10mm Stroker Piston Speed

[Legdragger]

Longer stroke = faster pistons speeds right.

So A big stroke motor is stressed more, but how much and at what legnth stroke is does it start to get to be a lot of work.

I ride a kawi and run the stock stroke crank which is 74mm. That is + 6mm over a Yamaha twin. So I am already dealing with higher piston speeds. However I never hold it wide open for more than a second or two. I don't race or try to get out to the surf fast. I usually just ride around 1/4 throttle until I get a wave. Do I need to stress out about piston speed as much as other applications would?

I would like to figure out if a 4mm stroker would be practical or not in my situation. I have heard opinions on both sides of the issue. This is equivilant to a 10mm stroker in a yamaha which is supposed to be pretty high maintenance.

So whats the deal? Can I run a 78 mm stroke or not? I think it will be a pump gas motor so that is a lttle less stress on the crank but will it run a season or two with out wearing itself out?

 

[robertg]

Download the two stroke tuners handbook, it has some information on piston speed.
http://edj.net/2stroke/jennings/

 

[SUPERTUNE]

I wouldn't have ANY problem with running that kind of set-up for how you will ride it. I would run Wiseco pro lite pistons for sure.
I use to run my 76mm stroker XIR kaw 785's motors about 8100 RPM's with Wiseco pro lites and didn't suffer much wear as we used Honda CR 250 rods in it for a little better rod ratio...
P.S. You can push the limits as stated in the two stroke handbook as piston and rod materials and technology are better today .

 

[Legdragger]

Thanks guys. Two posts with a lot of good information.

Mr Quenzler,
I am guessing those are a little longer rods? Longer rods have to swing less per stroke right? I don't claim to be a motor builder. I just get try to get by with what I know.

 

[SUPERTUNE]

Yes they were a little bit longer, would have to find some of my race notebooks from 94' to find out. we had Falicon build it, with billet T-bone flywheels.
The longer rod lessened the rod angle and helped the piston to bore wear with the huge exhaust ports we were running to get the rpm's

 

[Idok]

Thanks guys. Two posts with a lot of good information.

Mr Quenzler,
I am guessing those are a little longer rods? Longer rods have to swing less per stroke right? I don't claim to be a motor builder. I just get try to get by with what I know.

dont sell yourself short bro.......we have ALL seen your innovation into your X2...and might I add the OG round to square conversion designer..

 

[Legdragger]

Thanks for the support IDOK:biggthumpup:

 

[egbrig]

A 78 mm stroke @ 7800 rpm yields a piston speed of 3992 fpm. This is real close to the accepted limit of 4000 fpm. But why 4000 is the limit is up for debate. There are many more factors than just piston speed. You will find that things will wear out much faster and that you'll have to stay on top of your ring gap and piston to cyl. wall clearance.

 

[joesuperjet]

Lord Vader is correct.....


JAM

 

[MTRHEAD]

Guys the GPR , XPL, and the STXR are 78mm stroke

zillions of GPR and XPL's are out there turning 7500-8000 in superstock form with 80-90mm pistons.


The rods on the older Falicon cranks were Honda CR480 rods. I think they were .5 to 1mm longer than the SX rods. Problem is they use the old piston size (20mm) I guess its not really a problem.

 

[cambo au]

interesting thread i am only new to this two stroke stuff but when i was working with dragracing stuff my small block ran a 6 inch rod with a of set pistin pin to get the same tdc to slow the piston speed down dose any one do this with two stroke rods and pistons if not you would not think it would be long before someone starts manufacturing because of the increased popularity in stroker motors in fre style and free ride

 

[Legdragger]

Cambo:
Are you saying when you ran a stroked crank you ran longer rods with special pistons that the wristpin was higher to compensate for the added height of the longer rod and stroke at TDC?

 

[egbrig]

Yea, there are quite a few modded 78 mm stroke engines out there. The thing is how long do they run at peak performance. A real full s/s watercraft engine will only make peak power for a couple of hours run time, team Kawi and like aside (they got one or two mottos out of an engine). It's just a matter of how things are set up. Because a two stroke has a hole in the side of cyl. wall the piston gets a faster death. The shape, size, and design of the exhaust port have a biggest thing to do with life of an engine but for a freerider type of riding, it should be ok. I'm just curious of how long the cases will last after being bored out to clearance a 10 mm stroke. It not like yama cases are strong to begin with and there is a level of stress difference between freestyle and closed course.

 

[egbrig]

The offset of the piston pin is not for clearence issues, it's for stress reduction and sometimes yields a little more power. Any moter head book will discuss that setup.

 

[Watcon]

Stroke

Egbrig,

You are correct about the "offset" and also about the crankcase durability. If you want maximum horsepower, be prepared to pay the price in decreased crankcase, rod & piston life. When you increase piston speed and compression ratios, you are increasing the amount of stress on the crankcases also. The Kawasaki motors already have a problem with increased piston wear, at the rod angle tries to force the piston out the exhaust port.. (look at the Simon book for specifics). But who cares about increased wear and reduced life, as long as you are making awesome horsepower??

 

[cambo au]

leg drag
no my motor was a standard stroke 350 ci chev this would in stock form have a 5.7 inch rod lenth my motor had a 6 inch rod with a ofset pin height (still same deck height as standard) to reduce the piston speed because i was turning it to 8-8500 rpm all of the nas car motors do this in fact i think any decent small block that revs hard and has been built by a decant engine builder would have this the only thing i can see being a problem with a 10 mill motor the pin height might already be to close to the crown so you cant go up any more but that i am unsure of :dunno:

 

[Legdragger]

Cambo,
I am not quite a rocket scientist but I am trying, however I can't figure out how piston speed changes unless the stroke is changed at any given rpm. I can see the rod angle being more favorable with what you are describing though. It is less of a pengelum with the offset piston and longer rod. This requires a special crank to be built with special pistons to go with it right? That is really getting high tech..........

 

[cambo au]

i am not quiet sure either but i think the way it was described to me was that at the top of the duration the speed is decreased i think this makes sence in my head but i could be wrong this is a good thread tho it will be interesting to see what comes of it

 

[Powder]

I would run Wiseco pro lite pistons for sure.

:biggthumpup:

Pete shoot me your email address. I have a great spread sheet that might help you figure out piston speed and some other fun stuff. The old "limits" are just that OLD. The new materials are a lot better then when those "limits" were determined.

 

[wetwolf]

Download the two stroke tuners handbook, it has some information on piston speed.
http://edj.net/2stroke/jennings/

HOLY CRAP!!!!!! Thats the lord, god, king of 2 stroke shiznit. I haqd one years ago and lent it to someone, never got it back, and still pizzed. Good thing i wrote down most of the formulas, it got me thru te rough times.
I'm downloading it right now and i am on dailup.

I hope this takes all nite,,,,,,,, it'll be worth it. since that book was written, the composition and design of pistons has come a long way. 4000 fps is the bottom line of racing engines meant to go WOT all the time