FORGED vs CAST vs HYPEREUTECTIC PISTONS
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Published 2020-02-16
To really understand how forged and cast pistons differ from each other we will first look at how they are made. When it comes to cast pistons their name tells you how they're made - by casting. The manufacturing process of cast pistons starts with a molten alloy which is composed mostly of aluminum and a few other metals. Once the alloy is completely melted and liquid it is poured into a mold (mould). It then cures and solidifies. After this the mold which is usually composed of three parts retracts and the basic piston shape is complete. After this the cast piston requires very little machining and it's ready to be installed into an engine block.
How are forged pistons made? Their manufacturing process is very different from cast pistons and there is no melting of anything. A long aluminum alloy rod is cut up into billets. These billets are then heated up and put in a forging press that applies approximately 2000 tons of pressure onto the billet shaping it into a basic piston shape. After this the forged piston shape needs a lot more machining compared to the cast pistons. Forged piston production requires a lot more time which explains why forged pistons are more expensive than cast ones.
But the manufacturing process doesn't just play a key part in the forged piston price it also shapes something called grain flow. What is grain flow you ask? Grain flow is the directional orientation and distribution of metallic grains within the piston. For maximum strength you want your metallic grains to be coherent, uniform and rounded off. If you look at the grain flow of cast and forged pistons you will see that they are very different. Cast piston grain flow is random and can even have pockets of trapped air in the casting. This is because there is absolutely no pressure applied to the material during the casting process. On the other hand the grain flow of forged pistons is very uniform and coherent. This is because the forging press applies tremendous pressure onto the forged piston removing any traces of porosity and forcing the grains together into a tight uniform shape. This is why forged pistons are much more ductile than cast pistons. But increased ductility comes at a price, the price is greater thermal expansion. Forged pistons expand more than cast pistons when hot, which means they need more piston to cylinder wall clearance to account for this expansion. This increased clearance can cause piston slap when the forged piston is still cold and this can cause increased wear. Cast pistons expand less, which means there is no piston slap and ultimately a cast piston engine will survive more cold starts and last longer than a forged piston engine.
What about hypereutectic pistons? They are also cast, so are ultimately less ductile then forged pistons but are a step up from typical cast pistons. Hypereutectic used to be a biz buzzword in the 80s and 90s but what it really has to do with is the silicon content of the piston. Any piston with more than 12.5% of silicon in it is going to be called hypereutectic. Silicon is important because it improves the wear characteristics of pistons, it makes them harder and thus lighter but most importantly hypereutectic pistons expand even less than typical cast pistons which have on average 8-10% of silicon in them. This means that engine designers can specify extremely tight engine clearances when hypereutectic pistons are used, this in turn improves performance, efficiency, fuel economy and reduces emissions, which is something that every modern engine designer wants.
What about silicon in forged pistons? 4032 forged pistons have around 11% silicon in them while 2618 forged pistons have 1-2% silicon in them (some as low as 0.2%). This means that 2618 are the best alloy for extreme racing applications while 4032 pistons are forged pistons that aim for a compromise, trying to give you the benefits of forged pistons while also trying not to expand as much.
How often will you drive your car and what are your power goals are the two main questions you should ask yourself before deciding between forged, cast and hypereutectic pistons. In essence your are trading engine longevity for power potential. The more power potential the less longevity you have. It is important to note that both cast and forged pistons have come a long way. Cast pistons are not as weak as the internet wants to convince you, while modern skirt designs and coatings are helping forged pistons reduce wear.
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All Comments (21)
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This is an absolutely wonderful discussion. I live near Detroit, and one of my classmates is a piston design engineer. He loved this when I showed it to him. His only comment was that modern pistons are NOT poured when cast. Rather the mold is dipped into the liquid metal and a vacuum is created such that the liquid metal flows into the evacuated mold. There is no possibility of air pockets in the resulting piston. The narrator touches on this when he speaks of "modern" techniques at 11:50, but does not say it specifically. I was told that this process is used for two reasons, to exclude air pockets, and that liquid aluminum alloys will oxidize instantly, at their surface, when exposed to air, and this creates aluminum oxide (slag) that is horrible when inside the body of the casting. The older pouring methods depended on the slag floating on the liquid and not going into the mold. There was a certain amount that did get into the casting but until vacuum techniques were developed this was unavoidable.
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It is so refreshing to hear someone actually understand materials engineering and their application. Subscribed
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Without this video out there I would had bought a car, turboed it and ran forged pistons. Just to daily it, I'll pass. You are helping a lot people in their future builds. Thank you!
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"What piston is best for your engine" Me driving rotary : "why am I watching this?"
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So. You're telling me I probably shouldn't put those 1000 dollar forged pistons i bought yesterday into my 89 corolla wagon with 251000 miles? Dammit.
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I’ve Been a car guy since the early 80s. And this might be the best piston content I’ve ever seen. Well done!👍
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Anyone else here because they didnt know wtf a hyper eautectic something is
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I did NOT know any of that. I always thought forged was the way to go. Very good way of explaining this topic. Thank you for taking the time to put this together — I really benefited from it!
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I used cast hypereutectic pistons in my engine build. My engine is a mild 40% power increase over stock so I didn't need anything over the top. My next engine build will have custom forged slugs, expecting a 95% increase in output over stock, not huge but the high rpm operation calls for something a bit stronger regardless.
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I'm a 35yr career mechanic & this short but fascinating video was so insightful. It's actually making me reconsider my current engine build project! You've highlighted so many variables based on final choice. Many thanks indeed. Subscribed 😉👍🏻
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I've been working on cars most of my adult life, and I learned something today. Thank you, very well explained!
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Excellent discussion on the differences between these piston materials. Having had significant experience with all 3 in racing applications, the only thing I would add is that in comparison to forged, hypereutectic pistons are far more likely to shatter if anything significant goes wrong. For example, when a hypereutectic piston makes contact with a valve (say missed shift/valve float) that piston will often shatter and take every thing out--rods, block, crank, cam and heads. Often with forged pistons you are just dealing with a bent valve and a salvageable engine. Two thumbs up for this vid--you know your stuff.
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When I saw the hypereutectic pistons, I was like, "they are making pistons out of cast iron?", this phrase is used in cast iron metallurgy; you have eutectic and eutectoid, the amount of carbon being below or above 8% respectively, but hypereutectic means it's right at 8%, which has characteristics different from less or more carbon, so I was a bit confused on this video. You did an awesome job explaining all this! People need to know the what and why, before they spend big bucks on their engine, daily driver, a bit more than that, it a full blown racer
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I'll be honest, when I 1st saw the 1st few seconds of the clip, I thought that this would have been a waste of time… but this person is very knowledgeable and gave a lot of good information
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Great job explaining the differences and why sometimes throwing the most expensive parts at your engine build can be a waste of money.
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GREAT video, you give the FACTS about the most misunderstood component in a engine, most engines are DAILY drivers, while it might be "suped" up a bit w/ a bigger cam better heads you don't spend day after day at the track, another thing you have to ask yourself "How many times do you want to rebuild your motor?", I'm personally rebuilding my sbc 350 that has 439,906 miles on it, it still gets 17mpg and 12 if I'm loaded down(7,000gvrw+)the cylinders are a lil glazed but there is Zero ridge at the top of the bores but its starting to get more blow by on the rings than I care to tolerate. I'm upgrading my heads to better flowing ones along with my intake and exhaust(mid length headers to full length because they produce better power down low) but pistons will still be cast or hyper and I will use zero gap rings to further reduce contaminants in the bottom end getting into the oil. Yes most engine shows we all love to watch are dropping 1000.00 sets of forged pistons in like its nothing and flogging them on the dyno saying look at how much power I made at 6k rpm etc but this DOED NOT reflect the average guy who has a hot rod that likes to drive his car or suv on a daily basis. The true cost of doing a total rebuild is getting more and more expensive if you don't want imported cheep parts since our money is worth less and less every year, I personally like to drive my vehicles.
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I love that you're diving into these deeply technical topics. I'd love to see more on different car components and what their advantages/disadvantages are for people considering them. A lot of people "over build" their car for what they're really doing with it, and videos like this might help them save money.
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I really enjoyed your enthusiastic explanation, humor with your “Accent” on accuracy. As a retired ASE Master Tech who worked on factory turbo charged cars in 1980, then Grand National in 85, I used your information to reinforce my knowledge. Liked & subscribed!
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Perfectly explained. Well done. Now just get 7.5 billion people to watch it. I'll bet I spend an hour every day explaining the whys and wherefores of hypereutectic and forged pistons.
