COMPACTED GRAPHITE ENGINE BLOCKS
These blocks offer greater strength and stability, but be aware of the machining concerns.
by Mike Mavrigian
CGI engine blocks offer greater strength and superior stability with no weight penalty. In theory, a CGI block can be produced at near-aluminum-weight while maintaining needed strength. One benefit is that cylinder walls can be made thinner without sacrificing stability or strength. (photo courtesy Makino)
World Products is about to introduce a 426 Hemi block cast in CGI material. (photo courtesy World Products)
WHAT IS CGI?
CGI, or Compacted Graphite Iron, features a molecular makeup that creates a tightly-interconnected graphite during the casting process. CGI casts much like gray iron, but offers more hardness, more fatigue strength, superior ductility and greater tensile strength…sort of a happy medium between gray iron and ductile iron. The compacted graphite molecular shape, often referred to as “semi-nodular,” makes it less brittle and more stable than gray iron.
From a molecular standpoint, the basic difference between gray iron and compacted graphite iron lies in the graphite shape. The makeup of the graphite mix in CGI is more tightly compacted and “interwoven” (the molecular structure is similar to a bunch of tiny worms that are interlaced in a tight matrix). Gray iron’s makeup features graphite that’s similar to a mix of odd-shaped, larger and more widely dispersed flakes. Once you realize this difference, it’s not difficult to understand why CGI offers greater strength and ductility.
CGI is up to 75% stronger than gray iron. Although relatively “new” in the automotive field, CGI was actually conceived and patented way back in 1949, but its use was delayed until production manufacturing and machining techniques were developed to handle this stuff. Although not as strong as ductile iron, CGI is 500% more fatigue resistant than aluminum and 200% as resistant to fatigue compared with gray iron. In other words, it’s ideal for racing engine blocks that need to withstand tremendous pressures and thermal levels. Also, because of its increased strength, a block can be machined to reduce weight by as much as 22% compared with gray iron, if the builder so chooses.
By the way, in terms of producing the block castings, CGI is only slightly higher in cost per pound than conventional gray iron, with very similar casting techniques, so raw CGI blocks shouldn’t carry a big cost penalty. When you compare the cost difference (in base material) between aluminum and CGI, compacted graphite iron becomes even more enticing when you consider that CGI is more fatigue resistant at extreme temperatures.
In slang terms, CGI is often referred to as the “titanium” of cast iron because of its additional strength and rigidity without the detriment of added weight.
CGI blocks offer greater stability and strength, which is of particular benefit in engines that produce high cylinder pressures. CGI blocks are very popular in Pro Stock and NASCAR Cup applications. The blocks are more stable and offer much lower wear characteristics. Several OEMs are using CGI for their blocks. For example, Audi V8 diesel blocks are cast from CGI, and since the material has such excellent strength characteristics, the blocks are actually cast in place as part of the block, then laser-etched and fractured off to achieve a perfect cap-to-block fit (the same approach used in powder metal rods with fractured caps). Other OEMs (to name but a few) currently using CGI for their engine blocks include BMW, Jaguar, Rolls-Royce, GM/Opel, Suzuki and Aston Martin.
The CGI 5.8L blocks used in Toyota’s NASCAR truck program, as another example, weigh a mere 195 lbs. and feature 3mm-thick cylinder walls.
In theory, a CGI block can be final-produced to a lighter overall weight than an equal-sized aluminum block with equal strength/density (although it’s suspected that not many builders take advantage of this for fear of giving up the rigidity factor).
COMPARATIVE MECHANICAL PROPERTIES
To gain an understanding of the differences in attributes between gray cast iron, CGI and ductile iron, the list shown here (for illustration purposes) assigns CGI a 100% value. The percentages shown for gray iron and ductile iron enable us to better understand how CGI compares to the other two types of iron. For instance, when looking at fatigue strength, gray is only 55% as strong as CGI, while ductile iron is stronger than CGI. Gray iron is only 86% as hard as CGI, while ductile iron is slightly harder than CGI.
Engine blocks made of compacted graphite iron present their own unique challenges in terms of machining. In a nutshell, you need to run at half of your normal cutting speed and half of your normal feed rate. Also, CGI is both abrasive and “gummy” and requires the use of CBN tooling.
According to Sunnen’s engineering whiz Tim Meara, CGI blocks tend to hone more like steel than gray iron. It’s a much tougher material. Tim noted that CBN tooling was actually developed to handle CGI. If traditional stones are used for honing, a fairly course 150-grit stone is needed, yet soft to allow free-cutting because the material is so much harder than gray iron. With abrasive stones, you’ll see a smoother finish (usually 3 to 5 points smoother) than you would with gray iron. If you use diamond tooling for honing, you’ll notice very little difference in the final finish. CBN inserts must be used for boring and decking procedures.
If tooling and tool operation technique is correct, CGI machines extremely well. Since the compacted graphite iron makeup is not as brittle as that of gray cast iron (tight matrix instead of big molecular flakes), the material doesn’t fracture and tear as tooling begins its attack and as pressure is added (compared with gray iron). This means a more consistent machining operation and, in theory, more consistent and more tightly held results in terms of tolerance. In this regard, CGI machines more closely to the characteristics of steel.
However, CGI material is very abrasive, so if you’re drilling or tapping, it will wear your drills and taps faster (although drills are being developed specifically for CGI to reduce tool wear).
As we keep mentioning, CGI is a very hard material, so in general, all speeds and feed rates will essentially be cut in half, compared with the speeds and feeds normally used with gray iron.
World Products’ Skip Gladstone noted that CGI machining requires more of a tooling positive rake, compared with a negative rake on gray iron or nodular iron. Skip noted that World Products plans to introduce a CGI 426 Hemi block in the near future (possibly in late 2007). Several aftermarket block manufacturers are using or plan to use CGI castings, so you really should get together with your machine equipment suppliers to obtain proper tooling and procedural advice before you begin machining these blocks.
CGI is said to grind, cut and hone very well, but features a much greater amount of graphite content in the cutting swarf. Because of the reduced cutting time, CGI can take 2-3 times longer to machine compared with gray iron.
Basically, CGI is very speed sensitive. If you run your cutters too fast, you’ll wear out your tooling much faster.
Microscopic view of CGI. The tighter intertwined “worm” makeup features a closer-compacted grain structure which provides increased hardness, yet increased ductility.
(photo courtesy Atlas Foundry)
Ductile iron. Tighter matrix with random nodules. CGI is tougher than gray cast, but not as tough as ductile iron. CGI is a great medium between the two extremes.
(photo courtesy Atlas Foundry)
This is a super-closeup of a micro view comparing gray iron (left) and CGI (right). The CGI matrix features more “rounded” elements that are compacted closer together and wrapped together, as opposed to the large wafer/flakes in gray iron.
(courtesy Brazilian Society of Mechanical Sciences)
CBN cutters are definitely required for any boring, milling or cutting operations on a CGI block. (photo courtesy Makino)
Many OEMs are now using, or are planning to use, CGI for their engine blocks in order to reduce weight, maintain needed strength, and to save cost (compared with using aluminum). (photo courtesy Makino)
Here’s a CGI V8 block for BMW’s 7-series. (photo courtesy Makino)
Since CGI is stronger and more stable than gray iron, a CGI block can be bored to greater displacement oversize while maintaining the strength to get the job done. Want bigger bores without adding more block weight? CGI is one way to go. (photo courtesy World Products)
Block boring with newly-developed helical boring tool from Makino. (photo courtesy Makino)
Block boring with CBN cutters.
(photo courtesy Makino)