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Connecting Rod vs. Stroke Analysis
The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts. This ratio (normally represented by “n”) can be calculated as follows:
Ratio “n” = Rod Length ÷ Stroke
The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length. It’s not possible for the rod to be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and are not practical for most purposes (although used for racing).
The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of “n”) will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of “n”) will occur with a longer rod or a shorter stroke.
If the rod length is decreased, or the stroke is increased, the “n” ratio value becomes smaller. This has several effects. The most obvious is the mechanical effect. Motors with low values of “n” (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high “n” motors):
physically shorter top-to-bottom & left-to-right (more oil pan, header, and air cleaner clearance)
lower block weight (400 vs. 440, for example)
higher level of vibration
shorter pistons, measured from the pin center to the bottom of the skirt
greater wear on piston skirts and cylinder walls
slightly higher operating temperature & oil temperature due to friction
There are also differences in how the motor breathes:
intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response
on the negative side, a small or badly designed port will “run out of breath” sooner
piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)
Spark advance is also affected:
earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation
the motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)
Effects of Long Rods
Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.
They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.
Effects of Short Rods
Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.
Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.
Rod Ratio vs. Intake Efficiency
An “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. There will be instances where the choice of stroke or rod has not been made, but the intake pieces (carburetor, manifold, and head) have been selected. Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “n” value can be used to compensate for less-than-perfect match of intake parts to motor size & speed. The reverse is also possible: the lower end is done, but there are still choices for the top end. Again, the “n” value can be used as a correction factor to better “match” the intake to the lower end.
The comments in the following table are not fixed rules, but general tendencies, and may be helpful in limiting the range of choices to those more likely to produce acceptable results. Rather than specify which variable will be changed in the lower end, “n” values will be used. Low “n” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. High “n” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.
“n” = 1.45 - 1.75 more compatible with:
Large intake port volume vs. motor size (”J” head on 273)
Single-plane or 360° intake manifolds
(Edelbrock Victor, Torker & Torker II, TM7. Holley Strip Dominator. Offenhauser Equa-Flow, Port-O-Sonic. Weiand X-Celerator, Team G)
Large carburetor vs. engine size (273 with 750cfm)
Moderate engine speed (pick-up, RV, towing)
Tall axle ratio (2.76, 2.93, 3.23, 3.55 and/or with tall tires)
“n” = 1.75 - 2.1 more compatible with:
Small intake port volume vs. motor size (stock 452 head on 498” RB stroker)
Dual-plane 180° intake manifolds
(Edelbrock: LD340, CH4B, DP4B, Performer & Performer RPM, Streetmaster, SP2P. Holley Street Dominator. Weiand Stealth, Action Plus)
Small carburetor vs. engine size (440 with 600cfm)
High engine speed (peak power more important)
Short axle ratio (3.91, 4.10, etc. and/or with 25 or 26” tires)
Planning a 383 Motor
This engine is generally overlooked in selecting a high-performance project. The motor has an excellent bore to stroke ratio: 1.26-1 (similar to 327” SBC, better than 340). The short stroke allows high RPM without destructive piston speed (7100 RPM = 4000 ft./min., the accepted “safe” limit for piston stress). The large bore permits big valves (2.14” intake, 1.81” exhaust).
A potential method of increasing peak power is to substitute the longer 440 6.768” (LY) rods for the original “B” 6.358” rods on the original crank. This has the following effects:
Increases the rod ratio (“n”) from 1.884-1 to 2.005-1
Reduces the piston compression distance to about 1.525” for a useful weight savings
Slightly reduces piston acceleration
This should allow an advantage in peak power. For a start in piston selection, take a look at the KB224 for BBC: flat top, CD = 1.52” (just below zero deck), and .990” pin for more weight savings and moderate cost. There may also be “possibles” for the 400 (4.34” bore), but not discovered yet. Ideas?
Stroke vs. Rod Length in Common Automotive Engines
Motor Stroke Rod n” Ratio
Mopar LA 273/318/340 3.31” 6.123” 1.85-1
Mopar LA 360 3.58" 6.123" 1.71
Stroker crank 3.79” 6.123" 1.62
4.00” stroker crank 4.00" 6.123" 1.53
“B” 350/361/383/400 3.375" 6.358" 1.88
400 w/ 440 crank/rods (451”) 3.75" 6.358" 1.70 ( & standard rods)
400 w/4.15” crank & std. rods 4.15" 6.358" 1.53 (498”)
400 w/4.15” crank (498”) 4.15" 6.535" 1.57 BBC +.400” rods
RB 413/426Wedge/440 3.75" 6.768" 1.80 w/440 crank & rods
B 383/400 3.75" 6.768" 1.80 w/440 crank & rods
RB413/426W/440 4.15” 6.768" 1.63 (494”)
Mopar 426 hemi 3.75" 6.86" 1.83
The angle of the rod at 90° ATDC is a good indication of how much stress the piston and cylinder wall will be subjected to with a specific rod/stroke selection (this is not the angle of maximum thrust, which occurs when the rod’ beam axis is at 90° to the crank throw or journal, typically between 70-76° ATDC; however, the math is easy to do). Angles beyond 17° (where the rod axis is 90° to the crank throw at 73° ATDC) promote excessive wear at the piston major thrust surface, and piston breakage could be the result. Before you purchase connecting rods that are shorter than previous or increase the stroke of the crank, calculate the new rod angle. High rod angles will require quality rods that have been checked for cracks and have quality (ARP, etc.) fasteners. Piston selection will be critical for the life expectation of the engine; maximum skirt length below the pin is desired.
Sine of Rod Angle = Stroke ÷ (Rod Length * 2)
Sine of Rod Angle = .5 ÷ R/S
To make your own calculations using the Microsoft Calculator (every Win95/98/00/ME has it):
Double-click the “Calculator” icon to open it
Click “View”, then “Scientific”
Input the result from the formula above
In the left margin of Calculator, look for the check-box that says “Inv” - check it
Make sure the box marked “Degrees” (not Radians) is checked
Click on “sin”
The rod angle in degrees will show in the window
Connecting Rod Length Comparison
By Rick Draganowski
Piston movement was computed by simulating the crankshaft/connecting rod/piston assembly in several precise engineering drawings (DesignCad) and then determining the exact amount of piston movement for each of 256 divisions of one rotation.
The piston movement data was then used as an input vector in a MathCad program to calculate velocity, acceleration, and dynamic forces.
The simulation of an infinitely long connecting rod, which imparts true harmonic motion to the piston, is the starting point.
The motion generated by a finite length connecting rod is quite distorted by comparison. It has much more velocity and acceleration at the top of the stroke compared to the bottom. A graph of the movement is peaked at the top of each cycle and rounded and flattened at the bottom. This is caused by the rod angle increasing and pulling the piston down and adding to the motion caused by the crankshaft rotating down from top dead center. At the bottom as the rod journal slows the angle decreases. This retards the movement of the piston by subtracting the rod angle component that was added at the top of the stroke from the crankshaft movement component at the bottom of the stroke.
Compression and combustion pressures are in opposition to the inertial forces so the top of exhaust and intake strokes generate the largest forces on the rod.
1) Maximum Piston Acceleration
This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.
------infinite rod--------6.0" rod---5.7" rod---5.565" rod
5000rpm 1332G 1749G 1776G 1790G
6000rpm 1933G 2525G 2558G 2578G
7000rpm 2631G 3437G 3482G 3509G
Percent difference due to rod length in above table.
Difference between 6" rod and 5.565" rod 2.34%
Difference between 6" rod and 5.7" rod 1.54%
Difference between 5.7" rod and 5.565" rod 0.79%
This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.
------infinite rod---------6.0" rod---5.7" rod
5000rpm 1240G 1600G 1623G
6000rpm 1786G 2305G 2338G
7000rpm 2432G 3138G 3182G
2) Maximum Connecting Rod Dynamic Load (Tension)
This table is for a 3.75" stroke used in a 400 or 383 small block Chevy engine. The forces are based on the weight of the piston and pin assembly and do not include the percentage of force generated by the acceleration of the end of the connecting rod. The reference piston is the stock replacement Silv-O-Lite piston for a 400 engine.
------infinite rod-----------6.0" rod-----5.7" rod----5.565" rod
5000rpm 2249LBS 2938LBS 2976LBS 3000LBS
6000rpm 3239LBS 4232LBS 4287LBS 4320LBS
7000rpm 4409LBS 5769LBS 5834LBS 5849LBS
Percent difference due to rod length in above table.
Difference between 6" rod and 5.565" rod 2.34%
Difference between 6" rod and 5.7" rod 1.54%
Difference between 5.7" rod and 5.565" rod 0.79%
3) Maximum Rod Angularity
This is the angle the connecting rod makes with the axis of the cylinder bore at 90 degrees after top dead center (maximum excursion from bore axis. This measurement is for the 3.75" stroke of the 400 and 383 only.
6.0" rod-----18.21 degrees
5.7" rod-----19.20 degrees
5.565" rod-19.69 degrees
4) Cylinder Wall Load
Percentage of compression and combustion force against the top of piston transmitted to the major thrust face of the piston and then to the cylinder wall.
This table is for the 3.75" stroke.
This table is for the 3.48" stroke.
5) Piston Speed
Maximum piston speed for the 3.75" stroke at 5000 rpm.
Infinite rod---81.68 feet per second, 55.69 MPH
6.0" rod------85.64 feet per second, 58.4 MPH
5.7" rod------86.01 feet per second, 58.6 MPH
5.565" rod---86.20 feet per second, 58.8 MPH
6) Effective Stroke
Because of the mechanical advantage provide by the toggling effect of the rod the shorter rods act as if they were in a longer stroke engine at the top of the stroke. This effect would make the short rod engine rev faster from 2000 to 4000 rpm and the circle track people claim that acceleration out of the turns is significantly improved with the shorter rod. In all other factors the longer rod comes out superior...
Effective stroke as compared to the infinite rod model for the 3.75" stroke.
infinite rod-=- 3.75"
6.0" rod------- 4.20"
5.7" rod------- 4.23"
5.565" rod---- 4.25"
Note that the differences are subtle...
7) Dwell Time
This measurement is of the number of crankshaft degrees the piston is within 0.250 inches of top dead center. It is the subject of much conjecture and controversy in the automotive literature.
This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.
Infinite rod---59.853 degrees
6.0" rod------52.397 degrees
5.7" rod------52.071 degrees
5.565" rod---51.915 degrees
Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.626%.
Percentage difference in dwell time between the 5.7" rod and the 5.565" rod is 0.3%.
Percentage difference in dwell time between the 6.0" rod and the 5.565" rod is 0.928%. (Still less than 1 percent)
This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.
Infinite rod---62.188 degrees
6.0" rod------54.929 degrees
5.7" rod------54.605 degrees
Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.593% at the 3.48" stroke.
The data in this report seems to indicate that the differences between the rod lengths are exaggerated in the literature. In many (most) cases claims are anecdotal and represent the vested interests of the suppliers. I have seen no objective dyno testing of rod lengths but keep hoping for one.
There are real gains to be had by going to longer rods but they are small, usually a lot less than 2 percent. However, the hard-core racers are grasping at every tiny bit of performance and can justify the expense. For the more average rodder I would suggest staying with the rod length specified by the factory. Money would be far better spent on improving the heads, cam, and induction and exhaust systems. (and perhaps a supercharger..)
The study of piston velocity and acceleration provides some interesting insights into the operational challenges presented by a reciprocating engine. The following graph shows a plot of the piston travel (blue line) as a percentage of distance from TDC as the crankshaft moves through 360° of rotation. The graph also shows the instantaneous piston velocity (green line) and piston acceleration (magenta line) for one full crank rotation. The magnitude values are shown as percentages of the maximum calculated value. These graphs were calculated for an engine with a 4.0 inch stroke and a 5.91 inch rod length. All specific numbers quoted in this explanation are for that case.
(Note: If you still believe that installing longer connecting rods will increase an engine's stroke, there's no need for you to go any further here.)
NOTE: All the calculations and explanations on this page and the previous page assume zero piston pin offset. A non-zero offset will slightly alter the calculations, SLIGHTLY being the operative word.
Piston Motion Graph 1
You already know that if you position a crankshaft so that a given piston is at its Top Dead Center (TDC) location (absolute maximum upward motion), and then rotate the crankshaft 180°, the piston will be at the bottom of its stroke (Bottom Dead Center, BDC). It is less obvious, however, that during the first 90° of that 180° rotation the piston moves significantly farther than it does during the second 90°. Note that the blue line in Figure 1 above shows that during the first 90° of rotation, the piston travels 60% of its stroke. This characteristic is explained in detail on the previous page.
The green velocity line (Figure 1 above) shows the relative speed of the piston (as a % of maximum) at any point during one rotation of the crankshaft. Velocity with a "plus" sign is motion TOWARD the crankshaft; velocity with a "minus" sign is motion AWAY from the crankshaft.
Note that at TDC and again at BDC, the piston velocity is zero. That is because the piston reverses direction at those points In order for the velocity to go from a "plus" number to a "minus" number, it must be zero at some point.
Note also that the maximum piston velocity in this case occurs at about 73° before and after TDC, not at 90° as you might think. The velocity line also shows that the piston velocity at any rotation point after TDC and before max velocity is greater than at the same number of degrees before BDC. For example, compare the velocity at 30° after TDC (62%) with the velocity at 30° before BDC (34%).
The value of the maximum velocity varies directly with engine RPM. For the configuration used in this example (4-inch stroke, 5.91" rod length), at 4000 RPM, the peak piston velocity of 4402 feet per minute.
This asymmetric velocity profile is a result of the same geometry characteristics which cause the dissymmetry in piston motion (described above). The position of maximum piston velocity depends on the relationship between connecting rod length and stroke length, known L/R ("L over R", rod length divided by stroke length). (The graph is repeated below for easy reference.)
As the rod gets shorter with respect to stroke, two interesting things happen which can have dramatic effects on cylinder filling: (1) the point of maximum piston velocity moves closer to TDC, and (2) the piston moves away from TDC faster, creating a stronger intake pulse. The location of maximum piston velocity influences the design of camshaft lobe profiles (especially intake) in order to optimize the intake event in a particular speed range.
Piston Motion Graph 2
Figure 1 again
MEAN PISTON SPEED
There is another piston velocity which is used more as a "rule-of-thumb" in engine evaluations. It is called "mean piston speed", which is a calculated value showing the average velocity of a piston at a given RPM in an engine having a known stroke length.
Keeping in mind that the piston travels a distance equal to twice the stroke length every revolution, Mean Piston Speed (MPS) is calculated by:
MPS (ft per minute) = RPM x 2 x stroke (inches) / 12 (inches per foot) = RPM x stroke / 6
The Mean Piston Speed for the example engine (4.00 inch stroke, 5.91 inch rod, 4000 RPM) is:
4000 x 4 / 6 = 2667 feet per minute.
For purposes of rules of thumb, it is generally agreed that for an engine in aircraft service, 3000 fpm is a comfortable maximum MPS and experience has shown that engines having an MPS substantially exceeding that value have experienced reliability issues.
Piston acceleration is simply a measure of how fast piston velocity is changing. If velocity does not change, there is no acceleration. Conversely, if velocity changes very rapidly, there is a large acceleration (see Velocity and Acceleration).
The force it takes to accelerate an object is proportional to the weight of the object times the acceleration. From that it is clear that piston acceleration is important because many of the significant forces exerted on the pistons, wristpins, connecting rods, crankshaft, bearings, and block are directly related to piston acceleration. Piston acceleration is also the main source of external vibration produced by an engine.
Acceleration with a "plus" is caused by a force pulling the piston toward the crankshaft; Acceleration with a "minus" sign is caused by a force which is pushing the piston away from the crankshaft. Notice that the magenta total acceleration line (Figure 2 above) has a very different shape around TDC than it does around BDC. At TDC and BDC, the piston is reversing its direction of motion, so piston velocity is zero, but that velocity is changing very rapidly, producing large values of acceleration.
The maximum positive value of acceleration occurs at TDC (1216 "g"). Between TDC and 73°, acceleration is positive but decreasing toward zero (the piston velocity is still increasing but less rapidly). At maximum velocity (73°), the piston begins to slow down. At that point, the acceleration changes direction (from a "plus" number to a "minus" number), and in so doing, momentarily passes through zero.
The maximum negative acceleration (-643 "g") does not occur at BDC, but about 43° either side of BDC. The value of this maximum negative acceleration is only about 53% of the maximum positive acceleration seen at TDC. The acceleration at BDC (-601 "g") is only 49% of the TDC maximum. The acceleration from 73° to BDC is negative, and it is slowing the piston to zero velocity. Therefore, it might be (incorrectly) called deceleration. However, that same negative acceleration is applied to the piston after BDC and is causing its velocity to increase.
The piston motion caused by the vertical component of crankpin movement (explained on the previous page) is called primary motion. The piston motion caused by the horizontal component of crankpin motion is called secondary motion. Recall that as the crank rotates 90° from TDC, the secondary motion adds to the primary motion, and from 90° to BDC, the secondary motion opposes the primary motion. The same is true for the acceleration. The acceleration curve is the sum of the primary (blue) and secondary (green) curves, as shown in Figure 2 above.
The asymmetrical nature of all three curves is a function of the relationship between the length of the connecting rod and the crankshaft stroke. A longer rod with respect to stroke length tends to reduce the asymmetry of motion, reduce the peak acceleration at TDC and increase the peak acceleration at BDC, moving those two peaks closer to the same value.
Contemporary auto engines tend to have rod-length / stroke ratios in an approximate range of 1.5 to 1.9. Note that a rod / stroke ratio less than 1.3 is, for practical applications, not possible due to physical constraints such as the need for piston rings and a wristpin, sufficient piston skirt length, and the inconvenience of having the piston contact the crankshaft counterweight.
Here are two practical examples showing typical values. In a Lycoming IO-360 (and IO-540) the rod length is 6.75" and the stroke is 4.375", for a ratio of 1.543 (close to the low end of the spectrum in contemporary design). At the other end of the spectrum, the connecting rod on a typical (circa 2007) 2.4-liter Formula-1 V8 engine is about 4.01" long (what your average grease monkey would call a "very short rod"). The stroke is in the vicinity of 1.566", for a very large ratio of ratio of 2.56. The following graph (Figure 4) clearly shows the effect of large and small rod/stroke ratios, and the figures in this paragraph certainly reveal the absurdity of discussing rod length as an absolute.
It is clear that the engine with the "long rod" (6.75") has a very small L/R ratio, and produces the green and magenta curves in Figure 3, showing the substantially earlier velocity peak and the distinct reversal around BDC, indicating a substantial secondary vibration component. Compare that to the large-ratio ("short rod") black and blue curves, showing an earlier peak velocity (could that help intake tuning, perhaps ??) and a very clean curve around BDC, showing a substantially-reduced secondary vibration component.
Rod Length Relationships
You are invited to participate in this attempt to understand a part of internal combustion engines. I invite any/all criticisms, suggestions, thoughts, analogies, etc.-- written preferred, phone calls accepted from those too feeble or who have arthritis. Contributors are invited to request special computer printouts for specific combinations of interest to them.
In general, most observations relate to engines used for some type of competition event and will in general produce peak power higher than 6000 RPM with good compression ring seal as defined by no more than 3/16 CFM blowby per cylinder.
Short Rod is slower at BDC range and faster at TDC range.
Long Rod is faster at BDC range and slower at TDC range.
I. LONG ROD
A. Intake Stroke -- will draw harder on cyl head from 90-o ATDC to BDC.
B. Compression Stroke -- Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDC--may change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPM--measure CO, CO2? Find out!!
C. Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDC--allows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.
D. Exhaust Stroke -- see above.
II. Short Rod
A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.
B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.
C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.
D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.
A. Rod Length Changes -- Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%--ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.
B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.
C. Stroke -- Trend of OEM engine mfgs to go to longer stroke and/or less over square (bore numerically higher than stroke) may be a function of L/R. Being that at slower engine speeds the effect of a short rod on Intake causes few problems. Compression/Power Stroke should produce different emissions than a long rod. Short rod Exhaust Stroke may create more reversion--EGR on a street engine.
D. More exhaust lobe or a earlier exhaust opening may defeat a longer rod. I am saying that a shorter rod allows a earlier exhaust opening. A better exhaust port allows a earlier exhaust opening.
E. Definition of poor exhaust port. Becomes turbulent at lower velocity than a better port. Flow curve will flatten out at a lower lift than a good port. A good exhaust port will tolerate more exhaust lobe and the engine will like it. Presuming the engine has adequate throttle area (so as not to cause more than 1" Hg depression below inlet throttle at peak power); then the better the exhaust port is, the greater the differential between optimum intake lobe duration and exhaust lobe duration will be--ie. exh 10-o or more longer than intake Carbon buildup will be minimal if cyl is dry.
Short Rod -- Min Rod/Stroke Ratio -- 1.60 Max Rod/Stroke Ratio -- 1.80
Long Rod -- Min Rod/Stroke Ratio -- 1.81 Max Rod/Stroke Ratio -- 2.00
Any ratio's exceeding these boundaries are at this moment labeled "design screw-ups" and not worth considering until valid data supports it.
Contributors to Date: Bill Clemmons, Jere Stahl
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Connecting Rod Length Influence on Power
by William B. Clemmens
A spark ignition (SI) engine and a steam engine are very similar in principle. Both rely on pressure above the piston to produce rotary power. Pressure above the piston times the area of the bore acts to create a force that acts through the connecting rod to rotate the crankshaft. If the crankshaft is looked at as a simple lever with which to gain mechanical advantage, the greatest advantage would occur when the force was applied at right angles to the crankshaft. If this analogy is carried to the connecting rod crankshaft interface, it would suggest that the most efficient mechanical use of the cylinder pressure would occur when the crank and the connecting rod are at right angles. Changing the connecting rod length relative to the stroke changes the time in crank angle degrees necessary to reach the right angle condition.
A short connecting rod achieves this right angle condition sooner than a long rod. Therefore from a "time" perspective, a short rod would always be the choice for maximum torque. The shorter rod achieves the right angle position sooner and it does so with the piston slightly farther up in the bore. This means that the cyl pressure (or force on the piston) in the cylinder is slightly higher in the short rod engine compared to the long rod engine (relative to time).
Table 1 ROD LENGTH RELATIONSHIPS* (with Crank @ 90 deg ATDC)
Stroke Rod Length Rod Angle from TDC ATDC
3.5 5.70 17.88 2.025 72.12
3.5 5.85 17.40 2.018 72.59
3.5 6.00 16.96 2.011 73.04
3.5 6.20 16.39 2.002 73.60
Table 2 ROD LENGTH RELATIONSHIPS w/ CRANKPIN/ROD centerline @ 90o @ 7500 rpm
Stroke Rod Length Rod Angle Piston Distance Crank Angle Piston Accel
3.5 5.70 17.07 1.487 72.93 2728.35
3.5 5.85 16.65 1.494 73.35 2504.72
3.5 6.00 16.26 1.500 73.74 2324.26
3.5 6.20 15.76 1.508 74.24 2097.27
*data from Jere Stahl
Another concern in selecting the rod length is the effects of mechanical stress imposed by increasing engine speed. Typically, the concept of mean piston speed is used to express the level of mechanical stress. However, the word "mean" refers to the average speed of the piston in going from the top of the bore to the bottom of the bore and back to the top of the bore. This distance is a linear distance and is a function of the engine stroke and engine speed, not rod length. Therefore, the mean piston speed would be the same for each rod length listed in Table 1.
Empirical experience; however, indicates that the mechanical stress is less with the longer rod length. There are two reasons for these results. Probably the primary reason for these results is that the profile of the instantaneous velocity of the piston changes with rod length. The longer rod allows the piston to come to a stop at the top of the bore and accelerate away much more slowly than a short rod engine. This slower motion translates into a lower instantaneous velocity and hence lower stresses on the piston. Another strong effect on mechanical stress levels is the angle of the connecting rod with the bore centerline during the engine cycle. The smaller the centerline angle, the less the side loading on the cylinder wall. The longer rod will have less centerline angle for the same crank angle than the shorter rod and therefore has lower side loadings.
Classical textbooks by Obert ( ) and C.F. Taylor ( ) provide little guidance on the rod length selection for passenger or commercial vehicles other than to list the ratios of rod length to crank radiuses that have been used by various engine designs. Race engine builders using production blocks have done quite a bit of experimentation and have found many drivers are capable of telling the difference and making clear choices along with similar results from motorcycle flat track racers/builders.
Because of recent developments in computer modeling of the engine cycle by R.D. Rabbitt ( ), another factor may be critical in selecting a given connecting rod length. This new factor is the cylinder head flow capability versus connecting rod length over stroke ratio (l/r) versus engine speed. To understand this relationship, let us first review previous techniques used to model air flow during the engine cycle which as Rabbitt points out is founded on principles initiated in 1862 and refined in 1920. These theories are documented in Taylor's textbook ( ). To calculate air flow throughout the cycle these models use such parameters as mean or average inlet mach number for the port velocity and an average inlet valve discharge coefficient which compensate for valve lift and duration. In these models a control volume is used to define the boundaries of the combustion chamber. The air flow determined by the previous parameters crosses this boundary to provide air (and fuel) for the combustion process within the control volume.
However, this control volume has historically been drawn in a manner that defines the boundaries of the combustion chamber in the area of the inlet and exhaust valves as if the valves were removed from the cylinder head (ie. a straight line across the port). With the valves effectively removed, the previously mentioned average port flow and valve discharge coefficient (ie. valve restriction) are multiplied within current computer models to quantify the air flow (and fuel) delivered for each intake stroke. But, as Rabbitt points out, this approach totally ignores the effect of the air flow direction and the real effect of valve lift on the total air flow that can be ingested on each intake stroke.
Rabbitt reaches two important conclusions from his study. One, because of the direction of the air flow (angle and swirl) entering the combustion chamber, three dimentional vorticies are set up during the intake stroke. Two, that above a certain piston speed, density of the mixture at the piston face is a function of valve geometry and valve speed. Rabbitt further discusses the effect of the first conclusion as it relates to the mass of air that is allowed to flow through the port and by the valve. Vorticies can exhibit different characteristics and in general conform to two general types--large scale bulk vorticies that could be described as smooth in nature and small scale eddies that are highly turbulant.
If one can consider that the vacuum produced by the piston on its downward travel to be the energy that causes the air to flow through the port when energy losses throughout the intake tract (including losses at the valve) are at a minimum, the flow delivered to the chamber will be maximized. If the area between the piston face and the valve is also included in the consideration of flow losses, the effect of the type of vorticies created can be more easily understood. Large scale bulk vorticies comsume less energy than highly turbulent eddy vorticies. Thus, more of the initial energy from the piston's downward movement is available at the port-valve-combustion chamber interface with which to draw the intake charge into the chamber. Small scale eddies eat up energy which reduces the amount of the initial energy that reaches the port-valve-combustion chamber interface which in turn, reduces the port flow.
Rabbitt's second conclusion follows that at some higher piston speed, the vorticies within the combustion chamber (which are assumed to be large scale bulk type at low speeds) transition from the bulk type to the small scale eddy type. At this point the flow into the combustion chamber ceases to increase in proportion to increases in engine speed. It is theorized that this flow transition point can be observed on the engine power curve as the point at which the power curve begins to fall off with increasing engine speed.
As indicated earlier, piston speed is normally viewed as mean or average piston speed. Thus for a given engine, the mean piston speed increases as the rotational engine speed increases. However, in Rabbitt's model the piston speed of concern is the instantaneous piston speed during the intake stroke near TDC. For any given engine, changing the rod length to stroke (l/r) ratio changes the instantaneous piston speed near TDC. For the purposes of flow visualization, the type of vortex formed should not care whether a given instantaneous piston speed had been achieved by a given rotational speed or changing the (l/r) ratio and operating at a new rotational speed. As long as the instantaneous piston velocity is the same, the type of vorticies formed should be the same and the amount of air inducted into the cylinder should be the same.
If other factors influenced by rotational speed such as the time distance between slug of intake air flow and valve opening rates relative to the acceleration of the air slugs were ignored, one should be able to predict the location (RPM) of the peak power as a result of a change in the (l/r) ratio. Note, that even though power is a funtion of air flow and air flow should be roughly constant for the same instantaneous piston speed (neglecting the afore mentioned factors), the power may not be the same because of the lever arm effect between the crank radius and the connecting rod. (As we noted earlier, the shorter rod should have the advantage in the lever arm effect.)
In reality, the analysis must be viewed by stroke (ie intake, compression, exhaust, power) the selection of exhaust valve opening time combined with the exhaust system backpressure and degree of turbulance the exhaust port experiences. If the exhaust port has good turbulance control then you may run a shorter rod which allows you to use more exhaust lobe which reduces pumping losses on the exhaust stroke.