non OEM exhausts
 

Hi here are my thoughts on fitting an after market exhaust. The main reasons seem to be lighter wieght and sound. Personaly I want equal or more power and better sound. All standard exhausts are tuned to the engine but are all after market cans? I doubt it. An exhaust needs its first baffal plate to be at a set distance from the exhaust port which determines were the engine developes its power. Consider a two stroke the fuel air mixture passes through the engine and into the exhaust BEFORE ignition and is rammed back in by the rebound pulse from the baffal plate which has a specific sized hole which allows most but not all the exhaust gas to escape. The gas that cant get thro the hole piles up against the plate and bounces back up the pipe in effect super charging the engine. High performance four strokes use this system also. As the XT660 pops and bangs on the over run there must be some fuel air mixture in the exhaust and the above system must be in use. If I decided to fit an after market exhaust my first question would be has the exhaust been tuned to the bike on a dyno and where in the rev range has the HP and torque been increased. A louder exhaust could well be a too lean power robbing exhaust. This is not just a theory of mine I once designed an expansion chamber for a suzuki GT550 after studying gas flow through flat plate orifices at college during my engineering apprentice ship. I hope the info is of some use and food for thought.:blob1:

How right you are Keithy. I really do think that the Single can that I put on my Ten robbed the engine of power. It was toooo unbaffled and certainly was not tuned to the engine. Noisy does not produce power. The fuwl consumption went up as well. I went bakc to the standard silencer wnich Mr Yamaha in his infinate wisdom would most certainly have had his minions designing an exhaust to get the best from the standard set-up.
It's only us, when we start messing about that scrw things up. The popping is the AIS working, where a valve opens and allows clean air into the exhaust system effectively making it run very lean, hence the popping. It's for the emmission control. It's not that bad rally, compared to the single silencer that I took off.

Cheers Steve

I always remember my Dad talking about 'Back pressure' on engines.

Just a thought though, in your research Keithy2, was that rebound in the exhaust study with Catalitic converters in place or not? Would they also effect things before the pulse hits the baffle plate?

It would also be interesting to know how engines would be peforming now if manufactures didn't have to comply to all sorts of legislation but instead produce the motors to run how is best for them.

Another point to consider while I think of it, when altering you bikes plumbing regarding the exhaust is that a bigger diameter freer flowing pipe can effect the 'Engine braking'

I held back from a complete Yoshi on a DRZ as I found out from somewhere that the bend in the Header pipe was there to assist in engine braking which is useful in off road situations as opposed to stomping on the brake & locking up the rear wheel. The Yoshi header pipe had less bends in it which was good for acceleration, which suits the SM, but not so the E model DRZ.

exhausts
 

There were no such things as catylitic converters in those days. In a tenere the back baffle is what the converter is attached to i think, and the flow rate of the converter would have to be factored in, i think i have seen a picture of one and it looks like a cylindrical sieve. Talking of flow rates the diameter of the SMALLEST pipe in the whole system is the governing factor, big bore front pipes wont do much unless the whole system is modified to flow more air. Probly the best mod for a tenere would be keep the standard pipes and remove the catalitic converter from the back box.