rethinking expansion chambers

[Deleted member 12676]

I’m rethinking the wisdom of putting a normal expansion chamber on a motorized bicycle. Here are my negative observations:
1) they are extra loud and it takes a special silencer to quiet them (which no one sells). the noise is due to the small stinger “condensing” the exhaust pulse so it’s more explosive when it expands when leaving the stinger.
2) they create a 2000 RPM powerband at the top of the RPM range but for a span of almost 1000 RPM just below the band they lessen the power due to the baffle wave suppressing the intake and not contributing to the engine compression at that RPM range.
3) once the RPM gets higher than the pre-powerband dip in power then the engine wants to run away but then there’s no higher gear to shift to. it’s action is all in favor of a multi-geared bike.

So I want to try an expansion chamber without a baffle. Here’s my design idea. This would still aid intake via the diffuser suction wave but not cause all the power to be concentrated at high RPM. It would give a ride more similar to a 4 stroke which is much more pleasant on the road.

call me crazy, I'm used to it. I am always willing to try the "ridiculous" if there's any decent chance of success. Are any of you tired of the noise and the peakiness of normal expansion chambers? (If you are under 20 then don't reply. I know the young love them. the louder, the better, ha!)

 

[Deleted member 12676]

this design is only better than a stock pipe if the engine has a reed valve. That is because on a purely piston port intake the crank is sucking intake in while the piston has shut closed the exhaust port and so the return diffuser suction wave can't help the intake. Only the baffle wave makes any difference on engine power with engines with piston port only intake.

 

[Deleted member 12676]

Here is another version of it that is half way between a normal expansion chamber and the full out version above. The idea came from karting pipes that are like this which have the effect of allowing the engines to rev out farther than what a normal expansion chamber allows.

 

[Bike_Builder]

It's a dump pipe, you could just go with a 7/8" tail pipe like the MZ pipe has and the "dick-buster". Those pipes add some to the power without robbing before and allow for a longer roll-out. Most guys run the "stock" C.D.I. with them and get four-stroke like results. I think using a hollowed out stock pipe with a 16" header is just as good or better than the pipes I've seen used. I listen for scavenging on the videos I see and I don't hear any real results. I'm running an air-box to pick up the low-end slack, really a must for a piston-port. There's not a real noticeable transition from air-box to pipe operation. She'll lift the front on air-box pressure or pipe pressure, but that's scary on the pipe.

 

[Bike_Builder]

 

[Deleted member 12676]

My Suzuki 100 had a pipe like the MZ and instead of a baffle cone it had a "wall" with a hole in it. That would also create a return pressure wave but I think it was farther back and so its wave would only counter the intake process at very low RPM. I couldn't even notice a dip in power and the power delivery was smooth all the way thru the RPM range like a 4 stroke. Only reason I switched to a true expansion chamber is for the top RPM boost because I sometimes race on the street against 200cc four strokes. It's a good design. I wish I had recorded the distance between the end of the diffuser and the "wall".

 

[Deleted member 12676]

that bike looks like the exhaust is unmuffled. for some reason that megaphone type exhaust is very loud. My idea is to use the megaphone design but try to muffle the noise.

 

[Deleted member 12676]

here's the graphs showing pipes return waves in relation to BDC (bottom position of piston), TC (transfers closing as piston rises), EC (exhaust closing as piston rises). The rising piston causes a vacuum in the crankcase that draws in gas/air through the reed valve from BDC to TDC (top dead center). Only between TO and TC is the crankcase open to influence from the pipe with its return waves. When the return baffle wave peaks before TC then its pressure nullifies the vacuum that the rising piston causes. The return wave graphs below are of my Suzuki 100 which revs to 8500 RPM and has a pipe powerband from about 6000 - 8500. So from 5000-6000 the baffle wave interferes with intake which lowers power. But if I lengthen its belly from 110mm to 500mm then it interferes intake around 2700 and aids compression around 5000. So this is an example of what would happen with an MZ type pipe. Designing it so the intake interference happens at a very low RPM is also hardly noticed because the intake time from BDC to TDC is long enough that complete intake happens anyway as there's enough time for that to happen after the interference of the baffle wave.

 

[Deleted member 12676]

here's the last graph that pertains to the message above:

Even though it counters the intake till TC, after that there is still 7.4 milliseconds until TDC for the gas/air mixture to enter the crankcase which is plenty of time.

 

[Deleted member 12676]

here's what was stock on my Suzuki. At the time I couldn't figure it out but now I understand.