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Off the wall - YZF250 rings in XS650 Yamaha

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Posted by: cfinney

I have the idea that designing a piston for the XS650 Yamaha twin based on the YZF250 piston would be a good thing. If for no other reason than to lighten the piston, increase the rod length and learn from the experience.

The XS cylinders are cast iron, or some such, maybe centrifugal cast or nodular. The stock XS cylinder bore is 75mm, so pistons designed around the YZF piston and rings would be a 2mm overbore, the liners are certainly up to this. My question is this, what, if anything, should I contemplate doing to insure the longevity and performance of the YZF rings in the XS cylinder? I'm presuming the stock YZF cylinder is chrome, Nikasil or somesuch, and as such the rings may not fare well in the XS cylinder. Thoughts?

While I'm at this, what is the rod length on the YZF250?

Thanks in advance.

Charlie in Iowa


Posted by: SpDyKen

Hello everyone,

It was my suggestion to my good friend Chuck to post his question here on DRN.

Moderators, could you possibly move this to the "Mods & Performance Forum," please?

Some background here. I had the pleasure of working & racing with Charlie in the 70's & 80's. I've learned a lot as we both ask questions like "why" or "why not" about M/C related things. Chuck has since moved on to the IT business, so I rely on him for advice in finding information worldwide.

The bike Charlie is discussing is his Champion framed XS 650 that he raced in the 70's. It is now street legal, with Ducati inverted fork front forks, 320mm Brembo brakes, 17" spoked wheels (rear KX500 hub,) Ohlins rear shocks and weighs in at about 320 lbs., if my memory serves me right.

The motor is the unique part, though. The crankshaft has been "twisted" to fire at 270 degrees. To work the bugs out, (and this thing is a sweet ride, already,) Charlie has Mikuni carbs on it now. His main focus presently is finalizing the model aircraft programmable electronic ignition recommended to him by a friend of mine, and continuing to develop a fuel injection system.

He is looking ahead to improving the engine performance in the future by taking advantage of some of the 4-t developments in the last few years. Thus, the inquiry into the YZF 250 information.

I'll post some pictures later.

I know Charlie emailed Eric already, but I see he is in the middle of moving his shop again, so, probably kind of busy right now. Rich, any possibility you might be able to offer any advice to my good friend Charlie? We both would greatly appreciate it; Thanks!

Later, Ken W.


Posted by: Rich Rohrich

The first thought that comes to mind is how to get enough compression in the engine. The XS has a large included valve angle, with a large volume hemi type combustion chamber that requires a fairly large dome to get a decent compression ratio. . The YZFs have a fairly flat valve included angle, with a very tiny combustion chamber volume. They use a concave piston crown on the OEM piston with large valve cutouts. The piston crown depressions account for a major portion of the total combustion chamber volume. I'm not sure if you'll be able to find a forging that will provide enough crown material to get a usable compression ratio. If you did, you would likely have to re-engineer the piston pin bosses and the piston skirt to account for the huge weight differences.

You could fill in the combustion chamber to help minimize the need for a large piston crown, but then you would run into major valve shrouding issues that don't have any easy solution.

The Triumph dirt track guys run into the same issues. Maybe Ol89r will see this and weigh in. He has a huge amount of experience in this area.

The other thought that comes quickly to mind is getting Advance sleeve to make up some aluminum liners for the barrels and get them coated with Nicom by US Chrome, or have them at least Nicom coat the iron sleeves (they have mastered that process now).

The coating would last a lot longer and have better compatibility with the the current ring packages.

I would look to Ross pistons for help on this. They do a lot of custom work and could help get the piston crown correct for the XS head if anyone can.

I would take one of the XS heads complete with valves, coat the chamber with a light coating of Pam or some other release agent, and then fill the chamber with two-part epoxy to get an accurate and durable mold of the combustion chamber.

This will go a long way towards getting the piston right the first time.

I'll give this some more thought today and see if I can add anything useful later on.

Good luck with the project, it sounds like a ton of fun.


Posted by: Ol'89r

Quote:
Originally Posted by Rich Rohrich
You could fill in the combustion chamber to help minimize the need for a large piston crown, but then you would run into major valve shrouding issues that don't have any easy solution.

The Triumph dirt track guys run into the same issues. Maybe Ol89r will see this and weigh in.



Charlie.

I agree with Rich. The piston and the combustion chamber are not compatable. If you were to cast a piston with a large enough dome to properly fill the combustion chamber, it would have to have a fairly long skirt or the piston would be too unbalanced. If you were to weld up the combustion chamber it would shroud to valves too much. You would almost have to cast a new head and lower the seats to accomplish this.

You may want to look into a 750 kit. I ran a Champion framed XS 650 with a Shell 750 kit for a couple of seasons and the 750 kit made a big difference in performance. We had to machine the head into more of a hemispherical combustion chamber to fit the dome of the Shell high compression pistons. This resulted in un-shrouding the valves even more and made tons of HP.

You may want to use that design and plate the liner as Rich suggested. I can't remember who cast the pistons for Shell but the liners came from LA Sleeve. I think the pistons may have been Forge-True but I would have to look back in my notes to be sure.

If you decide to go that route, I still have the cutter for making the combustion chamber hemispherical and can machine it for you.

The XS650/750 engine is a great little powerplant. They make awesome HP and would make a great street tracker. They didn't work all that great on the dirt track because of the lack of flywheel. They didn't hook-up on loose tracks very well. Kenny Roberts was the only one that could ride one to it's full potential. I remember even on the mile tracks, you had to feather the throttle on the straightaway or it would just spin the tire.

Good luck and keep us posted.

Ol'89r


Posted by: cfinney

First, thank you to Ken Welty for getting the original question in the correct place. Second, thank you to all of you for your observations and suggestions.

The unit in question is already a 750 with the combustion chamber opened up. The idea is not to build a bigger/faster engine. The idea is to build a smoother quicker engine. To that end, twisting the crank 83 or 90 degrees, getting the piston weight down to 250 grams and pushing the rod length out to 148mm is the objective. This is just a toy bike to try ideas out on and see what the results are. If they seem really promising, then I'll push them into a half miler.

Over the years I've used Venolia, ForgedTrue, Wiseco, JE and whatever Shell has had for pistons. They are all very heavy, 380 to 440 grams with pins and no rings. It looks feasible to get a piston with 9/9.5 one compression using the design of the YZF from the deck down and a low dome above the deck. The displacement is of no particular concern at this time.

Ian Drysdale of Yamaha V8 fame indicated he has run the rings on a cast liner and service seems satisfactory, so I will try that approach first.

The YXF looks to have a rod/stroke ratio of just less than 1.75 and a mean piston speed of 4750 fpm. The XS looks like it will come in with a rod/stroke ration of 2.0 and a mean piston speed of 4370 fpm at 9000, 3600 fpm at 7500. So I'm thinking the short skirt will survive nicely, ring life will be acceptable and the piston pin loading will be reduced enough to survive as well.

The test bed can be seen at: cfinney3.netexpress.net/cimg4072.jpg

I'm open to any and all ideas concerning how to make the XS a better platform by applying modern lessons to the old technical base. Keep it up.

Thanks,
Charlie in Iowa


Posted by: cujet

Charlie,

As I am sure you are aware, the old school 2 valve air cooled dirt bike engines were often modified just like you describe. Only the pistons available were not as short as the modern ones. We would bore them out, big time and use a flat top piston. The CR was high enough for the fuel we used (around 10 to 1). There was no need for domed pistons, as the big bore was the source of the high CR.

If I were to make a guess, I would give your ideas a thumbs up.

Chris


Posted by: joey26

charlie,
not to change or get off topic but where in iowa are you located? i'm in webster city about an hour north of des moines.
Joe.


Posted by: cfinney

Bettendorf, east end of I-80 on the Mississippi.

Charlie in Iowa


Posted by: SFO

Sorry to joing this party so late.
I remeber there being some englishman running a 1000cc xs motored grasstracker on alcohol, I know this isn't an answer to your question but an interesting sidenote.
It would be nice to fit cr 5sp ala/yzf?
making those hemi heads work is a balancing act, might think about doing a down draft head?



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