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Practical Engine Building: Forged vs Cast Components

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Forged vs Cast Components


00:00 - When it comes to choosing components for an engine, you'll often hear the terms forged and cast.
00:05 But you may not understand what they mean.
00:08 So in this module we'll talk about the two and discuss their differences.
00:12 Essentially what we're talking about here is the process used to manufacture the components and how this process affects the properties of the material and hence the strength of the component.
00:24 When we're choosing between forged and cast components, we're usually talking about pistons, conrods, and crankshafts.
00:32 And obviously we want to be certain that these components are going to be strong enough to survive in our application.
00:39 Let's start by discussing the casting process used to produce a cast product such as a piston.
00:46 In this instance molten aluminium is poured into a piston shaped mould before being allowed to cool.
00:54 Once cooled and released from the mould, the piston is mostly formed and will just require machining to finish it.
01:01 As far as the manufacturer's concerned, the casting process is cheap, and cost effective for mass production.
01:08 The downside of cast products is that there's no real grain structure inside the material, and the process also results in a low material density.
01:19 This reduces the strength of a cast component, making them more susceptible to failure under high loads.
01:26 Cast components are also brittle and likely to crack.
01:30 A perfect example here with a cast piston, is that they're very intolerant of detonation, and often we'll see the ring lands fracture and break out as a result of the shock associated with detonation.
01:43 Forged pistons on the other hand start out from a raw slug of material that's forced under immense pressure and heat into the shape of a piston.
01:53 This process results in a distinct grain flow as well as a higher material density within the finished component, and the improves strength and reduces the chance of failure.
02:05 While I've used the example here of a piston, the same is applicable to conrods, and crankshafts.
02:12 For a high performance application, we generally want to use forged conrods and crankshafts unless the stock cast components are known to be reliable in the sort of application we're going to be using.
02:25 Fortunately at least when it comes to crankshafts we find that the majority of late model cranks that are designed for performance applications are forged from the factory.
02:36 We can easily tell if a component is forged or cast by inspecting its surface.
02:42 A cast product will have several thin sharp parting lines where it's been removed from the mould.
02:49 A forged component on the other hand, will have relatively wide parting lines in comparison.
02:55 When it comes to pistons, it's common for a factory piston to be cast.
03:00 And it's also common for these to be replaced with forged pistons in performance applications.
03:06 While there's nothing wrong with that, and certainly there's often the requirement to do so if our target power level is known to result in failure of the stock piston, there are also some downsides to a forged piston that are often overlooked.
03:22 In particular a forged piston will usually be manufactured from a material that has a lower silicon content than a cast piston, and this means that the piston will expand more as it heats up.
03:35 This requires that the piston to bore clearance is larger for the forged piston than that of the cast piston.
03:41 The result is that the forged piston can often be a little bit noisy, as it essentially rattles back and forth in the bore when the engine is initially started from cold, until the engine builds up temperature and the piston expands.
03:56 This can also result in increased blow by past the rings as well as increased oil consumption compared to a cast piston during cold operation.

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