I was looking at the different 3.00" cranks offered by GM in the 60's and was wondering which was the strongest.

There was the "small journal" 283 forged steel crank.

There is the forged steel 283 crank with the "medium journal" in motors before 1968.

And then there is the "large journal" forged steel cranks from the 68-69 302's.

Now obviously I would think that the large journal cranks would be the best (they are also the most expensive), but has anyone heard anything different?

Also, is it worth it to have to use a large journal block and pay more for the crank? Or are they all good for enough power that it wouldn't matter one way or the other. And are there any balancing issues with these cranks, or can they all be internally balanced?

Just to give you some goal info this crank wouldn't ever see more than 700hp, and that is definately on the high side, probably more like 600.

Thanks,
Jeremy

 

check out these sites they have a lot of info
http://www.chevy-camaro.com/
http://www.nastyz28.com/sbchevy/sblock.html#camaro
http://www.mortec.com/


Wade

 

Thanks Wade,

I've used all of those sites numerous times to get casting numbers and various part numbers. But what I really need to know is the strength of the cranks, or whether the difference is worth caring about.

I appreciate the help,

Jeremy

 

you can turn less RPMs and make more power with more cubic inches than building some small cubic inch high revver motor. It will be more reliable too.

 

Depends what you are building. Yes, a large journal crank is stronger. But OTOH, a small journal crank may be strong enough. And FWIW, small crank journals = less friction = more hp, but also higher bearing loading and decreased bearing life. Like everything with the damned IC engine, there is no such thing as a free lunch!

Best way to look at it is that the OEM's don't make design changes for the hell of it. Typically, they do so because the change makes it cheaper to produce an acceptable component or because it is needed to achieve a performance goal. A change like that from small to large journal would be in the latter category, IMHO.

Rich

 

Well thanks for your input, but i'm pretty familiar with the physics of it.


Rich,

Yeah it's true, building these motors is "give and take" all the way. Usually the more money you give the more abuse it can take.

And I thought about the fact that they used a large journal crank in the DZ302s, so you'd think they would be stronger. The question is how much stronger? I've personally seen 450rwhp on a small journal 327 in a 64 vette, and I could live with that. But if there is some fatal weakness in the small journal 3" cranks i'd like to know.

It probably will be strong enough. And if not, it was only $60 on ebay.

I guess i'll try it and if it fails, then we'll all know. I do hate being the guinea pig though.

Thanks,
Jeremy

 

I bet it will be fine. Most street or street/strip engines do not sufer crank failure unless subjected to unusual loads like a blower.

Rich

 

Unusual loads, huh?

Do 4-valve heads count?

Edit: You were probably talking about side-loading the crank with belt-tension?

 

Yeah, the commonest place I see cranks break is the snout when a SC is used. Obviously, under very high load (high boost race applications) they fatigue quickly. You don't see lots of outright failures 'cause very highly stressed motors get lots of maintainence and small cracks are usually found and the crank replaced before catastrophic failure. Or else they just replace them on a schedule. It's cheaper that way, believe it or not! My buddy with a blown alcohol Hemi gets 50-75 passes out of a $2,500 crank. By that time, small cracks have started to develop in the fillet area and it's time for a new one. Finances permitting he keeps one "in stock" balanced and ready to go.

Rich

 

That's incredible. I wish I had someone to finance all the ingenious (or insane) ideas that I have. There would definately be some cool cars running about.

Of course his racing is more likely to make money, so financing is probably less of a burden.

But this motor will more likely utilize a turbo or nitrous to make power. The combustion chambers are small, so nitrous would be the obvious choice. I did have a crazy idea to use some pistons from the old 350 diesel motors. I have to check on it but if the pistons are designed like many other diesels, where the dish is the combustion chamber, it might end up being a good fit.

We'll see how it all works out in the end.

Jeremy