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  1. #1
    Senior Member CJ7Pilot's Avatar
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    Big Bore Options...?

    As it turns out, I'm building two 225-based engines... one for me, and one for a friend of mine.

    One will will be a 229cc 10.25c/r engine, originally destined for my bike.

    The other will be a 249cc 9.5c/r engine, which is now destined for my bike, because I'm greedy.

    There was a question on these forums recently, as to which setup would be better....

    I intend to build both engines, and compare them... but I'm wondering which engine should I install on my own bike?!?

    I'm thinking the 249 would work better for me, because I'm running a larger than stock pumper carburetor... but the 229 higher compression engine, with a good stock carburetor might well end up being the better setup!

    I'll let you know how it turns out... but if you have any suggestions, I'd love to hear them!
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  2. #2
    Senior Member TW-Brian's Avatar
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    Quote Originally Posted by CJ7Pilot View Post
    I intend to build both engines, and compare them... but I'm wondering which engine should I install on my own bike?!?

    I'm thinking the 249 would work better for me, because I'm running a larger than stock pumper carburetor... but the 229 higher compression engine, with a good stock carburetor might well end up being the better setup!
    Here are my two cents:

    I have now done two TW225 6-speed conversions. The first was TTR225 engine with a small overbore to 226cc with a 10.25 Wiseco piston and a stock TW carb.

    The second was an XT225 engine with no engine mods (aside from Placerlodes's shaft extension) and a stock TW carb.

    Of these two bikes, the first one with the higher compression piston clearly has noticeably more power, at least according to my butt dyno. Although they say there is no replacement for displacement, I feel that the higher compression piston will give you much more bang than an additional 25cc's at the stock compression ratio.

    YMMV.

  3. #3
    Senior Member joeband's Avatar
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    having been side by side with elime on his 214cc w/ a wisco piston... my 226cc is challenged.
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  5. #4
    Senior Member Fred's Avatar
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    Basic engine dynamics would predict that the larger displacement will be torquier but rev slower due to greater reciprocating mass.
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  6. #5
    Senior Member CJ7Pilot's Avatar
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    Well, I'm going to stick with my plan of building the 249 for my own TW.

    I'm building it with a later model XT225 lower end (which originally came with a nikasil cylinder), so the case mouth is already large enough to accept an oversized cylinder sleeve.

    Also, somewhere in all my reading today, Sebastian mentioned that the 74mm Yamaha piston is 9.5:1 in the XT250... but since the TW200/TTR/XT225 head has a smaller combustion chamber, it is possible that the compression ratio might be a bit higher in our conversions.

    I could try to figure it out using elime's anti-freeze method... but I'm not sure it matters to me all that much.

    I know there have been several successful conversions using the 74mm Yamaha piston, so I'll move ahead with that plan.

    Besides, my buddy will be thrilled if I build a "better" engine for him than for myself!
    Now I must hurry on... for there they go, and I am their leader!

  7. #6
    Member TW-JMS's Avatar
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    CJ7 Pilot, I think you made the right decision. Before I state my thoughts on this, I want to point out that I am not a mechanical genius nor a mechanical engineer so please review my logic with some compassion...

    OK, I looked long and hard at various options and the pioneering work done by others. One of the things I noted was that in all cases those who went with higher CR pistons always seemed to go with the overbore as well. Sometimes this was minor (70mm to 71mm) but sometimes more pronounced (67mm to 70mm). I do not know if this was simply a case of piston availability, a desire for more displacement, or both. Given that everyone seems to think that an increased CR along with at least a minor increase in displacement increases performance, it was hard for me to deduce which was the bigger contributor. This is where I had to resort to math and maybe some questionable logic...

    First, let us assume that carburation, intake and exhaust flows, timing, and camshafts are not factors that affect the motor's ability to produce power. Let us also ignore any increase in piston mass and resultant momentum and any increase in ring friction due to the increase in piston circumfrance. Finally, let's assume that power increases linearly with displacement. I know, big assumptions, but please stay with me....

    I am basing this on an XT/TTR motor going up to a maximum of 73mm as this is what I am planning for my TW.

    I looked all over the web and withought going in to too crazy of math, I found that an increase in CR from 9.5 to 10.25 should give us an increase of about 3.5%. Now, if we do a 1mm overbore on a 223cc motor, we increase it by 2.5% to 229cc. Doing so with a 10.25 CR piston compounds this to a bit over 6%. Now, if we overbore our 223cc motor to 73mm we get 243cc, or about a 9% increase. Clearly, at least to me, going to 73mm at 9.5 CR makes sense if you are going to overbore anyway.

    If we were able to find a 73mm piston at 10.25 CR, we could theoretically make a compounded 12.5% gain. Using the same logic, I understand why the 250cc/11.0 CR big bore kit seems to make so much more power. BTW, everybody seems to look for an increase in CR via the piston. Has anybody thought of shaving the head a bit? I don't see how it should be an issue as long as there is sufficient valve clearance, but I'm not sure about any impact on timing. Piston/head interference shouldn't be an issue if it's safe to put a TW head on a 74mm bore jug. Just curious...

    Of course, there is a downside. This extra power will produce extra heat - there is no way around that. Not a huge issue for me due to where I live and the fact that I mostly drive on the street and on gravel roads/tracks. I also already have an oil cooler and plan to go with a ceramic heat dissipating coating on the head and cylinder jug to be on the safe side. For those who drive in the desert, on slow trails, or both, this could be a deciding factor to go "big bore". IIRC, there was a comment on one thread that 229cc with the 10.25 CR piston might be the practical limit.

    All of this to say... there is no replacement for displacement! Is I had a later XT case, I'd probably be going to 250cc. Regardless, I will go to 243cc because I already spent a crapload of $$ on the parts to do so...

    Apologies for any spelling errors - SFS at work. Also apologize for the long and pedantic post... inside every engineer lurks a frustrated teacher.
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  8. #7
    Senior Member Fred's Avatar
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    Well said. Guess if the big bore high compression overheats there is always Nitrous Oxide injection according to the web.

    " When you heat nitrous oxide to about 570 degrees F (~300 C), it splits into oxygen and nitrogen. So the injection of nitrous oxide into an engine means that more oxygen is available during combustion. Because you have more oxygen, you can also inject more fuel, allowing the same engine to produce more power. Nitrous oxide is one of the simplest ways to provide a significant horsepower boost to any gasoline engine.Nitrous oxide has another effect that improves performance even more. When it vaporizes, nitrous oxide provides a significant cooling effect on the intake air. When you reduce the intake air temperature, you increase the air's density, and this provides even more oxygen inside the cylinder."
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  9. #8
    Member TW-JMS's Avatar
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    Well, I did see a video where someone put a turbo on a TW... why not nitrous?? Then again, the additional weight of the system may offset any power gains achieved. As my goal is compensate for the non-svelte nature of both the bike and myself, additional weight might not be a good idea!😮😮😮

    Getting back to CJ's plan, I still think he is on the correct path. I know my numbers above may seem to be only minor increments, but those were in comparison to a 223cc. If we compare to the original 196cc mill, a 20-25% increase doesn't sound too bad. I can't wait to see how this works out for CJ7Pilot.

    I'm still very curious about milling the head. Used to work great on flathead Fords!😉😉
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  10. #9
    Senior Member Fred's Avatar
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    I was joking of course about the laughing gas for treating overheating.
    We've teased in the past about the little push-to-pass NOX bottle kits that give about 30 seconds of almost normal motorcycle acceleration to our plebeian mules but they are not really the solution. Nitrous injection into the helmet might result in more grins though.
    Upping to TTR's 223cc has been a good first step. A 71mm bore and Wiseco high compression piston may eventually happen but in meantime I just wanna ride!
    Will anxiously follow other's progress .
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  11. #10
    Senior Member TW-Brian's Avatar
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    Quote Originally Posted by TW-JMS View Post
    I'm still very curious about milling the head. Used to work great on flathead Fords!😉😉
    By milling the head, you can definitely reduce the squish volume and therefore increase the compression ratio. Easy to do for a flathead engine where you don't have to worry about the top of the piston smashing into the valves. The picture below shows a stock TTR225 piston on the left and a Wiseco high compression piston on the right. You will note that the stock piston already has reliefs cut out of the piston top to allow clearance for the valves. The Wiseco piston has additional material on the crown of the piston requiring that these valve reliefs be deeper than on the stock piston.

    It may be possible to mill some material from the head to increase compression, but my guess is that the piston to valve clearance is already close to the minimum allowable. Of course, you could always machine deeper reliefs into the stock piston head to offset the material removed from the head, but this is not something that I would recommend doing .

    100_4677.JPG

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