CR200X - Is it possible?

[SpeedyManiac]

Hey guys,
I've been toying with the idea of making an ultimate woods racer: CR125R with a new cylinder to bump up displacement to 200cc, then add all the normal woods goodies. A 200cc two-stroke is pretty much perfect, and the aluminum chassis is superior to steel (IMHO).

My biggest question is if it would even be possible to increase the displacement that much and still have a reliable engine? I'm guessing there would be many problems:
1) New rod to handle increased stress on it from more powerful combustion. Crankshaft too?
2) As the motor is a case reed motor, I'm thinking there would be major timing issues associated with getting the increased fuel charge into the combustion chamber.
3) Cooling. Increased radiator size or just increased impeller?

Goals of the project:
To create a lightweight, reliable 200cc two-stroke woods racer using an aluminum chassis.

For the record, I'm graduating with a Mec. Eng. degree in April so I'm hoping that I can actually do the design and building on a custom woods racer.

 

[blackduc98]

If you visit the green room (http://www.dirtrider.net/forums3/forumdisplay.php?f=6) you will find that a number of folks did this by stuffing a KDX200 or KDX220 motor into a KX125 frame. Here is one of several threads on this subject:
http://www.dirtrider.net/forums3/showthread.php?t=143514
I realize it doesn't fulfill your desire for aluminum frame - but it is in the same spirit as what you're thinking.

 

[SpeedyManiac]

Thanks for the links, but I'm not interested in doing another engine swap project. I'm looking at taking a CR125R, designing and manufacturing a new cylinder and adding offroad goodies. I'm just wondering if anyone with some experience might be able to chime in their thoughts about the technical details of increasing the displacement that much.

 

[XRpredator]

It's impossible. It just has to be.


Now, consider that inspiration and go for it. :)

 

[Matt Fisher]

Since you're talking about starting from scratch, I would suggest you do all your work on a YZ125 instead. 6 speed instead of 5, easier to access the carb (important since you'll be doing lots of jetting experiments getting it to work right), and since Yamaha hasn't thrown the 2-stroke completely out the window like Honda, parts should be easier to obtain as the years go by.

Good luck though.

 

[76GMC1500]

Why not just buy a KDX 200 or an EX/C 200?

 

[KDX607]

KTM has everything

200sx motocross
200xc h/s mx combo
200xc-w full on woods, save for spark arrestor and removed lights after '03

 

[91kx]

Why not just buy a KDX 200 or an EX/C 200?

Cuz a kdx is a swine and a KTM has no linky.JK!

 

[XRpredator]

maybe the kid wants the challenge. People wanting to beat a challenge is the reason AJ Wagonner built the CR500AF.

Now it's speedymaniac's turn

 

[mtk]

Is it possible?

Sure, anything is possible.

Just be sure to bring money. Lots and lots of money. Because you're going to need it.

 

[SpeedyManiac]

Pred's right, it's for the challenge. I already have the easy way - GasGas EC200, but I'm planning on upgrading to an EC250.

I think it would be cool to make a project bike and I'm biased towards Honda. I also think that a 200cc two-stroke makes the ideal woods bike size for most riding/racing. The Honda chassis is by far the best chassis to start off with too.

 

[blackduc98]

It does sound like a fun project. If I was doing this, I would start with an existing con rod and piston of the 200cc flavor, and see if it can be modified to fit the cr125 crank, which will most likely require increasing stroke, so you'll have to verify feasibility of stroking a cr125. The other big question is whether the crankcase volume of a cr125 is large enough to feed 200cc. Increased displacement will obviously increase stress on the crank, but if you reduce rpm ceiling that should compensate for it somewhat, which is OK for a woods bike anyway (reduced rpm ceiling, that is). 2-stroke Tuners Handbook by Gordon Jennings offers some basic guidance for determining fundamental engine parameters.

I would not want to build my own con rod or piston, but I would be willing to fab my own cylinder. You can find a bunch of info on DIY aluminum casting. You should be able to do rough machining of the cylinder, and then send it to USChrome for precision bore and Nikasil plate job to fit your piston, which will cost you roughly $200. You can start with a head from something like a KDX200. I would seriously look into building an ECU-controlled exhaust power valve and ignition. Megasquirt DIY ECU is a good base to start from. Since you'll be making your own cylinder and ECU, you might as well make provisions for direct fuel injection, but I'd get it running with a carb first. You might even investigate ECU-controlled solenoids that dynamically alter parameters of transfer ports in the same spirit as the exhaust power valve. Best of luck with your project.

 

[02rm250]

?????Sleeve a 250 down?????

 

[SpeedyManiac]

?????Sleeve a 250 down?????

Defeats the purpose. I want the weight of a 125, and a 200cc engine gives almost the power of a 250.

Blackduc, thanks for the advice. I'm taking a combustion engine class as well as a manual machining class (I'm in 4th Mechanical Engineering) so I'm hoping that will help give me a better idea of how to approach this project, if it's even possible.

 

[adam728]

How about a KDX engine in a CR aluminum chassis?
http://www.dirtrider.net/forums3/showthread.php?t=92288&highlight=hybrid

You get your Honda suspension and everything, but save bookoo bucks on the engine. There is a ton of engineering and trial and error (which is a big part of engineering) that go into designing an engine, you'd be very hard pressed to get it right the first time. I persoanally would start with an already existing powerplant and stick with proven mods. Breaking new ground is fun, but spending huge amounts of time and money and ending up with mediocore results will take all that fun right out of it.

 

[mtk]

I would not want to build my own con rod or piston, but I would be willing to fab my own cylinder. You can find a bunch of info on DIY aluminum casting. You should be able to do rough machining of the cylinder, and then send it to USChrome for precision bore and Nikasil plate job to fit your piston, which will cost you roughly $200. You can start with a head from something like a KDX200. I would seriously look into building an ECU-controlled exhaust power valve and ignition. Megasquirt DIY ECU is a good base to start from. Since you'll be making your own cylinder and ECU, you might as well make provisions for direct fuel injection, but I'd get it running with a carb first. You might even investigate ECU-controlled solenoids that dynamically alter parameters of transfer ports in the same spirit as the exhaust power valve. Best of luck with your project.

Considering that Bimota couldn't get direct injection to work with an actual factory development budget, I'd suggest you stick with one Mt. Everest-sized challenge at a time. Yes, I think Honda did make it work on a desert race engine, but Honda has more money in that project than you will earn in your lifetime, regardless of what career path you choose in life.

If you don't mind me asking, exactly how much cash do you have to devote to this project?

Someone mentioned casting a new cylinder. You'll also need to cast a new head. Both will need new internal water passages in them. Plus you'll have to reverse-engineer the power valve so you can graft it into your custom cylinder. Since your in engineering school you can probably access a copy of SolidWorks, which is good because you'll need it. In addition, casting a cylinder with water jackets as a DIY casting project is probably going to be a stretch. In fact, I'd go so far as to say that if you had a hope of pulling that one off you wouldn't be asking the question in the first place. The cores needed to cast something with internal water passages are going to be well beyond the average DIY foundry operator. They're also a one-shot deal in that if the casting is porous, or otherwise not perfect, you get to start all over again.

If I sound like a wet blanket, you'll just have to consider me the cold, hard, slap of reality. What you're proposing is a gigantic undertaking and honestly I highly doubt you have even the slightest chance of success unless you're independently wealthy, highly motivated, and have no desire to have a life outside of working on this project.

Consider this tidbit: No tuner offers anything close to 200cc as a big-bore kit for a 125 engine.

Before you embark on a major engine redesign project, I'd suggest you check out what is available from tuners for the CR125. Overbores to 144cc are common so maybe you can combine an overbore kit with a stroker crank and get it closer to 200cc that way without having to reinvent anything. Plus you'll have a far larger chance of suggess.

Either way, good luck with it.

 

[XRpredator]

a guy doesn't necessarily need to cast anything. Amazing things can be done with a CNC machine these days

 

[SpeedyManiac]

Right now the budget is small, but that's because I'm still in school. Realistically, I know I'll be very lucky to ever get this to work, but I want to try regardless.

Casting a cylinder is out of the question. CNC might also be out as it will most likely be a one-off cylinder which makes CNC VERY expensive. Direct injection is out too. I want to try to keep it fairly simple and so far direct injection has had very limited success.

As I stated above, I'm not going to just shove a different engine in a Honda chassis. It's been done before. If I did, I definitely wouldn't use a KDX engine, I'd go with a KTM or GasGas engine, but that's a mute point.

Thanks for the tips guys, any ideas are welcome. What was the last year that Honda had a 6 speed 125?

 

[adam728]

Why so against the KDX engine? With very few mods it has more hp and lowend than a KTM 200. And it's a pretty light engine. I remember someone doing the KDX/KX swap, and a complete KDX engine was only 6 lbs more than a complete KX125 engine. Cheap, widely available, and it's been done before because it works well. It sounds to me like you want to do something different just for the sake of being different, even if you know you can't pull it off. Why waste the money?

 

[mtk]

a guy doesn't necessarily need to cast anything. Amazing things can be done with a CNC machine these days

CNC can't create water passages inside a cylinder or head, period. Unless you want to make the part in multiple parts, in which case you now have even more engineering work to do.

 

[SpeedyManiac]

See what bike I currently ride? I do like unique/different bikes. Part of this project is to do something that hasn't been done before.

My main problem with the KDX engine is that it's not a great performer unless it gets some major modifications. The idea is to make a lightweight RACER. Sorry, the KDX engine is just not up to snuff IMHO.

As far as the money factor, once I buy the bike it shouldn't be too bad. All design/engineering work will be done by me, so it's just a matter of getting parts manufactured (this can get expensive, but I'll be doing my best to reduce manufacturing cost).

 

[mtk]

Right now the budget is small, but that's because I'm still in school. Realistically, I know I'll be very lucky to ever get this to work, but I want to try regardless.

Then my suggestion to you is to buy a bike and a Sawz-all. I have an 18V DeWalt and it works pretty well.

Take the sawz-all, cut the bike into bits, and throw the whole mess in the dumpster. That is all you're going to end up doing with this "project," so this will save you some time and stress.

This is a collossal undertaking and a shoestring budget just isn't going to get it done, no matter how much you hope it will pan out. One of the things you're supposed to learn in engineering school is what CAN be done and what CANNOT be done with the project budget at hand. So far it sounds like all you have is some hope and no cash. Projects take cash, not hope. Thinking otherwise is this: :coocoo:

The end result will be one messed up bike and a lot of wasted time and money (mostly in good parts you ruined as part of this exercise). You'll end up selling off the butchered wreck for a fraction of what you have invested in it.

I hate to sound like a wet blanket, but I've bought several of these types of "projects" and they all start out with the same lofty intentions, and lack of budget, and they all end up in the same botched state of completion. Then I buy it for pennies on the dollar and salvage the remaining usable parts out of it.

Tell you what, I just did a minute of research on the web and came up with this data for several bikes:

Bore x Stroke
CR125R: 54.0mm x 54.5mm
CR250R: 66.4mm x 72.0mm
KTM200XC: 64.0mm x 60.0mm

From that limited amount of data, I'd say that the odds of turning a CR125 engine into anything remotely close to 200cc just isn't going to happen. I'm reasonably sure that an increase in radius of 5mm is going to put you into the water jacket. By the same token, I doubt you'll be able to come up with 6mm more stroke out of a CR125 crank. You already admitted that a custom cylinder & head is out of the question. So exactly what is left to try?

 

[adam728]

My main problem with the KDX engine is that it's not a great performer unless it gets some major modifications. The idea is to make a lightweight RACER. Sorry, the KDX engine is just not up to snuff IMHO.

Major mods? You were talking about manufacturing your own engine a minute ago!

My bike sports Fredette Racing porting, recut head, bored carb, exhaust (FMF silencer & rev pipe), and some Boyseen reeds. That's well under $700 in mods and my bike has a nice wide power band and I have yet to find a 125 that can even come close in a drag race. Actually, I haven't even been beaten by a 250f (yet), I've lined up against an 06 CRF250 (stock) and an 05 KXF250 (with at least a full exhaust, don't know what else). I'm not saying the Gas Gas and KTM 200's aren't great, I'm just defending the KDX engine that I feel you have something against for no real reason.

I've let a lot of people ride my bike, and the comments are always the same, "smooth", "linear", and "a lot more power than I thought!". Only one guy didn't like it, said it was way too pipey with no low end, but he said the exact same thing about another guy's KTM300. :coocoo:

I understand the need to do something differently, but you are really looking to do things the hard way! How many KDX-powered Honda chassis are you going to run into in your life-time? Yes it's been done, but the hybrid-bikes are extremly far from common.

I really agree with mtk on this one. I too am a mechnical engineer. In college there was always all sorts of crazy projects being dreamed up and started, probably 2% of them were ever finished, the rest ended up as huge money pits like mtk said. Makes me think of some of the sand rails I saw in my trip to the dunes. Some guys run some pretty crazy stuff, like balls-out turboed EcoTechs. In the end they have the same power and 3 times the money into their rail as someone that is running a warmed up Chevy small block, not to mention the fact that everything is custom and hard to find parts for!

 

[SpeedyManiac]

I know this is a long shot, but I'm still hoping it will happen. Keep in mind that:
1) I am extremely motivated as far as bike stuff goes (I will have my own hop up business down the road).
2) It is not some pipe dream that I dreamt up the other night, I've been thinking about this since I started university.
3) The bike will not be my primary bike so I will have time to get things right.

By the way, I've ridden a KDX and wasn't at all impressed. Now that I've become accustomed to instant power (my GG EC200 is setup like a DE200) I really like that peaky power. Makes riding fun and a challenge. Sorry, but the KDX engine just doesn't do that for me.

I know the odds are against me, but I still think it would be damn cool if I pulled it off. Not to mention the experience gained in the process of building this bike.

 

[marcusgunby]

The cr 125 engine will not go beyond 144 and the yz 125 will go to a 167, so i would suggest you look at a yz 125/167 as a project.

The modern cr250 doesnt weigh that much more than a 125(compare a cr250 to a ktm 125) so a sleeved down version could be very cheap.