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  1. #1
    Member Teddy_Bear's Avatar
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    Head port

    Thought I'd share with you guys my recent experience porting the head on my 225. I originally set out to fix the infamous base gasket oil leak due to the unevenly machined engine halves, but noticed that the passages could use a better finish and decided to go a step further.

    IMG_20190310_174812.jpg

    Nevermind the carbon deposits (I am running rich and with a overfilled crankcase to keep the clutch temp down), a small step is visible in the passage past the valve seat. The intake valve is done the same way. Ideally these must be evened out at the factory, but it makes no sense for them to invest more time into a cheap product like the TW and they leave them like that, as it affects performance only marginally. However, being somewhat a perfectionist, I just could not tolerate it.

    Another issue I discovered is the sharp bend in the exhaust path at the point where the exhaust manifold attaches to the head. The path is straight, but the manifold attaches at an angle to clear the frame. Not good for flow efficiency (I will get into that later). My solution was to dremel the passage to enlarge it slightly and introduce a curve.

    Factory design:
    problem.png

    My solution:
    solution.png
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  2. #2
    Member Teddy_Bear's Avatar
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    So, the main factor at play here is turbulence, which is almost never good. Steps and abrupt changes in passage direction create turbulence and turbulence reduces max flow capacity, which limits the efficiency and power of the engine.

    I used my dremel to grind down the steps below the valve seats to an even surface. They are visible initially, but as you work on them you will need to use your fingers to feel if there is any unwanted material left
    IMG_20190310_183452.jpg

    IMG_20190310_183507.jpg

    Another area where i discovered a step is where the carb adapter bolts on to the intake side:
    IMG_20190310_171822.jpg

    Run your finger trough there and you may find that the diameters of the adapter and the passage are slightly different or misaligned. Make a note of where the step is, take the adapter off and grind it down. Be careful not to overdo it, lots of repeated bolting back, checking and taking off will be involved for a proper job.

    Finally, there is huge step where the expahust manifold attaches to the head. You can see the difference in the diameters of the seal and the passage here:
    IMG_20190310_171737.jpg

    THIS IS A DELIBERATE DESIGN FEATURE and you should avoid removing too much of this step. There is a process called scavenging, where the intake valve opens slightly before the exhaust valve closes and some of the intake charge is sucked into the exhaust to ensure the cylinder is completely free of the exhaust gas. This step prevents the exaust from getting back onto the cylinder.

    I had to take some of it off to introduce a curve into the exhaust path, and also due to the reason that i have a 225cc, but you should try to keep at least some of this step:

    IMG_20190317_154228.jpg
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  3. #3
    Member Teddy_Bear's Avatar
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    The photos I am posting are mostly in-process and not of the final state, in case you are wondering if that's the way the final result looks I had to rush when assembling back together, as I had a bike trip with my pals coming up and did not take many photos in the final stages.

    So this is the in-process photo of me making the passage curved.

    IMG_20190315_222542.jpg

    And this is the passge in the finished state. Initially i took too much material off the exhaust step and could not curve it as much as i wanted to due to the fear of grinding through the wall. Though it is pretty easy to estimte how much material is left, as both sides are visible.

    IMG_20190319_211036.jpg
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  5. #4
    Member Teddy_Bear's Avatar
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    Finally, I used valve lapping paste on valves and seats:

    IMG_20190325_220918.jpg

    The left (exhaust) valve has been lapped, the right (intake) has not. I used a minimal amount of regular (not diamond) paste and 10 back-forth movements by hand in each quarter (turned valve 90 degrees after 10 short back-forth movements)

    Here are the valves. Again, the exhaust has been worked on. You may notice the intake valve seat has some small scratches at 1 o'clock. The paste has removed those right off (me happy I was very worried about damaging the seats and have not rised removing the carbon from the areas close to the seats.

    IMG_20190325_221131.jpg
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  6. #5
    Member Teddy_Bear's Avatar
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    Overall the project took about 3 weeks of evening work, but that's a very rough estimate.

    I used steel bits to work on aluminium, stone to grind steel and rubber polisher for the finish. One word on the finish: there is a false rumour that you need to have mirror-like finish on the walls of the passges for minimal tubulence. Mirror-like finish does reduce turbulence, but it is detrimental for the intake passage, as some turbulence is needed to mix the petroleum with the air. As for the exhaust passage, the shape of it is much more important than the surface. The mirror finish accounts for no more than 1-2 percent of the peak power. If you the necessary tools to do it, it will not hurt, but there is no reason to be overly preoccuppied by it.

    The most noticeable effects on the performance for me are smoother throttle respose (subjective) and, for some reason, reduced fuel consumption when I don't push the bike (I measure how much mileage i get out of the fuel i put in). Before I would never get below 3.4 liters per 100 kms, but since the mod I was able to get 3.1 one or two times.

    I have not noticed any power gains and I have no accurate way to measure it.

    Hope you guys find this interesing and if you have any questions, I will be happy to explain
    buellzebub, Purple, Ken and 7 others like this.
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  7. #6
    Senior Member Rider21's Avatar
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    Thanks for sharing. Very interesting.
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    If it works good, it's beautiful.

  8. #7
    Senior Member troll's Avatar
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    wrap the header pipe.... exhaust density increases as the gas cools.... increased density slows the exhaust in the pipe.
    Converte gladium tuum in locum suum. Omnes enim, qui acceperint gladium, gladio peribunt. - JC

  9. #8
    Member Teddy_Bear's Avatar
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    Didn't know that, thanks for the tip. But I heard wrapping the pipe decreases its lifespan? As in it burns through or disentegrates from overheating?
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  10. #9
    Senior Member Peterb's Avatar
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    I think performance is all in the head more than displacement. Thanks for sharing.
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  11. #10
    Senior Member elime's Avatar
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    I have looked at the exhaust port on several TW heads and the exhaust port compared to the gasket were all different sizes. It appeared to be flashing left over the casting procedure partially ground out by hand and probably by many different people, hence the wide variation in port size.

    As I felt with my finger going from the ex. valve to the outside, it was as if there was a wall where the "flashing" was and it seemed to be the most restrictive point, the place that impeded the flow of exhaust gases the most.

    Your explanation of the huge step at the exhaust port preventing a reverse flow of gases is very interesting. I have never heard that before. Even with camshafts designed with huge amounts of valve overlap (both valves being open at the same time to promote scavenging) we always matched the port size to the header trying to create the least restrictive path, but then this was on big V-8's in my hot rodding days (daze?) from my misspent youth.

    To me it seems the valve overlap on the TW camshaft is minimal. From what I have read one never really knows if the porting is doing any good without a flow bench. With that in mind I bought a couple of really cheap TW heads off ebay and did my own porting.

    PB010062 (2).JPGfloorA.jpgHead on view.jpg

    I installed this head on my TW with the only previous mod being a 70mm Wiseco piston. The exhaust note has a definite pop that is sharper than stock and it seems to run better than with the stock head but then that could be "placebo" and/or wishful thinking. I wish I had access to a dyno.........
    Ken, Teddy_Bear, troll and 3 others like this.
    Long live the internal combustion engine!

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