This is my first post on this forum, so before I get into my topic, I'll tell a litle about myself. My name is ARN ATWOOD and I live in central Ohio. I am 36 years old. I have been an auto gearhead for nearly 20 years, but always loved Harley's. I finally bought me a new softail in 2004. Now my gearhead tendancies are infiltrating my mind on these bikes. I am planning on marketing some performance parts in the near future. But that is a different story. I enjoy the technical discussions the most and the ones that involve formulas, calculations, and numbers are my favorite.
Anyway, I normally read mostly and don't say much, but I thought some of you might find this interesting. I was reading another thread and a member was talking about how a motor with a longer stroke, but of equal displacement would produce more torque due to better leverage.
I have heard this opinion before and thought I might try to see if this theory holds water.
Most people understand that a stroker produces better torque due to increased displacement and increased leverage.
But a bored motor produces more torque due to increased displacement AND an increase in downward force.
The downward force is figured out by multiplying the psi inside the combustion chamber times the surface area of the piston.
So I got my calculator out and tried to see what would happen if I pitted two cylinders of equal ci against each other and see which one wins.
Cyl #1- 4" bore 3" stroke =37.7ci
Cyl #2- 3.465 bore 4" stroke = 37.72ci
For this comparison, we are going to say the heads flow Identical, the compression is the same and the same cam timing.
The psi inside the chamber is only a guess, but it does not matter, cause the outcome is always the same as long as the psi in both chambers is the same.
For this comparison, I'll use 500 psi with the piston half way down the cylinder.
Cyl #1 has 12.56 sq. in. surface area. 12.56 x 500 psi=6280 pounds downward force.
Cyl #2 has 9.42 sq. in. surface area. 9.42 x 500 psi=4710 pounds downward force.
To figure out torque, we must convert to lbs/ft. This is based on twelve inches of arm length. We must figure the percentage for each crankshft.
Cyl #1 has a 1.5" arm on the crank. so 1.5/12= .125 %
Cyl #2 has 2" arm. 2/12= .1667 %
Cyl #1 6280 pounds x .125 = 785 lbs/ft(torque)
Cyl #2 4710 pounds x .1667 = 785 lbs/ft
So we see no matter how the cylinder is configured, the theoretical torque output is ultimatly based on the total displacement. It is simply a trade between leverage and downward force.
Now take into consideration the larger bore motor can have better breathing heads, less piston velocity, higher rpm capability, less wear on the piston, rings, and cylinder walls(better rod to stroke ratio). It seems to me that the winner has to be the bore motor.
However, I have always believed you should get as much displacement as possible, by either means, and many times you don't have both options. So get the cubes however is is easiest an most economical.
Maybe the stroker motors get a reputation for low end grunt because on the average, the cylinder heads have smaller valves and tighter ports for that low rpm torque.
I thought some of you tech heads might enjoy this so I thought I might pass it on for discussion.
There you have it. My take on the old Bore vs. Stroke argument.
Then again there is always boost............
Anyway, I normally read mostly and don't say much, but I thought some of you might find this interesting. I was reading another thread and a member was talking about how a motor with a longer stroke, but of equal displacement would produce more torque due to better leverage.
I have heard this opinion before and thought I might try to see if this theory holds water.
Most people understand that a stroker produces better torque due to increased displacement and increased leverage.
But a bored motor produces more torque due to increased displacement AND an increase in downward force.
The downward force is figured out by multiplying the psi inside the combustion chamber times the surface area of the piston.
So I got my calculator out and tried to see what would happen if I pitted two cylinders of equal ci against each other and see which one wins.
Cyl #1- 4" bore 3" stroke =37.7ci
Cyl #2- 3.465 bore 4" stroke = 37.72ci
For this comparison, we are going to say the heads flow Identical, the compression is the same and the same cam timing.
The psi inside the chamber is only a guess, but it does not matter, cause the outcome is always the same as long as the psi in both chambers is the same.
For this comparison, I'll use 500 psi with the piston half way down the cylinder.
Cyl #1 has 12.56 sq. in. surface area. 12.56 x 500 psi=6280 pounds downward force.
Cyl #2 has 9.42 sq. in. surface area. 9.42 x 500 psi=4710 pounds downward force.
To figure out torque, we must convert to lbs/ft. This is based on twelve inches of arm length. We must figure the percentage for each crankshft.
Cyl #1 has a 1.5" arm on the crank. so 1.5/12= .125 %
Cyl #2 has 2" arm. 2/12= .1667 %
Cyl #1 6280 pounds x .125 = 785 lbs/ft(torque)
Cyl #2 4710 pounds x .1667 = 785 lbs/ft
So we see no matter how the cylinder is configured, the theoretical torque output is ultimatly based on the total displacement. It is simply a trade between leverage and downward force.
Now take into consideration the larger bore motor can have better breathing heads, less piston velocity, higher rpm capability, less wear on the piston, rings, and cylinder walls(better rod to stroke ratio). It seems to me that the winner has to be the bore motor.
However, I have always believed you should get as much displacement as possible, by either means, and many times you don't have both options. So get the cubes however is is easiest an most economical.
Maybe the stroker motors get a reputation for low end grunt because on the average, the cylinder heads have smaller valves and tighter ports for that low rpm torque.
I thought some of you tech heads might enjoy this so I thought I might pass it on for discussion.
There you have it. My take on the old Bore vs. Stroke argument.
Then again there is always boost............
