Engine Question - Forged Metal vs Cast Metal?

Factual Questions
1

G’day guys.

Some of you might be aware I own a '66 GT350 Shelby Mustang. As you can imagine, with a vehicle like that, you can easily go off into huge research adventures as you learn more and more about how engines were made in the 1960’s - and how far the technology has come since then etc etc.

I love the Hi-Po 289 which is in the car right now, but man, that friggin 715 cfm Holley simply stinks the car out it runs so rich. Any thing less than 75% open throttle and it truly is a car which simply pours unburnt fuel down the inlet valves.

So I’m considering keeping the original 289 in storage and whacking in a nicely built roller 347 with all the modern techno goodies like Edelbrock’s fuel injection etc etc.

Hence, the following generic questions regarding potential rotating assemblies. And fellow Dopers, please feel free to give me all the info you can possibly muster after you read the following stuff.

I read about hypereutectic pistsons, and how they’re nowhere near as good as “forged” pistons. I read about nodular iron crankshafts, and then I read bout 5140 chrome-moly crankshafts, and forged 4340 cranks. And “I” beam con-rods, and “H” beam con-rods etc.

Just what is the difference between a “forged” piece of metal, and a “cast” piece of metal?

And, when dealing with spanners, what is a “drop forged” spanner?

Why is a “forged” piece of metal better than a “cast” piece of metal?

Fellow Dopers? Please, get as friggin technical as you want. I’m a total sponge!

Thanking you in advance,
Boo Boo Foo.

2

This is a real crude and imperfect analogy, but it may give some idea.

Forged metal has a grain structure, in some ways similar to wood. In forged parts this grain flows around the contour of the part while in cast or machined parts it doesn’t. castings usually have only randomly distributed grain structure, not real strong, while a well designed forging has a grain structure that flows in such a way as to most efficiently carry the stresses the designer anticipates. Going back to the wood analogy, imagine a fork formed by the branch of a tree, then invasion one identical one carved from a board of the same type of wood, the natural fork is stronger since the grain follows the contour more closely.

3

I’m not much of a car guy, but I can answer the metallurgy questions.

First off, a forged piece of metal is one that has been shaped while in the solid phase. Metal can be ‘hot’ forged or ‘cold’ forged (or worked), but the result is often the same: the metal has been plastically deformed, and so has what is known as work hardening of strain hardening. This means that the metal is stronger in the engineering sense - it will support more stress before yielding. However, the temperature at which the metal is worked is important, since it affects how much residual strain remains in the metal. Also, it is important whether the metal has been annealed, since annealing is a process that )among other things) removes strain hardening. Typically, the increased strength is gained at the expense of plasticity.

Cast metal is meatal that has been melted into a liquid and poured into a mold. Naturally, the metal cools from the outside in, so to speak, so it has a set of features typical to this process: dendritic crystal growth and eutectic shift to name a few. This means that it has different behavior than forged or worked metal, typically being more brittle. (I should note that the features of cast metal are greatly dependent on the rate at which it cools.)

Ok, with that out of the way we can get onto the real question: is forged metal better than cast metal. There are a number of ideas out there, but really, cast metal isn’t better or worse than forged metal - they’re just different. As with all material and process selection, the use of one over another has to do with the relative cost of each (casting is often cheaper, but not always), the complexity of the part, and the behavior of the material. Cast metals are just great for a wide variety of applications, but so too are forged ones.

PS - “drop forged” has to do with the manner in which the wrench was forged. If I recall correctly, drop forged means that a wrench blank is heated and then inserted into a hot metal press where it is forced into a die.

PPS - If you want more detail I’ll be happy to get into grain boundary deformation and its modes and the quirks associated with casting various alloys. Just let me know.

4

Uh, not to be cranky, but I think I have to voice disagreement with 3.885AM. The grains in metal have pretty much nothing to do with those found in wood, even in appearance. You see, metals are crystalline so they’re basically made up of little crystals known as grains. Ths size, shape and composition of these grains determine many of the macroscopic material properties of the metal. This is vastly different from wood, where the grain is formed primarily by the vascular tissue and grown rings of the tree. In wood, the grain is a macroscopic physical structure that alters the properties of the wood, in metal the grains are microscopic structures that determine the properties of the metal.

NOTE: The whole discussion of crystalline grains is irrelevent when discussing amorphic metals, since they don’t have any. Instead, they are basically like glass, with a random internal structure. Because of this, they don’t behave like normal metals.

5

http://www.howstuffworks.com/question376.htm
http://www.forging.org/facts/faq.htm
http://www.utm.edu/departments/engin/lemaster/Machine%20Design/Notes%202.pdf (PDF)
http://www.ent.ohiou.edu/~raub/manufacturing/forging.htm#Board+Drop+Hammer

6

Sorry, I forgot to mention that unlike the grain in wood, the grains in metal don’t form long ‘chains’ that can flow around curves and so forth. They can be consisdently deformed to do something like that, but it’s not the same thing.

Also, cast metals have grains just like forged metals do, they are just of a different size, shape, and composition. They aren’t random.

And, machined parts are a bit different than either forged or cast ones, but not that different. In machined parts the grain deformation, and so the strain hardening, tends to be concentrated right near the surface of the areas that have been machined. But the metal still has grains. All normal metals have grains.

Finally, these processes are not exclusive. A single part can be, for example, cast, forged, annealed and then machined.

7

Hold on there with that engine!

It sounds like the carb just needs some adjustment. A few things that immediately come to mind:
Smaller jets, especially in the primary metering plate, since you say it seems to be good at WOT and rich down low.
Power Valve issues. What is your vacuums like - about right for the type of PV.
Or even just some vacuums leaks.

But then that 347 sounds cool too :slight_smile:

Hyperutetic…whatever sp??? type pistons are good pistons. They are not as strong as forged pistons but they are stronger than cast. The Hyper type pistons are no good belief have been attributed to some problems with Keith Black Hyper type pistons. I know lots of people, including myself, that have used Hyper slugs even in blown engines pushing 30 #'s of boost.

The different types of rods get there name from the shape of the beam. Able to get just as strong, possibly better, with an even lighter rod. The forged units are better than cast. But again, what are your power goals? If you are going for >500 then it is time to start thinking about forged rods. There are also powder formed rods – a hot new topic that I have no experience with but they sound good.

Here is another link that I saved a long time ago and have not yet read myself but it looks like it is right on target for some questions here:

http://victorylibrary.com/mopar/billet-c.htm

The SDMB official rules require you to report back on the state of tune of the 289 and/or of progress of the new engine build. :smiley:

8

Ahhh… well then, I should give you some background as to what I’ve found out regarding the engine thus far.

(1) Leak down compression is good on all 8 cylinders. PSI tests are showing 180 psi during crank tests.

(2) The elastomer between the inner and outer ring on the harmonic balancer has died and the timing marks have slipped on the outer ring as a result. To get 12 psi of manifold vacuum under idle the ignition has to be advanced as far forward as 42 degrees and obviously that’s ridiculous - hence, the harmonic balancer is going to be replaced with a top flight Romac steel/alloy 6 pound unit, along with the corresponding Romac timing chainset which has 9 separate camshaft timing lock points - each timing mark incremented by +/- 2 degrees. A very nice unit.

(3) The vehicle was imported by me into Australia from Miami in November last year. It has 51,000 original miles and it’s quite immaculate but someone, at some point, has done some work to the engine. Unfortunatley, I can’t contact the former owner to find out just what has been done, hence, we’re currently flying a bit blind here.

(4) The heads have Crane Gold needle pin roller rockers. That much we know.

(5) It revs like a fucking Cosworth. That much we know. However, a dyno test shows only 210 rwhp - and that’s WAAAAAY down on what a healthy Shelby 289 with good compression should be able to produce.

(6) It almost certainly has an aftermarket replacement cam in it, but without pulling the timing cover and inlet manifold off, it’s impossible at the moment to ascertain what the lobe timings are.

(7) There’s a significant chance that the camshaft has not been dialled in correctly.

(8) We’ve done a thoroughly professional resto and rebuild on the Holley carburettor with the local carburettor specialists. They’re very highly regarded. we’ve detuned the jets as far as the Holley will go. To get 12 psi of manifold vacuum, the engine is constantly hovering near a state of permanent pinging under even minor throttle settings. This leads us to believe that the camshaft is probably not dialled in correctly.

So, you see, this is the position I find myself in… persevere with the glorious GT40 spec 289 or simply shrinkwrap it and get a honking new, all aluminum DART 347 with Formula One Mahle pistons…

hmmmm… droool… Mahle…

Either way, as far as famous cars go, it’s the sort of problem you WANT to have, isn’t it?