Page 3

We'll be using Scat's forged I-Beam rods. Both 331 and 347 strokers typically use a 3.400" rod.
Rod Ratios
Another term that gets flung around when talking about strokers is the concept of "rod ratio". More appropriately termed rod-to-stroke ratio, the figure represents the angularity at which the rod forces upon the piston, as a result of the crank stroke and rod length. The angular force imposes a side load on the thrust side of the piston and cylinder wall, resulting in wear on the rings, pistons, and bore.

The consensus amongst engine manufacturers is that a ratio of 1.50" is the lowest acceptable rod ratio for a street motor. Realistically, rod ratios between 1.65 - 1.80 are ideal

One way to understand the concept of rod ratio and it's affect on the engine, is to think of the extreme case where the rod is nearly 90° to the piston. The crank rotation would essentially force the rod through the side of the piston and bore. Obviously this is not a realistic example, but it helps visualize the effect of increasing the stroke.

To diminish these side loads, a longer rod can be used. However a longer rod by itself would likely push the piston too far out. The result is to create a shorter piston. This is what leads to the issues regarding the pin placement on 347 pistons.

The chart below shows the effects of stroke and rod length on the rod ratio for popular 302 based strokers.

Rod to Stroke Ratio of 302 based Strokers (displacement based on 4.030" bore)
Probe 347*
Rod length 5.155 5.090 5.205 5.400 5.400 5.315 5.205
Stroke 2.870 3.000 3.076 3.250 3.400 3.400 3.500
Rod Ratio 1.796 1.696 1.692 1.662 1.588 1.566 1.487
*Probe 347 kit with oil rings above pin hole -accomplished with a shorter rod. Note the rod ratio gets worse.

A final note about rod ratio. Engine gurus, like Smokey Yunick, have showed time and time again that the higher the rod ratio (he likes to approach 2:1) the more power the engine will produce. However there are several other variables, such as piston speed and dwell time which affect the power equation. Many import engines use rod ratios as low as 1.5:1 in production engines, however due to the small bore diameter, the angularity of the rod is not as severe as typical 4.00" V8 bores.

Incidentally, the new 4.6L OHC motors have a rod length of over 2.5, due to the unconventionally long 8.9" rods.

Continue with Building a 302 Stroker

Tech Archives Project Cars Readers Cars Feature Cars