intake length and surface finish!!!

M

mech_engineer

New Member
A little bit of fluid mechanics!!!

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Velocity profile for a fluid as air in a pipe or intake : ) note "u"is the velocity vector the larger higher the velocity. Fluids have less velocity near the surface due to surface friction or shear. Imagine a lot of people trying to pass inside a hall, the ones nearest the wall will be in contact with it gets stuck to the wall because the friction between them,the ones in the middle will advance more faster. Here is the deal: the air velocity from 0 at molecular level at the wall to the max air speed at the center of the intake. Imagine the velocity vector like a cone inside the intake the top pointing to the direction of the flow. Remember the ideal flow do not exist and the "16v" theory that the air have more speed at the surface and less in the center is totally wrong. The only real situation that occur air velocity increase near the surface is due to acceleration of air cause by the negative pressure generated by an airfoil. Airplanes have negative pressure at the top wing creating lift for the airplane. In cars we use rear wing to accelerate the air at the bottom of wing generating lower pressure down force. But always 0 velocity at the surface.
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Here we got the three types of flow. The laminar flow is for smooth finish and short intakes including the ones with a normal surface that generate friction, the turbulent flow is for a very long intake or very small diameter intake like the factory one. transitional flow is the middle phase between laminar flow to fully developed turbulent flow. The best intake will be one with a not so smooth finish you can use sand paper 100 grid, the length need to be calculated to create a 25% (just my 2 cents) of turbulent flow to promote a better air and gas mixing without loosing too much air mixture velocity. The intake port must never be polished. For turbo application we can use 220 grid. The bad story is no way to put a turbo on a 66cc or 49cc or 69cc, during the 2008 and 2009 i worked at the sae supermileage car of the university of puerto rico mayagüez campus, we tried to put a smart car turbo( the smallest one available at that moment and i think still today but if i am wrong please tell me) to a 5hp or less B&S engine right now i can remember exactly the hp. The turbo is like a restriction because the turbine prop never move rapid enough to generate boost. END OF STORY NO TURBO HERE UNLESS SOME CUSTOM TURBO IS MANUFACTURED. The internal diameter of the intake must be 19-23mm also the carb this is for stay in the time area parameters you can find more information about this at my post "going case inducted"
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For hp
the runnermust be short and have a large diameter. This will provide the air flow needed at high rpm but narowing the power band. Torque is reduced due poor fuel mixing and low air velocity at low rpm. Remember hp means that you could reach a higher speedon your bicycle.

For torque
the runner must be long and have a smaller diameter. this will create a more turbulent flow and due to the friction with the runner surface and high flow the velocity due to a small diameter runner. hp is reduced because at high rpm the air flow became turbulent this will cause the flow velocity decrease


Note: the intake diameter, sand paper grid, intake length. Are just my 2 cents based on personal thinking.
 
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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.
 
You are wright engine load is what produce boost that why big turbos cars need retard timing to create a huge load that produce boost at no transmission induced load or boost at neutral that why the 2 step system.
 
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...

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The velocity stacks are on other chapter but is true narrowing a intake from larger diameter to a small diameter will increase pressure inside the intake and once it left the intake it will loose pressure and increase velocity like in a water hose if you put the finger to increase water velocity at the end
 
A very important thing here the only way to approach to find the best combination is with math this will reduce the failure index. Other wise you are going to do it like the old times try, fail and correct and try again all steps again and again.
 
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
 
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.
 
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