Tighter Valves Give More Power

When it comes to engine performance, your bike’s valves play a huge role – so it pays to get them as accurately set as possible. But is there any gain in running them tight? ‘Know it All’ Neil puts us straight…

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It’s important for an engine to maintain a small clearance between the cam base circle and its follower to allow lubricating oil to do its job and to ensure that the valve is allowed to fully close under all circumstances. Significantly, intake and exhaust valves tend to expand and contract with heat change at a different rate to the cylinder head. This is particularly true of the exhaust valve – a really hot valve in a still cold head actually has the potential to expand enough to completely eliminate all its clearance, even if its settings are within spec. If this happens, burnt valve, valve seat and valve train damage soon results. This is typically why every 10,000 or 20,000 miles the clearances need to be checked and adjusted back to spec. Or do they?

Manufacturers specify an acceptable range for valve clearances – typically 0.1 to 0.2mm for inlet and 0.2 to 0.3mm for exhaust valves. Looking at these numbers, you can see that there is a significant range between the tightest and slackest allowable clearance values. So what? I hear you ask. Well, basically, the tighter the clearance, the longer the cam can keep the valve open for – known as cam duration and the higher the valve will open – valve lift.

So as you might imagine, in the pursuit of power, if you’ve got to go to the hassle of adjusting your valve clearances, you should set them as close as possible to the tighter end of the spec’. That way you’re going to achieve longer cam duration and more lift. Theoretically, these two factors are going to get more fuel and air into your engine and you’re going to get more power out of it.

Ramping things up