More fun facts (primary inertia)
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More fun facts (primary inertia)
Having a little time on my hands I decided to reconsider whether or not I should bother having a set of rods made for the V. Conveniently there are calculations out there to help determine if the changes made to your motor will effect it positively, such as lighter piston, rod, etc. In short, the issue with reciprocating weight is what they refer to as inertia and it considers what happens at TDC and BDC when the piston stops and starts rapidly https://www.racepagesdigital.com/pis...e-your-engine/. The calculation to find primary inertia is:
.0000142 x Piston Weight (lb) x RPM2 x Stroke (in) = Inertia Force
If you trust my calculator and scale the inertial force of each piston assembly in an oem VTR is 4552 lbs at 10k rpm.
If you then figure the correction factor using this formula: Crank Radius÷Rod Length = .2598
“Because of the effect of the connecting rod, the force required to stop and restart the piston is at maximum at TDC,” says Fussner. “The effect of the connecting rod is to increase the primary force at TDC and decrease the primary force at BDC by this R/L factor.” This factor is added to the original inertia force for the upward stroke and subtracted on the downward movement."
So...after all that what we find is that at TDC the forces are 5735 lbs and at BDC they are 3274 lbs. Why does anyone care? Because with a new, lighter forged piston assembly from JE (which is lighter) the forces at TDC are 163 lbs lighter and at BDC 95 lbs lighter. That is the result of an overall decrease in weight of 15.8 grams per piston assembly. Now how does that help influence my decision?
Well, the rod is exposed to either tension or compression depending on which way it's traveling. The sudden transition at TDC causes tensile loads and is extremely hard on not only rods but rod bolts. So, if I visit ARP and getter stronger bolts that do not fatigue as easily as oem bolts and I have my rods cryogenically treated like everything else, I have not only reduced the tendency of the rods and bolts to fatigue, but I have reduced the inertial forces acting on them both. In other words; I made my stock rods stronger and reduced the amount of work they have to do and in consideration of that, I think I should keep my stock rods and save $500 bucks that I can spend elsewhere.
Note* This works only if you maintain the same redline. Raising redline raises inertia exponentially.
AV
.0000142 x Piston Weight (lb) x RPM2 x Stroke (in) = Inertia Force
If you trust my calculator and scale the inertial force of each piston assembly in an oem VTR is 4552 lbs at 10k rpm.
If you then figure the correction factor using this formula: Crank Radius÷Rod Length = .2598
“Because of the effect of the connecting rod, the force required to stop and restart the piston is at maximum at TDC,” says Fussner. “The effect of the connecting rod is to increase the primary force at TDC and decrease the primary force at BDC by this R/L factor.” This factor is added to the original inertia force for the upward stroke and subtracted on the downward movement."
So...after all that what we find is that at TDC the forces are 5735 lbs and at BDC they are 3274 lbs. Why does anyone care? Because with a new, lighter forged piston assembly from JE (which is lighter) the forces at TDC are 163 lbs lighter and at BDC 95 lbs lighter. That is the result of an overall decrease in weight of 15.8 grams per piston assembly. Now how does that help influence my decision?
Well, the rod is exposed to either tension or compression depending on which way it's traveling. The sudden transition at TDC causes tensile loads and is extremely hard on not only rods but rod bolts. So, if I visit ARP and getter stronger bolts that do not fatigue as easily as oem bolts and I have my rods cryogenically treated like everything else, I have not only reduced the tendency of the rods and bolts to fatigue, but I have reduced the inertial forces acting on them both. In other words; I made my stock rods stronger and reduced the amount of work they have to do and in consideration of that, I think I should keep my stock rods and save $500 bucks that I can spend elsewhere.
Note* This works only if you maintain the same redline. Raising redline raises inertia exponentially.
AV
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