Oh how it sucks to be cursed with PMR's. Ok, we know by now there isn't anything "safe" about modding one. Some people are nervous with stage 1's, some feel great with hybrids and big oil. Of course, there are huge differences in CP's between these trucks, as there are so many factors that contribute to failure, or the lack thereof. There has to be some quantitative way to measure what actually breaks them instead of saying ~401+hp, since the peak in the torque band could be anywhere from 1800-2800 (or someday 3400) rpm. Black widow's dyno chart of a TNBB with mini-me's shows crazy torque down low (a rod-crushing 1070 @ 2100rpm), I'm sure due to excellent spoolup at low rpm; but at a not too
-pmr-unfriendly 433 HP. Is this the rig that has been running hybrids for a few years on pmr's? I know "its all in the tuning", but geeze, 1070 @2100 is still 1070 @ 2100. So, when I hear ~401hp is red zone (and since most 7.3's peak [email protected]
~2700rpm), I assume ~802 tq. Why the HUGE variance?
I'm kindof a layman in mechanical engineering, but I do know destructive harmonics are the death of most machines. Most reports of PMR
failure have been someone going to the grocery store, or leaving the movie theater, not boosted launches and 3000 rpm doughnuts, though of course failures happen then too. Long stroke, high CP, low rpm, heavy truck all = crazy stress on rods. I think of it like standing on the pedals of your 18 spd mountain bike in 12th gear going up hill at about 5 mph; thats when your going to loose a shoe or bend a pedal. So, why didnt BWD's rods break? Are they randomly 30% stronger from the factory then joe schmoe's rod that broke at 700 ft lbs.? I would guess not, though im sure there is some random variance; 15% would seem like alot to me.
Harmonics seems like the culprit to me, which would involve temperature, rpm, CP, Timing, and actual down force on the rod, among others. Torsional vibrations feeding back from the crankshaft and tensile vibrations on the rod/wrist pin itself at just the wrong temp and BOOM! I guess what I'm theorizing here, is what is ultimately leading to PMR
death is not a purely a function of HP (since that is based on rpm) or direct torque (to a point), but the vibration levels on the rods and the components connected to the rods causing destructive harmonic resonance. I bet a fluid Damper would help greatly! How bout some other dampening devices? Anyone out there with precision instruments to measure micro vibrations? Cheaper to buy forged or billet, I know. Higher rpm torque peak seems like the band aid, but I dont tow at 2900 rpm, I'd rather be at 1900. If higher rpm helps, this must be a function of harmonics evening out at higher rpms, caused by the upward stresses increased with the rising centrifugal force, aka dampening! Your thoughts......'Leeter