The first piece of engine-performance-improving equipment for a Jeep I ever saw was an aluminum cylinder head designed by Vic Hickey in the early '50s before I had a driver's license when I was just a kid. It was the coolest thing on a Jeep I had ever seen. Later on, Brian Chuchua, also known as Mr. Jeep, sold a similar one with his name on it that was still listed in his 1971 catalog. Dual carburetor manifolds in the '50s were available for just about any engine you could imagine and an old-time company, Burns, made a log-type manifold that utilized two Stromberg Ford V-8 carburetors for the Jeep four-cylinder. I'm sure these helped the four-cylinder flathead's performance, but, still, the long-stroke 134.2ci engine needed a lot more than this. Based on a modified Whippet engine from 1926, the Go Devil struggled to put out all of 60 horsepower at 3,600 rpm and a questionable 105lb-ft of torque at 2,000 rpm. The bore/stroke ratio, or should it be called the stroke/bore ratio, was way out of place when compared to a modern engine. The extremely long stroke of 4 3/8 inches was a full 1 1/4-inch more than the bore, so along with the very heavy rods and deep skirted T-slot pistons, rpm potential was limited. The oiling system was also lacking. The only thing one could do was open up some of the passageways, increase the crank clearances a bit, make sure the oil pump clearances were at a minimum, and increase the pump's relief valvespring tension with either a stronger spring or shims.
When I built my first performance Jeep engine in a '43 MB, I used the timing gears and oil pump from a civilian four-cylinder along with the stronger rods that included larger cap bolts as well as the later crankshaft. The block was ported and relieved around the valves for better flow -- well, as best as I could with a grinding stone on the end of a 3/8-inch drill. Offenhauser made a special adaptor to use the Ford flathead V-8 Stromberg carb which worked good going up and down hills, but side hills were not in its favor. I never figured the compression ratio, but the head was milled a considerable amount. With a 16-year-old driver, this engine didn't last long, and when we pulled it apart we found a hole in a piston and the crank was broken at the center main.
The next step was to swap in a later model F-head engine. It wasn't technically an engine swap because it was a direct bolt in. Still the same basic engine, but a new cylinder head design with the intake valves upstairs in the head instead of the block. Depending on the compression ratio, which ranged from 6.9 to 7.8 (depending on year and usage), it was rated at 72 to 75 horsepower at around 4,000 rpm and torque was up to 114lb-ft. This one got a balance job, some performance pistons from Speed-O-Motive, and a Holley carb from a Falcon six. My new performance motor maybe made 100 horsepower on a good, damp day.
Harry Buschert, who owned a farm implement repair shop in Hemet, California, was a real innovator in design. He built up a very-modified, four-cylinder F-head that even had a Paxton Blower that he had salvaged off a Packard. I figured that some day I would have to duplicate this because I knew just where to find a similar blower off a Packard that we had cut down for a dune buggy, or perhaps it was a doom buggy. However, it never happened. (Paxton, which at the time was owned by the Studebaker Corporation, later offered a kit to supercharge your Jeep or Scout four-cylinder engine.) Harry also modified the stock four-cylinder exhaust manifold into dual outlets. As I look back on it, I doubt that this exhaust modification helped any. It maybe even hindered performance a bit, but it sure did sound different.
Jump a couple of years to 1962-1963 and a new CJ-5 for $2,242 that included tax, licenses and a white top, but still had the four-cylinder F-head engine. The Paxton blower was no longer available to me, but performance mods were a Chevy six carb, a milled head, several different styles of homemade headers, a magneto from an Jeep industrial motor, Packard 440 solid core wires, and the late Clay Smith reground the camshaft. Who knows what the horsepower rating was, but I know that it wasn't very driveable for slow-speed, off-road usage. It ran rich and the idle wasn't all that great. A new engine was in order, but what to go to?