We’re talking valves and how best to set them.
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…
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
Counterintuitively, the opposite of this is true. The reason is due to a peculiar design feature of engine cam shafts. Inlet and exhaust cams aren’t just metal bars with rounded triangles sticking out on one side. Cam lobes comprise a multitude of angles all carefully calculated to achieve rapid valve acceleration without over-stressing the valve train. And the most important of these angles are the transitionary ramps between the cam base circle and the lobe flanks.
These initial and final parts of the cam lobe are designed to gently accelerate and decelerate the valve train during the first and last few degrees of the lobe’s duration. These ‘quietening ramps’ gently lift and return the valves from their seats. If cams didn’t have this gentle opening and closing action, the cam lobe would tend to ‘strike’ its follower
once any clearance had been used up, then allow the valve to smack back into its seat as it closes. Not only would this make a lot of noise, it would also wear the whole valve train pretty quickly. The effect would be exactly the same as running excessively wide valve clearances.
Unfortunately, what is good for engine reliability and quiet operation, isn’t ideal for ultimate power. Essentially the first and last five degrees of cam lift is dedicated to gently accelerating and decelerating the valve train, rather than contributing to gas flow. This is why cam duration is always measured between the valves at 0.1mm off their seat as they open to 0.1mm off their seat as they close. Essentially, the quieting ramps just make your valve leak a little compression for about 10 degrees, or around 5% of the time that they’re open.
Too tighty isn’t righty
Now we can start to see how tighter clearances can have a negative impact on engine power: With a clearance of 0.1mm the quieting ramps will keep the valves just off their seats for longer – maybe a total of 15 degrees or so. But a clearance of 0.2mm means the cam needs to turn a little further before the quietening ramps take up the clearance. The same engine with 0.2mm clearances, now only has the valves moving slowly and gently for an aggregate of about seven degrees.
So how much difference does all this tinkering make to engine power output? Well, it depends on the engine, but peak power is hardly effected – I’d be surprised if the difference between valve clearances set at either end of their specs could be detected on a chassis dyno. However, at smaller throttle openings and low rpms the effect can be significant.
This is because at low loads, the actual amount of charge trapped in the combustion chamber waiting to be burnt is so small. Plus, at low rpm, 10 degrees of engine revolution takes longer, so there’s more time for gas to escape past those slightly ajar valves.
By setting clearances to the wider end of their spec, you may also find that a bike’s throttle response and fuelling improves. As most bike fuel injection systems derive engine intake air flow from several parameters, including manifold pressure, intake pressure fluctuations can confuse the engine management system. In fact, if one or more intake valves are leaking, or the intake valve clearances are too tight, the pressure pulses generated can fool the bike’s ECU into thinking the engine isn’t flowing any air at all!
What you should do
To sum up, check, or have checked your valve clearances in accordance to your bike’s service schedule. If it’s a modern Japanese or European machine, chances are that most valves will be in, or very close to spec anyway. If some aren’t, adjust them back to within the correct tolerances. The consequences for not doing so are too serious to ignore.
If you’re rebuilding an engine, or suffer from OCD, perfectionism or just have too much time on your hands, you might as well set all the clearances towards the upper end of the range specified by the manufacturer. Provided you stay within the manufacturer’s specs, valve trains won’t be over stressed, noise shouldn’t be appreciably louder and the exhaust valves and seats will run marginally cooler. Not only that, low rpm throttle response should be improved. In my experience there is no power advantage in running clearances greater than those recommended by the manufacturer. Excessively large valve clearances just lead to valve train wear, dropped valves and valve seat recession. You’ve been warned.