Step 8: Making the flame tube
The picture below is what your every day flametube looks like. From left to right, the hole patterns have special names and functions. The small holes to the left are the primary holes, the middle larger holes are the secondary, and the largest to the right are the tertiary or dilution holes. (note that there are also some additional small holes in this design to help create a curtain of air to keep the flametube walls cooler)
The primary holes supply the air for fuel and air mixing, and this is where the burn process begins.
The secondary holes supply the air to complete the combustion process.
The tertiary or dilution holes provide the air for cooling of the gasses before they leave the combustor, so as to not overheat the turbine blades in the turbo.
The size and placement of the holes is a mathmatical equation at best and a logistical nightmare at worst. To make the process of calculating the holes easy, I have provided a program below that will do the work for you. It is a windows program, so if you are on a Mac or Linux box you will have to do the equations longhand. The program, Jet Spec Designer, is a great program, and can also be used to determine the thrust output of a particular turbo.
For the long hand calcualtions of the flametube holes and an in depth explanation of things, please go to our website at http://www.badbros.net/jetbike5.html
Before making any holes in the flametube, you will need to size it to fit into the combustor. As our combustor is 10 inches long as measured from the outside of the ring ends one side to the other, you will need to cut the flametube to that length (make sure you cut to fit your combustor length). Use the posterboard wrapped around the flametube to square up one end, then measure and cut the other. I would suggest making the flametube almost 3/16ths of an inch shorter to allow for expansion for the metal as it gets hot. It will still be able to be captured inside of the end rings, and will "float" inside of them.
Once cut to length, get going on those holes. There will be a lot of them, and a "unibit" or stepped drill bit is very handy to have here. The flametube can be made of stainless or regular mild steel. Stainless will of course last longer and hold up to the heat better than mild steel.