Combustor and Flame Tube

To fabricate the new combustion chamber inlet, we needed a length of pipe bent to fit the curve from the turbo compressor to the combustor. A 3.5 inch diameter mandrel bent pipe was purchased from Ebay for this purpose. Mandrel bent pipes are much better than other bends, as the process used leaves a nice clean pipe which is the same diameter from one end to the other. Many other pipe bending techniques use a method which either crimps the pipe walls, or leaves a small inside diameter, both of which would not be good for our purposes.

Pipe bend rough cut

The pipe was rough cut by using the tried and true "just eyeball it" technique. Both ends of the pipe were cut, one cut to fit the compressor outlet, the other to fit the combustor.

Fitting to combustor

The angle of the end of the pipe nearest the compressor was easiest to cut. By figuring the angle and length that was to be removed, the pipe was cut simply and easily using electric tape wrapped around the pipe as a guide. The other end nearest the combustor needed to be hand fit to close all the gaps and make the best possible weld.

Pipe fit to combustor

After more than a few trips back and forth to the angle grinder, the pipe was a pretty good fit to the combustor housing. Although it is possible to get this completely perfect, a small gap will not hurt anything. Once the pipe has been welded on a porting tool will be used to smooth out the insides of the pipe to combustor union anyway.

Ground clearance

Keeping the ground clearance is a big concern so that the bike can get a full lean when making right hand turns. The new pipe sits right at the bottom of the combustor housing and should not have any interference. Also note in the picture above that the body panels of the bike will have to be cut to fit around the pipe as well. Having a good fit will mean better welds at this point. Since this part will be outside of the body panels, it will be more prone to scrutiny by viewers when the bike is shown at vehicle shows later.

Tight fit against the combustor

With all of the preliminary fitting done, there was only one small area to be removed at the very top so that the pipe would site completely flush with the combustor housing. The angle grinder and a flap disc were used to make quick work of the protruding part, and the pipe was a perfect fit!

Almost no gap

With the final fit, there was less than 1/16th inch gap around the pipe to combustor union. This is acceptable by my standards, and would fill in nicely with the weld bead.

View from the front

The engine is barely higher on the outside right as shown in this picture. This is to provide that ground clearance talked about in previous updates. The mounts for the motor have been modified too, so that the engine angle can be adjusted up and down easily.

Nice fit

The fit is good enough that it can be welded to the combustion chamber once the hole has been made in the side of the chamber housing. With the pipe in place against the combustor as shown here, a sharpie pen was run around the edge of the pipe to leave a mark where the hole would need to be.

Combustor and inlet pipe

The combustor and inlet pipe were removed from the bike to make the cut in the combustor housing. The inlet pipe was held up to the housing to make sure the fit looked good, and then the sharpie line was darkened to make it more visible.

Cut line

Although it is pretty hard to see in this photo, the sharpie line is near the top of the combustor housing. It is almost a teardrop shape where the pipe will meet the housing. The opening is so large and oddly shaped that I determined the best way to cut the housing would be to use a hole saw first, and follow that up with a cut off wheel in the angle grinder.

Hole saw and the now deceased drill

I used a hole saw to make the first opening in the side of the combustor, being careful to stay well within the lines drawn on with the sharpie. Those lines were drawn using the outside of the pipe, so they will have to be cut a little inside to make a good fit. Also of note is that our beloved drill "little orange" died during this procedure. He gave his life for the cause, and will be sorely missed. The magic left him at the same time all of the smoke did.

Teardrop shaped combustor opening

The hole was finished in the combustor by using the cut off wheel. The tear drop shaped opening is very clear in the photo above. The opening of the combustor is much larger than the opening of the inlet to the pipe. I have suspicions that this may actually work somewhat as a diffuser pipe, since following the airflow of the pipe you can see the cross section get larger before it enters the combustor. There is some debate about that between fellow builder Gary Richards and I, but only testing will see for sure.

Inlet pipe welded

After reattaching the inlet pipe and combustor to the engine, everything was lined up and welded together. A nice bead about 3/16th of an inch wide was used to close any gaps and get good weld penetration. The material on the pipe bend is thinner, so it is important to keep it from burning away. Getting a good bead of filler material in the weld joint is a must.

Inlet welded into position and cleaned up

The weld bead on the inlet pipe looks nice, and was cleaned up using a wire wheel on an angle grinder. Yes, I realize that there are way too many angle grinders in the shop, but it is much easier than constantly changing to different wheels.

Cleaning the threads

With everything welded in place, I wanted to clean up the threads of the nuts tacked to the end rings. Since flux core wire tends to produce a lot of spatter, this is really essential. If not done, the little metal spatters can cause the bolts to bind in the nuts. Not a good thing to happen. I ran the tap all the way through the thread in the nuts first.

Removing the tap

To save time, I just run the tap all the way in, and instead of backing it out I remove it from the inside of the combustor and pop it back in the tap wrench for the next hole. Sure enough, as I was tapping the holes I could feel little metal blobs inside the threads of the nuts. The tap cleaned them all out and everything was right as rain when done.

Attaching the end covers

Now that the combustor was pretty much complete, I wanted to get another look at it with the covers on. I could also take the bike off of the stand and see if I did get the ground clearance I was looking for.

Everything looks good

Everything bolted back together with no problems, and seemed to have the angle in it that I had been looking for previously. I am pretty satisfied with the look and fit of everything here, and can feel the time closing in on the first tests!

Engine in the bike

This is just a wider photo that shows how the engine will sit in the bike. Interesting to note, is the placement of the engine in that it is angled down towards the rear somewhat. It is a little hard to tell since the rear wheel is elevated on the stand right now, but if you draw an imaginary line between the front and rear wheels you will see what I mean. I believe that I will have to raise the rear of the engine in the frame anyway, so this is not a big problem.

Flame tube

With the combustor out of the way, we had to get the flame tube ready for the engine. To square up the ends of the pipe I wrapped poster board around the pipe and squared up the ends of the board. This gives me a line that will be a perfect cut around the pipe. I can then use the cut off wheel and a flap disc to bring the end of the tube right up to the edge of the paper template. One end of the pipe has been prepared this way already in the photo above. Once one end of the pipe is cut, it is easy to measure from that end to the length you need the flame tube and make a mark. You would then repeat the posterboard template trick to make a nice even cut on the other end.

Laying out the hole pattern

With the flame tube cut to length, I laid out the hole pattern using the template I created on the computer. The template is designed for a turbo with a 3 inch inducer, but will work with other turbos with inducers close to the same size.

Hole pattern template - Click to download

This is the template that I made for the hole pattern. You can click on the template image above to download the full sized template. Instructions are included on how to use it. If you make a successful engine using this template or any of my other information on the site, please be so kind as to post a link to me and give me some credit. Thanks!

Center punching and marking the holes

With the template wrapped around the flame tube, a center punch is used to mark the center of each hole by aligning the punch with the cross hairs on the template and giving the punch a couple of good whacks with the hammer. When all of the holes are punched, I use a sharpie pen to put a little circle around each hole so that I can spot them easier when at the drill press.

Drilling the flame tube

For this flame tube I used a step drill to make all of the holes. By drilling the first hole to the required size and then going slightly into the next larger step of the drill, a nice countersink was made. I then set the depth stop on the press to that point and all of the subsequent holes of the same size will be exactly the same. Using a step drill is much easier that using different sized drills and changing bits only to re-drill the same holes again and again until they are the right size.

Inside after drilling

This is what the inside of the flame tube will look like after drilling is complete. All those metal burrs have to come out, so I took the flame tube back to the drill press for a little clean up work.

Drum sander

I put a drum sanding attachment on the drill press and locked the spindle all the way in the down position. I can then use the drum deep down inside of the flame tube to clean up most of the burrs and other nasty metal snags left over from the drilling process.

Abrasive nylon brush

After sanding the inside of the flame tube, I use an abrasive nylon brush to clean up the remainder of the little nasty spots in there. I then use it on the outside of the holes as well to get them nice an clean, as well as deburred.

The finished flame tube

After a little final clean up with a wire wheel on the outside of the flame tube, the finished result is rather nice. It is hard to believe that the flame tube above is the same old ratty piece of metal that I started with. I did place the secondary air holes further back near the primary holes on this flame tube, as I was trying to keep the burn further away from the turbine side. Testing will show if this works out OK, and I will report what I find.

Flame tube going into the combustor

It was finally time for all of our hard work to get completed. The flame tube going into the combustor marked the final step in the process of rebuilding the engine.

Lining up the back side

Just like the smaller combustor before, the flame tube is supported at both ends by the rings. When almost in, you just give a little lift to the backside and it will slide right in.

A perfect fit

The flame tube fits right in, and there is just enough of an expansion gap at the end to keep it from pushing against the end cap when the engine is running.

View down the flame tube

This is the last thing those hot combustion gasses will see before they are macerated by the turbines whirling blades of doom! I don't think I would want to make this trip.

1/8th inch gap

As you can see above, the flame tube has a 1/8th inch gap at the end which allows the tube to "float" inside of the rings. If the need arises, this gap can be expanded to 3/16ths of an inch. If I did my math right, the expansion of the metal with the heat it should see will mean that the tolerances are just about right!

Next time around, we will be getting the test stand ready to fire up the engine. It has been a long hard road to get to this point, but we are almost there! Keep checking in for more updates, and remember to support the site!

Russ W. Moore
and
The Bad Brothers Racing Build Team

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