Mysteries of Bore Stroke & Rod length Part1
02-05-2015, 03:05 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
__________________
02-07-2015, 11:35 AM
#3
I just want to add some parasitic frictional numbers to the discussion here.
In a four inch stroke application each of eight pistons travels 8 inches per crankshaft revolution. From top to bottom and back up again.
8 X 8 X 100 (revolutions/second @ 600rpms) = 6400/12= 533 feet 4 inches
likewise a 3.622" crankshaft is:
8 pistons X 7.244" X 100 equals 5795.2/12=482 feet 11 inches
50 feet and five inches of total linear ring friction each second @ 6 grand.
Second stat is rod big end distance traveled.
The average con rod big end is say 425 grams plus 50 or so for a pair of bearing shells. Just over a pound each or 2 pounds on each of 4 rod throws
Circumference is Pi (3.1415927) times diameter of a circle
A 4 inch crank has each pair of rods travel 12.57 inches times 4 throws = 50.27 inches times the same 100 revolutions per second equals a distance of 5026.5 inches or 418.88 feet
A 3.622 crank X Pi X 4 throws X 100 divided by 12 equals 379.29 feet
Again roughly forty linear feet of travel distance (each second) that 8 pounds of rod big ends move.
One could even throw crank journal diameter or bearing speed into the mix as well.
All kinds of physics that come into play when trying to design the "BEST" combo for your application.
In a four inch stroke application each of eight pistons travels 8 inches per crankshaft revolution. From top to bottom and back up again.
8 X 8 X 100 (revolutions/second @ 600rpms) = 6400/12= 533 feet 4 inches
likewise a 3.622" crankshaft is:
8 pistons X 7.244" X 100 equals 5795.2/12=482 feet 11 inches
50 feet and five inches of total linear ring friction each second @ 6 grand.
Second stat is rod big end distance traveled.
The average con rod big end is say 425 grams plus 50 or so for a pair of bearing shells. Just over a pound each or 2 pounds on each of 4 rod throws
Circumference is Pi (3.1415927) times diameter of a circle
A 4 inch crank has each pair of rods travel 12.57 inches times 4 throws = 50.27 inches times the same 100 revolutions per second equals a distance of 5026.5 inches or 418.88 feet
A 3.622 crank X Pi X 4 throws X 100 divided by 12 equals 379.29 feet
Again roughly forty linear feet of travel distance (each second) that 8 pounds of rod big ends move.
One could even throw crank journal diameter or bearing speed into the mix as well.
All kinds of physics that come into play when trying to design the "BEST" combo for your application.