breakdown of bore/stroke help

[up the creek]

hi gang.

ive skim read and heard a few comments and it still makes no sense to me the effects of stroke and bore lengths..
so if i dont make sense then thats why im asking you guys. i read something about scavenging, effects on engine heat and crank/piston cylinder friction etc and it all effects engines in different ways depending on bore/stroke lengths.
my question is im looking at a 30hp 2 sroke, yamaha equal bore stroke of i think 59.5 and 59.5 respectively i believe suzuki tohatsu mercury all have a longer stroke then bore.
i really wanted the yamaha but more importantly i want an engine that runs kool has good scavenging and higher output doesnt really matter,..as it has 30hp which is plenty for me and i dont care if it goes harder...
can anyone roughly translate the above and what bore stroke config would suit me..i will be spending good money and just want to get it right.
thanks..

[Michael Wright]

The relationship of bore and stroke on a motor of that size is almost irrelevant. But generally longer stroke than bore will give more torque but wider bore than stroke gives a more responsive motor that will rev higher than a longstroke motor. My advice is to buy the motor that you want, all the name brands are pretty reliable these days


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[Noelm]

I don't think running cool is going to be an issue with any of those motors, the basic design has been around for decades, nearly all motors around that size will be near "square" meaning bore and stroke will be the same, or close to it, the basic concept only comes into play when you want very high RPM (like motor bikes and chainsaws) or need very high, low RPM torque (like commercial diesels) buy any of those motors with confidence, and don't over think it.

[up the creek]

The relationship of bore and stroke on a motor of that size is almost irrelevant. But generally longer stroke than bore will give more torque but wider bore than stroke gives a more responsive motor that will rev higher than a longstroke motor. My advice is to buy the motor that you want, all the name brands are pretty reliable these days


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hey mate thanks for your reply very helpful as i will put a 30hp on a smaller tinnie so more torque isnt that important, and as you say id definatley rather a more responsive motor, so the yammie at even bore and stroke will be responsive? dont need to rev higher but definately worth it for responsiveness..as for choice ive had genuine american mercs, tohatsu, evinrude i do really want my 1st yammie...cheers bro..

[up the creek]

I don't think running cool is going to be an issue with any of those motors, the basic design has been around for decades, nearly all motors around that size will be near "square" meaning bore and stroke will be the same, or close to it, the basic concept only comes into play when you want very high RPM (like motor bikes and chainsaws) or need very high, low RPM torque (like commercial diesels) buy any of those motors with confidence, and don't over think it.

kool noelm good to know they all stay kool, but i want to think a bit as ive learnt as you say they all run kool with only small bore/stroke differences, and michael telling me about more responsiveness is a seller point for me and you know some motors scream a little louder etc so i just want to hear whats thought about these things i definately want smoother and responsivness over raw torque and screamin rpms..

[Noelm]

Smoother is a product of design and the amount of cylinders, a single cylinder 30 HP will not be as smooth as a 2-3 cylinder 30, and most carby 2 strokes are not real smooth, it's just a fact of life with the simple basic principal of 2 strokes, they need to be under load to run smooth.