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      09-26-2013, 07:03 AM   #22
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Drives: BMW
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Originally Posted by mkoesel View Post
Thank you.

But, torque changes achieved solely by adjusting bore and stroke at a specific fixed displacement are of minimal significance in the overall picture. With so many other factors affecting the engine's output characteristics, attacking the bore dimensions simply to achieve a torque increase is not cost effective in general.

I am not so convinced. I disbelieve that if we built a 3.2L naturally aspirated I6 engine using the 89.6mm of the S55 at 360hp (a power figure which BMW did reach in the CSL), that there would be any valid claims of a torque issue. Such claims would boil down to the same rhetoric we endure with respect to the S65 and S85. And speaking of that, what happened to this over-square, high revving alleged high torque design of the S54 once they got ahold of the 98mm bore spacing? Surely we cannot honestly believe the huge increase in bore and huge decrease in stroke was all done at the expense of a supposedly superior higher torque design of the past I6, all while the ability to reverse the bore dimensions to under-square just happened to conveniently and coincidentally present itself.

Color me a very deep shade of skeptical.

Getting back to the S55, I accept that the new spray on lining allows for a larger bore. But I still think the bore center must be greater than 91 mm now and I am very eager to know if this is the case.
A oversquare design is better for a high revving engine as that keeps piston speeds down. Lower piston speeds is good for durability. One of the disadvantages of a oversquare (short stroke) is lack of/less torque than a similarly sized undersquare engine.

Remember that a long stroke means a longer arm from crankpin to crank center, and as we all know Torque = Force x Arm. So the longer the stroke, the longer the arm from crankpin to crank centre is and more torque is made.

However on a FI engine, boost can be increased to make up for the torque deficit. In a high performance FI engine then, a oversquare design makes a lot of sense because it has so many advantages over a undersquare design. So, no this didn't happen by coincidence. A large bore/short stroke gives many advantages in breathing (larger valves), less friction, less heat, lower piston speeds. In a high perf engine a large bore/short stroke will allmost allways be the best solution for high end power, high rpm and agility. With FI you can also compensate for less torque, which the old M3 engines couldn't.

The magnitude of torque depends on three quantities: the force applied, the length of the lever arm[2] connecting the axis to the point of force application, and the angle between the force vector and the lever arm.
An oversquare engine allows for more and larger valves in the head of the cylinder, lower friction losses (due to the reduced distance travelled during each engine rotation) and lower crank stress (due to the lower peak piston speed relative to engine speed). Due to the increased piston- and head surface area, the heat loss increases as the bore/stroke-ratio is increased excessively. Because these characteristics favor higher engine speeds, oversquare engines are often tuned to develop peak torque at a relatively high speed.
Undersquare engines

These produce strong torque at low to mid range rpm's because of the "leverage" advantage of a longer stroke. But, undersquare can be a negative trait, since a longer stroke usually means greater friction, a weaker crankshaft and a smaller bore means smaller valves which restricts gaseous exchange; however, modern technology has lessened these problems (explanation?). An undersquare engine usually has a lower redline, but should generate more low-end torque. In addition, a longer stroke engine can have a higher compression ratio with the same octane fuel compared to a similar displacement engine with a much shorter stroke ratio. This also equals better fuel economy and somewhat better emissions. Going undersquare can cause pistons to wear more quickly (greater side-loads on the cylinder walls) and can cause ring seal problems and lubrication problems; with increased loads on the crankshaft, pistons, the piston pins, connecting rods, and rod bearings (due to piston speed). In general, a longer stroke leads to higher thermal efficiency through faster burning and lower overall chamber heat loss. A longer stroke will have greater port velocity at a given RPM, more torque due to more leverage on the crank, will achieve it's greatest efficiency at a lower RPM. Smaller combustion chambers are also more efficient, with the flame front having a shorter distance to travel- this leads to being more detonation resistant, and having an advantage for emissions.

Oversquare engines

These are generally more reliable, wears less, and can be run at a higher speed. In oversquare engines power does not suffer, but low-end torque does - it being relative to crank throw (distance from the crank center to the crankpin). An oversquare engine cannot have as high a compression ratio as a similar engine with a much higher stroke ratio, and using the same octane fuel. This causes the oversquare engine to have poorer fuel economy, and somewhat poorer exhaust emissions. Breathing is an important advantage for oversquare engines, as the edges of the valves are less obstructed by the cylinder wall (called "unshrouded"). The big bore can fit larger (or more) valves into the head and give them more breathing room.

With shorter crankshaft stroke (and therefore piston travel) parasitic losses are reduced. Ring drag is the major source of internal frictionand the crankshaft assembly also rotates in a smaller arc, so the windage is reduced. Oil-pressure problems caused by windage and oil aeration are lessened.
Appreciate 0