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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............
 

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Welcome to the forum. I like you already. You think logically. What kind of parts are you thinking about making?

Bill
 

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A Atwood said:
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 3" 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............

Pssssst! you forgot to factor in the inherent increased piston speed in the strokers. This alters the downward force calcs that you refer to.
 

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GRock said:
Pssssst! you forgot to factor in the inherent increased piston speed in the strokers. This alters the downward force calcs that you refer to.
C'mon GRock, don't leave us in the dark:thumbsup:
 

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A Atwood said:
Cyl #1- 4" bore 3" stroke =37.7ci
Cyl #2- 3.465 bore 3" stroke = 37.72ci
Something's wrong here. Both have the same stroke but cyclinder 2 is smaller than cyclinder 1. No way they could have the same displacement.

V= pi x r^2 x h

cylinder 1 = 3.1417 x 4 x 3 = 37.7 ci

cylinder 2 = 3.1417 x (3.465/2)^2 x 3 = 28.29 ci
 

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wyodude said:
Something's wrong here. Both have the same stroke but cyclinder 2 is smaller than cyclinder 1. No way they could have the same displacement.

V= pi x r^2 x h

cylinder 1 = 3.1417 x 4 x 3 = 37.7 ci

cylinder 2 = 3.1417 x (3.465/2)^2 x 3 = 28.29 ci
Looks like a typo to me, with 4" stroke it works out to 37.7 ci. In the subsequent calcs he used the correct 'arm' to determine effective torque.
 

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Hot rod magazine did a lengthly article on this same subject a few months ago. Same conclusion..peak power and torque are for all practical purposes the same. The ability to manipulate the torque curve was "easier" with the stroker motor than the bore motor. I would rather have displacement with added bore ...mainly due to reduced piston speed.
 

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the problem with the worry about piston speed at the RPM that street harley operates at are nothing the average piston speed of a 4-5/8 stroker at 6000is only 4625 when todays piston tech and oils 5600 is on the edge where that 4-5/8 stroke reaches yhis at 7200 how many daily drivers ever see this ?????????????????????????? and if its built to see it it is not a touring bike:cool:
 

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This is why I like the 98" kit so much. It is close to being square and that way I don't have to make up my mind on larger bore or larger stroke. Puts it right the middle. :huh:
 

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IronButt
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There are other things to look at piston stablility, and cost of the process and yes piston speed. And piston acceleration is another factor.
 

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wyodude said:
Something's wrong here. Both have the same stroke but cyclinder 2 is smaller than cyclinder 1. No way they could have the same displacement.

V= pi x r^2 x h

cylinder 1 = 3.1417 x 4 x 3 = 37.7 ci

cylinder 2 = 3.1417 x (3.465/2)^2 x 3 = 28.29 ci
From Atwood's original post he had;
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

So who went and changed the 4" stroke on the second cylinder? The 37.72ci is correct when people don't change what the man had written.
 

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wkohn said:
From Atwood's origianl post he had;
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

So who went and changed the 4" stroke on the second cylinder? The 37.72ci is correct when people don't change what the man had written.

Last edited by A Atwood : Yesterday at 10:06 PM.

He edited his post and corrected the error
 

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ViennaHog said:
He edited his post and corrected the error
Ahhhhhhhhhhh
 
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