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Understanding camshafts: What do uprated cams actually do?

Understanding camshafts: What do uprated cams actually do? Classic and Performance Car

Always wondered how a camshaft actually works? John Simister explains all...


Mild, hairy, fast road, rally, peaky, full race… many are the adjectives applied to camshafts, depending on what they do to an engine’s demeanour. The history of sporting cars is full of camshaft lore: BMC Special Tuning’s 631 or 749 cams for hot Minis, a Cosworth A2 in a souped-up Ford, Isky cams in US hot rods. Underpinning all camshaft designs, though, are some undeniable truths.
 
More ‘overlap’ – the period when exhaust and inlet valves are both open – tends to boost high-revs power at the expense of low-down vigour. More valve lift makes everything better everywhere provided the ports and valves are big enough. More duration – the length of time a valve is open – allows more intake air in and gives a bigger chance for the exhaust to escape.
 
Up to a point, anyway; the valves have to be shut some of the time, otherwise the mixture will never be compressed and there’ll be nothing for the burning gases to work against when they’re forcing the piston down. That time can be short in a really revvy racing engine, which is why Cosworth’s Keith Duckworth dubbed his early Ford Formula Junior camshaft the Windmill cam after the famous theatre. As in: ‘We never close.’
 
So it’s all a compromise, slanted to suit the purpose. Knowledge changes, so do fashions, so do legislative requirements for emissions and suchlike, so do fuels. It was usual in past times for camshafts to be ‘symmetrical’ in their inlet and exhaust events, but clever people have found new ways of getting more out of old engines by opening minds, shunning needless convention and observing what designers of modern engines sometimes do.
 
Ken Newman, of long-established camshaft creator Newman Cams (set up by father David in 1967), describes the thinking. ‘There’ll be a shorter duration on the inlet valve than the exhaust valve, but more lift,’ he explains. ‘That’s actually the reverse of some very early engines, which tended to have a mild inlet lobe and a big exhaust one.’ Helpfully, more-volatile modern fuels make for a better tickover with ‘hotter’ camshafts, and mapped ignition or full engine management can make a tuned engine surprisingly tractable.
 
Camshaft design is highly complicated and calls for much mathematics. Or you can just rely on experience: ‘It’s still a bit of a black art,’ says Ken. Which, in this digital age, is strangely comforting.
 
Words: John Simister

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