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Torque is a function of stroke (crank) the bigger the stroke the more torque the motor will make. Cylinder pressure (compression) is the same at 500 rpm or 15,000 rpm
 

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Cylinder filling is inefficient at low RPM due to the low intake velocity. The intake and exhaust system tuning will affect where the highest dynamic compression occurs.

Torque output is a function of cylinder pressure, piston diameter, stroke length, crank offset, pumping losses etc.

Typically an engine of a given displacement and type will make more torque if the bore/stroke ratio is less oversquare. Two main reasons:
1) work = force*distance. Longer stroke -> piston is pushed further by combustion gasses.
2) mechanical advantage of piston rod against crankshaft. Longer lever -> more multiplication.

The downside is that there's more rotational intertia of the crank, and piston velocity and rod angular accelleration is much higher. This limits the maximum RPM, and therefore ultimate power output. Also, the longer the hot gasses are in contact with the piston, the more heat is absorbed by the engine. This is bad. Remember that the combustion gasses are hotter than the melting point of aluminum.
 

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Discussion Starter #4
Piston diameter, stroke, and crank offset are all constants yet our torque changes over the rpm range. That leaves cylinder pressure as the major variable. So is torque the greatest when cylinder pressure is greatest? Seems it pretty much has to be since it's the pressure pushing down on the piston that gives the force that results in torque.
 

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u are correct 10rpilot peak torque occurs at peak VE (volumetric efficiency)

peak cyl pressures occur at peak VE

cyl pressures do not remain constant they change with VE

static compression is the mathimatical difference from the relationship of the piston at BTC and the piston at TDC. this is the number manufactures give in the specs

dynamic compression is the actual compression the engine sees while operating and changes as the VE of the engine changes. dynamic compression is almost always lower that static as most N/A engines arent anywhere near 100% effiecent

F/I engines will see dynamic compression greater than there Static compression.
 

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10RPilot said:
Piston diameter, stroke, and crank offset are all constants yet our torque changes over the rpm range. That leaves cylinder pressure as the major variable. So is torque the greatest when cylinder pressure is greatest? Seems it pretty much has to be since it's the pressure pushing down on the piston that gives the force that results in torque.
More or less. Torque output at the crank is a net thing, so peak cylinder pressures might be at a slightly higher RPM than the torque peak.
 

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Discussion Starter #9
Then I am advancing my next hypothesis: The major mechanism by which the compression rings seal against the cylinder wall is combustion pressure against the back side of the ring. Therefore, it stands to reason that for fastest wear during engine break-in, keeping the engine at or near it's peak torque is most effective. Maybe not practical, but.......
 

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I once checked this out with an actual ring manufacturer. Their take was that the majority of ring seating happens within the first 20 seconds the motor is running.

The amount of mythology surrounding engine break-in is amazing.
 

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There is an optimal point at which the fuel mixture will burn most efficiently below this point and above this point there is either time left when the fuel mixture is done burning and the piston is still traveling down or the fuel mixture is still burning as it goes out the exhaust valve. The max torque will be when the best fuel burn is achieved under wide open throttle. Horse power will continue to increase even though there is less effect fuel usage because as the engine speeds up the cylinders fire more often and horse power is a function of work done over a time period.
 

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As for engine break in I have always felt that it is done quickly as well and that the best thing is forces that are not steady. Basically rolling on the throttle to almost wide open quickly then letting off of the throttle and letting the engine slow down the bike or car then repeat repeat repeat. This throws a heavy down load on the pistons and then a large suction in the chambers that pulls up on the piston. Doing this in all gears will also load up the clutch and transmission gears on both surfaces as well. Not to mention that while the engine is in the decel mode minute amounts of oil come up past the rings and lube everything.
 

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well you need to keep in mind thats rings actualy flex. meaning they tip in the piston grooves. some manufactures actualy "clock" the ring groves upward at an angle (usualy very minute, measured in seconds of an inch) to help keep the rings from tipping too much, and keeping a flat contact surface with the cyl wall for a better seal.

they mostly tip downward due to cyl pressure on the power stroke. if youve ever taken an engine apart with a few mile on in youl see most of the wear to the rings is on the top edge. this is due to the rings "tipping" so thats another reason changing your speeds is important. because on deceleration the rings wont be tipped downward as great and will break in more evenly.
 

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Discussion Starter #15
Captain Jack said:
The amount of mythology surrounding engine break-in is amazing.
That is precisely why I am pondering all this. It's hard to tell who to believe. I know a fair amount about engines, physics, metallurgy and thermodynamics due to my career (and ripe old age). I still like to discuss these matters though, both to validate my beliefs and possible gain new tidbits of info. Thanks fellers.
 
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