Automotive History: The Curious F-Head Engine

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– Posted on April 28, 2019

(first posted 9/2/2013) The recent Jeep FC-150 and Jeep Commando CC with their Hurricane Four engine that featured an F-head (Intake Over Exhaust – “IOE”) cylinder head gave me the impetus to re-visit that rather unusual cylinder head arrangement, and consider its place in engine history. If cylinder heads speak to your head, open your intake valve and let’s get sucked into the peculiar mysteries of the F-head.

Not wanting to insult anyone’s knowledge, yet taking nothing for granted, let’s briefly review the two basic cylinder head arrangements, as the F-head is a hybrid of the two.

The L-head and related T-head, more commonly referred to as a flathead, has both intake and exhaust valves in the block, parallel to the cylinders. This cylinder head is a very simple affair, basically a flat slab with a combustion chamber indentation, and the spark plug. It was extremely popular from the earliest days of the IC engine into the fifties, and is still used in some lawnmower engines. Its shortcomings in breathing and cooling will be the subject of another article, but they are substantial, and challenging to overcome.

In the familiar OHV cylinder head, the intake and exhaust valves are suspended directly over the cylinder. OHC engines are a variation of this, but the basic valve arrangement is the same.

In the F-head (pictured at top of article), the intake valve is directly over the cylinder, like in the OHV, but the exhaust is in the L-head location; a mixture of the two. Why? There were several reasons why a few manufacturers adopted it.

The real challenge to improving breathing through larger valves was that almost all engines prior to WWII were very undersquare (stroke length greater than bore diameter), in having very long strokes and relatively small bores. Those small bores spaced close together meant that there was a serious lack of physical room for larger valves in either the side valve or OHV engine. That is the fundamental reason that the hemi-head engine was invented; by canting the valves, their respective sizes could be increased.

peugeot L76

This resulted in the classic DOHC hemi-head design that was first successfully used by Peugeot in about 1912, for Grand Prix racing.

Because of the mechanical complexity, cost, noise and difficulty in adjusting valve lash, the DOHC hemi-head engine was primarily used in racing and high-performance sports cars, and generally avoided in passenger car use, where quiet running, low speed torque and low production cost and easy maintenance were priorities.

The F-head offered an alternative, because it allowed for a potentially huge single intake valve, as well as a larger exhaust valve, without changing the fundamental architecture of the small-bore long-stroke engine.

HD 1910 F head twin

A number of American motorcycle manufacturers, including Harley-Davidson, Indian and others used F-heads in their earliest years and into the twenties (1910 HD twin shown). That may in part have been to have a greater separation of the two valves in order to keep the hard-to cool exhaust valve on an air-cooled motorcycle engine away from the intake. After the twenties, they drifted away from F-heads to either L-head and OHV engines.

Hudson used an F-head briefly from 1927 to 1929. But the two main proponents of the F-head were two very dissimilar manufacturers: Rolls-Royce and Willys/Kaiser Jeep. But in both cases, their reasons were similar, if not quite the same.

They both were heavily invested in the manufacturing tooling to build blocks with very undersquare dimensions. That was in part due to the British taxation based on bore size as well as to achieve the effortless low-speed torque that was such a part of Rolls’ reputation, to avoid having to shift gears as much as possible. A new smaller RR engine for the twenties originally had a DOHC hemi design, but it was too noisy for its reputation also as a maker of silent engines. So the F-head was employed very successfully for decades on, their line of 4.2, 4.6 and 4.9 L engines used in the RR passenger cars until it was finally replaced by the OHV V8 in 1959, which is still in use by Bentley today. The B60 (shown above) was a smaller 3.9 L engine and used in the 4 liter Princess.

The B series, also made in four and straight-eight version for military vehicles, proved themselves to be both very quiet and smooth, as well as delivering “adequate” power levels. The larger six was also made in an eight cylinder version for the Phantom. The bigsix engine found its most illustrious home under the exquisite hood of the R-1 and S-1 Continentals, which had higher performance versions and were renowned for their effortless ability to cruise at one hundred mph.

The Willys 134 CID (2.2 L) “Go Devil” flathead four that powered the original military and post war CJ2/3 Jeeps was a rugged mill, and very torquey, thanks also to its very long stroke/small bore configuration. Rated at 60 hp, it made the quite light Jeeps quick and powerful for the times. But those conditions were changing, and by 1950, when Willys was pushing larger civilian vehicles like the Jeepster and Jeep Wagon, more power was needed.

The 1950 solution, the F-head F134 “Hurricane” engine, was expedient as much as anything else, because it allowed the original block to be retained with little change. The pushrod for the prior side-intake valve was extended into the cylinder head, where it now resided. Quick, easy, and cheap, and the results were decent, if not spectacular. Horsepower jumped from 60 to 75, and torque from 102 lb-ft to 114 lb-ft. The F-head F134 soldiered along until emission controls and even greater power demands doomed it in the beginning of the seventies.

The F134 was adopted in the CJ range with the 1953 CJ-3B (above), which had to have its hood raised very substantially in order to clear the much taller engine.

Now there is one other F-head engine we need to touch upon, the very curious case of the Rover engine. It’s a perfect example of the British eccentric era, combining creative engineering with certain overly-complex and expensive solutions that had no future in an increasingly pragmatic world.

F Head Rover 3 litre

The legendary Henry Weslake was involved, and the result was remarkable: an F-head with some of the characteristics of the hemi head and none of the disadvantages of the convoluted F-head combustion chamber, which was far from ideal in the RR and Jeep engines. Take a good look at this cut away (source: head2head.free-online.co.uk), it’s both so brilliant yet unnecessarily complex.

The intake valve is overhead, where it can be large. But the exhaust is canted at an odd angle, sticking out the side of the block and necessitating a complex casting and valve train. But the result is close to a hemi/pent-roof cylinder head, and the exhaust port’s pathway is delightfully unencumbered , unlike in the traditional F-head’s tortured ports.

Rover P5

The Rover engine was built from 1948 until the late seventies, and powered such handsome and delightful cars like the Rover P5 (above), the executive and government vehicle of choice, depending on one’s very carefully defined class level, automotive and otherwise. The final Rover 3.o six was eventually replaced by the Buick-derived 3.5 L aluminum V8. Detroit pragmatism triumphs again.