The differences between 8620 and 5160 steel core camshafts and why it’s so important

Words: Jeff Smith; Photos: COMP Cams, Jeff Smith

Bill Jenkins wrote a book in 1976 with Larry Schrieb, entitled The Chevrolet Racing Engine. This soft-cover book became the bible for thousands of enthusiasts who wanted to learn about building a high-horsepower drag race small-block Chevy.

Much of the information from that book is still useful today. Among the thousands of bits of information was a reference to 8620 steel core camshafts. This was nothing new in the cam industry, but few casual enthusiasts knew about this high-performance cam core.

For some, little has changed in nearly 40 years and there are many engine builders who claim that the 8620 steel core camshaft is the only way to build a race-oriented or even a hot street-performance camshaft.

Lately there have been many discussions on forums and message boards about the differences between 8620 steel “gold core” cams and “black core” 5160 steel billet cams. Much of the opinions offered in these forums contain barely a sliver of actual fact.

This story weaves a tale that involves a little bit of metallurgy, a touch of heat-treat technology, and an approach to building a quality camshaft core that will do the job without a lot of drama, and might even save you some money.

In talking with COMP’s Scooter Brothers, he explained that for multiple decades, most mechanical roller cams were built on 8620 cores and worked relatively well. Cam failures that did occur were often traced to issues with cam spalling, which is damage to the surface of the lobes.

Scooter says that about 30 years ago COMP commissioned an investigation to evaluate a better way to build a high-performance steel billet camshaft. The study employed metallurgists, heat-treating experts, engineers, and other sources who eventually revealed these cam problems were traced to issues with the depth of the heat-treat.

The issue with making a camshaft core is to build it with a surface finish hard enough to resist wear, yet retain ductility (the ability of the steel to bend rather than break), and also be able to be machined easily. These differing requirements are often at odds with each other.

For a long time, 8620 alloy steel billet camshaft cores met these tasks most of the time. But as valvetrain inertia loads have increased as engine speeds and power levels escalate, more problems with the 8620 core have surfaced. Nearly all of these problems can be related to the heat-treat process.

Another part of this investigation revealed that there was a better way to create a more consistent heat-treat. Specialty, high chromium alloy steels have become the answer for high-stress engine applications like NASCAR and sports car racing, but these camshafts are also expensive, often between $2,000 and $3,500 apiece. For Sportsman racers, these solutions are just not economically practical.

The problem COMP and other companies have encountered with 8620 steel cams concerned inconsistent depth of the surface hardness created by the carburizing heat-treat process.

Before hardening, a semi-finished cam is created with unground lobes (UGL). This UGL cam is then put through a heat-treat procedure. In carburizing, the entire 8620 cam is coated with copper and then the copper is removed from the lobes and the distributor drive gear. This leaves the main body and the journals of the cam coated with copper to protect them from the heat-treat process.

The cam is then placed in a large, sealed furnace and specific carbon-infused gases are introduced into the furnace. The heat opens up the iron-lattice structure of the steel, allowing the carbon from the gas to move through the structure. This creates a surface layer approaching 0.8-percent carbon that slowly diminishes down to 0.2 percent at the maximum depth. Then the entire cam is quenched and tempered.

Scooter says that in the best-case scenario, this heat-treat process extends the hardened case to a depth of 0.100-inch below the surface of each lobe. The cam is then finished ground to the required specs. Keep in mind that this finish grinding removes a certain amount of the hardened surface thickness.

While it may seem like increasing the time the cam is exposed to heat in the furnace would increase the depth, this attempt usually suffers from diminishing returns, because the carbon closer to the surface fills all the gaps in the lattice that were being used as flow paths to diffuse carbon.

The one result that often does occur is an increased risk of reducing the cam core’s ductility. The biggest issue with carburizing is that the process is inconsistent in creating this 0.080-inch hardened depth.

Scooter likes to use the analogy of building a road. If your goal is a hard, durable concrete road surface that will accommodate years of heavy vehicle traffic, you want a stable roadbed and a thick surface. You start by creating a solid sub-surface and then pour a consistent 12-inch concrete surface.

Now all kinds of vehicles can drive on it without suffering from cracking or potholes. But if that road surface is 12-inches thick in one area but only 6-inches thick in others, the thin areas will quickly crack and fail. Now substitute a camshaft lobe for that road surface and you should be able to see how important a thick, strong, consistent lobe surface is to create a durable cam lobe.

As mentioned, carburizing does not always create a consistent case hardening depth. The areas with thinner hardened depths will be where the failures occur, just like thin areas on a concrete roadway. In order to build a quality cam product, these inconsistent heat-treatment depths were unacceptable, creating the need to come up with a new way to make cams. The solution was to change to a different cam core material that could be heat-treated with a process called induction hardening.

Induction hardening uses high-current electric coils placed around the lobes of the cam to selectively heat-treat the lobes separately from the cam core, the journals, and even the distributor drive gear.

After much experimentation, COMP learned that this induction-hardening process could be applied to 5160 alloy steel with results that consistently produce a consistent hardness depth of a minimum of 0.140-inch — 40 percent deeper than what could best be expected from an 8620 core camshaft.
Induction hardening works better with high carbon steels like 5160. This steel uses 0.60-percent carbon, while 8620 is a low-carbon steel with only 0.20-percent carbon.

Another disadvantage with 8620 is that the deeper the thickness of the carburizing, the hardness tends to lose strength where induction hardening is far more consistent. This means the core can be hardened to one level by heating and quenching, which tempers the core to improve both its strength and ductility first, before the lobes are hardened.

The major benefit of this different process is that it allows COMP to create multiple lift, duration, and most importantly, lobe separation angle (LSA) combinations from a single UGL cam core. This reduces the number of cores required. This may not sound like a big deal, but when the company literally has thousands of cores for all the different engines, this small step reduces the cost of the finished cam. It’s a smarter and more efficient way to make camshafts.

Induction hardening creates a surface hardness that is every bit as hard as a carburized 8620 camshaft but benefits from a more consistent hardened case thickness. The Rockwell C scale target hardness for both cams is 58-61RHc.

Where some people have concerns might just have as much to do with how a cam looks as opposed to how it works. An 8620 carburized camshaft will have bright copper bands on the core of the camshaft in between the lobes. COMP’s induction-hardened 5160 cam is different, with a more consistent black case-hardened appearance on the core between the lobes.

This has given rise to concerns of the “black cam” being not as good as the “gold cam.” But now that you know about both processes, you have a much better handle on the differences.

One significant difference is with environmental concerns. The copper on 8620 is best adhered with a chemical process (using sodium cyanide), and that has serious environmental concerns. The carburization process also requires using a great deal of natural gas (for carbon), and that is being frowned on more and more going forward.

COMP still offers 8620 cams for custom or one-off applications, which saves a little cost for small runs. 8620 is almost foolproof on heat-treat, where any induction hardening of camshafts (like 5160) requires significant expertise and several cams have to be sacrificed to ensure the setup is correct.

Once the process is recorded for a particular camshaft (like the majority of catalog cams), the 5160 is comparable in price and is every bit as accurate and more durable than the 8620 version.

A further area of confusion also relates to the 5160 cam’s black appearance. COMP’s mild hydraulic roller cams for older, retro-fit small- and big-block Chevy, Fords, and Mopars (among many others) have for several years been created on what is called an austempered ductile iron (ADI) cam core.

These cores were selected because they offer a low-cost alternative for retro-fit roller cams for these older performance engines. These ADI cams are also induction hardened, giving them a similar appearance to the steel billet 5160 cams. But make no mistake, there is a huge difference in hardness and durability between an ADI cam and a 5160 steel core camshaft.

In a follow-up story, we will dive more deeply into the composition of different steels and give you additional information on some of the more exotic steels now used in contemporary race engines. But at least on an enthusiast level, you now have a much clearer understanding of the alloy steel-hardening process and why it’s so important.

Source

COMP Cams
compcams.com