Paccar MX-13 tuning

[Texasdude74]

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.

None of this means the ISX sucks. An engine doesn't know what brand it is. It just is whatever it was designed to be, and there's no free lunch in engineering. Every design decision has consequences. The ISX works fine at moderate power levels up to around 600/2050 and it will fit in every single truck and large rv in production today. If, for some reason you want(nobody needs) 7-800+ horse and 2400+ torque it's probably not the best platform for that.