How do tuned pipes work

This pressure wave travels through the exhaust gases that are in the pipe at the speed of sound.. Imagine a stream and you throw in a rock. The waves from that rock will travel down the stream faster than the speed of the water.

The length of the front cone and its distance from the cylinder header length determines the amount of time that the pressure reducing wave from the exhaust does it work in emptying the cylinder of exhaust gas and then assisting the fresh mixture up from the crankcase into the cylinder. It should arrive there when the pressure in the cylinder is low but there are still exhaust gases that need to be extracted. If the header length is too long then the wave is arriving later than optimum and the exhaust gases are not fully removed from the cylinder.

The front cone needs to be long enough to generate a wave to help the fresh mixture into the cylinder but it also needs to continue working long enough to allow some fresh mixture into the first part of the header. This is the mixture which will be forced back into the cylinder. If it is too short, then it does not allow mixture into the header. If its too long, then it reduces the length of the rear cone and that needs to be long enough to force all of the unburnt mixture in the header to be forced back into the cylinder.

The pressure wave continues into the rear cone and immediately sends a positive pressure wave laws of physics! The strength of the wave increases as the rear cone gets smaller and the length is made so that the returning pressure wave from its very end at the junction with the stinger coincides with the point of exhaust port closure. When this most critical length start of stinger to exhaust port is correct, then maximum power is achieved. If this critical length is too short then the returning wave forces hot gases back into the cylinder, dramatically increasing cylinder combustion temperatures.

If this length is too long the maximum power will not be achieved because maximum supercharging or cylinder filling will not occur, although power in the corners will be better because the tuned length will coincide more with the reduced rpm in the corners. In conclusion we can see that the front cone length and distance from exhaust port is very important to achieve maximum cylinder filling and to pull some mixture into the header and the distance from piston to start of stinger is extremely important to get maximum filling supercharging of the cylinder.

Here are some some simple calculations for gas engines where the exhaust gas temperature is not affected by nitro content. For these calculations you can measure the pipe length between whatever points you want to, but the norm is from plug to widest part of cone. If you take a boat that is running well and pipe length has been optimised then to alter rpm or exhaust timing the following calculations may help to find the new required pipe length.

Pipe length from manifold face to widest part of front cone. If rpm is 15,, exhaust timing is degrees duration and pipe length is mm then Firstly you work out the constant for your set up. If you want the engine to rev at 16,, the equation changes to.. The speed of sound within the exhaust system is dependent upon the EGT exhaust gas temperature. The higher the temperature the longer the pipe length must be for a given rpm..

EGT will vary with the following factors.. The tuned length L as shown in the diagram is the length that most people use as a comparison. This is OK as a comparison but the length that is most critical is TL. Many different pipes can be used on an engine but that tuned length TL will always remain the same within a few millimetres for a specific rpm if all other factors remain constant, nitro content, oil content, air density, temperature etc. This applies to all two stroke model engines , petrol gas or Glow powered nitro.

We know this from many many bench and on the water tests conducted on many different engines. To elaborate: If you were running a tuned pipe at its optimised length the length that is giving most power or speed and that pipe had no flat in the centre section and you wanted to change to a pipe with a flat in the centre or belly section.

You should measure TL on the old pipe and then set TL on the new pipe to the same length to give you a starting point for adjustment. The last part should remain almost the same whatever you do to the exhaust timing or rpm. Shorten the pipe length and rpm will be higher, lengthen pipe length and rpm will be lower. If you increase the exhaust timing and rpm stays the same then pipe length will need to be longer. If you increase the rpm but exhaust timing stays the same then the pipe length has to be shorter.

If you can measure the rpm of your motor and exhaust timing, then you can use a simple calculation to show how much you need to change the pipe length when altering ex timing and rpm.

STINGERS Stinger length should be separated from stinger diameter because although they are linked, in practice you would need to make a big change in stinger length to affect the backpressure. Stinger diameter is crucial to the pipes operating temperature and hence the power production.

If the stinger is bigger than optimum them making it even bigger will have little effect but by sleeving it down then you will be able to find the size that gives best power. Normally a smaller stinger will improve top end power because the exhaust gas temperature will increase which will have the effect of a shorter pipe length. It spits out a nice sheet full of information including cone lengths and diameters at each end. Using that info, you can then make the cone printouts using the cone software that came with the program.

Pretty slick! See the attached screen captures. Now that you know the dimensions of the pipe you are going to build, we need to make some patterns. The software I used came with a sweet little add-on that allows you to print cone layouts by entering the length of the cone and the diameters at each end.

It would then generate a flat layout that could be printed on multiple pages, cut out, and taped together. Once you have all of the cones and cylinder patterns, its simply a matter of tracing them on to your metal. I don't like making lots of cuts, so i tried to consolidate as much as possible by lining up long cuts with each other. Once it's layed out on the metal, it's time to cut!

This is where the Squaring Shears come in handy- its great for the long straight cuts see Picture 4. I used a set of electric shears to cut out the curves, but aviation snips would probably do the job just as well see Picture 5.

Once all of the parts are cut out, you are ready to start forming! Ready to bend?! There are two ways to form the metal for cones or cylinders. The best way for the cylinder is to use a Slip Roller like the one in Picture 1.

It gradually rolls the metal tighter and tighter until you have a nice cylinder, then the top roller lifts and you slip the metal off the end of the roller Pictures Finished cylinder, ready to weld! See Picture 8 This works really good for straight cylinders, but its a little harder to do with a cone that will have different diameters on each end.

I've been told a trick to this is to pinch the short end of the cone with pliers to hold it back allowing the longer side to get sucked through faster see Picture 9. This didn't really work for me because my rollers were to big for the cones I was making anyways.

I think with some practice, though, it would work really good. The other option is to "micro-brake"- see the next step! If you don't have a slip roller, don't be too worried because there IS another way! A sheet metal guy taught me this trick and called it "Micro-braking". It worked great. The theory is simple- You can take a flat piece of metal, put hundreds of small bends in it, and "curve" it around to make cylinders or cones.

Here's how: Mark the cone by measuring each curve and then dividing the length by a set amount. For example, I decided on one of my cones I wanted to bend it every 7mm. I set my dividers to 7mm and marked that edge every 7mm. You need the same number of marks on the short end, so the marks are going to be a lot closer together. In my case, at 7mm I had 45 marks on the big end see Picture 1. That means I needed 45 marks on the other side, which worked out to be a mark every 4.

Its pretty simple math because you have all of the diameters for each side of the cone from the printout, you just have to find the circumference which ends up being the length of the arc you are working with. So that may be kind of confusing. Lets put it this way- your markings will end up giving you a pie shaped section on the cone- always going down the center line of the cone.

Now we bend. Slide the metal under the fingers of the brake all the way until you can line up your first set of marks. Picture 2 shows a couple of bends done. You can see in Picture 3 and 4 that I'm bending it too far. Not a real big deal, its pretty easy to straighten out. The split needs to be as near perfect as possible to be welded.

If I had taken more time on this one thing, my welding would have gone a LOT smoother! Get it as close as you can! Picture 8 shows how close they should be for welding. Time to melt some steel! Something else I've always wanted to be able to do is weld thin metal. This project was my excuse to give it a try. I do not claim to be any sort of an expert in the field of welding, so please take my tips and suggestions with a grain of salt This is just what worked for me.

How to weld thin metal I feel the most important thing for making these welds work is to have the two pieces aligned as perfectly as possible. Doing butt joints like this means there is a lot of heat right on the edge of the metal, making it very easy to melt or blow holes through it.

Not fun, or pretty. I spent some time with some scraps getting my welder dialed in correctly see Picture 1 - take your time, practice, and figure out what works for you!

Not the best setup for this situation. Ideally, a wire feed welder with argon gas shielding should work better for thinner stuff, or if you have access and the skill to use it, a properly set up TIG welder would be the best option.

My first attempt was done with an Oxy-Acetlyne torch, mainly because I wanted to practice using it. It can be done, but some machines make life a lot easier! Once you've got your welder figured out, the first step is to tack the piece together. Clamp the ends tightly as shown in Picture 2 , then tack weld just a small spot to hold it together it about every inch as shown in Picture 3.

Now for the hard part. I tried a couple of different ways to weld this thin metal together, and the best way I've found so far is to "zappy-zap" Kind of a variation on "skip welding". Skip Welding see Picture 4 is when you weld a short strip, skip to a different section and weld another short strip, and keep repeating until the whole piece is welded. It worked okay, but I found that it would build up too much heat and melt holes in my project. Then I started just zapping it for a second, pausing for a second to let it cool slightly, zapping it for a second again just long enough to get a nice puddle to form to ensure penetration , then letting it cool, and just repeating the whole way down the joint.

This ended up working really well for me. Tips on welding thing metal 1. You travel faster and build up less heat, making it less likely to blow holes in your project. This way, the hottest part of the weld is over top of THICKER metal because you ended on top of your previous weld making it less likely you blow holes in it. If you do blow holes in your metal, using a filler rod for acetlyne or TIG welding along with your MIG welder can put enough metal in the hole quickly without getting it too hot and making the hole WORSE.

Just hold the rod in your left hand, keeping it over the hole, and weld over top of it with the MIG welder. Welding your expansion chamber Individual parts: Pick one of the cones or cylinders and start by welding up the joint as explained above.

I started with the belly of the pipe the center cylinder. Tack weld it together, then weld it up. You can do one piece at a time and then weld the pieces together, or you can weld up all of the pieces and then weld the pieces together. Joining parts: Again, I can't stress how important it is that the joints line up as perfectly as possible.

If you took your time during the layout step, they should be pretty accurate, but you will have to do some tweaking to make sure the ends you are joining are both perfect circles and that they line up with each other.

The stake table and grinder can be your friend here- just be sure if you grind anything it IS absolutely necessary. Once you are sure they are lined up, tack the 2 pieces together as shown in Picture 7 , then clean up the burnt flux if you are using a flux core welder, and start welding! See Picture 8. You can get things very close this way if you are patient Keep on adding pieces until you are done! See Pictures So you have a pipe How the heck is this thing going to fit on the bike!? Whatever machine you are building this for, its very unlikely that the port comes out of the engine in a way that will allow you a straight shot out to put your pipe on We have to "curve" the pipe around both to avoid obstacles AND to make it shorter- 'cause you really don't want 3 feet of pipe sticking out behind your bike!

There are two ways to do this- the easy way, and the hard way. Grahm Bell's book And it's HARD. Requires a lot of thinking, planning, and way more welding. Goes like this: 1. Decide where you want the "bend" in the pipe. Cut the pipe straight through. On one side, starting at the widest point of the circle the diameter in the direction you want the pipe to bend, cut off a sliver at the angle you want the pipe to go.

See Picture 2. Weld the sliver back on to the same piece of pipe, just on the opposite side. See Picture 3. Weld the other half of the pipe to the first half. Repeat for every bend As you can see, its very hard to get everything to line up nicely. I did the entire first cone this way, and was not pleased with the results. I ended up building a new first cone, and used the easy way on it.

I feel it turned out MUCH nicer- and while it is more angular it was so much simpler to do. Decide where pipe is to be bent and at what angle Picture 7 2. Divide angle in half angle as measured from the centerline of the pipe, see Picture 8 , cut pipe in direction you want it bent at half of the desired angle.

Rotate one of the cut halves of pipe degrees, weld together Pictures There are programs out there and if you enjoy math, you could sit down before you started building and figure out all of the angles for the cones and have done it in one fell swoop- basically eliminating this whole step. I don't enjoy math that much, and since this is a one-off project, it was easier to build the pipe, then cut and fit like this.

Just keep cutting, welding, and test fitting Pictures to the bike as you go to make sure the pipe is going the direction you want it to go. My pipe came out better than I had hoped, but I do still think it could have been a lot better as far as fitting to the bike goes.

Hey, its only my 2nd attempt, can't complain too much! You can see in Pictures How the pipe turned out. After I had experimented with both methods of bending the pipe, I went back and re-did the first cone of the pipe using the "easy" method, and it turned out much better. Mounting the pipe can be a little tricky. Two strokes in general vibrate a lot, at a high frequency. The pipe is also containing pressure waves- expanding and contracting rapidly.

This can cause parts to shake and metal fatigue, so you want to be sure the muffler you've spent so much time on is well mounted with strong brackets or "Hangers" to keep the pipe from shaking, fatiguing, and eventually breaking. To help absorb some of these vibrations, I built some "isolation" mounts. These are not true isolation mounts in that the bolt goes all the way through the rubber and bolts solidly to the frame. Basically, its just a large rubber washer that will hopefully absorb some of the vibrations.

Building the Isolation Mounts 1. Drill a hole through a rubber stopper- the hole needs to be big enough for the bolt and the stopper needs to be big enough for your application.

Cut the stopper in half- it doesnt need to be that tall! Building the Brackets Once you have the isolation mounts built, you can start figuring out where to place your pipe hangers.

According to the books, the best way to build the hangers is to use the same metal used for the pipe, but layer it so that it's twice as thick. This way, you also have two tabs to weld to the pipe- one on each side. You will need to build your bracket to fit where it needs to go- take your time and lots of measurements! If you are using isolation mounts, be sure to take into consideration that it's a lot thicker so the bracket will need to be out further.

Attaching the Brackets To attach the bracket to the pipe, tighten the pipe to the cylinder and make sure the pipe sits where you want it to be. Tighten the bracket to the frame. Tack the bracket on to the pipe, then remove the pipe and finish weld the bracket. If you are bending or forcing the pipe or bracket into position, things aren't going to line up well when you remove the pipe and it can put pressure on the pipe causing it to fatigue and fail.

Place brackets so that no large section of the pipe is left hanging freely- again, thats inviting metal fatigue both from the engine vibrating and hitting bumps while riding. In these pictures, the entire back half of my pipe is hanging free. I plan to add another long hanger behind the brake lever, as shown in Picture 7.

Now that you think your pipe is "done, it's time to test. Ideally, you dyno'ed your bike before you did all of this work, or you at least know how much horsepower your engine makes stock. Check the fuel, oil, and whatever else is important, and fire it up! VERY important Note! As was brought to my attention by maxpower49, I forgot to mention the domino of putting on a tuned pipe.

Since a tuned pipe will generally flow more air through it, the jetting in the carb needs to be changed to match. If more air flows through the engine with the same amount of gas, it will cause a lean condition which can be VERY bad for your engine! With a custom job like this, it's a process of trial and error that involves a lot of swapping of jets and lots of spark plugs. It's safer to guess rich too big on the main jet and then move smaller in little steps than it is to go larger in little steps.

Here's the quick version of how to jet your engine: Make an educated guess and guess on the rich or large size. Install the jet and a NEW spark plug. Start the engine, let it get good and warm, and then shut it off. It works best if you can actually ride it for a bit to put the engine under load. Remove the spark plug and compare to the chart in Picture 6.