Paccar MX-13 tuning

[dhirocz]

(01-11-2017 )Texasdude74 Wrote:  Yeah, I've always been told the rod ratio on ISX is the lowest of the 15 liters. But I've never measured the center to center length myself and divided it by the stroke. The differences are fractions of an inch. I seem to recall the ratio being listed around 1.507 or so off the top of my head. If my memory serves from my high school readings of Smokey Yunick and other engine building guides, a rod ratio of 1.5 was considered the absolute minimum for an engine to be halfway durable and 1.7 was the sweet spot. NASCAR and Indy shoot for 1.9 to 2.2. Although that's as much for increased dwell time as durability. If you're into hot rods ask an engine builder why road racers who run small block chevys go through so much trouble with custom pistons/pins and ring packs to run a 6" rod vs the factory 5.7"? Think you could eyeball the 3/10" difference? But it increases rod ratio from 1.52 to 1.6.

The side effects of too low of a rod ratio are increased wear on the thrust side of cylinder walls/liners, pistons that rock in the bore breaking wrist pins and fretting.
These effects get exponentially greater as power levels rise.

Yeah, that's about the bulk of it. A long rod is almost always better, certainly is for durability. It does have an effect on other things though, like piston speed, bore and skirt wear, torque production and cam selection. I'm rather surprised that Cummins didn't go with a longer rod myself, 1.5 is not horrible but far from optimal.

...then again, I'm not surprised either. I remember looking for a hot minute to see if one could mix and match rotating assembly parts to make a better bottom end...got close, but I came up empty handed.