Pushrods

Thursday, May 08, 2014

Tags :  valvetrain

 Pushrod engines are ‘old hat’, an anachronism in these modern times, but while the rest of the world has moved on and adopted the overhead camshaft, the users of big V8 engines have remained loyal to the old tech. That is probably an opinion shared by many engineers who have never had any dealings with pushrod valvetrains.

The fact is that there are countries – the US and Australia for example – where cheap gasoline makes daily motoring with a big engine perfectly affordable. While there is an increasing permeation of European and Asian cars into these markets, pushrod engines remain stubbornly popular. Engineers from Europe and Asia ought not to labour under the impression that the Americans can’t understand the benefits of overhead cam (OHC) valvetrains, because they do. The likes of GM and Ford know very well how to make OHC engines because they make them in their millions.

The fact is that overhead valve (pushrod) engines are very compact. From the crankshaft to the top of the cam cover, an OHC engine is much taller. When GM looked at new engines for its latest incarnation of the Chevrolet Corvette, it assessed OHC designs. GM is a huge company, with resources to match, and a lot of clever engineers there looked at the options and concluded that a big pushrod V8 was what they needed. The OHC V8 that provided the requisite performance, however, was simply too tall to fit beneath the desired bonnet/hood contour.

There are other alternatives though to allow an OHC engine to fit under that hood – a smaller, higher-revving engine might have done the trick, as would a turbocharged or supercharged engine of smaller displacement and dimensions. After all things were considered, GM came to the conclusion that the simplest and most effective engine that gave the required performance with the required dimensions was the pushrod engine.

Until legislation or economic factors force manufacturers into making big changes to their car designs and the engines that power them, people will still want big V8s while they can afford to run them. While there is sufficient demand for big-capacity engines, we can assume that pushrod valvetrains will remain a popular choice when people come to assess their power unit requirements, just as GM has done, mainly owing to their advantage in terms of engine height. Someone once said to me, “The biggest problem with a pushrod engine are the pushrods”; that may be true, but the engines still have a valid place in the market, based on sound engineering judgement.

Pushrod engines still do very well in open competition against OHC engines. In endurance racing for example, there has been a protracted duel between Corvette and Aston Martin for many years. The rules aren’t exactly the same though for both engines: there are breaks given to the Corvette owing to the fact that the engine has two valves per cylinder, but the breaks are relatively small. Of course this leads to some complaining, but of course the option is there for the complainant to use a pushrod engine of their own!      

Written by Wayne Ward

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