromberg two-barrel downdraft carb that stood above and outside the V-12's cooling system. Shoemaker took air from the fan cage, brought it up through a small, flat duct into an air cleaner, and then to the mouth of the carburetor.
Franklin advertised this intake system as "supercharging," but it wasn't supercharging in any conventional sense since it delivered a very weak stream of forced air that had very little effect on performance.
According to Tom Rasmussen of Odyssey Restorations in Minneapolis, Minnesota, Franklin's induction system might have increased power ever so slightly at low rpm, but at high rpm, the flat, narrow duct between the fan and the carburetor actually restricted airflow.
Rasmussen says that in his experience, the V-12 delivers eight to 10 more horsepower at high rpm with the "supercharger" ducting removed. The factory rated the 398-cid V-12 at 150 bhp at 3100 rpm.
To equalize cooling to all 12 cylinders, engineer John Rogers installed thermocouples to measure the heat at various locations on the heads and cylinder barrels. In a typical air-cooled engine, up to 75 percent of the heat exits through the head, while much less leaves via the cylinder barrels.
Thus, the Franklin V-12 took a lot of experimental work to determine the best size, location, and spacing of fins on the heads, plus trial and error with cooling-passage baffles to provide equal volumes of air to all cylinders.
Normally, the rear cylinders get the greatest volume of air, which "packs up" against the engine cover and then flows forward toward the front. Franklin engineers placed a small wedge-shaped baffle near the fore section of the vee to equalize airflow to all cylinders.
At 60 mph, the crank-mounted, 15-inch cooling fan shoved 5720 cubic feet of air per minute through the vee of the mighty Twelve. But even after the engineers optimized airflow, oil in the crankcase still heated up to around 340 degrees, this despite a 10-quart sump.
The concern was engine durability and longevity, so in the final test version of the V-12, Franklin engineers mounted a little oil cooler in the airstream. The cooler dropped oil temperature to around 240 degrees.
The engine was made up of a cast aluminum crankcase with an aluminum oil pan, individual chrome-nickel iron cylinder barrels and aluminum heads with ni-resist valve inserts. The heads and cylinders were liberally finned. Three test engines were built, of which two were installed in "mules" or prototype cars. Each of the mules was made from a stretched-wheelbase 1932 Airman.
Go on to the next page to learn about the new Franklin Airman.
For more information on cars, see: