The recent Jeep Commando CC and its Hurricane Four engine with an F-head (intake over exhaust) 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 (above), 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 (above), the intake and exhaust valves are suspended directly over the cylinder. OHC engines are a variation of this, but the basic valve arrangement is similar.
In the F-head, 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, 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. 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 cots and 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.
A number of American motorcycle manufacturers, including Harley-Davidson, Indian and others used F-heads in their earliest years and into the twenties. 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 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 the B60 engine (above), a six that eventually grew to 4.9 liters and some 200 hp, until it was finally replaced by the OHV V8 in 1959, which is still in use by Bentley today.
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 suitable for RR and Bentley. The B series 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.
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.
The legendary Henry Weslake was involved, and the result was remarkable: an F-head with many 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 hemispherical, and the exhaust port’s pathway is delightfully unencumbered , unlike in the traditional F-head’s tortured ports.
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.
So many ways to create and contain that explosive combustion that makes an “internal combustion engine.” I’ve always been facinated by the ways they’ve changed over the century plus of the automobile. Under-square, over-square, basically square… hp and tq being fairly equal (the way it was before the environmental regs hit), tq being much higher than hp (the way it was for most American cars of the “mailase era”), and now we have lofty tq and hp at lofty RPM with direct injection and all sorts of tricks. I’m still convinced that the IC engine will be with us for many more decades before we all buzz around in electric cars.
Harry Westlakes motor is probably the best rendition of F layout but apart from being complicated it would have been costly, Im pretty sure Rover didnt go more than 3 litres in the 6cyl P5, 3.5 was the V8 used in the P5B. Austin put out an upmarket model the Vanden Plas R using a RollsRoyce F motor in the 60s .Since then though the bean counters have prevailed and other than the wankel noone has made a new design just rehashes of old ideas.
Yes, that was meant to say 3.0 liter. Thanks for being my proof reader today
Thanks for filling in the gaps in my engine knowledge. Made for an interesting read.
An inline 6 version of the Willys was produced in Brazil until the mid 70s, and even found it’s way under the hood of Ford Mavericks built there.
Now that was an interesting engine indeed. But that old flathead six was converted to an OHC hemi-head, and had a long career even in racing down in Argentina.
Great site Paul, love that you cover Tatras to T-Birds.
One small point – the Rover P5 shown is a P5B (for Buick) V8, identifiable by the Rostyle wheels and “L” registration (1972).
These cars were offered in both saloon (sedan) and 4 dor coupe forms (lower rofline) and were used long after production ceased as government cars in the UK right into the ’80s as Mrs. Thatcher’s preferred wheels. Seems the “Iron Lady ” posessed automotive taste!
Thanks; and good noticing. Maybe we’ll keep that as our little secret?
Damn, I was about to post that! One thing to say about the P5 is that the 4 door coupe was one of the most bad-ass looking cars ever made. In black it was positively sinister. I would have one in a heartbeat….
‘Rover 3.o six’ 3.0 six! I make that mistake all the time too.
Even after this excellent article, I still don’t quite understand the geometry, particularly how the valves are actuated and the position of the pushrod and cam (or is it cams) relative to the combustion chamber.
I guess I will have to find one of these engines and take it apart.
Nicholas the Camshaft is underneath the exhaust valve (labelled in the first diagram), the intake valve pushrod can be seen running parallel to the exhaust valve in both F-head diagrams, however in the first it is identified as the rocker arm.
I have what I think is an experimental 6 cyl. Willys Hemi head. I am happy to email additional pictures to Paul…feel free to contact me.
I find the Rover F head interesting. I would like to see a cutaway of the valve train. Did the cam bear directly on the exhaust valve or via a rocker. Was it much more expensive to make? Perhaps I should forgive the writer’s poetic license, but I find the ending odd. I doubt if anyone in Buick would describe the Buick based 3.5 liter aluminum V8 as “Detroit pragmatism triumphs again” when it was being built in the U.S. That was an era of engineering chances and new directions with the aluminum air cooled flat six rear engine Corvair, the aluminum 3.5 V8 of Buick and Oldsmobile, the Pontiac with the flexible drive shaft and transaxle. Of them all, the Pontiac with it’s monster slant 4 was probably the most pragmatic as it was just a Pontiac V8 with one bank of cylinders removed. The triumph of pragmatism was when all this new engineering was eliminated and the Corvair’s role was supplanted with the cast iron 4 of the Chevy II. The purchase of the 3.5 l V8 by Rover was the purchase of Detroit’s now discarded optimism, a very pragmatic move by Rover.
Here’s link to a section drawing of the Rover engine, showing the short rocker with roller camfollower operating the exhaust valve. Also interesting is the head gasket angle and piston shape, to give good squish area between inlet valve and piston.
http://www.head2head.free-online.co.uk/Rover/pics/eng_tns.jpg
Source: http://www.head2head.free-online.co.uk/Rover/engine.htm
Thanks for that cut-away. I’ve added it, and given credit to the source.
My memories of these “alternative” engineering ideas makes me rub my fingers. Since the best way to adjust valve lash was hot, the location of these valves made for some interesting lessons in thermodynamics. I remember when 1 h.p./ cubic inch was a laudable goal. Now, 100/litre to go with 25mpg is becoming routine. I’ve never owned a direct-injection car and my only experience with DSG is an old Electric Hand Essex my Father owned. Primitive and complicated. Is the Weslake you note also the same who designed engines for racing in the 60′s?
I just discovered your excellent website today. My favorite F-Head engine was found in the Rolls Royce Phantom IV. This was essentiaslly the Rolls/Bentley F-Head 6 with two more cylinders tacked on. An elderly gentleman I met 40 years ago brought the Phantom IV to my attention. He said the combination of a straight 8 and the F-Head configuration gave unparalleled silence and flexibility especially for parade duty. I have never owed an RR but I owned several NSU Sport Prinz’s that used an air cooled twin mounted in the rear and had an overhead cam driven by 3 connecting rod like devices. The Sport Prinz had a Bertone styled fastback coupe body. The 8-Liter Bentley used the same cam drive. Any other NSU fans out there?
I an new to this website and I continue to be amazed by the knowledge I encounter in these articles. I am 55 yrs old, and owned a few of the classics of the 60′s and 70′s like the 68 Firebird 400, the 67 Camaro 327, and a 69 Dodge Coronet among others. The Firebird is the one that I wish I had never sold.
Welcome!
I love reading about these various valtrain arangements. The most remarkable that I know about are the Pierce Dual valve Six, a four valve flathead motor. The only other of it’s type that I am aware of was produced by McFarland that I beleive performed a similar feat. I do remember reading that few Pierce Dual valve Sixes survived due to their quiet running and subsequent demand for prohibition rum-running boat propulsion.
I have a 58 cj3, the hurricane engine has multi colors. Who were the manufacturers? Was there a different color from each manufacture? Do I make it yellow? I was told John Deere made them. Or Blue? because Ford made them.