regulating pipe back pressure for broadened powerband

Discussion in 'Performance Mods' started by jaguar, Jul 27, 2013.

  1. jaguar

    jaguar Well-Known Member

    Here is a drawing of the system I put on my pipe for my 55cc bicycle engine. It broadened the powerband by increasing pipe back pressure at mid range (for less loss of intake charge out the exhaust port) and lessening 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
    Fabian likes this.

  2. bahramu

    bahramu Member can't help the itch to keep improving these things,huh? That's awesome. This reminds me of exhaust cut outs for cars. I bet you could implement that in our exhausts too, like have the torque pipe optimized for low/mid rpm and hit a switch to run the pocket bike exhaust for top end. But it seems like your piston/valve design would be better than a cut out
  3. wheelbender6

    wheelbender6 Well-Known Member

    Great idea. You are throttling the exhaust as well as the intake, and without a computer.
  4. jaguar

    jaguar Well-Known Member

    yeah, why use a computer when you can use a self regulating mechanical system?
    Why do you think this mod throttles the intake?
  5. Fabian

    Fabian Well-Known Member

    Now that is a very clever and simple idea.
  6. jaguar

    jaguar Well-Known Member

    I guess I only imagined it increasing the powerband. That's typical for any modification we do. Wishful thinking. Well, I try to always double check my initial impressions with repeatable tests. 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.
    Last edited: Aug 4, 2013
  7. keatonx

    keatonx Member

    Good job making it work!

    This makes me think of a similar system I was planning on putting on my 2 stroke lawnmower bike. I was gonna stuff the crankcase, and drill an opening going to a separate chamber that lowers the CCR. Then I was gonna hook a valve that closes off the separate chamber to what USED to be the governor system. This way, CCR is lowered at low rpm as the chamber is open, but is increased at high rpm when the extra chamber is closed off by the governor
  8. jaguar

    jaguar Well-Known Member

    I tried to think up a good system like that but can't decide on what to use for a plunger. Probably something similar to this with an o-ring to seal it. Don't know how long that would last putting up with the engine heat etc. It would be an interesting experiment to see if any noticeable gains can be had with low or high RPM power.
  9. keatonx

    keatonx Member

    That's what I was originally planning to do, have a plunger/piston that changes the volume of the chamber. (wait, by plunger you meant a piston, not a slide valve, right?) This would also give the engine a quick boost of power when the plunger moves in and shoves all the chamber's volume into the engine as the RPM rises (momentarily supercharging the motor). But I thought this would be too complex, and have an extra place for air leaks, and as you said melting O-Rings. So I decided it would be better just to have a valve to close off the chamber at high RPM. This would control the amount of fuel that could rush in and out of the chamber in the time given, and close off the chamber completely at high RPM.

    I'm also considering just having a small hole that the fuel mix has to pass through, between the chamber and the crankcase. At low rpms a considerable amount of the fuel mixture would have enough time to pass in and out through the hole, but at high rpm there wouldn't be enough time for fuel to pass through the hole (simulating a higher CCR). It would work the same way a capacitor conducts high frequencies, but absorbs low frequencies.
  10. jaguar

    jaguar Well-Known Member

    That is a super excellent idea. And super easy to implement. Just drill a hole in the crankcase and bolt or weld on the chamber. Figuring out hole size is the only real challenge.