Side-clappers: as endearing as dated

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During a visit to one of our advertisers, we saw a Ford Fairlaine. A model from the last year of production that fitted Ford V8 side valves. Those side valves? That is Flintstone technology from the early days of combustion engines. But why Ford presented a Jubilee model with side valve while OHVs would be delivered the following year?

Side flaps: In the Beginne

Early four-strokes - in the first half of the twentieth century - were side valves. And they use technology that has a number of clear advantages and disadvantages. Side valve motors are - with the exception of some Made in China generators for economical DIY'ers - hardly any longer in production.

A side valve engine is a type of four-stroke engine where the inlet and outlet valves are located next to the cylinder.

The advantages of a side valve engine are simplicity, reliability, low number of parts, low production costs, weight and compactness. Eb the pleasant response to the throttle at low speed. They make little mechanical engine noise and are pleasantly insensitive to low-octane fuel. The absence of a complicated valve train allows for a compact engine that is inexpensive to manufacture, as the cylinder head is little more than a simple piece of casting. Is nothing more than a 'lid'. And so they pull nicely from low revs. Then the combustion can keep up a bit. These advantages explain why side valve engines have been used for many years in fuel-efficient cars, trucks, motorcycles and agricultural engines. While overhead valve designs were seen as less reliable high tech things for airplanes, luxury cars, sports cars and sports motorcycles.

"Any judgment…."

The main disadvantages of a side valve engine are poor gas flow, a low-efficiency combustion chamber shape and low compression ratio, all of which result in a low-speed, low-power, low-efficiency engine. Because side valves do not burn fuel efficiently, they suffer from high hydrocarbon emissions. And that is a very strong environmental thing these days. The reason that you can now fly to Africa for € 57 is only because budget airlines do not use aircraft with side-valve engines.

Poor breathing and hot flashes

In a side-valve engine, intake and exhaust gases follow a long trajectory, with low volumetric efficiency or 'bad breathing'. This is not least because the exhaust gases disrupt the integrating gas flow. Because the exhaust fumes take a long path to exit the engine, side valves tend to overheat thermally. Side flaps can turn quite a bit. But because of the bad breathing, those extra revolutions quickly no longer provide more power. Funny: a valve can 'get stuck' when 'rushing' or with other technical inconvenience. This can lead to significant engine damage with overhead valves. A side valve driver simply waits until the block has cooled down and the valve returns to its original residence and residence. And that happens in nine out of ten cases.

Because the side valve technology dates back to the early days, although not the earliest early days, from the combustion engine time, the technology and metallurgy were also sub-optimal. Side-clappers were good things that delivered limited power at very quiet speeds, but generated a pleasant couple. A 750 cc boxer engine of the BMW R71 delivered around 20 hp. A 3,9 liter Ford V8 side valve delivered about 90 hp and had a torque of something of 210 Nm. At the end of its development process, the Ford 'Flathead' delivered 110 hp. Still from 3,9 liters engine capacity.

Simplicity is good. But there were frayed edges on the rug.

So the biggest problem was the combustion chamber. The combustion chamber of a side valve engine is very ineffective. The part where the valves are is a kind of empty space next to the center of the engine; the cylinder bore. To get some sensible flame front in that space by effectively igniting the mixture there? With the spark from a 6V battery ignition coil ignition? This is comparable to setting fire to a wet towel with a damp match. And when it runs, then such a side valve is also thermally sub optimal.

The power of the side flaps was in the simplicity. When the much more effective headers appeared, they were seen as diabolical machines for the most daring racers. But that time soon passed. The head valves were the new standard.

But now side valves are simply endearing

When you see how compact such a 'flathead' V8 lazes around in an engine compartment? In addition, working on side valves is the ultimate in simplicity. If we are not talking about V8 blocks, but about motorcycle blocks, it can result in the best experiences. For example, everything is transient. Or in the case of 70 year old ex Soviet Union technology just metallurgically pretty bad material. What if an entire cylinder breaks down 'under' acceleration from zero to 60 miles? You call that to a friend who has the same hobby. An hour later you receive another cylinder and piston plus some packing paper on the verge. And in peace and satisfaction, you will have the case back together in about twenty minutes and running. Try that at a Ferrari Testarossa. Cadzand Bad has never been such a beautiful finish.


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  1. Flintstone technology from the early days of the combustion engines? Until 1973, Harley-Davidson used 750cc side valve engines in the servicar, the tricycle that was often used by police forces. I know that they are not always at the forefront of technical innovations, but that way the 750 was the longest in use engine of the HD factories.

  2. Old-fashioned is not the right word. Multi-valve technology with overhead camshafts was also used by the Americans in the 30s. For the requirements that were set at the time, it was fine.

  3. Regularly watch a guest on youtube (project farm) who performs meaningless tests and almost always uses a lawn mower to test things. As you describe it, that is also a side valve.
    Are quite nice tests with surprising results.
    Can we buy something like Seafoam here in NL?

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