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) |
| |
289
|
302
|
315
|
331
|
347
|
Probe
347* |
355
|
| 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.
|
