Exhaust Flow, Noise, and Design

By Terry Hayden

Exhaust Flow and Noise

Flow and noise do not have to be synonymous. According to my calculations, for best performance a muffler on the Max should be able to flow at over 300 cfm with a backpressure of no more than 0.2 psi.

Unfortunately, the straight through perforated bore with absorption wrap (eg glass wool jacket) that appears to be the most common free flow performance silencer is also one of the poorest designs for a motorcycle. This is due to the intermittent nature and high velocity of motorcycle exhaust.

To improve the effect of this type of exhaust it is recommended to have a smaller outlet than inlet. This encourages the wave motion in the jacket. It is also recommended to have a minimum of 1.25 inches of absorption material surrounding the bore. This equals at least a 4.5" can diameter for a 2" bore. If the reduction is kept reasonable (8% or less) there should be no noticeable loss in performance. So if considering a new exhaust, you might want to match the bore close to your collector/ tail pipe and chose a size of outlet tip slightly smaller to match the desired noise level. You then have the option of switching out tips vs. cans for performance changes...

If noise is a real problem, for now keep on the stock dual exhaust vs. restricting the single can. Yamaha did a good job on its design despite having to keep within noise restrictions. The collector box fools the engine into thinking the primary pipes are dumping into open atmosphere and smoothes out the pulses allowing a more continuous flow through the mufflers. The stock exhaust is heavy but it works fairly well....better than a choked off single can by far...(Creating just a single (1) additional pound of pressure, eg with too small of an silencer outlet, would reduce hp by 10 to 15%).

Having said all this, there is a real need in the motorcycle world for a free flow low noise silencer. A lot of work is being done in the automotive field directing exhaust sound waves through a muffler in a manner that they cancel each other out. Although more difficult some of this technology could be used for a m/c. As a minimum, a dual silencer system will provide the best opportunity to combine performance with real world daily street use.


Exhaust Design

The topic of m/c exhausts can fill books, but here is my theoretical two bits to the 4-1 vs 4-2-1 discussion.

There are two primary considerations involved in designing/choosing an exhaust. The first deals with the exhaust gas that travels at 200-300 feet per second. Here we are concerned with reducing the back pressure while maintaining flow velocity to get the best cylinder exhaust purging.  The second is exhaust pulses or finite-amplitude pressure waves which travel the speed of sound or at about 1700 feet per second.  These pulses can be tuned to create an effect known as scavenging (sucking additional exhaust out and intake charge in, with a properly timed negative pressure pulse).

Exhaust gas - General rule of thumb, a narrower diameter pipe will improve low rpm flow hence torque but may limit top end.  A wide diameter pipe will enable top end power but have poor low end velocity (eg great for the track but lacking on the street).   So choose the where you want your power band and set diameter accordingly.  Increasing or decreasing pipe diameter 1/8th will move the torque peak about 500 rpm up or down.  Varying the length of the pipe will fill in the power curve around the torque peak.  Shorter pipes fill above and longer pipes below.  A good starting point for exhaust pipe area is the circumference of the valves times the maximum lift  plus about 15%. A good starting point for the length of the primaries is 28-32 inches.

Exhaust pulses - When a positive exhaust pulse traveling down the pipe reaches an opening or increased diameter it is reflected back as a negative pulse. This negative pulse can be timed to reach an open exhaust valve to provide scavenging.  The rpm's that this occurs is at peak torque and continues in the upper portions of the bike's power range. However, there is a trade off. In the lower rpm's the valves are open longer and thus also allow a positive wave to hit. This usually occurs around 2/3's - 3/4's of the torque peak (between 4000-5000 rpm range on the Max) and is why you see a lot of dyno dips in the power band in this range.  The positive wave pushes exhaust back into the cylinder and in some cases, if there is enough valve overlap it will travel up the intake track and cause triple loading of the intake charge (air is drawn through, pushed back and drawn through again.)  Here is where the 4-1 versus 4-2-1 make a difference. The 4-1 will have a STRONG tuned pulse. Therefore it will have a higher top end (strong negative wave) but a bigger hole in the mid range (strong positive pulse hits while valve is open).  A 4-2-1 will soften and lengthen the pulses, as the 4-2 connection provides an interference wave in addition to the primary waves. A similar effect can be done with stepped pipes.  The top end effect won't be as great but it will be longer. The mid range dip will also be shallower hence, the adage that a 4-2-1 improves mid-range.

In a nutshell, the biggest decision for power is the diameter of the pipe and then the length. Choose these based on where you want your power band. Tuning the exhaust pulses with a 4-1 or 4-2-1 then should be done with the idea to compliment your first choice....and of course always have a high-flow muffler.