Rod Ratio (Rod to Stroke Ratio)
Rod Ratio or Rod to Stroke Ratio is the figure achieved
when dividing a motor's rod length by its' stroke. This is an
important calculation to  understand
since it informs us about a motor's rod angularity. A low Rod
Ratio yields a high rod angle. For example, a motor with
a 5.400" rod length and a 3.000" stroke yields a rod
ratio of 1.8:1. If we maintain the same stroke and shorten the
rod length to 5.000" we get a 1.7:1 rod ratio. The rod
angle has increased.
A high rod angle or low Rod Ratio creates a greater potential
for accelerated wear to cylinder walls, pistons, and piston
rings. The illustrations below show why this is so. Figure 5
is exaggerated for effect but clearly shows how an extremely
low Rod Ratio can drive the piston into the side of the
cylinder wall.
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Figure
4. Low Rod Angle. (High Rod Ratio)
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Figure
5. High Rod Angle. (Low Rod Ratio) |
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By lengthening the rod, as stroke is increased, we can offset
the increased rod angle. However, this requires further shortening
of the piston. The further the piston is shortened the more
likely the piston pin will intersect the oil ring groove, creating
a potential for increased oil consumption. See Figure 6 below.
Many piston companies however have engineered pistons to avoid
this problem with tighter ring packs and
| Figure 6.
Shortened Piston. The further the piston is shortened
the more likely the piston pin will intersect the oil
ring groove. |
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bridge rings.
Either way, there comes a point when you cannot shorten the
piston any further before dependabilty is compromised. As in
the discussion about offset grinding, we have reached a limit
to how far you can stroke a motor before some component or function
is sacrificed.
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. See the tables in the following section about stroker
kits. Notice how the Rod Ratio decreases as stroker displacement
increases.
Piston Dwell Time and Piston Speed
An often overlooked factor that contributes to the advantage
of a stroker motor has to do with piston dwell time, the amount
of time the piston remains at the top and bottom of the stroke.
The increased stroke and rod length of a stroker motor yields
a longer piston dwell time. Longer dwell time allows for better
flow of combustion and exhaust gases since the piston accelerates
slower in the transition between "up" and "down"
strokes. Intake gases have a longer time to enter the cylinder
while exhaust gases are given more time to escape. This translates
into more natural torque over a longer range of rpm.
Power and torque can also be enhanced with valve event timing
and cam profile.
Even though the piston accelerates slower in transition, the
piston ultimately reaches higher speeds to cover the additional
stroke. This increase in piston speed means greater component
strain. Another factor to consider before simply going with
the kit or components that give you largest stroke increase.
Stroker Building Considerations
As you may have guessed, there are certain issues which must
be addressed when actually assemblying any stroker engine. First
and foremost is the issue of clearances. Due to the increased
stroke and rod length changes, it is common for the rod and
crank to interfere with cylinder bore end, pan rails, piston
skirts, windage trays and other areas inside the block. Therefore
it is mandatory that you preassemble the engine components,
mark the areas needing grinding for clearance, dissasemble and
make the neccesary clearances, and then reassemble and check
again. As a rule of thumb you should have at least 0.030" clearance
between any interfering points. Another set of considerations
unique to stroker engines is rotating assembly balancing. Whether
the stroker kit is custom made, or off-the-shelf, the use of
new or offset ground cranks, longer rods, and stroker specific
pistons ensures that the assembly is not going to spin evenly.
Any stroker kit, even off-the-shelf ones, must be balanced by
a competent machine shop. Not doing so is a recipe for failure.
Always perform the balancing with the harmonic balancer and
flywheel you intend to use.
Stroker Kits
Many of the issues that arise when planning a stroker motor
are solved by using a kit that provides a crank, connecting
rods, and pistons. Rather than purchasing the components separately,
you can purchase predetermined safe combinations for your block.
You will get a thousand differing opinions regarding the best
stroker for your application. We urge you to gather opinions
from fellow enthusiasts and engine builders. Also use the information
about rod angularity in this article to make your decision.
Stroker displacements remain fairly consistent from kit provider
to kit provider. We have highlighted the most popular stroker
displacements for Ford blocks in the tables below.
|
289-302 based strokers (4.030"
bore - 0.030" over stock)
|
| Displacement |
289
|
302
|
315
|
331
|
347
|
355
|
| Rod Length |
5.155"
|
5.090"
|
5.205"
|
5.400"
|
5.400"
|
5.205"
|
| Stroke |
2.870"
|
3.000"
|
3.076"
|
3.250"
|
3.400"
|
3.500"
|
| Rod Ratio |
1.796:1
|
1.696:1
|
1.692:1
|
1.662:1
|
1.588:1
|
1.487:1
|
|
351W based strokers (4.030"
bore - 0.030" over stock)
|
| Displacement |
Stock
351W
|
383
|
393
|
408
|
418
|
426
|
| Rod Length |
5.956"
|
6.250"
|
5.956"
|
6.125"
|
6.200"
|
6.125"
|
| Stroke |
3.500"
|
3.750"
|
3.850"
|
4.000"
|
4.100"
|
4.170"
|
| Rod Ratio |
1.702:1
|
1.667:1
|
1.547:1
|
1.531:1
|
1.512:1
|
1.469:1
|
|
351C based strokers (4.030"
bore - 0.030" over stock)
|
| Displacement |
Stock
351C
|
383
|
396
|
408
|
426
|
-
|
| Rod Length |
5.778"
|
5.850" |
6.000"
|
6.000"
|
6.000"
|
-
|
| Stroke |
3.500"
|
3.750" |
3.850"
|
4.000"
|
4.170"
|
-
|
| Rod Ratio |
1.651:1
|
1.560:1 |
1.558:1
|
1.500:1
|
1.44:1
|
-
|
|
429 and 460 based strokers
(4.440" bore - 0.080" over stock)
|
| Displacement |
Stock
429
|
Stock
460
|
501
|
532
|
557
|
-
|
| Rod Length |
6.605"
|
6.605"
|
6.800"
|
6.800"
|
6.800"
|
-
|
| Stroke |
3.550"
|
3.850"
|
4.150"
|
4.300"
|
4.440"
|
-
|
| Rod Ratio |
1.861:1
|
1.715:1
|
1.638:1
|
1.581:1
|
1.531:1
|
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|
In reality there are many more displacements
possible for the blocks above if you decide a kit does not meet
your needs. This does however require a greater understanding
of custom engine building. By offset grinding, choosing custom
rods and pistons, or using rods and pistons from unconventional
donor engines you can build your own custom stroker.
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Posted by memoisbill, 11/15/11 07:08pm:
This article is very informative, and a great refresher on strokers.
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