ENGINE SPEED [RPM)
> Don't count on a cam change to yield significant torque changes. Once cam duration exceeds about 236 degrees (at 0.050), the engine won't make any more peak torque. The larger cams just raise peak torque to a higher point in the rpm range. Here, Westech Performance tested two Comp solid roller cams on an otherwise identical Ford 393 crate engine.
distance, as well as rotating assembly weight, will also change within the same engine block from any change in stroke. To max out an engine's capabilities, the base cam event timing versus the stroke, the rod length, and the head characteristics would also most likely change. For example, even if only the piston pin location is altered to accommodate the stroke increase without changing the piston skirt's overall shape, the piston ends up dropping further out of the bottom of the cylinder bore at bottom dead center (BDC). This could negatively impact piston stability, increasing piston rock, which in turn may call for additional piston-to-bore clearance. But for the purposes of this comparison, we'll assume all this stuff remains the same.
Dave Ebbert of DNE Motorsports Development has some old-school, DOS-based, analysis software that he says models with a high confidence level what happens due to a stroke change. He ran two cases for me: a small-block and a big-block. One cautionary note is that as stroke increases, so does the rotating assembly's mass engine speed. For any rotating assembly, there is a max safe theoretical rotating assembly rpm, the point at which what Ebbert terms the critical tension number is reached, when the loads imparted to the rotating assembly exceed 4,350 g (4,350 times the force of gravity). Peak tension always occurs at TDC because of the reversal of load. Above this point, even with very good parts, things tend to fly apart (such as the caps pulling off the connecting rod). The torque and power must peak below
Was this article helpful?