For discussion - rod length and stroke vs bore

[Senator]

..from the 2-STROKE TUNER’S HANDBOOK by Gordon Jennings

In Chapter 1, Fundamentals, Mr Jennings opens a lively examination of piston speed, offering ups and down (pun intended) of increased and decreased stroke length. We should discuss.

While on the subject of bore/stroke dimensions, I would like to say that there is much in favor of long stroke two-stroke cycle engines in many applications. They are not superior (as many people seem to think) compared to the present day short-stroke designs in terms of low-speed torque, as torque is entirely a function of displacement and bmep, and wholely unrelated to bore/stroke ratios. With a long stroke, there is (at any given displacement) a reduction in bore, and with it a loss of piston area against which gas pressure can exert its force, that exactly balances the loss of leverage in a short-stroke engine (which is, in turn, compensated by a gain in piston area). The only thing wrong with the long-stroke engine is that its crankshaft speed is limited by inertia loadings, and that in turn limits its absolute power potential as compared with the “modern” short-stroker. On the other hand, it is compensated by having a much more compact combustion chamber, which makes for more efficient burning, and by lower thermal loadings on the piston as a result of the smaller crown area into which heat from the combustion process may soak. Finally, there is an advantage in port area for the long-stroke design resulting from its relatively large cylinder wall area. This area increases in the long-stroke engine because displacement rises only in direct proportion to stroke, but is increased by a factor of 3.14 (the constant, pi) with enlargements in bore. These are very real advantages, but they are not enough, usually, to prevail against the short-stroke engine's sheer ability to rev. Crankshaft speed is the only thing subject to much juggling in the horsepower equation- and is a far more potent factor in determining power output than the relatively slight improvements in bmep obtainable with the marginally better combustion chamber and porting in the long-stroke engine. A 10-percent improvement in our Kawasaki F-5 engine's bmep (a large improvement indeed) would raise its output to 52.3 bhp; leave the bmep unchanged, but shorten the stroke and spin it 11,000 rpm and you would have 61.3 bhp. There is indeed no substitute for revs.

[Duneblazer]

interesting... but way over my head!!!

[timsyfz]

Yeah too deep for my brain puddle

[SHOWTIME]

thats actially very interesting.
where did you get that? id like
to read up on it.

[xTHUNDERCATx]

so is this saying your almost better off have a short rod and boring it because the higher rpms with the displacement can put out more power? or did i miss something

[Senator]

Originally Posted by xTHUNDERCATx

so is this saying your almost better off have a short rod and boring it because the higher rpms with the displacement can put out more power? or did i miss something

That's one of the downs to having a long rod, according to Mr Jennings. CC for CC, the motor math of it says you can spin the crank faster with a shorter rod, because it keeps piston speed to reasonable levels that won't detinate your motor. I'm learning myself, just thought I would take you guys along on the journey.

Hey showtime, Barnes and Noble or any other bookstore should have it. Or borrow it from your local library if you think you may never reference it again.

[FL Smitty]

BRRRRAAAAAPPPP Thats all I got. It makes sence to me.

[2000BansheeBoy]

so big bore is best cuz piston speed is optimized but i'd think that the increased piston size might make it harder to rev higher but the increased combustion must weigh that out...

here's a question though, according to him it'd be more efficient to get a big bore kit rather than a stroker kit?

and what book or whatever is this?

[anti bling]

think of it as spinning a yo yo over your head. the shorter the string the faster it spins around your head (more RPMs). the longer the string the slower it is. it takes a lot more force to spin the longer string (IE longer stroke) faster than the shorter one (short stroke) and with a 2stroke, increased rpm will tend to produce more power. sometimes bigger displacements are not all what there cracked up to be.

i have felt this with the different 2 strokes i have owned. my 250R revved to the moon and had the power band in the upper rpm range. my 500 zilla has low end torque but lacked in the upper rpm power. i guess it is a trade off in power bands.

[Senator]

I like this discussion.

And bansheeboy , the information on the book is

Title: Two-Stroke Tuner's Handbook

Author: Gordon Jennings

Copyright © 1973