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Valve overlap

I can’t remember precisely when I first saw an engine camshaft or heard the word ‘overlap’, but I can only assume it must have been while I was still in short trousers. I do remember though my father taking the engine out of our car, a 1950s Ford Popular, and dismantling it on the garage floor. I’ve also never forgotten the explanation my father later gave me about the internal workings of the side valve internal combustion engine.

It was much later in life that the word ‘overlap’ in relation to engines was to enter my vocabulary, and funnily enough, and as you might expect, it was to do with camshafts and timing of the engine valves. As a reminder, valve overlap is that period when the exhaust valve is just about to close and the inlet valve has started to open. In our theoretical engine, the exhaust valve would close instantaneously at top dead centre and the inlet would open at the same time. However, the laws of physics prohibit instantaneous movement (and infinite accelerations!) and anyway, having both valves open a certain amount at the same time has advantages.  

First, the delayed closing of the exhaust valve encourages the last remnants of exhaust gas trapped in the clearance volume, where the piston cannot reach, to exit down the exhaust port. And second, the opening of the intake valve before piston top dead centre allows a fresh charge to enter, helping the exhaust gas to flow out of the exhaust port and replacing it with an additional charge to be burned during the following cycle. Provided little or no fresh charge is lost through the exhaust valve, scavenging the cylinder in this way increases the total volume of intake charge and also removes the potential ‘heatsink’ effect, when heat from the following combustion process is used to heat up the trapped exhaust using heat that would otherwise be turned into power. If ever there was a case of a double whammy, this is it.

The amount of overlap shouldn’t be too great though, and it very much depends on the restrictions to the intake and exhaust flow, both upstream and downstream of the cylinder. At a given engine speed, a four-valve chamber for instance will usually require much less overlap than, say, a two-valve head since scavenging in the former is more efficient. On the other hand, a race engine more used to running up to 6000-7000 rpm may well require an overlap much larger than that of a road-based machine. In such cases, a race engine could demand well over 100º of overlap for maximum power, whereas our roadcar is quite happy with 15-25º.

For comparison, a few years on from my garage-floor lesson, the Coventry Climax FWMV V8 of 1965 revving to just over 10,300 rpm at the time used a constant 89º of overlap to produce its maximum power. Three years later, the Cosworth DFV revving to only 9000 rpm used 116º. In road-going cars of course, maximum power isn’t always the only target – also of primary concern is engine idle ‘quality’, the ability to exhibit a stable engine idle speed using minimum fuel and giving minimal exhaust emissions, which is why most modern road engines use variable cam timing systems to alter this overlap when the engine is running.

In this way the manufactures get to have their cake and eat it. The overlap is minimised for best idle quality but increased again as the engine speeds up to give optimum scavenging when maximum performance is demanded.

Things have come a long way from those early side valve days in my life, but the sight of those two valves sticking up at the same time out of the top of the cylinder block alongside the tops of the pistons is still lodged in my memory.

Fig. 1 - The 1965 Formula One Coventry Climax V8 valve timing

Written by John Coxon

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