Why does high rpm cause an engine to "let go"???
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Why does high rpm cause an engine to "let go"??? I understand why too much power causes this, just a matter of all that down-force on the rod.
But why does high rpms kill a part?
But why does high rpms kill a part?
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Pin wheel your arm as fast as you can and see how your shoulder feels. Inertia and load stress increases the faster a thing spins. The more moving parts (cylinders) the more stresses you have. Thats why smaller motors can usually withstand more revolutions. Pretty amazing the LS1 takes what it does out of the box.
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Originally Posted by Quickin
I understand why too much power causes this, just a matter of all that down-force on the rod.
But why does high rpms kill a part?
But why does high rpms kill a part?
As RPM increases, piston velocity increases in a linear fashion. Piston acceleration, however, does not. It increases at a far greater than linear rate.
With the increase in acceleration comes an increase in friction. Somewhere in the area of the cube of the square of the rpm. I can't remember off hand.
RPM = high forces at play, higher friction, high temperature, etc, etc, etc....
It is pretty easy for a piston to be pulling -200g's at TDC on the exhaust stroke.
The non-linear acceleration curve is the reason for the large difference in lifespan between 6500rpm and 7000rpm stock engines.
Last edited by DenzSS; May 7, 2004 at 02:50 PM.
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Originally Posted by 99C5JA
Pin wheel your arm as fast as you can and see how your shoulder feels. Inertia and load stress increases the faster a thing spins. The more moving parts (cylinders) the more stresses you have. Thats why smaller motors can usually withstand more revolutions. Pretty amazing the LS1 takes what it does out of the box.
A CART or F1 car spins to what, 15,000 rpm? Is it as simple as stronger metals, thats it?
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Originally Posted by DenzSS
Too much power very rarely kills an engine. Detonation and RPM are the biggest killers.
As RPM increases, piston velocity increases in a linear fashion. Piston acceleration, however, does not. It increases at a far greater than linear rate.
With the increase in acceleration comes an increase in friction. Somewhere in the area of the cube of the square of the rpm. I can't remember off hand.
RPM = high forces at play, higher friction, high temperature, etc, etc, etc....
It is pretty easy for a piston to be pulling -200g's at TDC on the exhaust stroke.
The non-linear acceleration curve is the reason for the large difference in lifespan between 6500rpm and 7000rpm stock engines.
As RPM increases, piston velocity increases in a linear fashion. Piston acceleration, however, does not. It increases at a far greater than linear rate.
With the increase in acceleration comes an increase in friction. Somewhere in the area of the cube of the square of the rpm. I can't remember off hand.
RPM = high forces at play, higher friction, high temperature, etc, etc, etc....
It is pretty easy for a piston to be pulling -200g's at TDC on the exhaust stroke.
The non-linear acceleration curve is the reason for the large difference in lifespan between 6500rpm and 7000rpm stock engines.
What's going on with detonation thats bad? The way understand it, as the piston is coming up towards TDC, the A/F ignites to soon, causing a force downward wile the piston is coming up. I see the strain on the rod because it has opposing forces being applied to it, but what usually fails with detonation?
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I watched a video recently of an "03 Cobra taking off fast, at about 70mph the motor came apart. Literally had parts come through the oil pan onto the road.
What failed during acceleration?
What failed during acceleration?
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Under detonation, any number of things can come apart. You'll find the weak spot. You can take out a piston, rings, ring lands, connecting rod, crankshaft, or bearings. The detonation can set of a chain reaction that will send a harmonic through the entire assembly. As the connecting rods and crankshaft flex, the bearings get hit, oil starvation kicks in and it all goes down in a slag of metal. It usually isn't that bad, but it can be.
As far as the Cobra goes, who knows. You'd have to do a post mortem on the engine to know what happened.
As far as the Cobra goes, who knows. You'd have to do a post mortem on the engine to know what happened.
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Originally Posted by Quickin
With your shoulder example, that seems like a bearing failure.
A CART or F1 car spins to what, 15,000 rpm? Is it as simple as stronger metals, thats it?
A CART or F1 car spins to what, 15,000 rpm? Is it as simple as stronger metals, thats it?
Lightweight, strong materials.
Incredible machining
Incredibly good tolerances
Exotic valvetrain
Exotic bearing material
etc.
There are some Diamler race engines that are machined so tight that they do not use gaskets.
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Rods can handle alot more compressive stress than tensile stress. As the rpms increase the tensile stresses go up dramicatlly causing the rods to fail. Also oiling may become a problem as the rpms go up.
The F1 motors have a very short stroke which means the piston doesnt have to travel as far each revolution as a long stroke engine. This lowers the tensile stress in the rods. Also with a short stroke, shorter rods can be used which are lighter and stronger. F1 motors are made from exotic materials but the short stroke design helps alot.
The F1 motors have a very short stroke which means the piston doesnt have to travel as far each revolution as a long stroke engine. This lowers the tensile stress in the rods. Also with a short stroke, shorter rods can be used which are lighter and stronger. F1 motors are made from exotic materials but the short stroke design helps alot.
F1 cars dont use pushrods! lol. MAJOR limitiation for us. they have neumatic (air) controled valves. that is why they sound like turbines more than internal combustion motors.
also never compare race motors to yours. they get torn down every race (or start to finish)
also never compare race motors to yours. they get torn down every race (or start to finish)
Yeah, its the ratio of rod length to stroke. Assuming the F1 motor has a 2.5" stroke it would only need a 4.2" rod length to keep the same ratio as a stock ls1 motor.
Look at the ford 5liter motors. 4" bore with 3" stoke and 5.1" rods. The motors were made out of cheap materials but were still strong by design. A lot of people were runnin 10's with a stock short block and boost. The short rods can handle crazy compressive loads (without detonation) but revin them past 7 they would break. It was the bolts breakin the rods were fine. If you think about it, rod bolts see little stress during compressive loads and arent a factor. But when revved up its the rod bolts that keep the rods together.
Look at the ford 5liter motors. 4" bore with 3" stoke and 5.1" rods. The motors were made out of cheap materials but were still strong by design. A lot of people were runnin 10's with a stock short block and boost. The short rods can handle crazy compressive loads (without detonation) but revin them past 7 they would break. It was the bolts breakin the rods were fine. If you think about it, rod bolts see little stress during compressive loads and arent a factor. But when revved up its the rod bolts that keep the rods together.
not to mention that some times it is parts that hit each other.....
P-V.....
Float a valve..or go fast enough to launch a valve...and you get piston hitting it....and at a high speed this could actually bust the piston...
some limited racers use the launch technique...they get a really agressive ramp rate and shoot the valve off to get more lift than they are allowed to run....
but usually its the mass/velocity thing that kills ya...
P-V.....
Float a valve..or go fast enough to launch a valve...and you get piston hitting it....and at a high speed this could actually bust the piston...
some limited racers use the launch technique...they get a really agressive ramp rate and shoot the valve off to get more lift than they are allowed to run....
but usually its the mass/velocity thing that kills ya...
