Cast vs. forged crankshafts

I'm looking into building a turbocharged 350 Chevy V8 for around 450ish HP. I've never built an engine from the ground up, and I'm a bit confused on how the parts are rated in the catalogs. For example, a new Eagle cast crank is rated for 500 hp, and the forged for 700. How accurate are these ratings, and does forced induction change how I should select a crank? I've heard of things like crank flexing and spider cracking, but I'm unsure of how these things are affected by the casting/forging processes and forced induction. Everyone I have to talk to around here doesn't know anything except for big cams and 4 barrels.

I think those power ratings are really just a marketing gimmick to simplify what is really a pretty complex issue. There really is no definitive power limit for a cast crank.

I believe the criteria for crank selection is not just power, but how the engine is going to be used.

If the engine is built for low and mid range power, driven on the street, and will not be revved too high, a cast crank is probably enough.

However, if you are building a race engine (drag or circle track) and/or you plan to rev the engine to the moon, then a decent quality forged crank is good insurance and worth the money.
IMHO go for forged, because likely you will have to tune the engine to suit the combination you have. You may find the engine responds to higher manifold pressure and more revs than you had initially thought. It would be nice not to have to worry about the bottom end when you are trying to get the most out of it.

I plan to use the car mostly on the street, but the occasional trip to the track would be nice.

First as far as the advertised specs go the HP rating per se' isn't the biggest issue.

In your application a good cast or forged internally balanced crank will be more than adequate, I suspect.

Cranks are under the most stress at 3 situations:

One is at max torque. On your engine this will probably be near 4000 RPM at WOT. (depending upon boost, etc.)

Next is at high loads at a constant RPM point for long periods of time. (For example on an 2 mile oval track at WOT. Harmonics can occur which can crack the crank. Probably not something you will encounter.

Finally, A hole shot especially with high traction tires. As the clutch is released the energy in the engine is released into the drivetrain, putting huge stress on the crank (and other components).

Clearly a forged crank is better than a cast one for most any application. However, in yours, a good balanced cast system will be more than adequate.

I've been offered a factory forged crank by my neighbor. How does a factory forged crank compare to aftermarket cast or forged cranks?

in all actuality theres not much of a difference forged is forged many diffrent ways of forging but really there is no real difference from factory and after market aside from it maybe being different type of steel you'd find in an after market crank

in all, its forged right?

if your goal really is 450hp, you don't need anything exotic at all for internals. Decent pistons are the only thing I'd advise.

The main difference between castings and forgings are the grains of the crystalline structure. In a forging, they are mostly aligned and in a casting they are mostly random. The reason is that castings use molten metal poured into the mold. Forgings use hot metal about the consistency of putty
and forced into a shape under great pressure. In a casting the crystals can form as they wish. In a forging, they are maintained and aligned.

The net result is that castings are hard and brittle. Forgings are "soft" and malleable. The difference is minimal, but in an engine it makes a difference. If you have a cast crank on the ground and you hit it with a sledge hammer, it will most likely shatter. Do the same thing with a forged crank, it will dent or bend. In this way, forged cranks aren't chosen for high performance use because they are appreciably stronger, its because they can take abuse by absorbing the forces. A forged crank will deflect and return. If you ask that of a cast crank, it will break.

Think of a crankshaft like a diving board. Let's say you have a kid and an adult, and one diving board is fiberglass (like normal) and the other is tempered glass. The kid can jump on the glass one all day and not break it, but the adult will stress it past its failure point and it will break off. The fiberglass board however will take both in stride; not because its stronger, but because it is designed to take more abuse from deflection.

grab your neighbours forged crank, get it balanced and linished and with 450hp you will have nothing to worry about.

I have seen cast stock crankshaft take 500 hp in a 440 bored.... the stock crank took every bit but a forged crank is diffenantly the way to go just for piece of mind and if your neighbor is willing to GIVE you one :crying: (I wish I had a neighbor like that) take it!

Well, he's not just giving it to me, but he's only asking a few bucks for it. We sell our junk to each other all the time. I'll go ahead and pick it up this week. Thanks.