Looking Back: The Flathead Engine

Trendy engines have their valves immediately above the piston (OHV = overhead valve), giving a compact, quick-burning combustion chamber of minimal quantity, permitting use of torque-boosting excessive compression ratios. It wasn’t all the time so—main automakers constructed hundreds of thousands of flathead (a.okay.a. “aspect-valve”) engines via the Nineteen Forties, with Chrysler (revered by scorching-rodders for its OHV “Hemi” V8s, starting in 1951) constructing its six that approach till 1959. Till the straightforward availability of gasolines of upper Octane Quantity, allowing OHV designs to make use of excessive compression, the flathead did just about nearly as good a job as wanted doing.


In a flathead, the valves are positioned beside the cylinder, stems down, with the aircraft of their heads in the identical aircraft because the piston’s crown at prime lifeless middle. They have been referred to as flatheads as a result of with out valves within the head, the top might be what the identify says—flat—and little greater than an inch thick.

The central attraction of the flathead concept was simplicity and sturdiness. With valve stems pointing down, the tappets and camshaft(s) could possibly be barely greater than the crankshaft itself. This allowed straightforward enclosure and lubrication, whereas OHV, whether or not by pushrods-and-rockers or by overhead cam (OHC), required putting the valves and their mechanism on the cylinder head, up too excessive to be conveniently lubricated. Valve gear of early OHV motorbike racing engines of the Nineteen Twenties needed to be lubricated by hand or by grease cup, and solely by about 1937 did producers grasp the complexity of circulating oil as much as cylinder head degree to lubricate valve gear.

One other essential benefit of flathead valve operation was its lightness and minimal variety of shifting elements, making excessive rpm reachable even with modest valve spring strain. There was the cam lobe, the tappet or lever that adopted it, and the valve itself, lifted immediately by the tappet. When the additional weight of pushrod and rocker was added, valve spring strain needed to primarily double (and lots of producers added separate tappet and rocker springs to assist present it). This added strain on cam and follower surfaces pressured a lot redesign.

The negatives of the flathead are:

  1. Cylinder head and combustion chamber floor space should improve by the quantity essential to surround the valves of their beside-the-piston place. This robs combustion fuel of its warmth.
  2. The place does that misplaced warmth go? Into the cylinder itself, distorting it and resulting in additional loss from combustion fuel leakage.
  3. A battle is fought between the necessity for top compression (which requires minimal combustion chamber quantity) and the necessity for beneficiant move paths for engine consumption and exhaust stream (essential elements of that are within the head).

At first it seems like a fourth unfavorable must be the size and awkwardness of the flathead’s combustion chamber—a type of shallow inverted bathtub that should cowl each the piston and its valves. That appears like an extended, sluggish means for the flame to journey from only one spark plug!

However no less than three innovators noticed a solution to flip this disadvantage into benefit. One was Charles B. Franklin, an Irish racer/builder with in depth expertise at Brooklands, England’s nice speedway. He later turned Indian’s chief engineer within the US, the place his Powerplus-based mostly Indian flathead racers typically defeated the “extra trendy” OHVs of rival makers. How?

A immobile right combination of gasoline vapor and air burns fairly slowly—too slowly to make the IC engine practicable. The important thing ingredient is combination turbulence, which stretches, tears, and whirls the spark plug’s flame kernel shortly to all elements of the combustion chamber.

What Franklin found—virtually definitely accidentally—was that if a part of the top have been situated very near the piston crown at prime lifeless middle (TDC), the combination between the 2 can be quickly “squished” out because the piston approached TDC, vigorously agitating the combination in the remainder of the chamber. With squish, Franklin’s flatheads burned so quick that they didn’t have time to develop knock (the irregular type of combustion that may shortly destroy pistons). Virtually definitely, Franklin’s flatheads have been so quick as a result of, nicely-defended as they have been towards knock (a.okay.a. detonation) by squish, he might safely increase their compression ratios larger than might tuners operating supposedly “extra trendy” OHVs like Harley-Davidson’s eight-valve racers.

Close to the top of the reign of Harley’s postwar Okay-mannequin-based mostly 750 flathead racer, a sociable afternoon on the store of California tuning legend C.R. Axtell resulted in an influence increase for that engine so appreciable that Triumph’s beforehand dominant OHV 500 twin was made out of date in a single day. What Neil Eager, Axtell, and a few others stumbled upon and refined was that Harley’s design emphasised compression ratio over move. When the a part of the top via which movement from the valves to the descending piston was enlarged, extra energy tumbled forth (the 1968 and 1969 KRs subsequently ran on absurdly low compression ratios slightly below 6:1. Trendy OHC sportbike engines sometimes run 12:1).

Simply this morning, studying on a flathead discussion board, I discovered that a clear thinker recognized solely as “Frank” had made an admirable associated discovery whereas racing 5-hp Briggs & Stratton flatheads in a category filled with guidelines. He reasoned that with the valves as giant as they have been and the required inventory head so near them, move was being restricted by valve masking. Leaving the top inventory, he moved it towards the valves by decreasing the shank diameter of the top bolts. Then he reduce the valve diameters down till they seated solely at their edges. Each of those modifications offered extra stream space across the valves and so made extra energy. As he says on his discussion board publish, “These two issues helped us get suspended from racing and requested to by no means return.”

For racers, having your innovation banned is the very best reward!

Innovation comes from having an issue to unravel, hardware in your arms, and concepts turning over continually in your head. What if we…?

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