Hi Max,
Here's the long post I sent a week or two ago. It took me a while to find where it was lurking on my computer. I hope the forum allows something this long, last time I had to split it into two.
Kim
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Here's some theory stuff to think about, but it might not have a lot of influence on your decision.
When the exhaust port opens and the gases are pushed out, two things happen. One is that there is a flow of gas down the pipe (you knew that). The other is that there is a pulse wave in the gas.
Remember that the pipe is already full of gas, just air to start with and exhaust gases after starting up. Therefore the flow has some restriction as it moves into the crowded pipe and tries to displace this gas.
You can think of the pulse in terms of a large flat-fronted truck on the highway. It pushes lots of air in front of it, so it has a high energy leading edge. And then the body of the truck fills the middle. And then there is an area of low pressure behind it. You can feel this when you tailgate. The exhaust pulse is like this, with a high energy front, the bulk of the pulse, and the low pressure area bringing up the rear.
When your exhaust valve opens that pulse rockets through the pipe and bounces off things like obstructions in the pipe. One of the things it bounces off is the air at the exit end. After all, the outside air is not moving as fast as the exhaust gas is travelling through the pipe, so for the pulse it is like running into a wall. If the header pipe flows into a larger pipe or muffler the gas flow also slows and makes a similar airspeed barrier which induces a return pulse.
When the pulse bounces off things, like the slower air in the muffler or the exit end of the pipe, it returns up the pipe towards the engine. So now we have one wave going out and a return wave coming back to the exhaust valve. That return pulse is called back pressu
re. It can be good and it can be bad for how your engine runs.
What happens if the high pressure leading edge of that wave hits your exhaust port just as the valve is opening to expell the next lot of gas? It blocks the flow of the next exhaust pulse and your motor can't rid itself of the waste, so the next intake stroke can't breathe in as much fresh fuel/air mixture as it wants. The motor suffers.
However, what happens if the low pressure trailing edge of the wave hits the exhaust port as the valve is opening? The low pressure area sucks the gas out of the exhaust port, just like tailgating behind a truck drags you along. This is called exhaust scavenging.
So what we have is the possibility that this return wave can inhibit your motor or make it more efficient. The irony of all this is that it depends on engine speed whether the pulse does you good or bad. This is why some exhaust pipes, especially racing two stroke engine pipes, are tuned to a certain size to give maximum scavenging effect in a narrow rev band.
Now I'll say something about back pressure in general. People say you need back pressure for the engine to work properly. This is partly true. It is not back pressure so much as controlled flow that you need. Imagine taking the exhaust pipe off the engine altogether so the exhaust port opens to the fresh air. This seems like an efficient system, but it isn't. There is too much back pressure in an open port like this. Is that a surprise? The thing about opening to fresh air is that the exhaust gas rushes from the port and hits a heavy wall of air which effectively blocks its passage from the motor. What the engine needs is to be able to feed the gases into a place where the flow is already happening, such as an efficient exhaust system. An efficient system generates scavenging at the exhaust port, not back ure return waves. But we generate the scavenging effect as a by-product of the back pressure wave that exists in the pipe.
Now let's get to 4to2 or 4to1 sys
tems for your bike.
When you combine two pipes into one you get a combined return wave from each cylinder coming up to both exhaust valves. This means that for every exhaust stroke of each cylinder there are two return pulses. At low revs this means that the likelihood of the return wave (back pressure) closing off the exhaust port is greater because the exhaust port is open for a longer time than at high revs.
Now let's think about 4 into 1 pipes. You now have four return pulses all interracting with the pipe and each pulse having an effect on each exhaust port. At low revs, when the exhaust port is open for the longest time, the likelihood of those extra pulses having a bad effect on the flow of fresh exhaust gas is considerable. And so what we get is relatively poorer performance at low revs.
When the engine is spinning at high revs you get a funny thing happen. The exhaust port is opened for the least time, yet it is flowing the highest amount of gas. This means that the gas is being pushed much faster out of the port, and the pulse is much "sharper" as a result. The effect of the higher speed of the gas,and the different "shape" of the pulse, and the quicker open/close cycle of the exhaust port, means that there is less of the bad effect of the return pulse.
Not only that but one effect of the "sharper" form of the exhaust pulse, and therefore of the return pulse is that it is more likely that the low pressure effect, the scavenging, of each of the four return pulses at each exhaust stroke will help your motor to better breathing.
The "sharper" pulse has a quicker attack and a relatively greater portion of low pressure to each wave. Imagine the truck on the highway again. At 100 miles per hour, the high pressure wave at the front will pass quickly but the tailgating zone behind it will increase.
When you get better scavenging at high revs like this you need to get more fuel/air mix into the motor as you can't move stuff out the exhaust if you are not getting
it in at the carby, so you need to look at having larger jets. And here there is no standard, you have to measure or test / run / test etc. If you don't rejet you run the risk of running too lean and can burn out exhaust valves.
So, where are we in all of this? Generally back at your point of decision. Do you get a 4to1 or 4to2? The most stable exhaust system is a 4to4, one pipe to each cylinder. However, this has problems of balancing just as one carby per cylinder requires balancing between cylinders.
The next stable setup is 4to2 as the flow of exhaust gas is not effected by so many pulses in the pipe. The engine will be relatively happy at low revs with this setup and will continue to be happy through the range.
The next option is a 4to1. This will lose some low and/or low/mid happiness but will give an slight increase in high end power due to the scavenging effect.
Remember that as engine size increases you will not notice a certain level of power increase or decrease. And on a big capacity bike you have ample power for most riding requirements. If you are riding a GS it means that the general power curve of the bike probably satisfies you, otherwise you would go out and get a high performance sport bike (like the current GSX-R1000) where everything is tuned to run between about 10,000 and 11,000 rpm. As the dedication to top end power increases, so the exhaust design becomes more critical and the rev range it prefers narrows. Of course, if you are into high end power, just ask Dom Scotto what system/jets he is running, that is a cool set of 4to1 pipes.
I hope this makes some sense. Even if you decide to put original pipes on the bike, knowing how a performance exhaust system works for best efficiency is worth the reading.
Kim
