intake length and surface finish!!!

This is very well known in the automotive and motorcycle industry when it comes to porting heads and intakes on a carburated engine, there must be some form of turbulance to keep the fuel mixed well with the air, and the better the atomization, the more readily and completely the burn.
The smallest turbo I know of is the IHI RB31 which can work on an engine as small as 200cc, how effective it is on that small of an engine I don't know. the biggest misconception of the turbo is that it uses the exhaust gases to propel the turbine... This is true, but you also need heat... lots of it. That's the whole key to making boost, as the exhaust gases heat up, they also expand, and from this heat expansion we can have boost, otherwise, we get out only what we put in and that's why a turbocharged engine will produce no boost in a steady state where there's no load on the engine. You can actually feel this happening when driving a turbo car and aproaching a hill, as you start to go uphill, you will feel the car slightly accelerate without giving it more throttle, the engine may still be producing vacuum but it'll be producing less vacuum partially due to the load applied to it and partially to the heat expansion of the exhaust gasses.
This is all referenced in books from the likes of Corky Bell, who I've had the pleasure of knowing personally and working with in the past, and Hugh McInnes.
Turbocharging a 2 stroke engine is still quite debatable, but has been successfully done. Due to the lack of an expansion chamber in a turbo setup, the boost level needs to be higher before seeing any gains, and the boost tends to get blown right out the exhaust port, the solution for the lack of an expansion chamber is to simply make more boost where there's a noticable power increase over stock power with an expansion chamber, then the power comes on very strong, and the resistance of the turbine wheel in the exhaust path by using a smaller turbine helps prevent the boosted charge from blowing right out the exhaust port... there's a little more to it, but that's the general concept.

Final decision here is that turbocharging a China girl engine would be impractical due to lack of availability of a small enough turbo, and we hadn't even discussed how to keep the bearings lubricated etc... not to mention the overall structural integrity of these to begin with...

Now going back to the way fluids pass thru a tube, we do need some turbulance on the intake side to keep the fuel atomized, a longer intake tube will give more torque due to the increased velocity of the gasses going thru, but even more velocity can be had by tapering the intake so the air coming in has to funnel thru a smaller opening on the other side, this is what velocity stacks do, and they're usually seen on the intake side of the carburettor... Short stubby stacks are great for top end power because they let a lot more air enter the system, but they take more air before they come into effect, now for torque on the bottom end, you'd want a narrower tube that's much longer so the air has more room to accelerate, there is some math involved here to "tune" the port size and length to get the best possible gain at a specific rpm, but if we were to add a tube the same size as the carburettor intake tapered 1.5 degrees and let's say about 6" long, you'll feel the power come on at a lower rpm than the same tube that's 3" long, or if you make a short stubby stack to funnel the air in that's flared out at a much steeper angle, the additional power would be noticed at a much higher rpm, this is why most people spend money on these short smaller velocity stacks and feel or notice no gains, simply put, the engine can't produce enough rpm for them to be effective. it's been my experience that the 1.5 degree taper is most effective with the most noticable gains, but we can also experiment here a little further...

I'm in no way trying to correct Mech Engineer's post, he's absolutely right. I'm just adding a little bit of what I knw to it and hopefully someone can experiment and benefit from some of this.

Found some pics with the sinet and outlet sizes that say 25m turbine inlet and 26mm compressor outlet, I'll have to look up the a/r ratio for the turbine and compressor, but this thing is tiny, it should be able to boost a 200 cc engine...

Ok... got the pics up.. ^^^

here's one more...

Yeah, when adding a turbo to anything that's not already turbocharged, the math is mandatory if you want it to perform... I'm thinking about putting one of these on a Buell Blast, basically it's a Harley sportster engine cut in half so it's just under 500cc. I'm also looking at the Garrett GT-15 but that one might be too big for that engine

http://turbochargerspecs.blogspot.com/2013/03/garrett-gt06-gt0632sz-32-trim-80-hp.html?m=1 smallest garret made.

I'm all for squeezing the most from our China Girls, but I think a high compression head & expansion chamber pushes the limits of a head gasket with only 10 lb/ft of head bolt torque. Plus putting a $200 blower on a $100 motor (minus all the Rest of the kit) doesn't make sense to me.

Back to the original topic. I've been toying with the idea of making a long tube intake for my bike. My idea involves using EMT conduit pipe available from Home Depot (or most decent hardware stores) and taking it to a gun smith to have it "Rifled" prior to bending & welding to the flange mount. I'm thinking that having numerous grooves spiralling down the length will help stir the mix, preventing fuel/oil droplet condensation & seperation. This should allow more power extracted from the given amount of fuel (homogenous mixture burns more completely, less waste=more power), better mileage, & cleaner emmisions(less unburned hydrocarbons).

For the sake of argument, would I want to sand my intake tube before rifling, after, or leave it raw rifled? And what size would y'all recommend? The sizes available to me are 1/2", 3/4" & 1". The 1/2" closely matches the intake & carb diameter, but may be too restrictive for top end speed if it is too long. 1" may flow better but may lose resonance pulse pressure & vacuum strength due to the extra volume. Your thoughts?

The rifling would probably help, but I don't think it would make enough of a difference compared to just running a 100 grit sanding roll thru to rough up the inside surface, it doesn't take much roughing on the inside to make it effective, just enough to cause a small turbulance at the surface will do the trick.

Do you have a way to bend this tubing in a 2" or 3" radius? If you can do smooth bends in the tubing instead of the typical pie slicing and welding technique, you may be able to pick up a few more rpm and more power on top if your engine is ported and you're running a decent pipe with a better head.

For the pipe size, you'll want to go 3/4" ID for piston port, and can go up to 1" ID if you are going to use a reed valve. Also for piston port induction, the length of the pipe does have more effect on low end and mid range power. You can start with a section of heater hose clamped to your original intake pipe and find the length that the engine runs the best at, then measure and make your intake pipe that length. Usually the length of the intake pipe will work best when it's volume is the same as your cylinder's displacement, but you'll still want to experiment a little longer and a little shorter to find what works best for your engine.

I would avoid using the 1" pipe without a reed valve, but with a reed valve, the length of the pipe isn't so critical and you won't see much gain with a carb bigger than about 20mm. The 3/4" pipe is right about 19mm so you could also use it on a reed valve engine with good results. If the tubing is measured by it's outside diameter, the inside will be smaller depending on wall thickness and would need to be measured to determine which size is best.

I have a dedicated pipe bender at work, you can get them at Home Depot, Lowes, Harbor Freight, Ace Hardware, etc... in the electrical section, usually at or near the conduit for about $30. I fill the pipe with sand & cap the ends before bending, helps to keep the pipe from pinching or distorting too much at the bends. Can't do a radius tighter than 3 1/2 - 4 inches, depending on the pipe diameter & bender being used.