Flathead engines, also known as "L" head engines are actually called sidevalve engines, because the valves are located on the side of the cylinder as opposed to over the cylinder bore like in an overhead valve (OHV) engine.
The Sidevalve Engine
Low Compression Ratios
The Long Combustion Chamber
The Dirty Flathead
The basic idea of a flathead is that both the intake and the exhaust valves are located on the side of the engine, and open up away from the crankshaft. The valves open up into a pocket in the combustion chamber that is off to the side of the cylinder. This means that the combustion chamber has a very long and irregular shape. The advantage of flathead engines is that the valvetrain is simple and the cylinder heads have no moving parts meaning the engine is cheaper to produce. With the valves riding directly on the lifters flathead engines also have very lightweight valvetrains which would be good for high engine speed operation. This advantage of a lightweight valvetrain is however mostly lost on the flathead because they don't flow well at high engine speeds. With the valves located way off to the side the intake air and exhaust must make a long convoluted passage with many twists and turns. First the intake air flows down the carburetor into the intake manifold, then the air flows longitudinally along the length of the engine, then the air turns into the side of the block, then the air turns up into the intake port and through the open intake valve and then the air turns sideways and flows across the long combustion chamber and then finally the air turns again to flow down into the cylinder. That is six right angle turns that the intake air must make to get into the engine where on overhead valve engines the air makes just one turn in the intake ports and perhaps another sweeping turn from the carburetor to the intake runners. When it comes to getting the exhaust out of a flathead the same six right angle turns are required. At lower engine speeds all these turns are only a slight hindrance to good flow, but as the engine speed is increased the rapidly moving air much more strongly resists all of these turns and a large flow restriction is created.
Another consequence of the elongated combustion chamber is that the compression ratio tends to be low on flathead engines. Most flatheads had six to one compression ratios, but the earlier ones were all the way down at four to one. It is not that that flatheads cannot be built with higher compression ratios, because they certainly can. When flatheads were used for racing in the early 20th century compression ratios were routinely bumped up to 9 or 10:1 using aftermarket heads. Somewhat ironically though increasing the compression ratio on a flathead interferes with how the engine flows. This is a bit backwards sounding at first, since higher compression ratios nearly always yield better flow in an overhead valve engine. In a flathead though the valves open up into the combustion chamber, making the combustion chamber smaller means less room around the valves for the air to flow in and for the exhaust to flow out. And that is not the end of the compression ratio and flow entanglement problems for flatheads. The first thing to do to get higher compression ratio on a flathead is to make the pistons come nearly all the way up to the cylinder head so that the combustion chamber is entirely over the valves. This leaves the maximum amount of room around the valves for best possible flow through the valves, but it does create another problem. With the piston coming all the way up to the cylinder head the first part of the intake flow as the piston moves down the piston is sucking hard against a vacuum but hardly any air is flowing. Then again when the exhaust is being forced out the last part of the piston stroke tends to compress the hot exhaust gas considerably to force it out the narrowing gap. What this adds up to is somewhat worse flow and increased pumping losses both on the intake and the exhaust strokes. Still though the best thing that can be done for a flathead is to get the pistons to come nearly all the way up to the cylinder head both because it allows a higher compression ratio and because it tends to alleviate the other really big problem with flatheads.
Flow problems restrict maximum power generation up at higher engine speeds, but the really bad problems with flatheads come at lower speeds and lower power output levels and are due to the elongated combustion chamber. Basically flatheads don't work well in full flame front travel mode because the combustion chamber is just so darn long. A three inch bore with inch and a half intake valves on the side means that the combustion chamber is nearly five inches long. This five inch long combustion chamber means that the time of flame front travel is at least as long as for a five inch bore engine, and it gets worse. The other problem is with the location of the spark plug. Flathead engines have had the spark plugs located anywhere from directly in the center of the bore to directly between the valves, and some flathead engines have totally irrationally had the spark plugs located on the outside of the valves. For full flame front travel mode operation the spark plug should be directly in the center of the combustion chamber, that is offset one inch from the center of the bore towards the valves for a three inch bore engine. With this centered spark plug location the three inch bore flathead can run at the same engine speeds in full flame front travel mode that a five inch bore OHV engine can. In other words even at their best flatheads loose nearly all of the advantage of small bores. Because flatheads work so poorly as full flame front travel engines there is considerable impetus to run them in late compression ignition mode as much as possible. Getting a flathead to run well in late compression ignition mode totally precludes the central spark plug location for two reasons. One is that the central spark plug location makes it much more difficult to increase the compression ratio on a flathead engine. The other reason that the central spark plug location does not work well for late compression ignition operation is that this central area of the combustion chamber tends to be skinny leaving little room for the large pocket around the sparkplug required for low compression ratio late compression ignition engines. The best thing to do for a flathead to get it to run well in late compression ignition mode is to locate the spark plug as nearly directly in the center of the combustion chamber over the valves as is possible. This usually means locating the spark plug just off towards the cylinder bore from the valves so that the valves can be directly next to each other. With the piston coming nearly all the way up to the cylinder head most of the combustion chamber is then over the valves, and the spark plug being located directly in the center of this valve pocket means that most of the fuel is close to the spark plug when the piston is at top dead center. These are the conditions required for an engine to be able to run in late compression ignition mode with a low compression ratio.
The low six to one compression ratio of most flatheads means that a large portion of the fuel is going to have to be burned in flame front travel mode before the temperature and pressure in the combustion chamber come up to where late compression ignition can take place. If too much of the combustion chamber is located out over the cylinder bore when the piston is at top dead center then a low compression ratio flathead simply will not be able to get into late compression ignition mode because not enough fuel is close enough to the spark plug. Because flatheads do have very low compression ratios they tend to run in full flame front travel mode under all reduced loads. Full flame front travel mode operation tends to be very inefficient on a flathead because the fuel has to be burned very early or the engine speed has to be very low. This is true even with a central spark plug location, but full flame front operation gets much worse when the spark plug is located out over the valves for best late compression ignition operation under heavy loads. Getting an engine to run as well as it can in full flame front mode means timing the spark so that the flame front follows the piston as it recedes down the cylinder bore. With a flathead this is more difficult to attain, and if the spark plug is located out over the valves it becomes essentially impossible to attain unless the engine speed is further reduced. Full flame front travel combustion always tends to be inefficient and somewhat dirty, but on a flathead these problems are accentuated. If the spark timing is too late then the flame front does not reach the far side of the combustion chamber before the piston has receded so far that the temperature and pressure have dropped off to the point where combustion cannot take place. If the flame front does not follow the piston down as it recedes in the bore then some of the fuel will be drawn down with the piston unburned and later blown out the exhaust. Getting all of the fuel to burn in full flame front travel mode in a flathead means very early spark timing, which is going to be much less efficient, or extremely low engine speeds which also tend to be very inefficient. On an OHV engine there is some narrow range of engine speeds where full flame front travel combustion works fairly well, and efficiency is just about as high as is possible with the low compression ratio and low power output. On a flathead though complete full flame front travel combustion requires even lower engine speeds, and this additional reduction in engine speed typically means even worse efficiency.
After many decades of experimentation with different designs of flathead engines one particular configuration was landed upon as ideal for efficiency and usability. This ideal flathead had a six to one compression ratio, a radically undersquare configuration with about a four and three eighths inch stroke, a three and a quarter inch bore and a central spark plug location for full flame front travel operation. These long stroke engines ran well at low engine speeds and were able to run along smoothly and quietly down to as low as about 1000 or 1200RPM. Of course it was under quite light loads that these four and three eights inch stroke engines ran well down to 1200RPM, and to support more substantial loads the engine speed had to be up above 1500RPM. With the very small three and a quarter inch bore and a central spark plug location full flame front travel was possible up to quite high engine speeds despite the long flathead combustion chamber. Under a medium load these ideal flatheads ran quite well from 1500RPM up to about 2000RPM. Even with the central spark plug location these six to one flatheads could also get into late compression ignition mode on the fastest and lowest pressure premium fuels. In late compression ignition mode a huge boost in maximum power output was available above about 2,000RPM. Even if late compression ignition was attained under a full load though maximum power output usually came at about 3,000 or 3,500RPM because the radically undersquare flathead engine simply would not flow well enough at higher engine speeds to overcome the considerable reciprocating losses of big heavy pistons and rods running on such a long stroke. The main thing that could be done to get more power at higher engine speeds on these engines was to install an aggressive camshaft. The lightweight direct acting valvetrain had little difficulty running on fast opening and fast closing camshaft lobes, and substantial performance gains were possible in the 3,000 to 4,500RPM engine speed range.