Power or RPM, what stresses a block more? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-11-2005, 06:50 PM
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Power or RPM, what stresses a block more?

I've always wanted to know what is considered the upper limit of what a block will handle as far as output, total power or highest RPM? What I mean to ask by this is, will a block be more stressed with an engine producing 400HP at 8000 RPM, or one producing 700HP at 5500 RPM, assuming they are equal blocks and displacements, and the rotating assembly is strong enough? I realize RPM seems to be the more widely accepted factor for destroying blocks due to the huge loads on the rotating assembly, but what about the enormous twisting force on the block during a high horsepower grunt?

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Old 09-11-2005, 06:55 PM
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I'm guessing that as long as the rotating assemblies are "perfectly balanced" and the harmonic damper is of highest quality that the 700hp at 5500 would be more stressful on the block itself. I just figure the higher cylinder pressures are what would cause a complete block failure. Crank walk could be a problem at extreme rpm I'd guess, but have no experience with an engine making 400hp at either 5500 or 8000.
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Old 09-11-2005, 07:10 PM
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Extreme rpm will destroy a block when a rod lets go and saws a hole, or the crank breaks, or a piston skirt breaks, a valvespring/valve/retainer breaks.....starting to see a pattern?

400@8000 won't put nearly as much stress on the block as 700@5500, it will stress parts that move.

The cylinder pressure/torque needed to make 700HP at that RPM puts an incredible amount of stress on everything, including the block. It basically tries to shove the crank out of the block.

Horsepower is a mathematical product of torque and RPM. Less torque and more rpm = horsepower with less cylinder pressure pushing down on the piston. So the the block, which doesn't move, sees a lot less stress.

For example, a 200hp diesel and a 200hp 4 banger. The 4 banger might make 200hp at 7000rpm, and it only needs 150ftlb of torque to do it. The diesel makes 200hp at 2000 rpm so it might have to make 400ftlb of torque to do it.

That's one of the reasons that diesel parts are extremely large, strong and heavy compared to the little 4 banger parts.

I hope this helps you a bit.

Larry

Last edited by coldknock; 09-11-2005 at 07:19 PM.
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Old 09-11-2005, 08:23 PM
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It's rarely a block that fails.................its the moving parts that force their way through it.
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Old 09-11-2005, 08:56 PM
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In a well balanced rotating assembly the crankshaft is doing nothing more than spinning on an axis and as long as you have good lubrication it could do that all day long.


That being said...I would think high horsepower would be more of a factor.
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Old 09-11-2005, 09:16 PM
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Yes, this helps me a lot...

Maybe I won't worry so much about my block coming apart from simply revving a good rotating assembly.
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