Although the rotary is a 4 Otto cycle engine it tunes like a 2 Otto cycle engine. And it is very sensitive to inlet runner lengths and diameters and exhaust lengths and diameters. So it tunes like a big dirt bike engine. You cannot make a huge gain in HP by doing it right, but you can leave a lot on the table by doing it wrong. Except for the RX-8 engine the intake and exhaust are connected during overlap at BDC and the exhaust can interfere with the intake to poor result.

 

The number one factor in exhaust design is that both primary pipes join at the same length and enter the collector at the same angle. Look at the Racing Beat Catalog pictures of the street headers. Note the extra bends in the rear tube to result in the same centerline length as the front tube. Any design that does not collect the primary pipes is a non-starter, HP wise.

 

Primary tube diameters are just a bit larger than the exhaust liner. In most cases 1 7/8" to 2" ID is what you end up with. Measure the liner ID at the face. I gave up on the exhaust gaskets years ago, and Just lay on a bead of 100% GE silicone around each port about 5/8" outboard of the hole, and bolt on the manifold before it sets up. Never had a leak.

 

Primary lengths in multiples of 10"-12" looking for 32" to 36" to the collector if you have the room and can stand the complexity.  Ceramic coated inside and out would be good. Heat wrap makes an astounding difference. You can touch the exhaust while the engine is screaming. The race car is not drivable without heat wrap on the muffler. The interior sheet metal turns from white to brown with paint blisters. We run heat tape and a shield over  a 4" ID stainless packed Borla.

 

After the collector ending at 2 1/2" then up to 3" for the run to the muffler if any. Only round will last very long. I thought the Spintech looked promising with all of the vanes supporting both sides. If the design allows for a large volume to let the pulses drop to subsonic, velocity is traded for pressure and generally the muffler is hammered to pieces in short order. After the collector there is little affect in differing pipe lengths. In the RS series and in the Racing Beat catalogue there are recommendations for lengths that include having the collector right at the

muffler 102" from the flange. Good for overall RPMs, but not particularly good at any specific RPM. Even with the Mazda racing muffler at the very end of a long system under the rear bumper, it was a good idea to wire up both ends of the muffler so when it comes apart you don't drop pieces on the track. 

 

If you have minimal back pressure, any leaks will result in air being drawn into the system with much popping and banging on spool-down.  

 

The system has to be stainless. You must shield either the pipe (Easy and light) or the equipment you don't want melted, (harder and heavier), or it will overheat. If the exhaust system can see it (line of sight) it will melt.

 

If the engine can move and part of the exhaust system is tied down tight, it will fracture a tube and set a fire going. Add or fabricate a flex joint. Allow for a length change with temperature changes.

 

That's a start.

 

Lynn E. Hanover