Manley Rods and Pistons

How Rod Ratio & Length Connecting Rod Ratio Effect

How Connecting Rod Ratio Effect Performance

Connecting Rod Ratio Effect
Connecting Rod Ratio Effect

Connecting Rod Ratio is the ratio of the length of the rod compared to the stroke. Its common knowledge that the idea rod ratio all things being perfect is 1.752 to One. This give the engine enough time to breath and as little leverage against the bores as possible.

Connecting Rod Ratio Effect

The ratio is expressed as the rod length divided by Stroke. With this you can figure the rod ratio for your engine and make changes that will help make more power in the rpm range the engine will be operated in. For the small block chevy the calculation is easy to come by and easy to change. Since the engine is so popular and allows so many different combination of bore and stroke many from the factory and many more from the aftermarket. You can actually make use of this information and use it when building your engine. To get the rod ratio for a small block chevy you simply divided the rod length measured at the pins of 5.7 by the stroke and you get 1.6379:1 or (1.64:1).

How Does the small block chevy compare.

Now it is clear that this is less than the ideal numbers for performance engines. With the chevy’s clear track record. We all know the engine works and works well. The rod length is not actually less than ideal it is in fact ideal for the design of the engine. Since they are designed to make most power between 2000 and 4000 rpm. Because of this the rod ratio is lower than a performance engine’s ideal but perfectly calculated for the engines stroke and rpm range.

Things can be a lot different when the rods are shorter

Run these same numbers for the short rod 400 and the numbers become grossly out of proportion. It is clear that GM never intended for this engine to be a high rpm screamer. But a low rpm crawler. When the engines are built for race use much longer 6 inch rods are used that greatly increase the engines rod ratio and power output.


Common effects associated with a Shorter Connecting Rod:

  • Increased rod angle will increase wear.
  • Increased Piston velocity and acceleration increasing stretching force¬†on the rods.
  • More low rpm power with shorter rods
  • Reduced time at top dead center or dwell time as the rod rolls over the top of the crank. This helps at low rpm but cost power at higher rpm levels. The rod will be moving away from the explosion before its made max power

Common effects of Longer Connecting Rods in an engine are.

  • Less rod angle decreases wear on the bores and rods.
  • Lower Piston speed and acceleration reduces stretching force on the rods.
  • The piston spends more time at top dead center and less timing is required to burn the fuel/air charge. This gives less chance for detonation with more dwell time at higher rpm the explosion is allowed more time to push against the piston.
  • This also means that compression can be increase slightly before detonation.
  • High rpm breathing is increased. Same power levels are possible with smaller intake runners.