It seems that I often refer to the superiority of the overhead valve engine design as compared to the side-valve or “flathead” engine design. My day job is as a consulting engineer (although not in the automotive industry) so I have an engineering interest in the technical aspects of automotive design. As a result, I thought that I would try to further explain some of the technical aspects of automobile design and performance.
In North America, the most common, or at least the most famous, side-valve engine was the Ford side-valve V8 or “Flathead” V8 that was in their North American cars from 1932 to 1953.
The Ford Flathead V8
As can be seen from the above image the name “Flathead” came from the rather flat exterior surface of the cylinder heads.
In order to try to explain this engine design, I have prepared the following sketch of a side-valve engine design.
A Sketch of a Side-Valve Engine
As can be seen from the above sketch, in a side-valve engine design the intake and exhaust valves are located in the engine block – not in the cylinder head. As a result, there is very little in the cylinder heads other than the spark plugs.
The cylinder head that I have shown in the above sketch is typical of a slightly high compression side-valve design. Typically the top dead center of the piston travel in these engines is right at the top of the engine block. For my sketch, I have shown a reduced clearance between the roof of the cylinder head and the top of the block in the area of the piston. Many side-valve engines do not have this clearance reduction and the roof of the cylinder head is at a common distance all across the cylinder head which reduces the compression of the air/fuel mixture compared to what I have shown. With the cylinder head design as I have shown it in the sketch, during the compression stroke the intake mixture is forced into the cylinder head volume above the intake and exhaust valves and directly in the spark plug area. By compressing the intake mixture in the location of the spark plug, the burning of the fuel mixture is enhanced.
The purpose of automotive engine design is to convert the energy in the fuel into energy that can be used to move the car forward. So, in general, the more volume of fuel that you can burn and the faster that you can burn it, then the more power that the engine will produce.
The disadvantage of the side-valve engine is that the air flow characteristics of the design are far from optimal. On the intake stroke, the intake valve will open and the suction caused by the downward movement of the piston will draw the air/fuel mixture into the cylinder and cylinder head. The intake air/fuel mixture flow path is not smooth. The flat surfaces and internal corners are not conducive to good air flow. There are too many obstructions that cause turbulence and air flow eddies that impede efficient air movement. The same thing happens during the exhaust stroke as the spent air is not efficiently removed from the cylinder head. Any spent air that remains in the cylinder during the next compression stroke takes up space in the cylinder that should be occupied by the air/fuel mixture.
This inefficiency of air flow is not a huge problem at low engine revolutions, but if the engine designer wants a higher revving engine, then one of things that has to happen is to get the air flow mixture into the engine as fast as possible and get the spent air/fuel out as quickly as possible. This is a problem in a side-valve engine design.
Another problem with the side-valve engine design was that after World War II, the available fuel had a higher octane rating. To use this fuel efficiently, it is necessary to increase the compression ratio. In order to increase the compression ratio, the distance between the top of the engine block and the roof of the cylinder head is reduced. This reduced height between two flat surfaces further impedes air flow. The basic engine design did not allow the side-valve engine to take full advantage of the newer fuel.
The other negative feature of the side-valve engine design is that the air/fuel mixture has to reverse flow direction for the intake and exhaust strokes. In my sketch above, it can be seen that the air/fuel mixture moves from right to left during the intake stroke and from left to right during the exhaust stroke. In future postings I will show how the air flow is improved in other engine designs where the intake and exhaust valves are located across from each other.
A Ford Flathead V8 Engine Block
If you look carefully at the above engine block, the intake manifold, and the exhaust manifold you can tell which of the valves are the intake valves and which are the exhaust valves. Note that the relative position of the valves changes along the engine. In upcoming posts I will also discuss the merits of various types of the intake and exhaust manifolds.
The important thing to remember about side-valve design engines is that they are suitable for low revving engines, but as the need for higher speed engines increased and as the available fuel became better, the side-valve design was surpassed by other engine designs.