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Updated: September 21, 2001 |
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: 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 | ||
Pro: | ||
» | 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. | |
Con: | ||
» | 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 | ||
Pro: | ||
» | 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. | |
Con: | ||
» | 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. |
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. | ||||||||||||||
|
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? |
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 |
Rod Angle | "n" Ratio | Examples | Comments |
13½° | 2.142 | High speed motor with small ports. Best breathing with small ports | |
14 | 2.067 | ||
14½ | 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) | |
15½ | 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) |
16½ | 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 |
17½ | 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 |
18½ | 1.576 | Limited street use | |
19 | 1.536 | Chevy BB 454 | Good power due to large intake port |
19½ | 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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