Rod stretch and rpm
04-05-2006, 01:57 PM
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Staging Lane
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Rod stretch and rpm
This was brought up in another thread. I found it very interesting. It would be great if the author could ellaborate some more.
"... I've tried to convince people that the rods in Gen 2s are snapping, which is a sign of stretch caused by piston speed, not overpressure caused by detonation. Keep the revs down, the motor will live. IMO."
I have considered lowering my shift points by a couple hundred rpm when I step up to a KB or Whipple. At the least I won't raise my shift points. A couple hundred rpm won't hurt much, and if it helps the engine live it's good insurance.
"... I've tried to convince people that the rods in Gen 2s are snapping, which is a sign of stretch caused by piston speed, not overpressure caused by detonation. Keep the revs down, the motor will live. IMO."
I have considered lowering my shift points by a couple hundred rpm when I step up to a KB or Whipple. At the least I won't raise my shift points. A couple hundred rpm won't hurt much, and if it helps the engine live it's good insurance.
04-05-2006, 02:17 PM
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Sure! I'm almost always happy to ramble...
Rods of any sort, by design, are very strong in compression and do a pretty decent job of resisting torsional (twisting) forces.However, what they don’t do is resist stretch particularly well. As the rod slows the piston down on the exhaust stroke, and speeds it up on the intake stroke, it’s working basically in a vacuum (compared to the counteracting forces during compression and power). The energy (and inertia) the rod has to control goes up geometrically with RPM (That whole KE = (.5)mv^2 thing from high school physics – as velocity increases, the amount of energy increases by the square) so a relatively small change in RPM creates a very large load change in the rod.
The load is most critical at the neck of the rod just below the pin head and where the rod wraps around the crank.
All the Gen 2 motors I’ve personally seen fail have done so because a rod snapped, not because a piston ventilated (detonation) or a rod “banana-ed” (excessive cylinder pressure). This type of failure usually leaves a nice vent in the side of the block and a rod or two in pieces. And I'm not saying they don't fail because of that, but that failure is a lot more rare than the failures caused by piston speed.
Personally, I think that increasing the boost and dropping the rpm is a great way to build a long-living stock bottom end. Matter of fact, I use that theory on both of my trucks, turning them very few RPMs compared to some other combinations.
One thing about turbocharged engines that helps their survivability (you guys may be familiar with Frank5L’s ~730 rwhp on a stock long block turbo truck) is the effect of the pressure on both sides of the exhaust and intake stroke counteracting the stretch in the rod. The exhaust backpressure means that there is some amount of force acting on top of the piston to help slow it during the exhaust stroke, and boost pressure does the same (helping the piston accelerate) for the intake stoke.
Rods of any sort, by design, are very strong in compression and do a pretty decent job of resisting torsional (twisting) forces.However, what they don’t do is resist stretch particularly well. As the rod slows the piston down on the exhaust stroke, and speeds it up on the intake stroke, it’s working basically in a vacuum (compared to the counteracting forces during compression and power). The energy (and inertia) the rod has to control goes up geometrically with RPM (That whole KE = (.5)mv^2 thing from high school physics – as velocity increases, the amount of energy increases by the square) so a relatively small change in RPM creates a very large load change in the rod.
The load is most critical at the neck of the rod just below the pin head and where the rod wraps around the crank.
All the Gen 2 motors I’ve personally seen fail have done so because a rod snapped, not because a piston ventilated (detonation) or a rod “banana-ed” (excessive cylinder pressure). This type of failure usually leaves a nice vent in the side of the block and a rod or two in pieces. And I'm not saying they don't fail because of that, but that failure is a lot more rare than the failures caused by piston speed.
Personally, I think that increasing the boost and dropping the rpm is a great way to build a long-living stock bottom end. Matter of fact, I use that theory on both of my trucks, turning them very few RPMs compared to some other combinations.
One thing about turbocharged engines that helps their survivability (you guys may be familiar with Frank5L’s ~730 rwhp on a stock long block turbo truck) is the effect of the pressure on both sides of the exhaust and intake stroke counteracting the stretch in the rod. The exhaust backpressure means that there is some amount of force acting on top of the piston to help slow it during the exhaust stroke, and boost pressure does the same (helping the piston accelerate) for the intake stoke.
04-05-2006, 07:55 PM
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Staging Lane
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Thanks, that was good info.
Currently I'm using the stock shift points with a tune/4# lower. What shift points do you recommend when I run a KB with 14#? Is 4,900rpm good?
Currently I'm using the stock shift points with a tune/4# lower. What shift points do you recommend when I run a KB with 14#? Is 4,900rpm good?
04-05-2006, 08:24 PM
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Originally Posted by FMOS Racing
Sure! I'm almost always happy to ramble...
Rods of any sort, by design, are very strong in compression and do a pretty decent job of resisting torsional (twisting) forces.However, what they don’t do is resist stretch particularly well. As the rod slows the piston down on the exhaust stroke, and speeds it up on the intake stroke, it’s working basically in a vacuum (compared to the counteracting forces during compression and power). The energy (and inertia) the rod has to control goes up geometrically with RPM (That whole KE = (.5)mv^2 thing from high school physics – as velocity increases, the amount of energy increases by the square) so a relatively small change in RPM creates a very large load change in the rod.
The load is most critical at the neck of the rod just below the pin head and where the rod wraps around the crank.
All the Gen 2 motors I’ve personally seen fail have done so because a rod snapped, not because a piston ventilated (detonation) or a rod “banana-ed” (excessive cylinder pressure). This type of failure usually leaves a nice vent in the side of the block and a rod or two in pieces. And I'm not saying they don't fail because of that, but that failure is a lot more rare than the failures caused by piston speed.
Personally, I think that increasing the boost and dropping the rpm is a great way to build a long-living stock bottom end. Matter of fact, I use that theory on both of my trucks, turning them very few RPMs compared to some other combinations.
One thing about turbocharged engines that helps their survivability (you guys may be familiar with Frank5L’s ~730 rwhp on a stock long block turbo truck) is the effect of the pressure on both sides of the exhaust and intake stroke counteracting the stretch in the rod. The exhaust backpressure means that there is some amount of force acting on top of the piston to help slow it during the exhaust stroke, and boost pressure does the same (helping the piston accelerate) for the intake stoke.
Rods of any sort, by design, are very strong in compression and do a pretty decent job of resisting torsional (twisting) forces.However, what they don’t do is resist stretch particularly well. As the rod slows the piston down on the exhaust stroke, and speeds it up on the intake stroke, it’s working basically in a vacuum (compared to the counteracting forces during compression and power). The energy (and inertia) the rod has to control goes up geometrically with RPM (That whole KE = (.5)mv^2 thing from high school physics – as velocity increases, the amount of energy increases by the square) so a relatively small change in RPM creates a very large load change in the rod.
The load is most critical at the neck of the rod just below the pin head and where the rod wraps around the crank.
All the Gen 2 motors I’ve personally seen fail have done so because a rod snapped, not because a piston ventilated (detonation) or a rod “banana-ed” (excessive cylinder pressure). This type of failure usually leaves a nice vent in the side of the block and a rod or two in pieces. And I'm not saying they don't fail because of that, but that failure is a lot more rare than the failures caused by piston speed.
Personally, I think that increasing the boost and dropping the rpm is a great way to build a long-living stock bottom end. Matter of fact, I use that theory on both of my trucks, turning them very few RPMs compared to some other combinations.
One thing about turbocharged engines that helps their survivability (you guys may be familiar with Frank5L’s ~730 rwhp on a stock long block turbo truck) is the effect of the pressure on both sides of the exhaust and intake stroke counteracting the stretch in the rod. The exhaust backpressure means that there is some amount of force acting on top of the piston to help slow it during the exhaust stroke, and boost pressure does the same (helping the piston accelerate) for the intake stoke.
04-06-2006, 07:23 AM
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I believe that if you keep the RPMs near stock that your motor will live a long and happy life. You have to be prepared to resist the power curve temptation - seeing that the power is still up at 5000 rpms requires some degree of self control to not rev it to 5500 rpms and so on.
A 10% change in RPMs creates a ~25% change in the load the rod has to deal with...
A 10% change in RPMs creates a ~25% change in the load the rod has to deal with...
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