In a recent post I described the Side Valve, sometimes known as an “L-Head”, engine which was the first mass produced iteration of the internal combustion engine that used petrol or diesel. Humans being very inquisitive started to figure out how to improve this engine to make it more efficient and reliable. The Overhead Valve engine (OHV) was an evolution of the Side Valve.
David Dunbar Buick was the first to design a working OHV engine. He was born in Scotland but arrived in the US when he was a toddler and eventually founded the Buick car company after several attempts at developing engines and manufacturing processes for the early automobile industry.
Most early cars – and by early, I mean pre World War 1, had Side Valve engines which had the valves built into the engine block. Buick, however, developed the Overhead Valve engine by merging the valves into the cylinder head and using a camshaft that was at the bottom of the engine near the crankshaft. This revolutionary design developed much more power than the earlier engine and became the industry standard. Today GM, through its Buick heritage, is the largest producer of OHV engines with their LS series V8s still using this method of input and output. The Chrysler Hemi is also an OHV engine.
The concept is simple. At the bottom of the engine is the crankshaft delivering power through the prop shaft to the wheels. The crankshaft is connected by a timing chain to the camshaft, which is positioned somewhere in the engine block – it could be at the bottom of the “V” on a V8 or V6 or near the crankshaft or in the side of the block. As the cams move they push rods (hence the name “pushrod” engine) up the inside of the block that moves a spring loaded rocker arm that opens and shuts the valves.
The whole design is smaller than the newer Overhead Cam engines that I’ll discuss in a later post but they do have some limitations. One is that it is harder to get the timing spot on because of all of the moving parts and there is a limitation on the speed of the engine. Typically they can only spin up to about 8,000 rpm for road use or 10,000 in a race car. At higher engine speeds, the pushrods take such a beating that they have been known to bend under pressure! If a rod bends, its not going to open its valve very far – if at all, so power is lost. In addition, because the engine uses rocker arms, there often isn’t enough space to have a multi valve head, thus they can’t get more fuel mix in and exhaust gases out.
Today most OHV engines are found in run of the mill cars aside from the LS or Hemi motors. Most sports cars and higher performance cars use Overhead Cams. It is typically the American based manufacturers like Ford, GM and Chrysler that continue to build OHVs. The European and Japanese manufacturers have now typically standardised on the Overhead Cam design due to their need to squeeze more power from a smaller capacity motor.