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I've heard of these for years and not fully understood the benefits, can someone elaborate? Frequently the vehicles with forged internals have serious boost pressures, but I've also heard of naturally aspirated and / or carbureted engines that have been upgraded.

Thanks

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    Welcome to Motor Vehicle Maintenance and Repair! This is great first question. I'm sure somebody will chime in on this one. Give it a half day or so. – zipzit Jun 2 '16 at 2:08
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Forging is a process where hot metal of the rough shape is then very forcibly squished by exact shape dies, severely compressing the metal or alloy molecules. There are internal tensions created within the structure, which ultimately resist sheer and tension stresses by having a reserve of counterbalanced "forces" due to the myriad of tiny forge-created stress nodes inside. Dies are not necessary for forging, as any blacksmith or swordsmith is creating similar stresses when beating hot metal with a hammer as it cools. However, if the part (or sword) is reheated again, much of the gain can be lost as the molecules relax their cramped positions.

There is also quenching, which is taking a metal part that is very near a eutectic (phase change) point and suddenly cooling it in an oil or water bath, perhaps even liquefied gas. This creates similar strength properties, but should not be considered forging as the gain is shallow and typically does not extend through the part. This typically followed by "annealing", which is a very controlled heating and slow cooling process which relieves SOME of the surface stresses that might lead to cracks or shattering. If the part is irregular in size (like a connecting rod with a big end and small end) the quenching process is also likely to cause warping which must be removed, typically mechanically by bending. I'm not aware of this being used much at all for engine parts.

Most mass-production of engine parts are done with casting, which is unlike forging in that molten metal is "simply" poured into a near-shape mold and allowed to cool. Molecules of the alloy are allowed to move around as they need to, and the part has little internal stress. This is a less-costly process than forging, as molds are much cheaper to make than forging dies--as well as many other reasons. Forged parts often start out as castings. In high-horsepower applications and other cases where RPM and/or cylinder pressures (BMEP) are very high, the added expense of forgings is well worth it.

BMW, Porsche, Corvette, Ferrari, etc. will use forgings where cost is less of a concern vs. performance and perception. Another cruical bonus, is that parts can be forged with less material for a lower weight, while still being stronger than an apples-to-oranges casting. This is HUGE in something like a connecting rod (plus a piston), where most failures are NOT due to compression from boost, but from tension stresses fatigue from changing direction after Top Dead Center. Each entire reciprocating assembly might only weigh a few pounds, but image the stress if you had to throw that away from you and instantly jerk it back to you -- 15000 times a minute, 250 times a SECOND on a literbike or modern F1 engine. THIS is why high compression, high RPM, high boost engines use forged internals... not so much because of the pressure, but because of the higher tension forces created by higher-weight castings -- forces that are (I think I recall) CUBED with respect to RPM, and squared with respect to weight. [that recollection might not be exact] ... Going any further in weight reduction suggests materials like titanium or exotic nanoparticle carbon metal composites yadayada, which are far beyond answering your question or my wallet.

ALL of these processes require exact machining afterwords to achieve the proper exact dimensions for bearing fitment and threads for assembly.

Metallurgy is incredibly interesting. 6000 years ago artisans were making swords that still have not been fully recreated to this day. Despite incredible gains in alloy and metal working technology, I still feel the only huge difference between the art of alchemy and the science of metallurgy, is that metallurgy actually works!

I'd love a set of forged Manly or Crower rods for my Saabaru EJ205 project, but $1000-$1400 is likely 4 or 5 times the cost of perfecly serviceable stock parts for a stock vehicle. I won't even mention the cost of a forged crankshaft. And then there are these piston things... [sigh]

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  • Thank you for the great background on forging and explanation of some of the benefits of forged internals. When you compare cost to stock, are you essentially saying that a new stock short block alone would cost $1800 and would replace almost all catastrophic failures which forged pistons/rods/crank might defer? Example, if a rod fails at high rpm and permeates the block. – voxobscuro Jun 2 '16 at 15:47
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    Not sure exactly what you are asking. A entire forged reciprocating set (pistons, crank, rods) is likely to be upwards of $3500 on a 4 cylinder boxer that I'm familiar with. You can buy an entire "short block", which includes all those parts (castings) plus the block and some other bits, factory OEM shiny brand new, for probably $1800. There were a few Sti Subaru impreza versions that the 2.5L came with forged rods from the factory, but in general, production cars get cast parts. – SteveRacer Jun 2 '16 at 17:08
  • That's exactly the comparison I was after. I'm speculating here, but I get the feeling that forged pistons (for high boost) and/or forged rods (for high rpm) aren't worth it in terms of dollars per performance. When the cast internals engine throws a rod and punctures the block, a second block + pistons & rods + crank is still less than the cost up front of forged internals. It feels like for the average joe, the dollars might be spent on other performance changes, unless riding the extreme high rpm + high boost is otherwise important. – voxobscuro Jun 2 '16 at 19:44
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    Yes. very roughly, if your stock engine is a sound design, say 200hp, you can probably run cast parts to 300-350hp. Add forged parts and play in the 400+hp range. But today's tuners are getting 750hp from a 2.5 Subaru ... which mandates everything must be perfect and balanced, and the astronomical cost of building such a beast is repeated every 5000-10,000 miles, – SteveRacer Jun 2 '16 at 19:59
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A bit of experience from the turbo dodge world (84-93 2.2/2.5l turbo engines)

There were cast and forged connecting rods. Cast were said to hold about 200hp, where forged would hold about 400hp without issues.

There were cast and forged cranks, but I don't remember of ever hearing about crank issues.

The engines all came with cast pistons. If you got into high boost and had detonation, you could easily crack a ring landing. Forged pistons could stand up to a bit more abuse.

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Castings are more likely to have manufacturing flaws than forgings or, in other words, there are more process variables in casting. Castings are more likely to have coarse grain than a forging; That can be fixed with extra heat-treatment ( homogenization + normalize ) but that costs money. Some castings may need carbon restoration , also more money. For decades many parts have been made with powdered metal and sintered, some contain a little copper and possibly other alloy ( I am not familiar with that process). As an old guy ,I have a hard time understanding camshafts, pistons and rods made from powder , but most manufacturers use them. Forgings are not without problems ; the grain flow needs to be oriented correctly relatively to the stress orientation. Some complex parts can not reasonably made by forging . So, it depends on the component and its cost as to which is best.

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