Newb with a crank question

For starters, I have a 385ci SBC in a 72 Chevy SWB pickup and I have decided to apply some form of boost (probably a centrifugal supercharger). I am, however going to have to upgrade my rotating assembly to do so. I have found a heck of a deal on a 4140 forged kit with h-beams and 9:1 forged pistons. I have also found a similar 4340 kit that is about $600 more than the 4140, with the same pistons and rods. It will be a street truck with 10-15 psi boost, running below 6000 rpm. So, the question is 4140 or 4340?

Hi aucheyenne14, and welcome to SCH.

This is the typical engine-builder's dilemma. The fact that the components are forged is the most important aspect - forging is what gives these parts their strength, since it facilitates better control of the metal's microstructure; casting provides very little (if any) control in that respect, which is why cast parts are weak in comparison to forged.

Given the amount of boost you're looking to run, chances are that the 4140 kit will be sufficient. However, if I were in your shoes, I would spend the additional money and go with the 4340 kit. And I would do so for a number of reasons:

1. A forged 4340 crank is the strongest of-the-shelf component you can buy
2. While you're targeting 10-15psi, power is like money: no matter how much you have, you'll always want more so going with a 4340 crank will facilitate (any) future power increases
3. The additional $600 cost for 4340 is a small price to pay for ultimate strength
4. Peace of mind knowing that there's very little you'll do to that engine that it won't be able to handle.

Quick story: a friend of mine has a 383cid SBC in his '73 Corvette. This engine had all kinds of work done to it - ported and polished heads, forged con rods, forged JE pistons with 10:1 compression, a radical cam (I don't remember the exact spec but it was really lumpy), ported intake, and a Holley 850 double-pumper carb. Although he didn't have any kind of forced induction (super or turbo), his car would easily break into the 10s, and he would regularly beat brand new C6 Z06 Corvettes by SEVERAL car lengths at the stop-light drags.

A few weeks ago, I had heard through the grapevine that he was experiencing a problem with his car, but the details were a bit shady. When I ran into him at a local car show shortly afterwards I asked what had happened to his car. Wouldn't you know it: broken crankshaft!!! Sheared just aft of the rear main seal! Before I even had the chance to ask what kind of material the crank was made of, he said, "It's nodular cast iron, so I'm not surprised".

The point of this story is this guy had all kinds of forged parts and lot's of high-end items on this engine, yet when it came to the basic foundation - the cornerstone of his engine, the crankshaft - he went with a stock nodular cast iron crank (probably to save money). He didn't put his money where he should have and now he's looking at a huge repair bill to remove the engine, inspect everything, and (if he's lucky) replace the broken crank with a new unit. He's already declared that he's going with a 4340 forged unit because he doesn't want to have to go through replacing a crankshaft ever again. Of course, that's assuming all he'll have to do is replace the crank - if a rod is bent or a bearing has spun...? Well, it'll be a lesson learned either way.

I'd hate to hear a similar story after you've put your engine together. So learn from the mistakes of others - put your money in the right place and have the peace of mind knowing that you won't have any problems or issues in the future.

Hope that helps. And if it didn't, I at least hope it was an entertaining read.

Thanks! I was thinking about it, and for the peace of mind, if nothing else, I'm pretty sure I'll go with the 4340. I have tons of other questions so I'm sure you'll be putting up with me indefinitely. I'll post pics of the truck later on. Thanks again for the help!