Quote:
Originally Posted by Stan Weiss
Can you explain why / what about a longer rod causes that?
Stan
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Simple physics, Stan. As a piston travels up the bore on a long rod, the primary thrust face of the piston “unloads” on the primary thrust face of the cylinder. The longer the rod, the more the unloading occurs. In low rpm engines it is manageable, but once this starts in a high rpm engine it can be catastrophic. This is WHY shorter and lighter pistons are used in coordination with long rod/long stroke combinations. Ever seen a long rod, long stroke F1 engine or IMS engine? What works in street and drag racing engines doesn’t work in forms of competition where high rpms (8,000 or higher) for extended periods are very common. Back in the “old days” of NASCAR long rod/short stroke engines were preferred on short tracks while longer stroke longer rod engines were preferred on super speedways (back then Daytona, Talladega, Michigan and Pocono). WHY? The long rod short stroke combination gave great torque out of the turns onto the straightaways. On a bad day, you might see 7,000 rpm a few times( remember, we are taking about BBCs, Elephant Hemis and FE Fords/ Boss 385 big blocks). On the super speedways engine speeds in excess of 7,000 RPM were common as were cylinder wall collapses. WHY? The longer stroke/longer rod combinations were unloading into the primary thrust face of the cylinder walls. At a continuous 7,000+ rpm it doesn’t take all that long. Today, NASCAR mandates a 3.310 maximum stroke with a 6.200 maximum rod length(?...haven’t seen a NASCAR rule book in a while); a relatively short stroke with a relative short rod( the opposite of our beloved Pontiacs). You can learn a lot listening to old NASCAR/ARCA and IMS/USAC engine builders. Consider this: in 1970 Plymouth Superbirds and Dodge Daytona were running Hemis with 7.200 c-to-c connecting rods on the super speedways(3.750 stroke). That is 3/8 of an inch LONGER than we typically use in modern Pontiac stroker builds today. Now, try to find a NASCAR Hemi block that hasn’t been sleeved due to a cylinder wall collapsing...