The more resistance an exhaust on a 4stroke has, the more torqie the bike gets at lower RPM's.
But it has a disadvantage on the higher RPM's because it cannot get rid of the exhaust gasses because of the more resistance.
This is pretty common knowledge to much 'small' 4 strokers, Take a Honda C50 for example, you have 25mm and 35mm exhaust for it if im correct, the 25mm provides lots of torque on the lower rpm's, but isn't good for high speeds/rpm's.
The 35 to 40mm is the other way round.
In theory every 4stroke bike does have a 'right' exhaust, depending on use etc.
Just to keep it simple.
The Concept of Back Pressure...Just for S$#^ts & giggles I took my Gold Wing (fuel injected) out for comparison of 3/4 to WOT. At 3000-4000 rpm WOT in high gear uphill on a long grade, I could feel no difference in acceleration when I backed off from WOT to 3/4 throttle. I guess you can attribute that to the enging not reving enough to be able to use the max volume of air at that RPM. My 750K used to gain power when I backed off WOT till I richened the mains. I would suggest trying the WOT plug chop, or you could pull the baffels and buy your riding buddies some ear muffs! If you have lost 20MPH on the top end, that is a significant difference. 66 dream, 78 cb750k, 02fz1, 09 wing
Vince, you discovered what most people never find out and that is that a bike will normally make as much power at 3/4 throttle as wide open. Back it off to 1/2 and teh chances are it won't slow down much and sometimes you can cruise at 80+ on not much more than 1/4 throttle on some bikes.
The reason is often that the carb size is not what is limiting power. For example, say your bike had a 20mm exhaust system. That tiny exhaust would effectively become the restriction at say 1/4 throttle because teh motor could not get rid of the exhaust gas and so the combustion chamber is still partially full of hot stale gasses when the intake valve next opens. The bike would be constipated. Example 2, large exhaust no restriction there but cams with low lift or short duration. At higher revs/load the valves are not open long enough or far enough to get enough combustible mixture into the motor. In that case the cam would be the restricting factor. In most of our old motors it's actually poorly shaped ports that have bottlenecks or are "out of balance" ie the changes in section do not allow for maximum flow but actually impede flow. In your specific case the question seems to be something changed and how do we get back the lost power. It would appear that the current exhaust configuration is less efficient than the prior set up at ridding the gas or it is setting up pressure waves that are detrimental to good cylinder filling. Jensen is right that to get the fueling right on a bike with non-standard intakes and exhausts the best way is on a dyno with gas analysis (a sniffer). That will optimize the current set up. Yamaha (and others) have tinkered with this endlessly with a valve in the exhaust designed to create an increase in back pressure up to a certain RPM and then it opens. The goal was to improve low end torque for street riding but have less back pressure at higher RPM for more top end horsepower. I had an FZR400 with an exup valve and another FZR400 with the exup removed (for track.. where low rpms didn't matter but weight did).. the difference was huge. Jep, the theory of this is pretty simple.
I expermented with this on my small CB80 (just a cb50 with a 80cc setup) . It's pretty fun to experiment with this on a small bike, I also have a Honda PC50 with a 24mm exhaust on it, the bike is slow , but there is no other moped that can beat the torq it has. 'Of course I've already taken a very modest position on the monetary system, I do take the position that we should just end the Fed.' - Ron Paul
This could go on forever, but it's not back pressure that you are changing with different sized pipes - it's primarily gas velocity and pulse speed a,d of course the ability of a certain size of pipe to allow the gas to get out.
A small pipe will let a small amount of gas through and as revs rise the point will be reached at which the pipe becomes and obstruction. Gas travels at a certain speed down a pipe so it takes time to clear. Raise the revs and it needs a bigger pipe or the gas has to travel faster. There is a point at about mach 0.5 at which teh pipe cannot carry any more gas, so it chokes the motor off. At a lower speed that small pipe generates higher gas velocity and pulse speed and that aids extraction. I have even seen people trying to measure pressure in an exhaust system and they soon find that mean pressure is not the issue and as a rule they cannot measure the wave activity. Eventually the pressure will rise as the pipe becomes choked. But it's really an issue of pipe diameter and length and not pressure as such.
The pic of you on your bike in a hard right turn shows short straight pipes and no cones. ????????. ......lm
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