For discussion - rod length and stroke vs bore

[Wildcard-1]

Jenning's book is a very good wealth of information for novice 2 stroke tuners. But some of the information is slightly outdated with advancements in materials ect.

On the subject of rod length and stroke vs bore:
While adding bore adds more displacement and theoretically more available power per mm than stoke, the addition of stroke allows for more linear power stroke per deg of exhaust port open time. This allows for more aggressive porting while still maintaining or improving the amount of low to mid power. While adding bore can and does in most cases add a noticeable power gain it can in many common two stroke applications rob you of valuable transfer port volume. The gain of adding bore to an engine is largely dependent on that particular engines paramaters. As far as rod length is concerned, long rod motors tend to rev slightly slower than the short rod variants. But, they are more reliable due to the decrease in piston side loading they create. But to that point a certain ratio of rod length per stroke must be kept, so optimum rod length is also dependent on an engines perameters.

[2000BansheeBoy]

awwww thanks for clearing that up.

i'm gonna go try and find that...

[Senator]

I was hoping some of the more advanced builders and tuners would get in on this. Thanks Wildcard.

[SHOWTIME]

yea this is great discussion!
thanks for gettin it rollin' senator!

[Wildcard-1]

You can do a search for Gordon Jennings PDF and get a downloadable version right online for free if you hate spending the money for it. The newer revisions also have some add-ins by Eric Gorr. Pretty good reading if you want to learn a lot about the fundamentals of two strokes. Some other good reading is A. Graham Bells book, trying to remember the title.

I mentioned earlier how technology has affected some of Jennings theories. Two that come to my mind the most at the moment are advancements in reed induction, which he covers very little and piston ring materials/design. Reed petal and cage designs have come a long way since the 60's and allow for a much more controlled and larger intake charge. Most of Jennings work discussed is on piston port engines. As far as piston ring design, Jennings mentions standard exhaust port widths of 60% with extreme cases being in the 70% range with very carefull considerations on port designs. Today we can realize standard port widths in the 65-70% range with the extreme (drag) port widths in the 75% range without the amount of ring failures seen in the past. This is one reason I believe so much emphasis back then was placed on larger bores being the way to go. The larger the bore the wider you can make the effective exhaust port and thus the more power you can produce. Horse power is a product of many variable, engine speed being one. But the best method is to find ways without increasing rpm, to increase the BMEP of the engine. In other words increase it's volumetric efficiency through more efficient porting, pipes, head chamber design ect. Performance two strokes have come a long way since the days of Jennings studies.

Very good topic Senator....Thanks!

[Senator]

Thank you, Wildcard, for all of your input

[anti bling]

very interesting stuff..... i must research this topic more.... my girl is a librarian so I'll have her find more info for me. makes me want to look into trying to fuel inject my 2 stroke. i wonder if that would work well? EFI and two stroke power! hmmm...... now i need to look into this a lot more.

[Wildcard-1]

Throttle body type injection systems have been used on sleds for years and there have been a few adapted to the atv market. They work well in that once you get them dialed in you don't have to mess with jetting/tuning so much, but a good carb tuner can get the same kind of performance. Now, there has been much work as of late with DI and that presents a whole new performance advantage being able to inject the fuel at just the perfect time to optimize the combustion process. Very interesting stuff for sure.

On a note: my earlier posts are very generalized from what I have found in my own work. What works well for one may not be the power ticket for an other. And I want to re-emphasize that to bore vs. stroke is largely dependant on the parameters of each individual engine.

[volfan537240]

good reading for sure!