Spannies !

For those who don't know, this is how they work.

Expansion Chambers.

The expansion chamber exhaust systems that are fitted to two stroke engines is a very clever idea. They were invented if you like by Walter Kaaden back in the 50's when he was working for the East German company MZ. Walter worked out that by using the pressure waves in the engine's exhaust you could in effect super charge your two stroke engine.

In very simple terms, this is what happens. When the spark plug ignites the fuel mix in the cylinder, it sends the piston down the cylinder bore. When the piston uncovers the exhaust port there is a rush of exhaust gas pressure out of the exhaust port and into the expansion chamber. If we look at the diagram of a Suzuki TR750 chamber below you will see that the first three sections of the expansion chamber (from the piston) look rather like a megaphone. Because it is this shape, when the exhaust pulse enters and travels into this megaphone and expands it creates a negative pressure or vacuum behind it. This vacuum sucks all the exhaust fumes out of the cylinder, but more importantly it also sucks the fresh fuel/air mix up the transfer ports and into the cylinder ready for the next firing stroke. If that's all the chamber had to do it would be easy. Looking at a TR750, each of it's cylinders are 250cc's. So if your expansion chamber could pull 250cc's of fuel/air mix into a 250cc cylinder that would be ok. Now here is the clever bit! What if the megaphone could be designed so that it could pull 375cc worth of fuel/air mix up the transfer ports? It can! But the problem is that you would have 250cc's worth of fuel/air mix in the cylinder and the extra 125cc' s worth of fuel/air mix would have been sucked out of the cylinder and into the first part of the megaphone. Obviously that 125cc's of fuel/air mix would be totally wasted unless it can be pushed back into the cylinder. This is where the second half of the expansion chamber comes into play. The second half of an expansion chamber looks rather like a reversed megaphone and what it does is actually bounce the exhaust pulse back up the pipe. By doing this it can use the reverse exhaust pulse to push that 125cc's of extra fuel/air mix back into the cylinder. This is effectively putting 375cc of fuel/air mix into a 250cc cylinder. Supercharging!

This is a very simplified explanation of how they work. To make them work effectively they have to be designed to work with a specific engine and the porting specs of that engine. The return exhaust pulse has to reach the exhaust port just at the correct time. Too early and it will also push some of the burnt exhaust gasses back into the cylinder before the piston closes the exhaust port, too late and some of the fresh fuel/air mix will still be in the expansion chamber when the exhaust port is closed by the piston and will be wasted. The dimensions of an expansion chamber therefore are critical. The problem lies it the fact that one set of chamber dimensions will not work for all the rev range of the engine. At low revs the chamber will be pushing burnt exhaust gasses back into the cylinder because the piston will be slow in closing the exhaust port. At high revs the piston will be travelling much faster and will close the exhaust port before the chamber has pushed all the fresh fuel/air mix back into the cylinder. Hence the reason a two stoke takes off when it comes on pipe, the rev range when all the porting and pipes are working in harmony.

Nicked from elsewhere, obviously!

Al, nip out and buy 3, right now!