Why Cracked Rods?
The rod/cap allignment is critical to making sure the 2 half circles of the bearing are properly alligned in a circle...if one half of the circle is slightly missalligned with the other half, there will be major issues with the way the oil shears between the bearing and the crank, as well as with the way the oil comes out of the side of the rod, and it'll usually result in bearing problems really quickly.
since this is an interesting and informational thread. as far as rod and cap alignment. why not have dowels or a guide pin that align the cap exactly where its supposed to go? i understand that, the purpose of the cracked rod is for cap alignment as said in a previous post.i guess the way im understanding it is, the rod alignment is key to properly support the bearing on all of its surgace? or am i misinterpreting it.
Lower chance of failure.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
No it doesn't...it starts to stretch at a certain RPM level.
Think about what a rod bolt does...when you combust the mixture above the piston, you push the piston down against the pin, in turn the rod, in turn the crank...that puts little to no force on the rod bolt at all.
HOWEVER, when you finish the exhaust stroke, and begin the intake stroke, the piston wants to continue heading upward towards the head...the crank pulls down on the rod cap, in turn the rod bolts, in turn the rod...pin...piston.
People tend to get misconstrued in this regard because more RPM usually means more power...you go to a different cam, it makes torque at a higher RPM, so you rev it to there, power is a function of RPM...so at that higher RPM, you have more power.
Find out what RPM some big power LS1 was at when the rod bolts failed...then rev a stock LS1 just as high just as often...it'll go boom too (if it can rev that high with the stock heads/cam).
The forces involved in pulling a piston back down are the hardest on the rods and rod bolts in most engines.
Worrying about what happened to the material during the cracking process is just a lack of information about the powdered metal, as well as about how it's treated prior to installation in the engine...last time I did a compression test on a similar powdered metal to whats in our rods...I broached it right through the hardened tool steel pads (I made them and hardened them myself) on the compression tester...and last time I tried a tensile test on the same powdered metal...I broke the gripping mechanism on the tensile tester...stock rods are VERY VERY good rods, they just lack the length I wanted in my new engine build, and they lack a good bolt capable of handling high RPM from the factory. If the length works for you, put quality bolts in them, have them checked for all necessary dimensions, and use them (modify them for floating pins if you want to).
Think about what a rod bolt does...when you combust the mixture above the piston, you push the piston down against the pin, in turn the rod, in turn the crank...that puts little to no force on the rod bolt at all.
HOWEVER, when you finish the exhaust stroke, and begin the intake stroke, the piston wants to continue heading upward towards the head...the crank pulls down on the rod cap, in turn the rod bolts, in turn the rod...pin...piston.
People tend to get misconstrued in this regard because more RPM usually means more power...you go to a different cam, it makes torque at a higher RPM, so you rev it to there, power is a function of RPM...so at that higher RPM, you have more power.
Find out what RPM some big power LS1 was at when the rod bolts failed...then rev a stock LS1 just as high just as often...it'll go boom too (if it can rev that high with the stock heads/cam).
The forces involved in pulling a piston back down are the hardest on the rods and rod bolts in most engines.
Worrying about what happened to the material during the cracking process is just a lack of information about the powdered metal, as well as about how it's treated prior to installation in the engine...last time I did a compression test on a similar powdered metal to whats in our rods...I broached it right through the hardened tool steel pads (I made them and hardened them myself) on the compression tester...and last time I tried a tensile test on the same powdered metal...I broke the gripping mechanism on the tensile tester...stock rods are VERY VERY good rods, they just lack the length I wanted in my new engine build, and they lack a good bolt capable of handling high RPM from the factory. If the length works for you, put quality bolts in them, have them checked for all necessary dimensions, and use them (modify them for floating pins if you want to).
Look at the mating surfaces on a cracked rod sometime and try to approximate the actual square inches of intimate contact surface between the cap and the rod. Compare that to any other method of aligning these parts accurately. Sometimes what appears to be the "cheapest" way is also a much better way.
Jon
I have seen some high end Aluminum rods precision machined to serve essentially the same purpose.. More surface area to interlock, to keep everything where it needs to be.
Ford does the same thing in their mod motors. My Lightning had cracked rods.
Ford does the same thing in their mod motors. My Lightning had cracked rods.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
Tensile force on rod and rod bolts Yes, this thread is a good read...
In the attached graph of piston motion (which is for the rotation dynamics only and does not include forces due to combustion/compression) look at the curves for piston acceleration wrt crank angle... (L is rod length, R = crank radius = half stroke)...
around TDC the acceleration of the piston is negative (pulling the piston downward)... for the red curve (4" stroker) it is about -2.7 in/rad²...
multiply the acceleration curve by ω² (where ω is angular velocity) to get acceleration in in/s², then divide by 12 in/ft to get ft/s², then divide by 32 ft/s²/g to get g's...
at 6000 rpm (i.e. 100 rev/s), ω is 628.3 rad/s;
the red curve says -2.7 in/rad² at TDC, so piston acceleration is -2775.8 g's (check my math)... at TDC this is the g force seen by the rod (and pair of rod bolts) purely due to the rotational dynamics.
Multiply the g force by the sum of piston mass and half rod mass to get the tensile load on the rod (and rod bolt pair)...
if you spin the motor higher then ω is higher and the tensile load is higher.
In the attached graph of piston motion (which is for the rotation dynamics only and does not include forces due to combustion/compression) look at the curves for piston acceleration wrt crank angle... (L is rod length, R = crank radius = half stroke)...
around TDC the acceleration of the piston is negative (pulling the piston downward)... for the red curve (4" stroker) it is about -2.7 in/rad²...
multiply the acceleration curve by ω² (where ω is angular velocity) to get acceleration in in/s², then divide by 12 in/ft to get ft/s², then divide by 32 ft/s²/g to get g's...
at 6000 rpm (i.e. 100 rev/s), ω is 628.3 rad/s;
the red curve says -2.7 in/rad² at TDC, so piston acceleration is -2775.8 g's (check my math)... at TDC this is the g force seen by the rod (and pair of rod bolts) purely due to the rotational dynamics.
Multiply the g force by the sum of piston mass and half rod mass to get the tensile load on the rod (and rod bolt pair)...
if you spin the motor higher then ω is higher and the tensile load is higher.
Yes, this thread is a good read...
In the attached graph of piston motion (which is for the rotation dynamics only and does not include forces due to combustion/compression) look at the curves for piston acceleration wrt crank angle... (L is rod length, R = crank radius = half stroke)...
around TDC the acceleration of the piston is negative (pulling the piston downward)... for the red curve (4" stroker) it is about -2.7 in/rad²...
multiply the acceleration curve by ω² (where ω is angular velocity) to get acceleration in in/s², then divide by 12 in/ft to get ft/s², then divide by 32 ft/s²/g to get g's...
at 6000 rpm (i.e. 100 rev/s), ω is 628.3 rad/s;
the red curve says -2.7 in/rad² at TDC, so piston acceleration is -2775.8 g's (check my math)... at TDC this is the g force seen by the rod (and pair of rod bolts) purely due to the rotational dynamics.
Multiply the g force by the sum of piston mass and half rod mass to get the tensile load on the rod (and rod bolt pair)...
if you spin the motor higher then ω is higher and the tensile load is higher.
In the attached graph of piston motion (which is for the rotation dynamics only and does not include forces due to combustion/compression) look at the curves for piston acceleration wrt crank angle... (L is rod length, R = crank radius = half stroke)...
around TDC the acceleration of the piston is negative (pulling the piston downward)... for the red curve (4" stroker) it is about -2.7 in/rad²...
multiply the acceleration curve by ω² (where ω is angular velocity) to get acceleration in in/s², then divide by 12 in/ft to get ft/s², then divide by 32 ft/s²/g to get g's...
at 6000 rpm (i.e. 100 rev/s), ω is 628.3 rad/s;
the red curve says -2.7 in/rad² at TDC, so piston acceleration is -2775.8 g's (check my math)... at TDC this is the g force seen by the rod (and pair of rod bolts) purely due to the rotational dynamics.
Multiply the g force by the sum of piston mass and half rod mass to get the tensile load on the rod (and rod bolt pair)...
if you spin the motor higher then ω is higher and the tensile load is higher.
