regulated pipe back pressure for broad powerband

[jaguar]

Here is a drawing of the system I put on my pipe for my 55cc bicycle engine. I hope it will broaden the powerband by increasing pipe back pressure at mid range (for less loss of intake charge out the exhaust port) and lessen the pressure at top rpm (to keep it from decreasing the diffuser return wave thats needed to increase intake at top rpm). The complete writeup on this experiment is at dragonfly75.com/motorbike/stingersizing.html

 

[jaguar]

While testing I noticed that top speed was lower with the regulated stinger, so for my engine the regulated 1.5psi is worse than the original .9psi. 2nd test was for 4000-5000 RPM going uphill. Results:
Minimum speed needed at base of hill to be able to climb it
16.5mph regulated stinger
14mph reed valve stinger
13.5mph straight stinger
So the straight stinger producing .9psi at top RPM was the best for mid range and top range RPM for my engine. I can't say that would be true for all engines. Someone wrote me that had tested different stinger sizes, using an expensive pressure gauge attached via 1 foot steel braided line to the belly, on his big bore snowmobile (that is used mostly at top RPM of around 8400RPM). He said that his engine preferred 2.5psi. He gave me the specs to his pipe and engine and the measured exhaust gas temperature and my pipe analyzing program revealed that the baffle was so far back on the pipe that it never had the baffle return wave arriving back at the cylinder at top RPM, only around 6000 rpm. So I think the 2.5psi was acting in place of the return wave at top rpm.
Since it's hard to completely predict outcomes it is almost always best to test different setups to see what works best. But these test results surprised me. I thought the additional pressure at mid range rpm would help lessen intake charge loss out the exhaust port and therefore boost delivery ratio, and therefore boost power. But the truth is that the back pressure also has a negative affect on intake since it puts a positive pressure into the crankcase that the piston doesn't nullify until it has traveled up a significant amount, to cause a negative pressure to start drawing in gas/air mixture from the carb/reeds. So that lessens the amount of intake drawn in to the crankcase which negatively affects delivery ratio.
I know the reed valve on my engine has a smaller than desired flow area which would limit intake at top rpm but I thought the longer intake time at mid range rpm would make that detail inconsequential. I could be wrong. (ha! like I'm never wrong!) Maybe on an engine with great intake flow area at the reeds the engine may like a higher back pressure. I don't know. If anyone wants to do their own testing and give me some feedback I will receive the info with open arms. The only other factor I can think of which may be important is the crankcase compression ratio. I think mine is about 1.4 to 1, slightly less than the most common 1.5 to 1.

 

[flyinzuki]

No suprise, FMF makes an SST pipe with a choke at the end of the stinger, its purpose is to increase pressure which increases temperature, which allows the sonic waves to travel faster to tune the pipe for a higher RPM. I have one of those on one of my RM's and it is awesome, but does want Octane. KTM went to a larger diameter stinger/silencer in 2011 for their 250 and 300's most people agree the new pipes work better, most of those users are riding offroad. I have a Dyno Port pipe on one of my RM's that has a huge stinger/silencer it is a beast off the bottom