The 11"-12"  22"-24" 33"-36" is for primary lengths into a collector. 36" is about as much as you can fit into a race car. The next primary length with any data is 102" inches to the collector, and in that case the muffler is under the rear bumper. Any length that will have 2 full exhaust pulses inside before the collector is the one of interest. I have read that the high frequency stuff tends to move along the outer wall of the pipe, so even a steel pack or glass pack muffler can work for a while, with the really annoying part of the sounds.

The 90 degree cut off the pipe is the loudest, any angle of cut away from 90 will be quieter. You can see this morphing to a fish tail muffler, which works well. The hammering will disassemble a fish tail unless it has some blades to support the sides but the long angle cut needs no such support. Removing heat also quiets the sound. If the headers have no cooling air supply they will glow orange and heat anything that can "see" the orange part. A thin wall bit of stainless stood off 1/2" or more, from the pipe will break the transmission of heat. 

Lynn E. Hanover

 

Here are some pics from today.  I have to safety some wires, hang the prop, fill the fluids, then I can make some noise.  If everything goes well, I can start her up this week.

 My exhaust questions:  You can see in the picture, that I stuck a 150* piece on the end of the muffler.  This isn't permanent, I just put it there for the radius.  I remember Lynn writing something about the exhaust length should be a multiple of 11 to 12 inches.  Is this for 6500 ish rpm?  Where do I start measuring?  I measured from the front rotor exhaust port to the back of the muffler and got 30".

 I can run a short bend and let the exhaust exit out the side of the cowling, right where it shows in the picture. But I am worried about the exhaust being too close to the fresh air intake.  The other option is to turn it inboard and down, so it passes over the engine mount tube and exits out the bottom of the cowling.

What is the appropriate angle for the end of the pipe as it exits the cowl so I don't have any back pressure? 

 Chris