Exhaust Questions

Since I started to build supermonos I have been curious as to the science of 4-stroke exhaust tuning, especially now since compromises can be covered up with idealized jetting.  However, in taking a look at the EX650 exhaust, it is so unlike anything I have seen before that Kawasaki either finally figured out how to properly create an exhaust, a "lightbulb moment" after 40 years of racing, or, more likely, it is a packaging or aesthetic compromise.  Basically, the whole system appears far too short in length.  The primary headers snake around a bit in order to find some length, and the old Leo Vince slip on system does a full 360 with the tailpipe in order to get some length...

Take a close look at this pic and you can see the tailpipe turning full circle!

In doing the reasearch, I stumbled across this website that is based on the famous tuner A. Graham Bell.  http://www.mez.co.uk/mezporting/exhaust_length.html  If you go there, you can plug in information about the characteristics of your engine (single to multi) which will then spit out the ideal header length and diameter; proper collector length; and the length of the tailpipe PRIOR to fitting a muffler or slip on.  I punched in some scnearios and proofed it against Bell's book, and it did check out.

So, how did the EX650 do?  After getting the BBDC and ATDC figures, and putting the idealized hp peak at the 9000 rpm mark here is what is suggested ( based on 1 of the 2 cylinders):

Header length:  27"
Header ID:  1.5"
Collector Length: 6"
Tailpipe Length: 18"
Tailpipe ID: 2"

Sounds pretty typical.  I went out the to the EX and this is what I measured (based on the Muzzy exhaust system I currently have mounted:

Header length:  26.75"  CLOSE!
Header ID:  1.5" (note this is the OD, but CLOSE again!)
Collector Lenght:  4" (a bit short)
Tailpipe ID:  2" (again, this is OD, but a negligable difference)
Tailpipe Length:  2" (YIKES that is short) -- that is the length of the tailpipe before it begins to expand into the megaphone exhaust.

As you can see, the exhaust is quite stubby; according to the tuning guru, I need an additional 16" of tailpipe, over 1', before the muffler should even be added.  Makes for some interesting concepts for experimenting.  Would it make a difference?  Again, with proper tuning, it should.  A smaller tri-oval muffler could be tucked away, mounted close behind the RHS peg for ground clearance.  Perhaps a project for the winter?  We'll see.