Powervalves: the open and shut case
IT’S more than a decade since the term EXUP (Exhaust Ultimate Powervalve) entered the bikers’ vocabulary. Back then, most riders knew it was something to do with the exhaust and that it helped power.
And they knew it must have been pretty good, because it was used on the best sports bike of the time – Yamaha’s FZR1000 EXUP. The firm stayed faithful to EXUP – an updated version is on the R1 – but the other manufacturers didn’t follow suit. So was it just a marketing gimmick, a cool name to make a bike appear more high-tech? Or did it have a more fundamental purpose?
If it was so good, you’d have thought Yamaha’s rivals would have jumped on the technical bandwagon and come up with their own EXUP systems. The truth is they wanted to, but couldn’t, because of patent rights. Recently, however, those rights lapsed – enabling both Honda and Suzuki to develop their
own powervalves. And it doesn’t come as much of a surprise, because to get the most from their engines they really don’t have a lot of choice. The EXUP philosophy works and it works well.
Honda’s H-VIX (Honda variable intake/exhaust) went into production on the 2000 FireBlade, the Suzuki SET (Suzuki Exhaust Tuning) system emerged on the GSX-R this year.
The principle sounds unlikely at first, as the point of the valve, sited inside the exhaust pipes in the region where the four downpipes converge into two, is to upset the exhaust system’s harmonics.
Exhaust " harmonics " refers to the movement of low and high-pressure pulses within the exhaust gas, produced by the opening and closing of the exhaust valves in the cylinder head (don’t confuse these with the exhaust system valves we’re talking about...).
In any exhaust, a high-pressure lump of gas comes to the end of an open pipe and a low-pressure pulse is reflected back in the opposite direction. If the pipe is the right length, this low pressure pulse arrives back at the exhaust valve just as it’s opening again, and it helps to suck out the next lump of exhaust gas.
But this only works within certain rev ranges – you also get high-pressure pulses moving up and down exhaust systems, and if one of these arrives at an opening exhaust valve it makes it much harder for the gas in the cylinder to escape. The result is a loss of power, and with most exhaust systems you can’t do much about it. You can choose to have your exhaust set to operate best at high revs and suppress low-rev torque – or vice versa.
It’s this either/or situation which Yamaha’s EXUP and Suzuki’s SET valves are designed to eliminate. You set the exhaust up to work well at higher revs and then the engineering sorts out the bottom end. The systems alter the timing of the high-pressure pulses as they head back up the system towards the cylinder head, but only within the rev range where they’re going to reduce power (at lower rpm). At higher revs, where the exhaust system’s harmonics work in favour of higher power production, and where the flow of exhaust gas is greater, the EXUP and SET valves open up and interfere as little as possible with the exhaust gas.
Yamaha’s device uses a rotary gate powered by cables and a motor, while the Suzuki has a butterfly valve, operated in a similar way.
There’s some sophistication in the control available as the strength of the pressure pulses is varied by how far the valve moves to restrict the exhaust. But when the valve is fully open – on the R1 that’s in the 5500-7000rpm range – there is no restriction to the exhaust flow. The exhaust system and the valve timing can be fine-tuned to work best in this range. Normally that would cause problems from tickover to 3000rpm, but the valve – in its closed position – sorts out the problems here.
The SET valve operates over the same rpm scale, offering the maximum restriction up to around 3000rpm then gradually opening to full aperture at about 5000rpm. Suzuki claims the result is a six per cent increase in torque and a 17 per cent reduction in emissions.
Slowing down the exhaust gases means the valve also has a silencing effect, which allowed Suzuki to reduce the volume of the silencer on its GSX-R1000.
Honda’s H-VIX also uses harmonics to improve torque, but in a different way. A rotary valve is positioned in the exhaust where all four downpipes feed into it. At less than 2900rpm it directs the gases from cylinders one and four as well as two and three into the pipes from cylinders two and three. Pipes one and four are closed off downstream of the valve, but because the volume of exhaust gas at these revs is low,
power is unaffected. With this set-up, the system has the harmonic characteristics of a low rev-friendly exhaust, boosting torque.
From 2900rpm to 7000rpm, the valve turns to a second position, letting the gases pass straight through as if it wasn’t there. Pipes one and four are paired up (in a Y-junction farther down the system), as are pipes two and three (in a second Y-junction), effectively changing the exhaust system into a new configuration which favours higher rev power. Its efficiency ceiling is about 8000rpm, so at 7000rpm the valve turns to a third position, opening up two exit channels for each entry pipe.
Gases from entry pipe one, for example, can pass through into exit pipe one as well as being vented into pipe two. The latter 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, reducing back pressure and allowing designers to use an extra set of pressure pulses in the exhaust to play with to help them scavenge the cylinders more efficiently. Where the EXUP and SET valves are like having two different exhaust systems, so Honda’s is like having three.
But just understanding it is exhausting...