Motorcycle Exhausts - effect on Performance

I was browsing a few motorcycle related forums this morning and came across a question I have seen asked many times - does a performance exhaust really have any effect on my motorcycle's performance? Well the answer is yes - particularly if you have a two stroke. I thought I would make an attempt to explain why - I ain't no professor but hopefully will be able to make some sense of it or provoke some discussion at least. I will start off with four strokes as that's what most bikes are these days.

Think of an engine as a pump - it sucks an air / fuel mixture in, burns it and lets the exhaust gasses out before  starting the whole process over again. Valves control when the gasses are allowed in and out - those valves and the timing of them are controlled by a cam shaft. All pretty straightforward so far. Thing is that physics gets in the way and we have to consider how those gasses expand and flow from the inlet side of the engine to the exhaust.

Many years ago some clever bloke found that if the exhaust valve was open for a while when the inlet valve was also open the hot gasses flowing out of the engine would pull the unburnt volatile air / fuel mixture in from the inlet side. This practice became known as port scavenging. Thing is that the inbound mixture has mass and because it has mass it also has inertia. That means that when the inlet valve opens there is a tiny time delay before the mixture actually enters the cylinder - this is significantly reduced if the exhaust gasses can pull that mixture in. The more you open the throttle the more mixture is made available, the more mass it has them more you need too pull it in. If, however, you leave the exhaust valve open for too long the unburnt mixture flows straight out of the exhaust and all you are doing is wasting fuel and killing rain forests. 

One of the ways of taking advantage of this effect is the use of a power valve, which changes the exhaust characteristics so that the optimum flow occurs over a wider rev range. ideally you want the fuel air mix to rush in to the point where the combustion chamber is full but no mixture has entered the exhaust before it is burnt. This is where you get your power band - that wonderful point in the rev range where everything is in perfect sync and your bike comes alive.

Clearly if you mess with the exhaust you will change the flow characteristics and you will alter the bike's performance. Most after market exhausts on four stroke bikes can only move the point at which the power comes in, their effect is fairly marginal - maybe a max of 4bhp power increase may occur at peak, often a power loss at another point in the rev range is usually the pay back. Manufacturers spend a fortune perfecting their exhausts to work in harmony with the bike, it's fuel system and ignition control. Modern fuel injected bikes need retuning of the ecu too really get any benefit at all, the limiting factor is always emissions and noise - some countries now have made it illegal to change exhausts from standard.

Now, if you have a two stroke the whole thing is a hell of a lot more complex, although the principles are the same in part. the exhaust is way more critical on a two stroke as there are no valves to control the flow of gasses, it's all down to transfer ports, primary compression and exhaust gas expansion. it's broadly speaking to complex for my head but it's easy to see how the exhaust on a two stroke is so critical and what a huge difference it can make to engine performance. 

In a two stroke engine the fuel / air mixture does not go direct from the carb in to the engine's combustion chamber - first it has too enter the crankcase where it is compressed - this is called primary compression. there is then a port called a transfer port that allows the compressed fuel / air to enter the combustion chamber, the port is opened and closed by the piston moving up and down. The exhaust port is also opened and closed by the piston, the movement of gasses is therefore far more critical in a two stroke - hence the tendency for two stroke engines to have much more noticeable peaks in power - known as a power band - than an equivalent four stoke engine. I forgot this last year and broke two bones in my foot when the two stroke I was on hit the power band and I lost the back end. Idiot.