bore vs stroke

[firko]

desaxation(there you go firko)

Nice effort Loz.....now all I have to do is find a use for the word in my day to day work.
Now, get back into my engine!

[matcho mick]

didn"t some 6HR guys get busted for boring a kwaka 4 (desaxe),was permissable to overbore,but not all to the front of the barrell ,

[Nathan S]

 10+ YZF450 has offset gudegon.

[Lozza]

01+ YZF450 has offset gudegon.

They don't count.

[Tim754]

Umm sorry I am a little familiar with the workings of Nitromethane fuelers    Just being a serial pest again.... Cheers Tim754

[vmx42]

01+ YZF450 has offset gudegon.

They don't count.

And I think the YZF450 has its barrel offset from the crank centreline. Trying for the same effect [I assume] in a different way.

[pancho]

 A shorter rod which increases conrod angularity while the piston descends the stroke increases the piston speed which in turn takes advantage of the rapid pressure rise occuring in spark ignition piston engines, resulting in increased torque. Torque plus rpm =horsepower.
 Similar situation occurs to a lesser degree in those engines where the gudgeon pin is offset [to quieten things down a bit], if you reverse the piston to put the offset the other way for a power increase. Ask the old "stock car" drivers who got sneeky extra power this way.
  Torque plus rpm=horsepower.
This explains to those who wonder why, when one looks at a rather large capacity V8 car engine sees it putting out 200hp, where as a motor cycle high performance engine puts out  half that figure from less than a quarter of the capacity.
 

[holeshot buddy]

what about a 2 stroke then

seeing thats what 90% of us ride

[Lozza]

A shorter rod which increases conrod angularity while the piston descends the stroke increases the piston speed which in turn takes advantage of the rapid pressure rise occuring in spark ignition piston engines, resulting in increased torque. Torque plus rpm =horsepower.
 Similar situation occurs to a lesser degree in those engines where the gudgeon pin is offset [to quieten things down a bit], if you reverse the piston to put the offset the other way for a power increase. Ask the old "stock car" drivers who got sneeky extra power this way.
  Torque plus rpm=horsepower.
This explains to those who wonder why, when one looks at a rather large capacity V8 car engine sees it putting out 200hp, where as a motor cycle high performance engine puts out  half that figure from less than a quarter of the capacity.
 

Think you got yourself all befuddled there pancho, most high performance engines run >2.1 rod to stroke ratio. Aprilia GP 125 and TM/Pavesi 125 shifter kart engines(all over 50HP) run 120 and 118mm rods for a 54.5mm stroke. There isn't much pressure rise as the piston decends.

[Nathan S]

Long rods reduce peak piston acceleration, and reduce the angularity of the rod relative to the crank (meaning that the piston is marginally more efficent at pushing on the crank). They also hold the piston near TDC for longer, meaning that the rate of increase in compression pressure is more gradual, and that ignition timing is less critical.
This is all good stuff.

In theory, a long rod can make the engine more likely to detonate, as the additional time the piston spends at/near TDC can prevent the combustion gases expanding as fast as they want (poor explaination, but hopefully it makes sense). In reality, it is very difficult to find anyone who even claims to have experienced this phenonema - if it exists at all, it has far more to do with large bore sizes (4"+) requiring lots of time for the flame front to travel across the combustion chamber.

Long rods also increase the height of an engine, which is rarely desirable but not often a problem.

The trend in the last thirty-odd years for rod:stroke ratio, has been clearly toward longer rods in all sorts of engines.
Four strokes have also had a clear trend toward shorter pistons - the reduced side loading of longer rods has allowed this.

[lukeb1961]

The trend in the last thirty-odd years for rod:stroke ratio

mmm.. is that because of fuel quality or other factors?

[Lozza]

It's because the longer rod 'works' better. Even a stock Banshee with just the 115mm rod swap feels torquier to ride. Something to consider for your PE

[JC]

Longer rod, when retro-fitted, changes valve/port/ign timing, increases dwell around TDC, & in 2T increases crankcase volume (unless compensated by piston w reduce deck height).

I waited a fair while for some-one to mention bmep

Torque, HP & BMEP are mathematically related to ea other by simple formulae.  Its hardly rocket science.


Getting back to the threads original question, one of the other effects/benefits of a long-stroke design is more compact/efficient combustion chamber (lower surface-area/volume ratio).

You can only go so far w oversquare 4T designs for better breathing via more valve area & higher rpm, till the size/shape of the combustion chamber becomes a problem (hi surface-area/volume ratio) & you start losing the gains.

In a 2t, its more win-win cos of more compact combustion chamber (lower surface-area/volume ratio) & more cylinder surface area allowing bigger/better ports (already mentioned) in long stroke/undersquare design.

[pancho]

 First of all let me say I enjoy this kind of discussion for the opportunity for me to learn more, I believe that one should never ever stop trying to learn more.
 As a 73 & a 1/2 year old I find the older I get the less I know [specially about 2 strokes]
  Lozza re.pressure on the piston. As I intimated on an earlier post, spark ignition engines are described as 'constant volume' which as I understand it means the fire goes BANG and the pressure rise is instant and short lived,which is the same as your post re 'not much pressure rise'.
 A short rod does have the disadvantage of more severe side thrust on the piston but conversely has a better straight line push on the crankpin.
 Added to this is the more rapid descent of the piston taking advantage of the short but rapid pressure increase from combustion. Less detonation?
 Going back in history [nothing new under the sun] 350 Gold Star BSA made up oval flywheels to allow a short conrod for a torque increase. More horsepower, from more torque same revs.
 Obviously rapid engine bore and piston wear follow.
 I believe the main reason engines in England, both car and m/cycles went to small bore - long stroke [to the detriment of good design] was the fact that the gov't taxed engine size on piston area times No. of cylinders for years so manufactures increased stroke to gain capacity.
cheers pancho

[Lozza]