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Discussion Starter #1
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! :eek:ccasion14:
 

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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.
 

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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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Basic engine dynamics would predict that the larger displacement will be torquier but rev slower due to greater reciprocating mass.
 

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Discussion Starter #5
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! ;)
 

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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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Well said. Guess if the big bore high compression overheats there is always Nitrous Oxide injection according to the web.:p

" 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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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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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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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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Fred - No issues. I figured the :p emoji was indicating a humourous intent. Sadly I was not joking about the non-svelte issue... :(

TW-Brian - my thoughts exactly on potential issues with the stock head. Mind you, with a flathead you could run into valve/head interference issues! Actually, cutting larger/deeper valve reliefs in pistons is not that uncommon in the hotrod world - usually done as a result of higher lift cams, higher ratio rockers, or larger valves. Given the fact that we can easily go with a higher lift cam without piston worries, I would think that with a stock lift cam there would be ample room to mill the head. Of course, that would negate the ability to use a performance cam. Interesting how all of this is inter-related. This might require some investigation during my TTR engine build next winter...

CJ7Pilot - apologies for the hijack. Once again, good luck with your project. I will continue to watch closely.
 

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Wouldn't surprise me if elime or someone else hasn't already done a squish test to determine squish band thickness.
 

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When I was figuring out compression ratios, I found that the Yamaha XT250 piston provided much LOWER compression in the TW/TTR/XT application.

In the end I think I've
decided that as much as I love modifying things, the fact that the gains will never make the bike where I would really like it to be has made me focus on just playing to the bikes strengths like increasing traction, more range, better suspention etc. rather than trying to fix really the one thing its lacks.

I think the real answer would be to fit a TW swingarm and maybe triple tree to an already more powerful bike. The new Himalayan might be a good choice--400cc with gobs of torque to turn a big knarly rear tire, fuel injection, etc. Fred's shaft extention could really help where most other attempts have reached a barrier.
 

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Discussion Starter #14
When I was figuring out compression ratios, I found that the Yamaha XT250 piston provided much LOWER compression in the TW/TTR/XT application.
Do you remember how much lower? :dontknow: That could really take the steam out of this build. :(

Sebastian seemed to like the XT250 piston, but he also did a lot of custom work to his cylinder heads, so it's not an apples to apples comparison.

In the end I think I've
decided that as much as I love modifying things, the fact that the gains will never make the bike where I would really like it to be has made me focus on just playing to the bikes strengths like increasing traction, more range, better suspention etc. rather than trying to fix really the one thing its lacks.
That's true, and admittedly, I'm doing this for the fun of it... but I'd still like it to be worthwhile! The real benefit to this conversion is the six speed transmission, with a lower low, and a higher high.

I think the real answer would be to fit a TW swingarm and maybe triple tree to an already more powerful bike....
Yep, I've said this a few times myself... fitting big tires to another bike might well be easier than fitting a big engine in to the TW.
 

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https://www.tw200forum.com/#/topics/32010

The info is somewhere in there. Regarding 6 speeds vs 5, I think the real value is in being able to keep the TW in its power band more so than top speed. When I used a gear vs speed calculator I think the higher 6th netted me like 2mph, and that's if you have enough grunt to make take advantage of it. The XT 225 and TTR225 both share a wide ratio transmission, but I beleive the TTR230 has a close ratio. I'm staring to think if I was going to so a 6 speed then that would be the one I would look for.

Another concern I've developed with overbores and more importantly higer cr's is that if I'm not mistaken, the TW is based on a blown out 125 to begin with. I just wonder how much the cylinder is up to cooling depending on the situation. It seems like lots of people have had luck with the 10.25 though so that's good.

Component stacking is another issue I have. To really make the most of more displacement or compression you are talking carb, exhaust, cylinder head, and cam mods. The $ adds up quick, but you already have the better carb so that's a plus. If you really want to go the 250cc route (and it sounds like you do, so I think you should) I would suggest trying to get the specs on the 11:1 74mm piston from the XRs only big bore kit and figuring out what needs done to drop it to 10:1. I think a minimum order from JE Piston (who made them originally iirc) was 4. I wouldn't think you would have trouble finding 3 others. Heck, I might even get one just in case I change my mind down the road.
 

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Discussion Starter #17 (Edited)
Well the XT250 piston (74mm) is dished, whereas the TTR225 piston (70mm) is slightly domed.

Both pistons were designed to give a 9.5:1 compression ratio in their respective engines.

Clearly the XT250 piston is not going to give a 9.5:1 compression ratio, using a TTR225 head.

The dish is similar to the YFM250 piston (71mm), which gave an 8.7:1 compression ratio.

Why did ATVs use a lower compression ratio? :dontknow:

If I can't find at least a flat topped 74mm piston, I'll probably give up on the 250, and use the 71mm Wiseco 10.25:1 piston on both engines.

EDIT: After playing around with the compression ratio calculator, I can see that the compression ratio increases more that I thought just by increasing the bore!

If I could find a piston just like the stock TTR225 piston, but 74mm in diameter, that alone could increase it from 9.5:1 to 10.5:1.

Maybe using the dished piston would keep me closer to the stock compression ratio that I thought.

It looks like I've got some measuring to do!
 

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Was the YFM 250 piston the one that was used in the Timberwolf? I just happen to have a 71.5mm one of those. The top is dished out and the valve reliefs are flat -- I guess the valves in the Timberwolf head aren't angled.

PB160018 (2).JPG
YFM250; 10.25:1 Wiseco; stock yamaha

Taken from a different angle one can see the dished aspect of the piston as well as the valve relief cuts.
P9160063.JPG

I bet some talented person could take a TIG welder and fill that dish in.

An alternative to milling the head maybe be to get a thinner head gasket made. I think Cometic will do it but it ain't cheap however they are reusable -- I think.

BTW, CC'ing the heads and calculating the dome volumes is very easy to do and surprisingly accurate.
 

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Elime, that is intetesting, because I am planning my build on a 73mm bore with a Wössner piston. Having said that, the piston I ordered was 8659D200 which is what Sebastien said to order. The Wössner site says it is for a 00-04 YFB250 Bear Tracker. The pics I have for it also show "flat" valve reliefs. Having said that, my rebuilt YFB250 Timber Wolf head definitely has canted valves à la TW. I'll have to dig some, but I'm pretty sure Sebastien said that YFB/YFM heads (non-raptor) were interchangeable with TW/XT/TTR heads. I'm now somewhat confused... Good thing my BB is a longer term proj.

EDIT: Found Sebastien's statement at post #69 of the BB Mother-thread:

https://www.tw200forum.com/forum/performance-customization/13306-list-parts-you-will-need-upgrade-your-tw-214-225-up-250-cc-7.html

Looks like no issue for the head. I still don't understand the "flat" valve reliefs though.
 
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