bore vs stroke
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Cargo Master
Joined: Apr 2003
Posts: 3,476
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From: Frequently frozen MN
bore vs stroke
I've read in a couple of places that for the same displacement, the engine with the longer stroke is more powerful. The theory was something about how the piston stayed close to TDC for a longer time.
I've thought about that and I'm not sure that that's the case. I'm too rusty in math to look into it further, but I have a couple of thoughts.
1. the crankshaft rotational speed will be approximately constant at a given rpm
2. That implies that the piston speed will also be constant.at a given rpm
3. The piston speed per rpm will be higher for a longer stroke because it has further to travel. Maybe that results in higher torque-- I'm not sure.
When I consider those 3 things together I can't see the piston spending increased time near tdc..
Opinions are welcome. This is just some conjecture. Would be great to hear from someone who actually knows.
Thanks,
hj
I've thought about that and I'm not sure that that's the case. I'm too rusty in math to look into it further, but I have a couple of thoughts.
1. the crankshaft rotational speed will be approximately constant at a given rpm
2. That implies that the piston speed will also be constant.at a given rpm
3. The piston speed per rpm will be higher for a longer stroke because it has further to travel. Maybe that results in higher torque-- I'm not sure.
When I consider those 3 things together I can't see the piston spending increased time near tdc..
Opinions are welcome. This is just some conjecture. Would be great to hear from someone who actually knows.
Thanks,
hj
Frmr Ford Trans Engr
Joined: Aug 1999
Posts: 24,739
Likes: 2,669
From: SE Florida
The longer stroke results in a larger lever arm on the crankshaft. This gives a higher torque multiplication when changing the straight line force from the piston/rod into rotational force on the crankshaft.
For example, if there is 100 pounds of force in the rod and the journal on the crankshaft is 0.1 feet from the center that is 100 x 0.1 = 10 lb-ft of torque into the crankshaft. Now if the force remains 100 pounds and the journal is 0.2 feet, this will give a longer stroke and also 20 lb-ft of torque in the crankshaft.
For example, if there is 100 pounds of force in the rod and the journal on the crankshaft is 0.1 feet from the center that is 100 x 0.1 = 10 lb-ft of torque into the crankshaft. Now if the force remains 100 pounds and the journal is 0.2 feet, this will give a longer stroke and also 20 lb-ft of torque in the crankshaft.
.
Super Moderator
Joined: Jan 2008
Posts: 40,039
Likes: 1,523
From: Maine, Virginia
This seems to be where Ford and Chevy engines are very different. As I understand it, Ford engines have a longer stroke than GM engines but GM engines have proven to be just as good and in many cases better than some of Ford's offerings.
It seems to me that GM engines are preferred for racing. Does the shorter stroke allow it wind up faster? Is there a stronger aftermarket for GM engines or better overall parts availability?
It seems to me that GM engines are preferred for racing. Does the shorter stroke allow it wind up faster? Is there a stronger aftermarket for GM engines or better overall parts availability?
Scope Wizard
Joined: May 2019
Posts: 6,530
Likes: 25
you might find this interesting.... you can watch the video too.
https://www.motorauthority.com/news/...kes-more-power
https://www.motorauthority.com/news/...kes-more-power
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