Scat Explains Why Your Engine Might Not Need Forged Parts

As I pulled my ’67 Valiant out of the garage this past weekend, I was immediately informed by my wife that I really need to do something about the extensive smoking issue. I know it smokes, but to be honest, the engine has earned the right to use a little oil. The 360 small-block was built way back in 2005, and to say it has been beaten to within an inch of its life is an understatement. It’s made countless blasts down the strip — some with a healthy dose of nitrous — and an untold number of high-RPM burnouts. It’s definitely ready to take a much-needed break.

To remedy the situation, I have decided it’s time to build another engine, and this time, make it a little bigger than the original. To do this, I decided to start by using one of the Mopar 340 Resto blocks I have sitting in the garage. This block features a beefier lifter valley area, thicker cylinder bores, and a stronger bottom end with four-bolt mains.

When built, I plan to have a small-block displacing 416 cubic inches, run a cylinder squeeze in the area of 11.5:1, and still be able to drive using Premium pump gas. As far as horsepower, I’m not one to make guesses, so we’ll all have to wait and see.

Scat

After speaking with Tom Lieb of Scat, I understood that the choice of crankshaft material relies less on horsepower numbers and more on how the engine will be used.

As previously mentioned, one thing I will be doing this time around is making the engine bigger. To do that, I am adding a longer stroke. This is a great way to increase torque and power, and is almost a requisite with many enthusiasts when building a new mill. To accomplish this, I will be using Scat Crankshaft’s PN: 1-980112BI. This rotating assembly uses Scat’s 9000-series cast crankshaft with a 4.00-inch stroke, 6.123-inch 4340-alloy I-beam connecting rods, and forged flat top pistons with two valve reliefs. With this assembly, the compression ratio is 12.3:1 when using a 62cc cylinder head, 11.5:1 with a 68cc head, and 11.2:1 with a 70cc head. Depending on which heads I use, I will be adjusting the compression ratio with the help of MLS head gaskets.

Yes, A Cast Crankshaft Is Sufficient

As you look at the part number of the rotating assembly I intend to use, I know what you’re thinking, “why use a cast crankshaft”? For starters, it’s the one the guys at Scat recommend for this application. Secondly, the Scat 9000-series cast-steel material delivers a tensile strength of 105,000 psi. This is a fairly stout number for a cast crankshaft, especially when you figure a seasoned, factory-forged crank typically has a tensile strength in the 110,000-psi range. However, since you asked a very good question, I reached out to Tom Lieb of Scat to get a solid answer.

“We offer complete rotating assemblies based on the user’s expectations,” says Tom Lieb of Scat. “We begin by asking the user what the intended usage will be for the engine (racing or street), and what horsepower they would like to make. Based on that information, we can safely recommend the right kit with either a cast, forged, or billet crankshaft. At the end of the day, we want to make sure the consumer has the kit that will do what he or she wants it to do.”

If I understand him correctly, the choice of crankshaft material relies less on horsepower numbers and more on how the engine will be used. Since the engine I am building will see very limited track use and the tach might only tickle 6,000 rpm once in a while, Scat is confident its 9000-series cast crankshaft should live a long and happy life.

At the end of the day, we want to make sure the consumer has the kit that will do what he or she wants it to do. – Tom Lieb, Scat Crankshafts

Beam Support

Moving on to the connecting rods, if we take a look at a traditional small block’s production-focused I-beam connecting rod, the construction is typically made of 1045-grade steel. This alloy offers what metallurgists call an ultimate tensile strength (UTS) of 82,700 psi. The UTS is defined as the material’s ability to withstand a load attempting to pull it apart. The Scat Pro Series I-beam connecting rods are forged from a 4340 chrome-alloy carbon steel that is generally rated to around 145,000 psi. In other words, it is roughly 75-percent stronger than an OE rod. Scat’s H-beam connecting rods are even stronger. So, I had to ask, when is an I-beam a good choice and when is an H-beam the necessary choice?

The Pro-Series rod features a reinforced big end, the beam itself is machined to remove any stress risers from casting, bushed wrist pin area, and ARP bolts.

“Again, before we can make a recommendation, we need to know the engine’s intended usage,” affirms Lieb. “In the case of the 416 small-block Mopar you are building, the Pro I-beam is a great choice. This rod features a reinforced big end, and the beam itself is machined to remove any stress risers from the casting. The benefit to having a machined or polished beam is that you remove the source of potential stress fractures, reducing the risk a crack will develop. If you were building an engine to go racing or adding forced induction or nitrous, then an H-beam would be the better choice.”

Scat

Scat’s rotating assemblies have the option of arriving on your doorstep completely balanced and ready to go. Since the kit comes with everything in the rotating assembly, including bearings, the team at Scat will balance all the components in-house before shipping.

Scat Keeps You Balanced

When Mopar built its engines, some were internally balanced while others were externally balanced. For many, this causes some confusion when buying parts. Since I am building a complete engine, I actually have the option to choose either type of balance.

In case you were wondering, engines that are externally balanced are typically done that way because of clearance issues within the block. For instance, in the case of the Mopar 360 small block, the rotating assembly could only achieve balance with counterweights so large they contact the block. In order to balance the rotating assembly, counterweights are made small enough to allow clearance while rotating, and the flywheel/flexplate and damper/balancer must have added weight to accurately achieve the needed balance.

Since we are building an internally balanced engine, certain parameters need to exist in order to make this happen. For starters, the pistons and connecting rods we’re using are lighter than OE. This allows the crankshaft counterweights to be smaller and still have the ability to handle the job of offsetting the reciprocating mass of the pistons and connecting rods while not contacting the block.

Solid Slugs

Automotive pistons are an extraordinary thing. Within a matter of seconds, these metallurgical marvels realize temperatures reaching 1,000 degrees, only to be immediately subjected to a blast of “cold” air with every intake stroke. In an instant, they can be asked to reach speeds of around 7,000 rpm and withstand side loads that try to push each one through the cylinder wall. To say a piston is abused is an understatement. When one of them fails, we are quick to blame the piston, but maybe, just maybe, it was not the piston’s fault. Maybe it was asked to do something it was not designed to successfully accomplish.

For instance, if you place a cast piston in a situation where nitrous or forced induction will be part of the package, I guarantee you will eventually see small pieces of it exiting the engine at some point. Before you choose incorrectly, you need to know just what type of piston is going to be a good choice for your engine.

…before we can make a recommendation, we need to know the engine’s intended usage. Tom Lieb, Scat

When it comes to choosing pistons, let’s start with what I’ll refer to as low-cost replacement pistons. These cast pistons are considered to be an OE replacement. If you plan to build a performance-based engine, most piston manufacturers will not recommend these pistons. If you plan to build a low-RPM, daily-driver type of engine, these might be a good fit.

If you want something a little better, but not quite as expensive as a forged piston, the hypereutectic piston is a great option. This is an option many consider suitable for mild-performance usage. While still technically a cast piston, its alloy makeup gives it more strength. While cast-piston alloys use silicon to increase wear resistance, durability, and thermal characteristics of aluminum, there is a limit to how much silicon can be utilized. When the alloy gets to this “eutectic” point, no more silicon is used.

Scat

What’s the difference between a cast and a forged piston? The difference is in the manufacturing process. With a cast piston, you basically melt down aluminum alloy and then pour it into a mold and quickly cool it. A cast piston is more affordable and weighs less. By looking at the underside of this cast piston, you can see the parting lines where the mold separates after the aluminum cools.

Hypereutectic means the forging process has forced extra silicon into the alloy and it exceeds the normal amount that can be simply blended into the metal. Although hypereutectic pistons are technically still a cast piston, the extra silicon affords it the ability to deliver a few benefits over conventional cast pistons. First, the piston is inherently stronger, with increased thermal characteristics, lubricity, and scuff resistance. It is also more resistant to corrosion, has more controlled expansion characteristics, and has better high-temperature strength.

A hypereutectic piston might be stronger than a traditional cast piece, but if pushed beyond its survivable limit, it will break, much like a conventional cast piston. Cast and hypereutectic pistons are ideal for engines that witness a lot of street use with very occasional weekend dragstrip visits. If adding a power adder like nitrous, turbo, or supercharger, you will want to avoid using a cast or hypereutectic piston.

Scat

Forged pistons are a serious upgrade to cast and hypereutectic pistons. This is because, instead of being melted and poured into a mold, blanks for forged pistons are created from a billet of extruded aluminum alloy, which is then formed into a rough shape under significant pressure. The forging process results in a denser, more ductile piston, with a better grain structure. This denser, more ductile material means that a forged piston is inherently stronger and more forgiving when its limits are exceeded. There are many different alloys used in forged pistons and you can read about those options by clicking here.

scat

Forged pistons are created from a compressed, and then machined, piece of aluminum rather than by aluminum being melted and poured into a mold. The raw billet is compressed in heavy forging dies that create the rough shape of the piston.

The forged pistons in the Scat kit I am using are from Icon. The FHR series (Formed Head Relief) line of pistons delivers a high-quality piston without breaking the bank. FHR pistons utilize valve reliefs that are formed into the piston as the forging blank is pressed, as opposed to being machined in after forging. If your FHR pistons are domed, many of the FHR dome pistons feature a solid dome. This affords engine builders the flexibility to machine the dome to achieve the desired compression ratio.

These Icon FHR pistons utilize an 11-percent silicon-infused 4032 aluminum alloy. One major benefit of the FHR pistons is they can be installed with tighter piston to wall clearance than Icon’s other offerings made from 2618 aluminum. Doing this delivers quieter operation and increased piston stability. In addition, the use of a skirt coating decreases the friction created during engine operation, this results in extended piston life and reduced wear.

Our build is definitely not an OE reproduction, so a cast piston is not in the cards. The Icon pistons that came with the assembly are a 4032 alloy. The use of this aluminum alloy, along with UEM's M42 skirt coating, results in extended piston life and reduced wear.

In many circles, a discussion about engine internals will inevitably lead to a debate about why a forged rotating assembly is needed. While those parts are probably a great choice for any application — overkill for some — Scat has given some solid knowledge to understand that you can have a reliable engine built with less-expensive parts that will be the perfect mill for your ride.

Now you have an idea about what the plan is for our latest engine build and how we’ll be getting there. Check back often as we get started with assembly and we detail the process in a way that any enthusiast can feel comfortable building their next engine in their home garage.

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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