Exhaust design

Hello again, as I’m making headway on my xs650 projects I find myself asking what exhaust is needed? I have a parallel twin but to use an o2 sensor will I need to run 2-1 exhaust to get full advantage of the wide band o2 system? Currently I have 2-2 non-crossover types. I can make x-pipes, or keep a factory h-pipe setup but that puts the o2 about 28-30" behind the port.

Hi Shaggy! I did my old 650 like yours, separate and loud! However, my on TRX 850, I made new head pipes and an H-pipe connector for my wide band sensor. We usually measure distance from the exhaust valve, not the port, but the concern is heat, so the measurements are a rough thing. Mine was 38"+/-. That was so I could weld the bung in a spot that would be open for the sensor. I think further back might be better, if possible.
I have a link here to Tech Edge who have a quick guide that might help.
"Tech Edge Wideband Oxygen Sensor Installation Guide"

Microsoft Word - LsuInstal.doc (wbo2.com)


I have had a similar thought, mine being on a Honda XR400.
Adding in an O2 sensor will surely cause a restriction to exhaust flow, as they aren’t designed to have a sensor in there, so I was thinking of adding in a FMF power bomb or mega bomb, basically an expansion chamber, and putting the O2 sensor in there.

Hi alby, just saw this. Two things in response to consider. First, the 02 sensor in your case, would be installed at or after the collector. (Twin head pipes?) Even with a single pipe, the sensor bung does not impede into the pipe and the sensor is designed to the same length. Thus, no restriction.

Secondly your power bomb idea is a good one…yet would need to be at that same location. The collector. I have only seen them installed into a single pipe. You might be able to find a drawing, but they are not truly an expansion chamber as one might think. They are built more like a resonator, with pass through section of the pipe ID and holes drilled into the pipe to allow flow into and out of the chamber.

It is a very good thought, if you put the bung/sensor at the belly of the bomb, however, may not add any value versus just a bung at the collector.

The thing to be wary of is that a wide band 02 sensor is more sensitive to heat than a narrow band. I recall that some sensors require installation after the collector ~6 inches, maybe more, but also need 18+ inches of pipe rearward to prevent oxygen intrusion. My old TRX 850 had an AFR setup that put the bung at the collector…and that was just after the rear of the engine, quite far from the head.