Hi, I'm thinking of playing with my 999 exhaust, and making a more custom solution. I've been reading up on exhausts and mufflers and back pressure but wanted to see whether anyone can speak from experience on a v-twin ducati? the question: If properly compensated with custom fuel mapping, would removing the muffler or replacing with a simple small baffle tip cause a loss of power or any issues? i'm thinking of removing the under-seat muffler and redirecting the top cylinder pipe to go down under the bike and just outlet the exhaust through something like this: Cafe Racer Motorcycle Shorty Short GP Exhaust Muffler Stealth Pipe 1.75" BLACK | eBay If the "back pressure" is an issue I could 2 into 1 link them versus installing 2 of the above. What do you guys think? anyone with experience in similar projects?
You shouldn't really need back pressure on a 4-stroke. I have no restrictions or baffles in my 750 zorsts..........and if you think about it, not many piston engine aircraft had silencers either.......Spitfire for example....... Drag racers, another example...... The object of a four stroke zorst is to get the gases away as quickly as possible...... But I'll bet you have to richen the mixture to compensate.
Don't forget that the horizontal link pipe has a catalytic converter built into it. This would need removing or the whole pipe replacing with a non-cat version. The fuelling would then need addressing, best by a custom tune.
Thanks, so based on what you're all saying, as long as I adjust the fueling for the additional flow, there should not be any loss of power due to the removal of the muffler or shortening of the exhaust pipes? I understand the difference between a 2 stroke (where this would matter) and a 4 stroke engine, however from my research there seems to be quite a lot of opinions on the subject, as some say that even though the 4 stroke engine does not use the pipes, or the 'expansion' chamber to revert the fuel back into the cylinder, the back pressure is still needed for low and mid range power. I have not been able to find any facts about the subject just a lot of 'opinions'.
Further reading on that can of worms, err subject can be found by googling "what does an EXUP valve do"
I think you'd still need to tune the length to some function of the frequency of exhaust pulses to ensure negative pressure on the exhaust stroke. Erm. Somewhere from memory, and it may be mistaken, i recall the exup (yamaha) almost readjusting the length of pipe for better flow. In a clever kind of way.
Pipe length must be important as Ducati make the link pipes for each cylinder to be of equivalent length by making them volumetrically equal.
There's an awful lot written about exhausts, and seemingly a similar ammount of confusion. When we think about gas flow in an exhaust pipe, there are two seperate things going on. The flow of the exhaust gas is the main one, the speed of which depends on the quantity ejected by the cylinder, and the diameter of the pipe carrying the gas. The narrower the pipe, all things being equal, the faster the gas needs to go. Secondly, the pressure pulses which bounce around inside the pipe. These travel at the speed of sound, and exist as compressions and rarefactions. When the exhaust valve opens, the pressure gradient across the valve causes a positive pressure shockwave, which runs down the pipe, and then is reflected as a negative pressure pulse when it reaches the open end of the pipe. In an ideal world, you need to make this negative pressure pulse arrive back at the exhaust valve the next time it opens. The phrase 'back pressure' when applied to a four stroke isn't meaningless, but refers to tuning the physical properties of the pressure pulses in the exhaust to work in harmony with the engine, and not fight it. Proper exhaust design is a properly big subject, because you need to be able to model what is going on inside the pipes, taking into account the flow and pulse characteristics, which in turn are influenced by cylinder volume, and gas temperature, and any junctions between cylinders causing bifurcation of the pressure pulses. Best of luck!
Thanks Chewy, surely there is a lot of math involved and engineering in creating the perfect solution. Even more so, the question is posed in stock pipes how much of how the design is engineered is due to best performance and how much due to emission standards and sound standards. I think none of us are creating any quote on quote 'perfect' bike here, and my question would be more so towards, how much difference would it actually make? Are we talking about wearing the cylinders significantly quicker if everything is not perfectly designed? are we talking about possibly losing 20bhp? or are we talking about simply the design not being ideal but maybe making a 1bhp difference in the end? I'm guessing the latter as if the first was true, all these bikes with after market slip-ons and custom exhausts would give out quickly as most people just buy some after market exhaust, at times one that has not even been designed for their bike, and shove it on.
