Mysteries of Bore Stroke & Rod length Part1
02-05-2015, 03:03 PM
#1
TECH Apprentice
Thread Starter
Mysteries of Bore Stroke & Rod length Part1 What Bore? Stroke? Rod Length ?
What makes a better combination for a given cubic inch engine combination? A 1.4 rod ratio or a 1.9 rod ratio?
What’s better: a big bore / short stroke or a small bore / long stroke?
It really depends on your engine application, the load and RPM range. If you need torque down low, say 1000 - 2500 RPM, you may want a 1.3 rod ratio. If you want a engine that operates at 9000 - 10.000 RPM you may want a rod ratio of 1.95 like the Sprint Cup cars, or 17,000 - 20,000 RPM like Formula 1 the rod ratio is about 2.4.
Here are a few thoughts about Bore, Stroke and Rod length (rod ratio).
These are exaggerations and it is not likely you will have a combination like these but here are to examples.
If you have two engines both the same 376 cubic inches both have the same rod ratio of 1.68
Example #1 has a B x S x R of 4.065 x 3.622 x 6.098
Example #2 has a B x S x R of 3.86818 x 4.0 x 6.734
Both are 376 cubic inches displacement.
How are they the same? Both have the same swept volume that means that every degree of rotation the cubic inch per degree is the same. That means that they should take the same cam specs for a given application. 1500 - 6000 RPM.
Both would have the same torque to the crank at every degree of rotation with the same cylinder pressure all the way through the cycle.
So how are they different?
Example #1, with the big bore, short stroke, the piston will have less acceleration, less velocity less movement 3.622.
Example #2, with the smaller bore, longer stroke, will have a higher piston speed and higher piston acceleration and more movement 4.0.
If two engines are the same CI with the same rod ratio like these two 1.68 it does not matter if one has a longer stroke or one has a bigger bore they do the same.
Yes I know the smaller bore will not let the head flow as much but there is not that much difference for a low rpms like this. More later
What makes a better combination for a given cubic inch engine combination? A 1.4 rod ratio or a 1.9 rod ratio?
What’s better: a big bore / short stroke or a small bore / long stroke?
It really depends on your engine application, the load and RPM range. If you need torque down low, say 1000 - 2500 RPM, you may want a 1.3 rod ratio. If you want a engine that operates at 9000 - 10.000 RPM you may want a rod ratio of 1.95 like the Sprint Cup cars, or 17,000 - 20,000 RPM like Formula 1 the rod ratio is about 2.4.
Here are a few thoughts about Bore, Stroke and Rod length (rod ratio).
These are exaggerations and it is not likely you will have a combination like these but here are to examples.
If you have two engines both the same 376 cubic inches both have the same rod ratio of 1.68
Example #1 has a B x S x R of 4.065 x 3.622 x 6.098
Example #2 has a B x S x R of 3.86818 x 4.0 x 6.734
Both are 376 cubic inches displacement.
How are they the same? Both have the same swept volume that means that every degree of rotation the cubic inch per degree is the same. That means that they should take the same cam specs for a given application. 1500 - 6000 RPM.
Both would have the same torque to the crank at every degree of rotation with the same cylinder pressure all the way through the cycle.
So how are they different?
Example #1, with the big bore, short stroke, the piston will have less acceleration, less velocity less movement 3.622.
Example #2, with the smaller bore, longer stroke, will have a higher piston speed and higher piston acceleration and more movement 4.0.
If two engines are the same CI with the same rod ratio like these two 1.68 it does not matter if one has a longer stroke or one has a bigger bore they do the same.
Yes I know the smaller bore will not let the head flow as much but there is not that much difference for a low rpms like this. More later
02-05-2015, 08:22 PM
#2
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This stuff keeps me awake at night. Thanks a lot...I'm going to be worthless at work in the morning
Question. Does the faster piston velocity of the 4" stroke have an affect on intake port velocity compared to the slower moving 3.622" combination?
Question. Does the faster piston velocity of the 4" stroke have an affect on intake port velocity compared to the slower moving 3.622" combination?
02-06-2015, 08:39 AM
#4
TECH Apprentice
Thread Starter
Like the example above it does not if the rod ratio is the same and the CI are the same. I will give you an example. They both have a 1.68 rod ratio so 20 degrees ATDC the the 4.00 stroke piston is down .155 the 3.622 stroke is down .141 but they both have displaced 1.82ci so not if the rod ratio is the same.
02-06-2015, 08:55 AM
#5
TECH Apprentice
Thread Starter
The 4.0 stroke has a 1.53 rod ratio the 4.125 has a 1.484 rod ratio so the 4.125 will make a little more torque but a little less power up top. Is it were mine I would go with the 4.0 stroke.
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02-06-2015, 02:12 PM
#8
If you had a solid roller and supporting AllPro heads/single plane intake/Ti valves the proper springs, Jesel rockers/lifters, Magnum 4" stroke/ultra h beam rods and 4.190" bore...
What RPMs would you spin this to with some longevity in mind. Meaning is 8k from time to time fine.... And/or is 8500 really going to shorten the life. Road race focus but street driven. I'll ping you in a couple weeks once I get my pistons ordered thru Wiseco Kip!
What RPMs would you spin this to with some longevity in mind. Meaning is 8k from time to time fine.... And/or is 8500 really going to shorten the life. Road race focus but street driven. I'll ping you in a couple weeks once I get my pistons ordered thru Wiseco Kip!
