Rod Length and Piston Compression Height
Many companies like Crower, Eagle, Lunati, Scat, and others sell stroker rods that use cap
While purchasing stroker kits is the best way to eliminate expensive mistakes, it's still worthwhile to know how various combinations of connecting-rod length and piston compression heights are achieved. Rod length is simple to visualize as the distance between the centerlines of the big and small ends of the rod, and piston compression height is similarly easy: It is the distance from the wristpin centerline to the top of the flat portion of the piston. Piston domes or dish dimensions are not considered in the compression-height distance.
To determine if a given stroke, rod length, and piston will fit, the math is also easy. All you do is add half the stroke, the rod length, and the piston compression height. This length should not be more than the block deck height. As an example, let's take a big-block Chevy with a block deck height of 9.80. Let's say we already have a crank with a stroke of 4.250 and a rod length of 6.385 inches. What should the compression height be for our piston? Let's add half the stroke and the rod length (2.125 + 6.385 = 8.51) and subtract this figure from the block deck height (9.800 - 8.51 = 1.290 inches). Most piston manufacturers produce pistons with slightly shorter compression heights to accommodate milling the block to ensure the deck is flat and parallel to the crank centerline. JE Pistons, for example, specs all its big-block Chevy pistons with a 0.020-inch shorter 9.780-inch deck. Subtracting 0.020 from our 1.290-inch figure gives us 1.270 inches, which is exactly what JE lists for a piston for this specific application. It's that simple.
When determining what parts to run, it's always best to mock up your engine first and meas
There has been more written about rod length-to-stroke ratios than the subject deserves. To condense it to a few sentences, longer rods offer no power advantage but do offer some durability enhancements. The easiest way to look at this relationship is with a rod length-to-stroke ratio (R/L). Merely divide the rod length by the stroke. A 454 Chevy with a 4.00-inch stroke and 6.135-inch rods has a R/L of 1.533:1. Conversely, a 302 small-block Chevy with a 3.00-inch stroke and 5.70-inch rods enjoys a R/L of 1.9:1. According to David Reher of Reher & Morrison Racing Engines, his years of Pro Stock engine development have revealed no power advantage to longer (or shorter) rods, but longer rods do reduce the side loads that push the piston into the cylinder wall on the power stroke. While there is no rule of thumb, a R/L of 1.55 to 1.6:1 is a good ratio. One of the quandaries facing stroker-motor builders is that a longer stroke often leaves little room for longer rods in production-deck-height engines, which is why tall-deck blocks are available.
Effect of Stroke on Static Compression Ratio
Adding stroke to any engine will increase the static compression ratio even if no other variables change. This is because compression ratio is calculated by comparing the volume of the cylinder with the pistons at bottom dead center (BDC) versus the volume of the cylinder with the piston at top dead center (TDC). If we increase the length of piston travel by adding stroke, this increases the volume of the cylinder with the piston at BDC. Even though the volume at the top of the piston travel does not change, the piston has compressed a larger cylinder volume into the same space, increasing the compression ratio. This is why short-stroke engines need really small chambers or a piston dome to increase compression. Just as a point of reference, increasing cylinder bore also increases static compression ratio, since we're now working with a larger volume due to the increased bore.
This chart uses a big a big-block Chevy as an example because this engine offers such a tremendous range of stroke length. Variables that we used to compute compression ratio include a 4.250-inch bore, a 118cc combustion-chamber volume, a piston deck height of 0.010 inch, a compressed head-gasket thickness of 0.042 inch, and a 30cc piston dome volume.
|Stroke ||Static Compression Ratio (inches) |
|3.76 ||9.73:1 |
|4.00 ||10.29:1 |
|4.125 ||10.58:1 |
|4.250 ||10.87:1 |
|4.375 ||11.16:1 |
|4.500 ||11.45:1 |
|4.750 ||12.03:1 |
|5.00 ||12.62:1 |