rethinking expansion chambers

[Deleted member 12676]

Randall, here's the dyno comparing a pipe with a normal baffle and one with holes in it (perforated). Notice the perforated pipe also doesn't have the classic power dip before the pipe powerband.

 

[Wolfshoes]

A near to spec expansion chamber made years ago attached to a engine with stock carb and compression seemed to have some added pulling power at mid range, but not much of an improvement. The hand made expansion chamber had a saxophone shape ahead of the motor as pictured in a past thread, leaving room for silencer options. As part of a more complete performance mod upgrade I'm thinking of adding a stock chrome or dual black mufflers as a silencer since the layout leaves room for such a configuration.

 

[Deleted member 12676]

"near to spec expansion chamber" doesn't tell me anything unless you convey which specifications

 

[Wolfshoes]

Near to your Jaguar website specs for a 66 cc engine as my simple metal forming would allow. If the metal forming techniques, making a shape that fits the bike, create results, the time could be spent to make a more accurate to detail copy. The stock muffler is adequate to silence the engine. It seems to me, the only issue is back pressure.

 

[Deleted member 12676]

Stock engines have piston port only intakes which can't take advantage of the diffusers return suction wave because when the transfers are open and connecting the crankcase to the combustion area, the piston skirt has closed the intake port. So the only advantage for them is the return pressure wave from the baffle to increase dynamic compression and limit the loss of intake charge out the exhaust port. For full advantage from an expansion chamber you need a reed valve.

 

[Wolfshoes]

Along the way I picked up that obtaining the benefits was a package deal; including the reed valve and other changes.

 

[Bike_Builder]

Stock engines have piston port only intakes which can't take advantage of the diffusers return suction wave because when the transfers are open and connecting the crankcase to the combustion area, the piston skirt has closed the intake port. So the only advantage for them is the return pressure wave from the baffle to increase dynamic compression and limit the loss of intake charge out the exhaust port. For full advantage from an expansion chamber you need a reed valve.

If you open up the case the piston port can hold a big charge for the pipe to draw. My case is so empty that the air-box and pipe do all the work.
Having pressure on both sides is the real key to proper performance. And a circling valve works way better than a reed, it never fails either. The Russian engines have one, it'd be neat to have one of those engines!
( I can hear draw on my intake side when the exhaust is open)

 

[Deleted member 12676]

with the piston closing the intake port the vacuum wave reaches the crankcase thru the transfers but can't draw anything more than a very small amount of fuel mixture into the cylinder because it is drawing on a sealed space. But with a reed valve there can be flow thru the valve, to the crankcase, and on to the cylinder.

 

[Frankenstein]

with the piston closing the intake port the vacuum wave reaches the crankcase thru the transfers but can't draw anything more than a very small amount of fuel mixture into the cylinder because it is drawing on a sealed space. But with a reed valve there can be flow thru the valve, to the crankcase, and on to the cylinder.

So by pulling more fuel from the carb up through the valve and transfers means you loose more fuel that the motor failed to scavenge.. Not sure if that means any more power... Just means you can pull more fuel through the motor at a time without indication that the extra was burned...

I do though know an expansion chamber will work regardless if there is a reed valve, the chamber simply pushes back unspent fuel, which 2 strokes can generate a lot of. Unless you're saying that a piston timed intake results in 0 waste in regards to unspent fuel then the theory that you need a reed to benefit from an expansion chamber just doesn't hold enough water..

Honestly if you have already filled the chamber with fresh fuel and are loosing some to the exhaust portion of the stroke then what's the point of drawing even more on through to get lost... The point is that it's probably a side effect... Yes by having a reed the vacuum effect the chamber produces can suck additional fuel through and has the advantage of not being slowed down as much by the pull (we are talking about pressure waves here, you put a stronger vacuum behind that wave and it will slow down) but that simply means the pipe is being helped by the reed valve. Without it the cycle of loss and having a pressure wave shove it back in there is still the same, and so the flat line basic upgrade of an expansion chamber can still help you gain more power and even select a particular range of rpm to produce the most power at.

We know that the same pipe on a reed valve engine will have a different power band than one that doesn't use a reed valve, but otherwise they both work.

I would say that the opportunity to better fill the chamber more completely with fuel and at a closer to ambient pressure than not would be good if the chamber produces enough force to push a large portion of the lost fuel back in, that would yield higher compression while that effect was taking place.

Mind you you should be sure that the crankcase pressure (which is ultimately connected to the transfers) is low enough that the pressure wave traveling towards the end of the chamber is actually being held back, if it's not actually pulling anything extra and just simply running at the same speed the crankcase is decompressing into the cylinder then it would basically be a bouyent system, and neither one are actually forcing eachother to do anything, and for all intensive purposes aren't interacting if that was the case.

Think of it like putting air in a balloon, fill it enough to be capable of breathing it all in one motion. If you inhale too slowly (such as a slower moving pressure wave, not generating much vaccum force) the balloon deflates slower than it normally would if you just let it go. If you breath in quicker then it can be possible to suck the air out faster than it normally would deflate, if the balloon had more air then more air could be pulled continuously through, like it would with an expansion chamber in jaguars theory, leading to a larger "breath." You can also breathe in exactly at the same rate the balloon deflates, in which case the balloon isnt forcing air into you, nor are you attempting to remove it faster than the balloon would otherwise do, and so you aren't actually affecting eachother in terms of pressure other than you getting lightheaded from huffing balloons for science all day you sick freak.

No but seriously, are you certain that the cylinder pressure is actually getting lower than the pressure at the beginning of the pipe? If you can determine that then you have a solid answer, otherwise additional power generated is just circumstantial, it has to happen because that's the math of the system, by having a valve you give the pipe and yourself the opportunity to take advantage of the additional fuel that can be dumped back into the lungs of the beast.

 

[Deleted member 12676]

I like that you are using your noggin to have an opinion although it's in disagreement with me. Getting the brain cogs rolling again is the first step towards an advanced intellect.
I'm sure of what I have said but it's a bit difficult and painstaking to fully explain to expel all doubts. Maybe if I get motivated I'll make a video explaining it. In this day and age of youtube and all other kinds of video entertainment people are losing the art of imagination of what is being conveyed by the written word. When I was a kid I loved getting a bunch of books from the library during vacation. I think that is what developed my ability to imagine. It was said that Tesla could fully imagine all the parts working together of any invention he was developing in his mind. For those that don't know he came with the idea of alternating current as the common power source and also he invented different types of electric motors among many other things.