In RET 138 (April 2022) we discussed the use of aluminium alloy con rods for power adder drag racing applications. Steel also has a place in the highest categories of drag racing, especially in naturally aspirated applications.
It would easy to assume that steel con rods would be far heavier than aluminium counterparts but that overlooks the higher strength of the base material. Note that the steel rods used in drag racing are made from high-strength alloys such as 4340 chrome nickel molybdenum, which is quoted as having a tensile strength of 145,000 psi. For comparison, the high-spec aluminium alloys used in rod manufacture have a tensile strength of more than 100,000 psi, and the ultimate tensile strength of mild steel is quoted as being between 58,000 and 79,800 psi.
A pair of forged steel con rods manufactured by Auto Verdi (Images courtesy of Auto Verdi)
Why use steel?
The previous article noted it is possible to increase the mass of an aluminium rod by a factor of three without exceeding the weight of a steel rod. Adding mass resists deflection but it can cause clearance issues.
NHRA Pro Stock is a good example of an application best suited to steel rods. The fact that steel rods can be made much more slender, giving more clearance, is a major benefit.
Service life is a major benefit to steel rods when compared to aluminium. Aluminium rods must be lifed very carefully: Top Fuel rods might do 10 runs or less; supercharged methanol rods may go 20-30; Sportsman applications may go 50-75 runs. By contrast steel rods can have a service life four times longer, and in some cases can also be inspected, resized and rod bolts replaced before being put back into service.
Using steel rods it is not necessary to allow extra deck clearance for expansion in rod length, and big-end and pin bores also have more dimensional stability compared to aluminium rods. This is especially useful in naturally aspirated engines, where every horsepower counts.
NHRA Pro Stock teams were using aluminium rods until the mid-1990s, when steel rods were introduced by an engineer involved in NASCAR racing. Within 5-6 years all the top teams had switched to steel rods.
H-beam versus I-beam
Steel rods are often shaped along their beam, primarily to save weight, as opposed to being ‘solid’ on all four sides like some aluminium rods. The two most common shapes are H-beam and I-beam.
H-beam rods are often heavier than I-beam rods owing to their broad sides, and the wider design can increase crankcase windage. However, they are generally considered to be stronger, so are often used in power adder applications.
An advantage with I-beam rods is that material can be removed from the piston pin end, in continuation with the removed material on the sides. Owing to the lighter design of I-beam rods, they are often used in high-revving naturally aspirated engines.
A special variant of the classic I-beam design. Auto Verdi has removed weight on the pin end and added strengthening ribs to the beam of the rod
A-beam and X-beam rods are variants of the I-beam design. There are many different opinions on which shape best suits different applications. Ultimately, the shaping is concerned with removing mass from the rod, and location of the material removed will dictate the strength characteristics.
Stefan Verdi of Auto Verdi, which makes forged steel rods for applications including Pro Stock, comments, “The choice between H-beam or I-beam is very much application-specific, and is not just a question of whether an engine is naturally aspirated or has a power adder. Internal packaging constraints are also important, but ultimately it is about having material in the correct places.”
Forged rods
Steel rods are much like their aluminium counterparts, in that forged as well as billet designs are common. Forging steel rods are manufactured slightly differently to aluminium ones. Steel rods are drop-forged at 1150 C, as opposed to around 370 C for aluminium.
Many forged steel rod manufacturers use a hammer forging method, three to four blows are used in the ‘pre’ forging process (which uses a primary die that is a rough shape of the rod with large radii), and four to five blows are used in the finishing process (using a secondary die in the finished shape of the rod).
Auto Verdi uses the hammer forging method for small-batch work, but for volume production it has a forging press which uses a large, high-inertia flywheel connected to a crankshaft to apply the pressure. One stroke of the crankshaft presses the hot material into the pre-forging die, and the second stroke presses the part-forged rod into the finishing die (the rod is transferred directly from one die to the other while still hot).
Many forged steel rods are made from 4340 alloy. Auto Verdi uses a base material similar to 4340 ,with a tensile strength of 188,500 psi. However, its strongest rods are made from a type of tool steel.
An advantage with the forging process is the compression and realignment of the grain structure into the rod shape. Not only is the grain direction in line with the direction of highest stress the rod will sustain along its beam, the grain structure is also forced around the big-end bore, creating a hoop stress. This increases strength in this area, where cross-sectional area is low.
A disadvantage with forged rods is the initial production cost, owing to the dies needed. This cost will be spread over the production period until the die is worn out, but it does mean that custom, small-batch rods that do not fit the constraints of an existing die can be more economically produced using the forged billet method.
Billet
Billet steel rods, much like their aluminium counterparts, are machined from a flat stock of part-processed material. Billet steel rods are however machined from forged plate rather than extruded.
The forged plate, much like an aluminium extrusion, has its grain structure flowing in one direction, so the machining process cuts through the grain structure. It can be argued that a billet steel rod is therefore not as strong as a true forged rod, although this trend is shifting, as materials technology now provides alloyed steels with extremely fine grain structures. With finer grain structures within the material, grain flow direction does not have the same importance as with larger grain materials.
The CNC-machined billet rod manufacturing process offers advantages in terms of flexibility. It does not rely on retooling to create rods for new applications, so rod attributes including basic dimensions such as centre-to-centre length, and pin and big-end bores and widths, can be specified to the customer’s requirements. Shape as well as mass reduction in the form of pocketing can also be customised, affecting overall weight and strength.
A completely custom rod can be designed on a CAD system, post-processed in CAM software and machined on the same day, something that is impossible within the constraints of forgings. Billet steel rods are generally H-beam in shape.
Both manufacturing techniques use a similar machining process. The big-end cap is cut off, drilled and serrated, the rod body is serrated, drilled and tapped, then machined into what will become the final product. Once these processes are complete, the rods are fitted with the specified cap screws and then the final big-end honing and pin fitting is carried out.
Post-processing
Auto Verdi uses a heat-treatment and stress-relieving process before machining. The heat treatment is done at 850 C, then the rods are cooled in oil and annealed. Stress relieving process is done at 350 C.
Auto Verdi then uses a shot-peening method to post-process the rods. This uses metallic media, fired at the surface of the rod with enough force to generate plastic deformation. The shots fired create multiple indentations that result in a compressive stressed layer on the outside of the part, relieving surface tension and work-hardening the outer layer of the material.
Shot peening can extend the service life of a component, because the induced compressive stress layer increases resistance to fatigue while also helping to prevent the propagation of cracks through the material. The process is carried out as a finishing operation after machining.
Hardware
High-end steel rods generally all use cap screws rather than studs and nuts. Radial or circular serrations on the rod cap mating surfaces negate the need for dowels and provide a greater location accuracy for the rod cap.
ARP fasteners are the most popular choice for cap screws, often in ARP L19 or ARP 2000 variants. Auto Verdi uses ARP L19s, and Verdi has clear reasoning for this. “I prefer the L19 bolt not only because of the higher strength, but also because it has a polished finish that resists galling,” he says.