Rotax exhausts and matting 

Tuned pipe exhaust

In the chapter entitled ‘Mixture flow through the two-stroke engine’ you will see that the exhaust system can be designed by the engine manufacturer to incorporate specially tapered diffusers and baffles.  If the manufacturer hits the sweet spot with the correct selection of the angles and lengths of them i.e. a ‘tuned pipe’, one ends up with an engine and exhaust system that produces far more power that what would be expected from the same engine with no exhaust at all.

The Rotax engine is no exception, and apart from the Micro and Mini Max classes, incorporates a tuned exhaust that was designed to provide the correct back pressure to divert as much unburnt exhaust gas as possible back into the engine to maximise potential power.  This can easily be seen by comparing the pictures below of the tuned systems used on DD2 and Junior/Senior Max respectively, and that of a non-tuned Micro/Mini exhaust.  That said, the parallel muffler portion which is attached at the end of each of them and does the job of killing much of the noise generated by the engine, also looks very similar in layout for all three categories.

DD2      / Junior & Senior Max / Micro/Mini Max

  

Why matting is important

The majority of exhaust maintenance centres around the matting insert used to deaden the noise level.  Remember that the Rotax exhaust muffler despite having the appearance of being sealed by virtue of the pop rivets one sees at the exit end, is outside of the sealed engine and hence, like the carburettor, it falls on the Competitor to ensure that it complies with the relevant specifications.  The matting inside the exhaust silencer has a service life of around 20 hours, and because of this relatively short lifespan and the fact that it can’t be checked from the outside of the system, the maintenance aspect can easily be overlooked.

A few years back, once the matting started to burn away on Rotax engines fitted with a power valve (PV), the engine began to lose performance and ran louder and sounded ‘unhealthier’ than other competing karts.  This was largely due to the burnt matting that resulted in an increased pressure build-up inside the muffler, which in turn was responsible for causing the PV to open earlier than desired.  The nett result was a drop off in power, and hence any serious competitor was constantly putting in new matting to stay ahead of the curve so to speak.  With the advent of the EVO configuration, the opening and closing revs of the PV essentially became one that was controlled by the ECU and the matting problem became a thing of the past.

The fibreglass matting covers the muffler perforated insert tube (a.k.a. the ‘stinger’) and as mentioned, is used to lower noise levels.  Due to the heat within the exhaust system, the matting burns and degrades over time at the end closest to the engine.  This can sometimes be identified by the engine running leaner, and you may even hear some popping sounds occurring at certain times in the rev range.  A broken stinger tube was a common problem particularly in the Mini and Micro Max categories, but Rotax now supply a redesigned one that has hopefully overcomes this problem.  That said, on any category it’s a good idea to check the exhaust and all mounting points for any wear or cracks, and replace the pipe, mounting rubber bushes, attachment springs, and brackets as required.

Over and above the fact that you could have the engine’s power adversely affected, you may not be aware that there is a requirement in the current technical regulations that specifies a minimum mass of 140g of the ‘used’ matting to be present at all times (more especially should a technical check be conducted).  This value is reduced to 110g on the Mini Max as it uses a slightly different size of matting when new compared to all of the other classes.  Please be aware that any specific values and/or part numbers quoted were correct at the time of writing – therefore, it is always best to consult the most current technical specifications and any supplementary technical regulations that were subsequently issued to confirm the latest details applicable to your class.

  

Micro and Mini Max exhaust headers

After much experimentation over the last few years, Rotax have overcome the problems of exhaust matting burning away quickly in the Micro and Mini categories, and by making these changes the performance of the engines in the same class are now very consistent.  The header pipe on the exhaust and also the stinger itself have some externally visible identifying marks that set them aside from the older versions that are no longer legal to use.

The current exhaust header for these classes is shown above with its older counterpart to the left.  Note that it has differently shaped hooks (refer arrows), a 1mm wall thickness (was 1,5mm) and uses a ball shaped socket connector at the manifold end.  Only the newer version on the extreme RHS is now allowed.  Differences in the stingers are discussed in the next section.
  

Matting and stinger details

The only matting for legal use in the Micro, Junior, Senior Max and DD2 classes has a size of 480mm x 270mm (±10mm on both dimensions), has a mass of between 176g and 238g when new, and must be supplied via Rotax on part number 297982.  For the Mini Max class, the size changes to 490mm x 180mm (±10mm on both dimensions), it has a mass of between 119g and 163g when new, and has a part number 297985.  The reason for the smaller size of matting on the Mini becomes evident once you compare the stingers of the three classes when you will notice that the Mini has a much larger diameter of perforated tube than the others.  So, in order to wrap a decent amount of matting around this tube and still fit it into the muffler housing that is essentially the same size for all of them, the width of the matting in the direction it is rolled onto the stinger needs to decrease i.e. it is only 180mm compared to 270mm for the others.

There are 3 different stinger tubes in current use as shown in the diagrams below, viz.

  • Part 273210 for use on Junior Max, Senior Max and DD2 mufflers.  This tube is perforated all the way to the end closest to the engine.

  • Part 273211 for use on Mini Max.  The stinger has a welded-on portion closest to the engine end and is easily recognised externally as it has an ‘X’ stamped onto the flange (refer arrow in the picture below).  Furthermore, the perforated tube diameter is also larger than the one used on the Micro Max.

  • Part 273212 for use on Micro Max.  This insert is not perforated all the way to the end closest to the engine as denoted by dimension ‘d’ in the sketch below.

Matting replacement procedure

Should the matting not have been changed on a previous occasion, your muffler will still have its original rivets in place.  These need to be drilled out with a hand drill and a 4mm drill bit starting with the rivet closest to the top.  On the Junior, Senior and DD2 mufflers, loosening off the springs holding the muffler itself to the exhaust will allow you to rotate the muffler to gain access to the last two rivets.  Unfortunately you cannot do this on the Micro and Mini mufflers as they are one-piece welded systems connected to the exhaust pipe, so you need to rely a bit on your contortionist skills to get to the ones on the underside.

If you are working on a hot exhaust be careful you don’t end up getting scalded in the process on your hands or arms, so the advice is to wear suitable leather gloves if the exhaust is hot.  In the drilling process try to angle your drill very slightly to the rivet axis and also use a set of vice grips or pliers to hold the inner end of the rivet in place to prevent it rotating.  Once the rivets are all removed, put the springs back (if removed) in order to hold the muffler in place as this will make your job a bit easier.

The next step is to remove the stinger perforated tube inside the muffler itself.  Use a pair of pliers inserted into the very end of the muffler tube just after it makes a 90° bend.  Give the insert a slight twist whilst pulling on it to release its flange from the muffler tube joint where the pop rivets were installed.  Remove the stinger and set it aside.

You will now be able to see the old burnt wool matting inside the muffler tube.  Use a pair of long nose pliers and grab the matting firmly with the pliers.  Then ‘wind it up’ on itself by a few turns on the pliers and continue to rotate it whilst applying tension on the matting to extract it completely.  If you work carefully and methodically you’ll be able to pull out the mat in one piece, which is highly desirable, rather than removing it in bits and pieces.

You then need to ensure that all of the old matting bits that may either have burnt off, or broken off in the removal process itself, are removed from inside the muffler tube.  The best way (apart from making a heck of a noise by starting the kart’s engine and getting the exhaust gases do the job for you) is to use a torch to see where these bits are located, and then use a long piece of threaded rod to ‘hook out’ any remaining bits of matting.

Having cleaned up the muffler tube as outlined and inspected the whole of the exhaust for cracks, etc., you’re finally ready to install the new matting.  Inspect the stinger for visible cracks and replace it if necessary.  To obviate any skin irritation from the fibres, you may wish to use some gloves during the process of rolling up the new matting around the stinger as tightly as you can.  Also, to get it to fit nicely around the stinger’s tube might require you having to re-roll it a few times.

Hold tightly onto the outside of the matting and stinger to prevent the matting from unrolling whilst sliding the assembly back into the exhaust.  The assembly may be a bit of a tight fit, so use a bit of a rotary action (in the same direction as the matting was rolled on) whilst pushing it into the muffler housing.  The last few millimetres could be a bit of a struggle, so use your fingertips to get any squeezed up bits of matting into the muffler as the flange of the stinger enters the tube – a soft mallet may assist the process of this final engagement.

Next, you need to line up the rivet holes ensuring that the exhaust outlet pipe is facing in the correct direction.  The Rotax rules allow you to replace the rivets with M4 bolts or cap-screws and nyloc nuts.  You need to use a bolt or cap-screw that is at least 10mm long to provide the proper grip length for the nyloc nuts, and whilst doing so may also need to ream out the holes slightly with a drill bit to get the fasteners to fit easily.  Tighten everything up, and its job done.

EGT probe

Rotax allows the Competitor to fit an exhaust gas temperature (EGT) probe into the header pipe just after the exhaust port.  To do so, a socket needs to be welded onto the header pipe at a distance of between 50mm and 80mm from the ball joint of the header pipe, and this threaded socket allows the fitment of the EGT probe.  Whether you choose to opt for this addition or not is entirely up to you.  Some Competitors find it extremely useful for monitoring the health of the engine and also use it as a guide for the choice of main jet i.e. their mixture setting.  Note that the fitment of an EGT probe is illegal for use in the Vortex Rok series.


Emile McGregor - MSA Technical Consultant