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Updated: September 21, 2001

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 rods 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. Its 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. 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.

Best Combinations of "n" Values & Intake Characteristics
"n" = 1.45 - 1.75 more compatible with: "n" = 1.75 - 2.1 more compatible with:
Large intake port volume vs. motor size
("J" head on 273)
Small intake port volume vs. motor size
(stock 452 head on 498" RB stroker)
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)
Dual-plane 180 intake manifolds
(Edelbrock: LD340, CH4B, DP4B, Performer & Performer RPM, Streetmaster, SP2P. Holley Street Dominator. Weiand Stealth, Action Plus)
Large carburetor vs. engine size
(273 with 750cfm)
Small carburetor vs. engine size
(440 with 600cfm)
Moderate engine speed
(pick-up, RV, towing)
High engine speed
(peak power more important)
Tall axle ratio
(2.76, 2.93, 3.23, 3.55 and/or with tall tires)
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
Mopar LA 340 with 3.79" stroker crank 3.79 6.123 1.62
Mopar LA 340 with 4.00" stroker crank 4.00 6.123 1.53
Mopar "B" 350/361/383/400 3.375 6.358 1.88
Mopar "B" 400 with 440 crank & std. rods (451") 3.75 6.358 1.70
Mopar "B" 400 with 4.15" crank & std. rods (498") 4.15 6.358 1.53
Mopar "B" 400 with 4.15" crank & BBC +.400" rods (498") 4.15 6.535 1.57
Mopar "RB" 413/426W/440; "B" 383/400 with 440 crank & rods 3.75 6.768 1.80
Mopar "RB" 413/426W/440 with 4.15" crank (494") 4.15 6.768 1.63
Mopar 426 hemi 3.75 6.86 1.83
Small Block Chevy 302 3.00 5.70 1.90
Small Block Chevy 327 3.25 5.70 1.75
Small Block Chevy 350 3.48 5.70 1.64
Small Block Chevy 350 with 6" rod 3.48 6.00 1.72
Small Block Chevy 400 with std. rod 3.75 5.45 1.45
Small Block Chevy 400 with Chevy 350 rod 3.75 5.70 1.52
Small Block Chevy 400 with 6" rod 3.75 6.00 1.60
Big Block Chevy 396/402/427 3.76 6.135 1.63
Big Block Chevy 454 4.00 6.135 1.53
Ford 289 (Windsor) 2.875 5.156 1.79
Ford 302 (5.0, Windsor) 3.00 5.156 1.72
Ford 351W 3.50 5.98 1.71
Ford 460 3.85 6.605 1.72

Angle Limitation
    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 is at 90 to the crank, typically between 70-80 ATDC; however, the math is easy to do). Angles beyond 17 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)
    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 (it's also: .5/rod ratio)
  • 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

  • Rod Angle "n" Ratio Examples Comments
    13 2.142   High speed motor with small ports. Best breathing with small ports
    14 2.067    
    14 1.997   Long rods for good breathing with small ports
    15 1.932   Long rods to help breathing with small ports. Responds well to stroke increases ("n" value too large for intake port size)
    15 1.871   Responds well to stroke increases ("n" value too large for intake port size)
    16 1.814 Mopar 383/400 Approximate "ideal" compromise between stress & breathing (1.81-1)
    16 1.760 Chevy 327 Good choice for motors with good breathing
    17 1.710 Mopar 360
    Ford 302, 351W, 460
    "Safe" limit for thrust angle. Approaching practical limit for street motors
    17 1.663   Approaching practical limit for street motors
    18 1.618 Chevy BB 396/427 Approaching practical limit for street motors. Good power due to large intake port
    18 1.576   Limited street use
    19 1.536 Chevy BB 454 Good power due to large intake port
    19 1.498   Not practical for street use due to short pistons
    20 1.462 Chevy SB 400 Poor peak power. Longer rods are used in any serious application

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