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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.
   
Figure 4. Low Rod Angle. (High Rod Ratio)
Figure 5. High Rod Angle. (Low Rod Ratio)

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.
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
-

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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