Exhaust tuning length

[hovel]

and provide sufficient backpressure at the exhaust (exhaust length is WAY more critical than intake length)

Not to hijack the intake tuning thread but Rusty reminds us of a very good point and I wonder if others have given this some thought. Look at the total length of the exhaust systems on the early XS. As years went on, the systems got shorter. We know that longer pipes produce more power than shorter pipes at low rpm's and vice versa for max rpm. Gary Hoos's street pipes are 42"-43"long and there is probably some science in that. Aside, I read some time ago that the ideal exhaust system was one single pipe per cylinder. This may have been written before we learned the benefits of proper length headers and correctly built collectors. My source may have been Phil Irving or maybe even Harry Ricardo - way too long ago. Look up "bundle of snakes".

 

[5twins]

BMW discovered back in the '70s running some of their R-bike twins in the Battle of the Twins race series that their twin ran best with a 2 into 2 system featuring long, gently tapered megaphones. This most likely applies to most other twins as well.

 

[arcticXS]

A proper exhaust system is definitely high on my priority list as well. If any modification does not improve performance in one way or other, it is not very useful imho.

 

[RC4MAN]

Joined header systems, ie. 2 into 1, 4 into 1 etc. have proven to improve scavenging of exhaust gases because there are more constant impulses pulling the latent gases out. it's probably less effective with v-twins or phased engines due to the uneven pulses. As an observation you'll note that none of the v-twin race bikes use them, Harley, Indian.
However nothing looks as sweet as twin pipes sweeping back with megas, heck anywhere, each side, together down low, up high, whereever

 

[Jim]

Look at the total length of the exhaust systems on the early XS. As years went on, the systems got shorter. We know that longer pipes produce more power than shorter pipes at low rpm's and vice versa for max rpm.

Good point.... and something that always seems to get lost in these discussions.
Tuning for exhaust scavenging, as a rule... applies to a specific rpm... or rpm range if you will. Race bike exhausts are tuned to give best scavenging where horsepower is greatest.... at the very top of the rpm range. That means low and mid ranges are actually off on power a bit. That doesn't matter in a racing application because the engine is seldom ever run down there.
So, there's no one size fits all silver bullet. Tune it for low or mid range grunt, and the top end will suffer.... and vice versa. I know this is obvious to those of you that's studied this, but thought I'd throw it out there anyway.