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Yamaha’s EXUP exhaust system on the new Apex sleds consists of a valve, located where the exhaust header pipes join the collector, which can vary the pipe’s cross-section at that point. Its purpose is to damp-out exhaust pipe waves that would otherwise return to the cylinders and push exhaust gas back into them during valve overlap.
Valve overlap is the period near Top Dead Center at the end of the exhaust stroke when the intake valves have begun to open, and the exhaust valves have not yet closed.
To understand the problem that EXUP was created to solve, consider how a properly designed exhaust pipe boosts torque. As a cylinder’s exhaust valves begin to open just before Bottom Dead Center, a pulse of exhaust gas is released into that cylinder’s header pipe. At full throttle and high torque, gas pressure remaining in the cylinder at the moment of exhaust valve opening is roughly 100 psi, so the resulting pressure pulse is large.
The pulse travels down the header pipe at the local speed of sound, and then encounters the sudden enlargement where the header joins the much larger collector pipe. The gas expands at this point, and it does so in all directions – including back up the header. This reflected wave of expansion (low pressure) travels back up the header, then into the cylinder through the still-closing exhaust valves. There, it has the effect of causing spent exhaust gas above the piston to be drawn into the pipe. The lowered pressure in the cylinder acts through the rapidly-opening intake valves to accelerate a fresh fuel charge into the cylinder. This in effect gives a head start to the intake stroke.
If cylinder volume is 250cc and compression ratio is 12-1, the volume of exhaust gas in the clearance space above the piston at TDC is 22.7cc, so from the standpoint of making torque, it is well worth our while to remove this volume of exhaust gas. A well-designed pipe, combined with enough overlap timing, can do this.
Making waves We can see how a well-designed pipe can affect things on a dyno printout in the region of peak torque. Experienced tuners know they can make this peak somewhat taller by scissoring the valve timing to make overlap longer – in effect “opening the window” to let in more of the helpful pipe wave.
Every helpful wave action in the engine also has a corresponding un-helpful action. In other words, positive and negative waves alternate continuously in the intake and exhaust pipes. At lower rpm, the exhaust waves move just as fast but the pistons move more slowly. Therefore instead of the helpful low-pressure wave arriving during overlap, you get the following wave of high pressure. This pushes exhaust gas from the pipe, back into the cylinder, and may even push it out through the intake valves into the intake pipe and the airbox.
When the intake stroke begins, the first gas drawn into the cylinder will be this blown-back exhaust, so the resulting charge contains a large percentage of inert gas instead of a fully fresh charge. The result is that torque falls. This is the dreaded “pipe flat-spot” that every user of 4-1 exhaust pipes has either felt or seen on a dyno printout.
It was to suppress this flat-spot that pipe builders switched from simple 4-1 designs to 4-2-1 designs. The second enlargement in a 4-2-1 pipe can be located so its low pressure cancels the positive pulse coming from the beginning of the collector.
Sadly, just as increased valve overlap timing can boost torque at the peak; it also pulls the flat-spot deeper. The result is the engine has a region of steeply-climbing torque up out of the flat-spot and onto the torque peak. While high torque is certainly desirable, that steeply-climbing torque may disqualify the engine from some uses.
If the engine is carbureted rather than fuel injected, the result is even worse, as carburetors don’t care which way air passes through them – they add fuel either way. This makes the flat-spot even worse by seriously enriching the mixture.
How EXUP helps Just as the barely open valves of a narrow overlap timing act to damp out waves that pass through them, so too can the EXUP valve damp out pipe waves by being partly or nearly closed at appropriate times.
By killing the positive pipe wave that would otherwise stuff the cylinder with exhaust gas, the nearly-closed EXUP valve increases torque at the flat-spot rpm. It is programmed into the valve controller to close the valve to a small aperture at the rpm that flat-spots would normally appear.
With EXUP nearly closed, a positive pipe wave cannot delay the intake process by stuffing the cylinder and intake pipe with exhaust gas, so the engine airflow increases. Naturally EXUP programming is done on the dyno by experiment.
Recently Aprilia used a like application on its RSV4 Superbike using the header valve as a “Jake brake” for rider Leon Camier, who likes a lot of engine braking. To generate engine braking the valve is closed with ignition and fuel injection in cut off, and the throttles are opened just enough to create the desired effect.