To start this is all hypothetical. I'm just trying to understand what determines the RPM range of a given engine.

Lets say you have a shortblock that is completely built but you have know idea who the builder was.

After doing a complete tear down you find you can say that you know all the parts, mass and clearances.
In this case lets say that the rods and crank are forged so we have a starting point.

What variables from the above will give you the needed info to determine RPM potential of the engine?

 

The limit is usually the valve train anyway.

Rich

 

I agree 100% but what determines the shortblocks limit?

Maybe it's a mute point on a forged LT1 setup? Maybe the valvetrain just caps the potential.

 

I think it would be weight of the RA and also the bearing clearances that are run. I don't think it really matters honestly cause you can spin it as high as you want without a problem on a machine, but when you add the cam/valvetrain and then the HP potential of the engine you will be limited before you start breaking stuff.

 

As a general rule you don't want to rev past 6800-7000 with hydraulic lifters, anything past that you need to switch to a solid lifter.

 

For the rotating assembly, inertia loads on the rod are generally the rpm limiting factor. The worst case is approaching TDC where the rod is decelerating the piston/pin/rings which are trying to pull the rod apart (tension loads). As it passes TDC the rod now has to accelerate that same mass so it again experiences high tension loads. The rod is much better at absorbing the compressive loads around BDC, and combustion loads....well, with the exception of Top Fuel/Funny Car.

Issac said F=Ma so the force on the rod (crank/block) is the mass of the components times the acceleration (generall spoken of a "Gs"). The G's depend on the rpm and the stroke, and a tiny bit on the rod length. If you cared to calculate the loads, you can get piston Gs from an engine simulator like EA Pro.

It is quite possible to use low-mass pistons, pins and rods and reduce the loads on the crank. In one recent example, a 355 was built with light but strong parts so that inertia loads @ 7000 were no more than stock loads @ 6100 or so. IOW, the crank won't know the engine is going 7000.

If you want to get deeper into things, loads on the crank are transferred to the block, so higher inertia loads which tend to twist the crank can also distort the block, perhaps to the point where the bearing journal bores distort. IOW, a strong forged bottom end might be better than the block into which it's bolted, and the block becomes the weak link.

As others said, forged RA in an NA LT1 is not often the limiting factor. It's generally valvetrain. Of course, if you have rugged (heavy) RA components and are boosted, AND spin it to the moon, you'll probably be block limited. Here's where you might look at crank torsional rigidity if you insist on using an LT1 block @ high hp + rpm. It could be the little bit that saves the block.

I like a quote from Mark Rigg of Custom Crank and Engineering concerning crankshaft flex under load: "You would get excessive vibraton which would damage the bearings and the structural integrity of the crankcase, ultimately leading to the crankshaft physically breaking." That's Brit-speak for "running over your crank".

 

That can hurt, when you have a catastrophic "oil pan failure" like that

Rich