Here's a 2 stroke question... Pipe/turbo/blown

[miedosoracing]

Got thinking about pipes because of a post on vital. So, if a 2 stroke was turbo'ed or blown, would the pipe need the return pressure that we all know works on 2 strokes? Or could you run a straight pipe, since you wouldn't need back pressure or would you still need back pressure? Pretty sure some of you tech guys will know this...

[2T Institute]

Just run the turbo at the end of the pipe where the tailpipe is. That's the way turbo sleds are set up.
Next question.

[CCOADY454]

It would definitely have to be a straight pipe with a turbo, because otherwise the returning gasses would try to spin the turbo backwards on the return cycle from the typical expansion chamer.

[CCOADY454]

Oh, yeah....unless you run it at the end of the pipe...like he said.

[miedosoracing]

You guys aren't understanding my question. I understand you can run a turbo with the typical pipe. My question is, do you need to use a typical?  Because normal 2 strokes need the back pressure to pull old air out and new air in basically. But since the turbo is pushing the air at all times, will it need that??? 
Here's my thoughts:  The turbo would send a backpressure until it was charged up and moving, so it would be adjustable for low end, where you place the turbo. You could move it from the front, to back and test at each location. Also, the RPM's would be out of this world. 

[2T Institute]

Still need a chamber. They don't rev any harder that is a mechanical issue limited by bore and stroke rod design etc etc.

[factoryX]

http://video.google.com/videoplay?docid=2683135400295297703# 

[TMKIWI]

That dosn't sound like any Vespa I have ever heard. 

[JETZcorp]

I would use a centrifugal supercharger if it were up to me.  Not as efficient as a turbo, but it would be a lot easier to figure out what your exhaust is doing without something doing weird things to your exhaust pressures, temperatures, etc.  Besides, the last thing you need on a motocross track is turbo lag.

I think you would still need an expansion chamber, because the function of the expando isn't so much one of back pressure, but of resonance waves.  When the exhaust port opens, the escaping gas creates a shockwave that travels through the pipe.  When it reaches the diverging cone (start of the chamber) the expansion causes a low-pressure wave to be sent back to the exhaust port, which acts to suck exhaust gases out.  But then when the original wave hits the back end of the chamber, it's reflected back to the exhaust port as a high-pressure wave.  This acts to keep the fresh air/fuel mix from leaving the cylinder and escaping out the pipe.  For a brief period, the flow of exhaust through the exhaust port can actually be reversed by this wave.  Unless your turbo would be able to perform both of these functions at the correct timing, I don't see it replacing an expansion chamber.

You are correct that the forced induction would make scavenging happen more quickly and that would lessen the need for the diverging part of the chamber, which is there to assist the flow during scavenging.  However, if you're running any sort of boost pressure at all, it's going to scavenge so thoroughly that you'll be blowing tons of air and gas out the pipe before it ever gets a chance to be burned, and that's true of a turbo or a supercharger.  So you'd have to very dramatically change your porting, you'd need a completely differently tuned expansion chamber, and I would probably want to see a trapping valve of some sort put on the exhaust port.

[Chris2T]

I would use a centrifugal supercharger if it were up to me.  Not as efficient as a turbo, but it would be a lot easier to figure out what your exhaust is doing without something doing weird things to your exhaust pressures, temperatures, etc.  Besides, the last thing you need on a motocross track is turbo lag. I agree about the turbo-lag thing, but if you did use a turbo the pressure coming out the stinger end would be stable enough to spin the blower without affecting combustion.

I think you would still need an expansion chamber, because the function of the expando isn't so much one of back pressure, but of resonance waves.  When the exhaust port opens, the escaping gas creates a shockwave that travels through the pipe.  When it reaches the diverging cone (start of the chamber) the expansion causes a low-pressure wave to be sent back to the exhaust port, which acts to suck exhaust gases out.  But then when the original wave hits the back end of the chamber, it's reflected back to the exhaust port as a high-pressure wave.  This acts to keep the fresh air/fuel mix from leaving the cylinder and escaping out the pipe.  For a brief period, the flow of exhaust through the exhaust port can actually be reversed by this wave.  Unless your turbo would be able to perform both of these functions at the correct timing, I don't see it replacing an expansion chamber. Agreed. and for high boost applications i'd go with double-walled thickness on the expansion chamber ;-)
You are correct that the forced induction would make scavenging happen more quickly and that would lessen the need for the diverging part of the chamber, which is there to assist the flow during scavenging.  However, if you're running any sort of boost pressure at all, it's going to scavenge so thoroughly that you'll be blowing tons of air and gas out the pipe before it ever gets a chance to be burned, and that's true of a turbo or a supercharger.  So you'd have to very dramatically change your porting, you'd need a completely differently tuned expansion chamber, and I would probably want to see a trapping valve of some sort put on the exhaust port. DKW used 2 stroke supercharging on their racing bikes back in the 1930's. Fuel consumption was stunning. Their solution? Humongous gas tanks ;-)