Originally Posted by Biker Bry
You guys are missing a serious element of motorcycle tuning and I see it a lot. Especially when people have common "super big" exhaust problems.
There is also another reason to have back pressure.. (Or more restrictive exhaust). Keeping positive pressure in the exhaust keeps the pressure high enough to allow the exhaust gas to make it through the pipe without atmospheric pressure being allowed to mix with the exhaust gasses that are exiting the pipe. Air from outside the pipe cannot mix with the exhaust gasses, this causes backfires. Less pressure... more air and when it gets enough air to detonate it backfires. Remember you "fire" triangle... it applies here as well.
What happens to the idiot that decides to put big exhaust on his bike to make it "loud"? He gets pops and backfires when letting off the throttle. Then some "genius" decides that he is "lean" and rejets or remaps his bike to "adjust out" the backfire problem. The issue here being that the bike was already at optimum Stoichiometric air/fuel ratio... Yes, there is only ONE optimum air fuel mixture... 14.7:1
so you have a bike running down the road with gas almost pouring out of the tailpipe to stop the bike from popping and backfiring when the PROBLEM is the pipe that was installed, NOT the A/F ratio at all.
Too many times I hear people say "The bike runs lean from factory for emissions" - If you think that, you need a lesson on internal combustion theory severely. A bike runs best at 14.7:1... PERIOD.
Yes and no...
Any internal combustion gasoline engine runs most EFFICIENTLY at 14:7:1. This means that per gram of fuel burnt you get the most miles out of that gram of fuel..
That being said, 14:7:1 is TOO LEAN to make the MOST POWER an engine can make. For the most power generally you'll be in the 13:2-9:1 range.
At 13..... whatever ratio you won't have gas pouring out the tail pipe. In fact, the bike will run and start much better in the cold, especially if it is carbureted, don't believe that, try it for yourself.
And, for an air cooled engine, 13.... a/f ratios prolong engine life by burning core, therefore reducing engine temps.
Adding an open exhaust DOES NOT in any way hurt the engine if the engine is tuned correctly after that. Also, if you tuned it back to 14:7:1 the bike would run virtually the same as it did stock, but with more power, likely. I would argue that there is no OPTIMAL fuel ratio, only one that suits the needs of the vehicle, riding style, and mechanical setup the best.
Talk to the professionals on here, Hillsidecycleco, dave63, schmidtys and they will all tell you the same things I said, or close to it. I am no professional but I have taken the combined 100+ years of experience they have and try to make worthy posts based on fact and good reasoning, not hearsay and disgust at fellow riders and their exhaust choices.