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Post by brett762 on Sept 24, 2009 3:26:46 GMT -5
Sometimes whilst enjoying a cocktail a question pops into mind, and for the life of me I can't recall if I've asked it before. Has anyone had any experience D porting the intakes on a ported and polished head to increase intake velocity? If so, what size carbs would be best suited to this particular mod, 36?
Thanks,
brettj
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Post by dpmulvan on Dec 14, 2009 13:21:42 GMT -5
have seen it done,I myself would like to learn more;however it seems to be a secret that most dont want to share
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Post by skimmer on Dec 14, 2009 23:00:15 GMT -5
What happens is, the short side radius of the port becomes the flat side of the D. This results in more flow as there is more area on the short side, the shorter side being the quickest route to the combustion chamber. My descriptive powers are less than perfect so I hope this sheds some light.
Skimmer
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Post by xcafes on Dec 15, 2009 5:54:41 GMT -5
...exhaust flows along the roof, not the floor, the stock roof is radiused upward to where the step is...exhaust flow turns abruptly causing some turbulent resistance... fix by filling the gap...path is smoother... extends the port for a stronger pull and reduces port volume a little...flow advantage... www.xs650.fi/jukka/pohja_files/history_2005.htm#10michaelmcbroom.com/bikes/cafe/pnp/www.650motorcycles.com/XSProPorts.html...comments from bud askland......The head is a poor design and suffers from a layout deficiency that causes the ports to follow paths that satisfied casting and ease of production concerns, but ignored performance considerations. For this reason the head is limited in what it can flow. Recognizing this, the OU-72 project replaced the stock head for a purpose-designed casting with more favorable valve angles, etc. Bud emphasized that the engine (like most others) can only be correctly modified by taking a system approach. He noted that C.R. Axtell, Mike Libby and all the other premier builders and tuners address enhanced engine performance in the same manner. In other words, the suggestion that any particular component, like just adding a cam or bigger carbs, will transform the engine’s performance is simply not going to work as well as desired. Every modification and addition to the engine has to be designed to work properly with all the other components. Save your money and don’t just throw parts at it. There is no magic engine bit that will suddenly give you world beating performance. He said that all things considered the ports in the XS engine are a bit too large to begin with, and lesser is better if you are going to do any porting. The lesson is that the tuner should not remove metal except to recontour. Do not “open ‘er up so she’ll really flow.” If you have the skill and experience, you do have the option on the intake to use epoxy to do a more comprehensive reshaping and obtain higher velocities, but that option doesn’t exist for the exhaust (although of course you can always add material by welding). So, the bottom line is that the tuner has to walk a very fine line between gas velocity and shaping the port walls and valve pocket areas. It is very easy to overdo it and actually make the ports flow worse than stock. |
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Post by Tom Graham on Dec 15, 2009 15:33:01 GMT -5
Long ago I had the same question about D ports. What the heck are they really? I read everything I could but never quite got it. I then can to the conclusion that the people writing about D porting in the magazines and books didn't really know either so I figured it out for myself.
A port, no matter how bent, should flow like it is straight. The straighter the port the better the flow. How do you get a port with a bend in it to flow like it is straight? If one looks at a bent pipe from the side (mind experiment) you see the air has longer to go around the top than the bottom of the bend. this differnce in distance leads to the air shearing and turbulance. If the bottom were gradually widened to the middle of the bend the air would slow on the bottom and match the speed on the top. As the widened bottom would taper back after the bend, the air on the bottom would match the speed of the air on top and the port would flow no different than a straight pipe. This concept uses the elasticity of air.
Another flow concept is a straight shot into a larger volume then a straight shot out of the volume at another angle. This utilises that the slower flowing air in the volume has greater pressure than the air in the pipe flowing out. Look at the K&N intakes. These swell into a D shaped volume and show both concepts.
A tapered pipe flowes better than a straight pipe. Even a slight taper flows better.
The edge of the valve and the valve seat are a very important part of the port and change as the valve opens. I think most professionals work to get the valve seat to flow at low lifts as this is where it makes the most difference.
The valve and the top of the combustion chamber can be concidered a continuation of the port.
A valve opening quickly acts as a pump. I wish I knew how to exploit this but I'm lost.
Now go forth and ruin some ports.
Tom Graham
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Post by xcafes on Dec 15, 2009 16:58:38 GMT -5
- this is correct...think about where the fastest and slowest velocities in a river are and why...the fastest is in the middle of the channel just below the surface in a straight section and on the outsides of bends...paths of least resistance...the slowest are wherever theres contact with the bed or banks in straight sections and the insides of corners...shaping ports is a matter of understanding how resistance and shape affect flow
- complicating this is that fluids-air and exhaust are fluids-like to flow sinusoidally...as energy disperses so does the desire of fluids to flow in a non-convective nature
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Post by weekendrider on Dec 16, 2009 4:00:26 GMT -5
Tom, Good luck with the mangling and THANKS for the post.
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jack
Junior Member
Posts: 16
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Post by jack on Dec 17, 2009 21:29:18 GMT -5
Sometimes whilst enjoying a cocktail a question pops into mind, and for the life of me I can't recall if I've asked it before. Has anyone had any experience D porting the intakes on a ported and polished head to increase intake velocity? If so, what size carbs would be best suited to this particular mod, 36? Thanks, brettj Yamaha produced a high velocity head for the XS1 long rod motor,totally different intake port shapes to compensate for the lower low pressure draw on the intake port as the piston accelerates from TDC. In either case,you could flow the early and later version and see little differences in flow + - 5 to 8 CFMs is this something you'd be able to notice on a street bike,very doubtful. Where caution needs emphasis is at cross section area where the spigot manifold bolts to ,this is where velocity will be gained or lost at the short turn. The XS head is what is and there's little that can be done to change it do to the valve angles,unless you've access to a flow bench to start changing port shapes to alter the direction of air flow to speed it up. The best person to contact on reshaping the intakes would be 3M at 650 central. Good luck to you! |
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