A rolled fillet does NOT make a crank "stronger". It makes the crank more resistant to failure due to metal fatigue, which eventually forms a crack. Typically, a crank (with no obvious defects) will fail after many, many cycles due to fatigue in a high stress area. Typically, this will be in one of the fillets, since these areas act as stress risers in the material.
By rolling the fillets, the material in those areas end up with a very high amount of compressive residual stress. By doing this, the material in those areas will last longer before it fatigues. The modern nodular and cast iron crankshafts that I've been around all had rolled fillets. The fillets were rolled not to make the crank "stronger", but to improve the life of the crank in the engine.
To put this more simply: A N/A crank and a turbo crank will be able to take the about the same load, all other factors being equal (casting quality, material properties, etc.). However, the turbo crank with rolled fillets will last for more passes, because it will take longer for the material to reach its fatigue limit in the highly-stressed fillet areas.
Somebody had earlier mentioned that their N/A crank failed while they were driving, and that a crack had formed in one of the fillet areas. This is a classic crank failure due to metal fatigue.
One other thing - metal fatigue is a very random, hard-to-predict phenomena. Two apparently identical cranks can have very different lives in an engine, especially if they are highly stressed. Crank #1 could last for 50 passes, and crank #2 could last for 500 passes. Rolling the fillets raises the average life of a crankshaft, hopefully to a life high enough where failure never occurs.
So, if you're the type that rebuilds your engine every couple of years, an N/A crank would probably be fine. If you only want to build your engine once for a long, happy life, you're better off paying the extra $$$ for a turbo crank.
For what it's worth...
By rolling the fillets, the material in those areas end up with a very high amount of compressive residual stress. By doing this, the material in those areas will last longer before it fatigues. The modern nodular and cast iron crankshafts that I've been around all had rolled fillets. The fillets were rolled not to make the crank "stronger", but to improve the life of the crank in the engine.
To put this more simply: A N/A crank and a turbo crank will be able to take the about the same load, all other factors being equal (casting quality, material properties, etc.). However, the turbo crank with rolled fillets will last for more passes, because it will take longer for the material to reach its fatigue limit in the highly-stressed fillet areas.
Somebody had earlier mentioned that their N/A crank failed while they were driving, and that a crack had formed in one of the fillet areas. This is a classic crank failure due to metal fatigue.
One other thing - metal fatigue is a very random, hard-to-predict phenomena. Two apparently identical cranks can have very different lives in an engine, especially if they are highly stressed. Crank #1 could last for 50 passes, and crank #2 could last for 500 passes. Rolling the fillets raises the average life of a crankshaft, hopefully to a life high enough where failure never occurs.
So, if you're the type that rebuilds your engine every couple of years, an N/A crank would probably be fine. If you only want to build your engine once for a long, happy life, you're better off paying the extra $$$ for a turbo crank.
For what it's worth...
