Ok, i have a 2U crank i picked up for a good deal already ground and ready to run. I also have a 391 crank thats already ground and all it needs is the Snout/rear flange work to make it compatible.

How much HP are the 2U cranks reliable for? I called the local shop and he was hessitant about doing the work saying that "the bolt sizes are different and it may make it too thin if its turned down" And he never heard of anyone doing it or has he ever done it.

So i'm pretty much just looking to see what the normaly HP range for the 2U crank is. Or if i should find somone to do the work on the 391.

I would swap the timing chain and everything over but Its super hard to find stuff for them and i want a new balancer.

Should i stop in at some non engine machine shops to see if they can do the work? i have the other crank they could copy?

 

Update, i called another shop. He's going to check his shop that does crank (he doesn't do them in house) and call me back. He's been at it for probably as long as i have been alive, and unlike the other shop he actually knew what i was talking about, but he has never done it.

 

It would help to know what you're intentions are - how much HP are you aiming for? And more importantly, RPMs?

Remember, the 391 is EXTERNALLY BALANCED - so you can't just swap that 391 crank in place of a 390 crank. And if you're looking for high horsepower and/or high RPM, it should be internally balanced. Which means even more machine work, Mallory metal ($'s), etc.

 

I'm not exactly sure of my intentions. I would say to start I would like in the area of 350-400 hp. But my main thing is this. I only want to build the bottom end one time. That way if 2 season's from now i want to go for a more aggressive cam, better race ported heads, and intake to match I don't have to worry about going from a 4500-5000 RPM ( may even be higher just guessing) range into the upper 6k's if she decides to pull in a higher class. Plus i would rather have the insurance of the forged crank over the cast in any situation.

NO matter what crank the 2u or the 391 it will be balanced internally. So thats not really a problem since i was planning on that anyway.

 

400 is not really a big number for a 390 (2U). There are plenty of people making more than that without problem. I would keep the rpms to 6200 max. But that's JMO. The crank has never been the weak point.

 

Sleepy is right, the cast crank can handle a LOT - if you're worried enough to use a forged steel crankshaft, you're going to need cross-bolted mains too, at the very least.

 

So the 2u will be able to handle anything a 30 over 390 will hand it? Given that the R's are kept at or below 6k? And anything over 6k or so you suggest crossbolts?


Edit: what are some of the higher numbers you have heard of 390's making? and what were they running?

 

Here's a link to a 475hp crate motor.
"390 Ford" With 475 HP Crate Engine

10Gs!
If you are looking for big power from a 4.05" base block, I'd go with a stroker setup. Thats what I did. The more ci you have the milder the manners with the same power. There is no replacement for displacement. My total cost was about $5200. I dont have the aluminum Edelbrock heads though. I have C8AE-H heads with CJ valves and some mild port work mostly on the exhaust side. Edelbrock RPM intake. Headers and compcams xe274h cam. Holley 750cfm carb. Diamond pistons 17cc dish makes 9.84:1 CR. It's making about 460hp/540ft lbs.

 

I mean, correct me if I'm wrong (and I have been) but the stock 390 crank can handle just about any naturally aspirated setup unless you are really pushing the revs past 6500 w/high torque. Put in a set of good "regular" 390 rods with ARP rod bolts, and you're good to go.

This is assuming everything has been checked - as in, magnafluxed, and checked for straightness. AND BALANCED.

 

Thats the way I understand it. ^^^

 

Must, the cast crank will be fine. As far as the strength of the cast crank, you're okay till at least 750 hp, maybe more. And rpms....you're good to at least 8300. Think about the high riser 427 Ford ran in '63 & '64, before the forged cranks came out for the Medium riser engines- SAE papers show Ford testing the 427 HR on the drag strip and turning 8300 rpms, and the cranks were fine. Those 600-750 hp 4.25 stroker engines that Barry R and others are building are using a cast iron crank, made just the same way. The crank is about th last thing I would worry about. If you are going to be above 550 hp or lbs ft of torque, you could consider adding crossbolted mains. But the cast crank is tough enough to laugh at anything most of us will ever ask of it. And find a new machinist who knows what he is talking about. The rear flange on the FT 391 crank does not need to be reworked, and the snout is just fine when turned down to the smaller passenger car/pickup size. There enough meat around the bolt to hold just fine, and the steel is even stronger then the iron. But balancing the steel cranks can be exspensive. DinosaurFan