Zim - these were both great articles, lots to absorb. Here's some interesting (to me) excerpts from the "rod" link. There's some cam/piston interaction voodoo also I left out as it made my "Brane Hert"...
Maybe ACE has some insights here? I think the stock Pre-Unit rod ratio is right at 1.9. Looks like the Bullet builders were looking for longevity & workaday power.
From:
https://www.chevyhardcore.com/tech-stories/stroker-engines-long-short-connecting-rod-length/Excerpts:
Any additional weight incurred by using a longer connecting rod has less of an effect on counter balance weight because the connecting rod is both reciprocating and rotating. Reciprocating weight requires more weight to offset than rotating weight. The difference in connecting rod weight is split between rotational and reciprocating while differences in piston weight is only applied to reciprocating weight. Using a lighter piston will allow for lighter crankshaft counterweights and may not require any additional weight to be added externally. When this is the case, the rotating assembly is considered to be internally balanced.
Stability of the piston should also be considered. A longer connecting rod will keep the piston further up in the cylinder bore when at BDC for a given stroke. The small end of the rod, which is connected to the piston pin, is further up the cylinder bore with a long rod as compared to a short rod. Therefore, the piston also moves up in relation to the bottom of the cylinder, adding distance from the center of the pin to the bottom of the cylinder wall.
Rod Angle - As the crankshaft rotates the big end of the connecting rod, the small end is moving up and down. This creates an angle between the cylinder wall and the connecting rod. The severity of the angle is determined by the ratio of rod length to stroke (rod ratio). Rod ratio is determined by dividing the rod length by the stroke. A shorter rod will decrease rod ratio, while a longer rod will increase the ratio for the same stroke. As the ratio decreases, the rod angularity, or angle between the connecting rod and cylinder wall, will increase. The maximum achieved angle always occurs at 90 degrees before and after TDC. Increasing rod angularity (decreased rod ratio) increases the amount of thrust acting on the cylinder wall, and the result is increased frictional loss and wear on the piston skirt and cylinder wall in some cases.
Peters suggests using, “As high a ratio as possible,” citing less rod angularity, reduced reciprocating weight due to a shorter compression height piston (remember, although a long rod will weigh more, the difference is not as significant because it is split between rotating and reciprocating mass), and reduced piston rock as benefits.
A common error that is made regarding peak piston speed is assuming that it occurs at 90 degrees of rotation — which is not true. Peak speed actually occurs somewhere around 70 to 75 degrees BTDC and ATDC (depending on rod ratio) due to the angle of the rod affecting piston speed and location. Peak piston speed is higher with a short rod compared to a long rod (stroke being the same), because the shorter rod creates a greater angle.
Rod length changes both the physical and dynamic properties of the engine. Factors such as assembly height, engine balance, piston ring location, and cylinder length are physical features that must be considered, while rod angle and piston speed are dynamic characteristics affected by rod length. The dynamic characteristics will change engine performance based on their relationship to camshaft events.
As an engine builder, it is important to take all aspects into consideration, and understand how one component will affect the overall combination. Rod length alone cannot be generalized as providing a certain change to every engine. Rather, any change in engine performance is due to the rod length’s role in changing the dynamic properties of the entire combination.
A few formulas you need to know when building a stroker engine:
Displacement in cubic inches = Bore x Bore x Stroke x Number of Cylinders x .7854
Assembly Height = (Stroke / 2) Rod Length Piston compression height
Rod Ratio = Rod Length / Stroke
Mean Piston Speed (feet per second) = (2 x Stroke x RPM / 60) / 12
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