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Published by Christopher J. Holley | Mopar History & Tech | June 2025

What is Rod Ratio?

Rod ratio is the length of the connecting rod divided by the stroke of the crankshaft:

Rod Ratio = Connecting Rod Length / Stroke

For example, if your connecting rod is 6.123 inches long and the stroke is 3.31 inches:

6.123 ÷ 3.31 = 1.85:1 rod ratio
(which is what the 426 HEMI and 340 small blocks had)

Why Rod Ratio Matters?

Rod ratio affects how the piston moves through the cylinder. Specifically:

High Rod Ratios (1.75:1 or higher)

• Less piston side-loading (reduces friction and wear)
• Higher RPM capability
• More stable combustion at high speed
• Better rod angle geometry = smoother motion

Low Rod Ratios (around 1.5:1 or lower)

• More piston side thrust against the cylinder wall
• Higher friction and heat
• Good for torque at low RPM
• Often results in a taller piston or a compromised combustion chamber

Chrysler’s Engineering Philosophy

Chrysler’s engineers were extremely focused on mechanical geometry, especially in the 1960s and early 1970s. Here is why they preferred higher rod ratios:

1. Durability

Chrysler engines, especially race engines like the 426 HEMI, were built to survive extended abuse, NASCAR, drag racing, and endurance use. High rod ratios meant less stress on pistons and bores, reducing failures.

2. High RPM Capability

Engines like the 340 small-block and 426 HEMI were designed to rev higher and harder than many factory engines. A higher rod ratio helps keep the piston speed manageable and improves stability at high RPM.

3. Efficient Combustion

The 340, for instance, had a 6.123″ rod and a 3.31″ stroke (1.85:1 ratio), giving it a long dwell time at top dead center (TDC). This allows more complete combustion before the piston starts moving down again, helping both power and fuel efficiency.

4. Engineering-Driven vs. Marketing-Driven

Chrysler engineering led the design room in the ‘60s. Chevrolet and Ford, meanwhile, were often more cost- and packaging-driven. Chrysler took extra care to maintain optimal rod ratios, even if it meant taller blocks or longer rods, especially in performance builds.

Examples of Chrysler High Rod Ratio Engines

Engine	Rod Length	Stroke	Rod Ratio
340 Small Block	6.123″	3.31″	1.85:1
426 HEMI	6.86″	3.75″	1.83:1
383 B-Block	6.358″	3.38″	1.88:1
400 B-Block	6.358″	3.38″	1.88:1
440 RB-Block	6.76″	3.75″	1.80:1

These are high ratios compared to many Chevy or Ford V8s, which often dropped closer to 1.5:1 to fit packaging constraints or increase torque down low.

Trade-offs

While high rod ratios are good for power, durability, and revs, they come with challenges:

• They require taller engine blocks, which are heavier and harder to fit in small engine bays.
• Engines with long rods and short strokes often make less torque down low, needing more RPM to shine.
• They may cost more to build due to longer rods and custom pistons.

Why Others Didn’t Care as Much

Chevrolet and Ford focused on mass production efficiency, cost, and packaging. Rod ratio was just one of many factors. For street cars running under 5,000 rpm, the difference was minor. Chrysler, however, took a “no compromises” approach on key engines, especially in the HEMI and high-performance small blocks.

Final Thought

Rod ratio is a window into how much thought and care went into an engine’s design. Chrysler’s emphasis on high rod ratios helped their engines survive and thrive at high RPM, one reason Mopar motors were so respected on the street and track.