When discussing the “length” of a connecting rod, it is not simply the total length. Instead, I am referring to the distance from the centerline of the crankshaft pin bore (the big end) to the centerline of the wrist pin bore (the small end). Precision rod-length specialty tools are designed to precisely measure this. However, if you want to perform a rough check of an existing rod, you can use a long caliper with a dial or a digital caliper with a range greater than the length of the rod.
Using either method, you measure from the bottom of the wrist pin bore (6 o’clock) to the top of the big-end bore (12 o’clock). Record this distance. Then measure the diameter of the wrist pin bore and the diameter of the big-end bore. Record these diameters. And plug the figures into this formula:
Rod Length = A + 1/2B + 1/2C
Performance aftermarket cranks are usually stamped or etched on the face of the front counterweight indicating the crankshaft stroke. In this example, the crank stroke is 4.750 inches.
Where:
| A = | distance from bottom of the wrist pin bore to top of the big-end bore |
| B = | diameter of wrist pin bore |
| C = | diameter of big-end bore |
Although this isn’t the most precise way to measure, it gives you a rough idea of your rod’s center-to-center length, for rod identification purposes. Trying to measure rod length with a ruler by guessing at the two bore diameter centers is a waste of time.
Today’s quality aftermarket connecting rod manufacturers (such as Scat, Lunati, Oliver, GRP, Crower, Callies, and more) produce rods at extremely tight tolerances for center-to-center length. It is very uncommon to find a set of rods that are not precisely at the specified length and all the rods in a set are not matched.
Although you can rely on performance rod dimensions in general, when blueprinting, you don’t want to assume anything. Measure each rod for length, small-end diameter, and big-end diameter. Knowing exactly what you have is better than guessing or assuming. If you’re using OEM rods, you must check all dimensions due to the greater potential for tolerance deviations.
Rod length has a direct relationship to engine performance characteristics. Granted, the rod length is part of the TDC dimension (from centerline of the crank rod journal at TDC to piston dome location relative to the block deck), but the rod length can be selected in combination with crank stroke and piston compression height in order to tailor the engine for certain performance characteristics. A shorter rod is slower at the BDC range, but faster at the TDC range. A longer rod is faster at the BDC range but slower at the TDC range.
Here’s an explanation from Stahl Headers: “With a longer rod, the intake stroke draws harder on the cylinder head from 90-degrees after top dead center (ATDC) to BDC. On the compression stroke, the piston travels faster from BDC to 90-degrees before top dead center (BTDC) with a longer rod; but travels slower from 90-degrees BTDC to TDC, which may change the ignition timing requirement. It is possible that a longer rod could have more cylinder pressure at 30-degree ATDC but less crankpin force at 70-degrees ATDC.”
Rod dimensions: A) center-to-center length; B) bend and twist; C) rod width; D) rod offset; E) big end bore; F) small end bore; and G) bearing tang location.
Check each piston location at TDC, relative to the block deck (after the block has been decked). If there is any deviation from bore to bore, you may be able to swap rods (if there is any CTC length deviation in your rod set) in order to equalize all piston deck heights.
On the power stroke, the piston is farther down the bore for any given rod/crank pin angle. At any crank angle from 20- to 75-degrees ATDC, less force is exerted on the crank pin with a longer rod. However, the piston is higher in the bore for any given crank angle from 90-degrees BTDC to 90-degrees ATDC, so cylinder pressure could be higher. A longer rod spends less time from 90-degrees BTDC to BDC, which allows less time for exhaust to escape on the power stroke and forces out more exhaust from BDC to 90-degrees BTDC. If the exhaust port is not efficient, a longer rod helps produce peak power.
Connecting rod length always refers to the distance from the center of the wrist pin bore (small end) to the center of the crank journal bore (big end). (Illustration Courtesy Lunati)
In order to place the piston at or near the block deck on TDC, the rod and crank stroke combinations can include a shorter stroke crank with a longer rod or a longer stroke crank with a shorter rod.
Long Rods
A longer connecting rod provides a longer dwell time at the TDC range. This helps to extend the compression state by keeping the combustion chamber volume small, which is good for mid- to upper-RPM torque. A longer rod reduces the rod angle, which helps to reduce friction. Also, with a longer rod, you can run a shorter piston compression height (that means a lighter piston), which helps to gain RPM.
However, longer rods are less efficient at promoting volumetric efficiency at low engine speeds. The piston moves from TDC (downward) at a reduced rate, gaining its maximum speed at a later point of crank rotation. Longer duration camshaft profiles tend to reduce cylinder pressure during the closing period of the intake cycle. Longer intake manifold runners with slightly smaller port volumes may be needed. Longer rods also pose more of a clearance issue (camshaft, bottom of cylinders, and pan rails).
Short Rods
Shorter rods provide higher intake and exhaust speeds at lower engine RPM, which improves low-end torque (and promotes higher vacuum). Shorter rods increase piston speed as it travels from TDC on the power stroke, which increases chamber volume. This delays the point of maximum cylinder pressure, which is a good match for forced induction (supercharger, turbo, and nitrous injection). Shorter rods also allow more radical camshaft timing. However, since a shorter rod increases piston travel from TDC, at high RPM the piston can run away from the flame front faster, which can decrease total cylinder pressure.
Most high-quality aftermarket rods have a high degree of precision machining, with center-to-center lengths dead on or within 0.0005 inch. Even so, always measure each rod, each crank rod journal stroke, and each piston to verify dimensions and to avoid a stack-up of tolerances. For example, if one rod is 0.0005 inch shorter than the other