More than just a length of plumbing with a can on the end

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LOOK at any four-cylinder sports bike and you’ve got four lengths of pipe and a big can at the end to keep things quiet. There you go, one exhaust, job done... well, not quite.

Bung any old set of tubes on a four-cylinder bike and it’d run, only it might not run very well, while the chances of getting the most from the motor would be very small indeed. That’s because the exhaust does a lot more than keep the noise down and make a handy escape route for hot gases. Provided it’s designed correctly, it can make big improvements in performance and iron out flat spots in the power curve.

Designers take advantage of pressure pulses in the system when they create a new exhaust. Gases aren’t just spewed out in a constant stream – instead they come out in " lumps " as the exhaust valve on each cylinder opens and shuts.

These lumps are actually high-pressure pulses of gas which travel down the pipe at high speed. At the same time a low pressure pulse is reflected back up towards the engine. Choose the right length of pipe and this low-pressure pulse will arrive just as the exhaust valve is opening, helping to suck out the next lump of exhaust gas.

On a multi-cylinder engine you can use the low-pressure pulses from one cylinder to help suck out, or scavenge, the exhaust from another cylinder.

Sounds straightforward, and it would be if only engines didn’t change speed. But they do, which means that a low-pressure pulse which reached an exhaust valve at 4000rpm might only be halfway towards it at 8000rpm. So you get two basic types of four-cylinder exhaust, known as 180° and 360° systems. The figures refer to the crankshaft angles between the pairs of cylinders whose exhaust pipes are being matched up.

In a typical four-cylinder motor, the firing order is one-two-four-three, so a 180° system on this engine has the pipes from cylinders one and two paired up, along with cylinders three and four. This tends to work well at low revs. In a 360° system, those cylinders which are 360° apart in the firing order, ie one and four, plus two and three, are paired. This works best at high revs.

So, exhaust systems either work well at high revs or at low revs, but not both. As a rule, touring bikes have one sort of system, sports bikes the other.

The 2000 Honda FireBlade, on the other hand, effectively has three types of exhaust thanks to a rotary valve in its H-VIX (Honda variable intake/exhaust) system. With the engine spinning at under 2900rpm, the valve directs the gases from cylinders one and four into the pipes from cylinders two and three respectively. Pipes one and four are closed off completely after the valve, but this doesn’t affect power because the volume of gas at these low revs is quite small anyway. And the system has the characteristics of a low rev friendly 180° exhaust – boosting torque at the bottom of the scale.

From 2900rpm to 7000rpm, the valve turns to its next position, letting gases pass straight through as if it wasn’t there. This leaves pipes one and four paired up, plus pipes two and three, changing the exhaust into a 360° type – which is best at high revs.

The very clever bit (as if all this wasn’t brain-stretching enough already) is that at 7000rpm, the valve turns again, this time opening up two channels for each entry pipe.

So for example, gases from entry pipe one can pass straight through into exit pipe one as well as being vented into pipe two. Two also goes into one and two, while three goes into three and four, and so does four. This increases the area of pipe the gases can flow through, which reduces back pressure overall and also allows the designer to use a whole extra set – helping scavenge the cylinders more efficiently.

The system is now behaving like combined 360° and 180° systems with pressure pulses hammering about all over the place (and Honda’s exhaust genius knows exactly where they are... ).

But this lot was too simple for Honda. So the same motor which turns the valve in the exhaust also operates a flap in the airbox. At low and medium revs (up to 7000rpm) the flap remains closed and restricts the flow of air into the engine. This might not sound like a good idea, but you might have seen the effect Honda was after when trying to dam a stream as a kid. When the stream is wide the water flows relatively slowly, but as it’s made narrower, the water flowing through the gap moves faster and faster. The same applies with air flowing into the engine. At low revs the demand is less and the air normally flows relatively slowly. The problem is that this is not very efficient at filling the cylinders during the brief period the inlet valve is open. So the H-VIX airbox flap speeds up the flow of air by partially closing off the intake. Above 7000rpm the engine’s demand for air is much greater, so even when the airbox flap is open the air flows in fast enough to fill the cylinders efficiently.

Thinking about it can fill your head rather too quickly, but this is the bottom line: For the sake of a valve in the exhaust, one servo motor, some cables and some negligible extra electronics (the bike has fuel injection anyway, so the control system is incorporated in that), the Blade effectively has three different exhausts and two airboxes.

Which is why it goes like stink.

Honda might have the most complex exhaust valve system, but it wasn’t the first. Yamaha debuted exhaust valves in the OW01 race bike, then more famously in the 1989 FZR1000R – known as the EXUP after its Exhaust Ultimate Power valve system.

Much the same EXUP valve is fitted to the R1. It’s lighter and more compact, but does the same job of partially blocking the exhaust at low rpm. The R1 version is more sophisticated, as its movement depends not just on rpm but throttle position and even the speed at which the throttle is being opened. It also takes into account which gear is being used.

But the principle is unchanged. At low rpm the valve rotates to partially block the exhaust (at a point just after the four separate pipes converge). This blockage reflects high-pressure pulses back up the exhaust, timed to arrive at the exhaust valves just before they close. These pulses force some of the escaping gases back into the combustion chamber. And if that seems like an odd thing to do, it actually allows the engine designer to have a longer valve overlap period – the time when both exhaust and inlet valves are open together. At medium and high revs, this overlap boosts efficiency, but at lower revs it can allow some of the unburnt fuel/air mix to pass straight through. The high-pressure pulses keep it where it does more good.

The strength of the pressure pulses is varied by how far the valve moves to restrict the exhaust, but the main benefit of the valve happens when it’s doing nothing at all – in the 5500 to 7000rpm range. This is because both the exhaust and the valve timing can be fine-tuned to work best in this range without having to worry about the fact the engine would not run well from tickover to 3000rpm. The EXUP valve compensates by letting the engine " see " a low rev exhaust up to 3000rpm, then a second system beyond that.

It can’t match Honda’s three exhausts, but two is better than one. And the R1 isn’t exactly slow, either.

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MCN Staff

By MCN Staff