What is the rpm limit on cast vs forged crankshafts? I am looking to do a 383 setup and I am wondering if the cast will be okay or if I should buck up and get the forged. Car will only see about 6000rpms max because the cam I will be putting in should only pull to that rpm but I drive the car hard and it is a 6speed with 4.10 gears. Is rpm limit determined by strength of parts installed or by setup of engine? Thanks.

 

I think most will say that the cast is good for 6500 max. The theoretical rpm limit is determined by hp/torque curves. So you either buy components that meet your requirements, i.e. I want 600hp at 8000rpm, or you build to the components you can afford, i.e. I have a cast crank thats good for 6500rpm so I will build the rest of the engine to peak at 6000.

 

Actually the RPM limit is dependant on piston speed, piston G's and component mass.... physics.

If you are only going to 6000rpm you will be fine.

Bret

 

some people get the forged crank as insurance against failure or just in case they decide to add on in the future. some will say the cast will be fine and some will say to get a budget forged so you won't have to worry if the crank will fail or not.

 

I haven't seen a cast one fail yet... all really depends on the power and RPM levels. Most guys going NA don't need a forged one.

Bret

 

I built my motor fully up, in the anticipation at somepoint i would throw juice on the car

 

I was refering to where you as a driver shifts or wants to shift, not structual limit.

I also read that up to it's structual limit cast has greater longevity. Will take more pounding over it's life time then forged.

 

Where did you hear that? Forged cranks are stronger and more durable than a cast crank.

Bret

 

A cast ductile iron crank may have a bit more "damping" than a forged crank and that might contribute to a little bit less vibration transferred to the engine during normal running. A good forged crank, especially one made form a high quality alloy like 4340 (Nickel-Chrome-Moly) has a higher "structural limit" and will take more abuse, like the pounding from detonation than the cast crank will without catastrophically failing.

Big over-the-road truck engines, which work hard for millions of miles, or billions of revolutions (1000 rpm @ 60 mph x 1 million miles = 1 billion revs) are forged, not cast.

If you keep a metal part below it's "endurance limit" load, it will not fail from repeated cycles ('longevity'). The amount that you exceed the endurance limit load and the number of times you exceed it determines when the crank (or other metal part) will fail from use. There is a load that will fail the part the first time it is applied.

Cast or cheap forged or expensive forged is a decision that should be made from the loads the crank will see. If your loads will not exceed what a cast crank will endure, paying for the mega$ aircraft-quality 4340 forged crank is money you could spend more wisely elsewhere in the engine. Of course some folks feel better with the added security....like wearing a belt AND suspenders, or two condoms.

 

LOL