Connecting Rods: So Many Choices
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Aluminum, Steel or ???
Aluminum is a good material for connecting rods because of its lightweight. Reducing the weight of the rods reduces the mass of the rotating and reciprocating parts and allows the engine to rev faster and rev higher. In addition to good throttle response, aluminum’s lighter weight can reduce vibration and stress on the crankshaft. Lighter rods also allow the use of heavier, stronger pistons.
Almost all Top Fuel dragsters and funny cars use aluminum rods in their motors. So do many ProStock racers. But aluminum rods can have a limited service life depending on how they are used. The rods can stretch, and may fatigue and fail after they’ve been subjected to one too many runs down the strip. Top fuel race teams tear down their engines between every race, and typically replace their rods after 8 to 10 runs. ProStock racers may replace the rods after 20 or 30 runs. In the lower drag racing classes, a set of aluminum rods may last 100 to 200 runs or longer. It all depends on the load, the rpm, and the quality of the rods used.
But the average racer may not be able to afford a new set of rods so often, even if they cost less than steel rods. Many racers want their rods to last as long as possible, so for this type of customer steel rods are probably the most economical choice. A set of quality steel rods will cost more than a set of aluminum rods up front (say $900 to $1,600 depending on the brand and quality, though some sell for as little as $500 to $600 a set). But the longer life of the steel rods will more than offset the cost difference over the long run. Because of this, some ProStock racers who have been using aluminum rods have switched back to steel rods.
Aluminum rods are not used much in circle track engines because of rule restrictions, although light rods are a plus in engines like sprint cars that are constantly on and off the throttle.
On the street, old myths about aluminum rods are slow to die. Some say aluminum rods won’t last and must be replaced after 15,000 to 20,000 miles if they are used on the street. However, makers of aluminum rods say a set of high grade forged aluminum rods can last upwards of 100,000 miles in a street application. It all gets back to cost and weight. If a customer wants throttle response, or has a high revving engine, light rods of either aluminum or steel would be a good choice. But for a high torque, high load motor with a limited rpm range, steel rods would probably be better.
Titanium rods are another option for racers who want an extra edge. Titanium is both lightweight and strong. A titanium rod weighs about 22 to 24 percent less than a steel rod of comparable strength, and has approximately the same durability. Titanium rods are a good choice for applications that need quick throttle response like sprint cars, road racers and also drag racers. But due to a surge in the worldwide demand for titanium, the price of the metal has skyrocketed beyond the reach of many racers. One supplier of custom-made titanium rods said titanium rods typically cost $425 to $450 each, or about two to two and a half times as much as a set of quality steel rods.
Powder metal rods are used as original equipment in many late model engines because they can be manufactured at less cost than steel rods. Metal powder is pressed into a mold and heated to high temperature to melt the powder into a solid mass (a process called sintering). The result is a near-perfect rod that requires minimal machining to finish. The caps are usually cracked, which saves time and additional machining. Powder metal aftermarket rods are also available for certain engines, and are a good upgrade over stock rods. But for high horsepower or high rpm applications, steel, aluminum or titanium rods are usually preferred.
Most high dollar steel performance rods are often shot peened and/or cryogenically treated (frozen to minus 300 degrees F in a nitrogen bath) to alter the metallurgy of the grain structure, and to relieve internal stresses for better durability. Magnetic particle inspecting and/or sonic testing are additional steps that many rod manufacturers perform to ensure their rods are perfect before they go in the box. Quality manufacturers will also match rod weights (big end and small end) as closely as possible to make engine balancing easier.
Some rod manufacturers analyze their rod designs with Finite Element Analysis (FEA) software that shows where the rods are weakest and strongest when they are under a simulated load. This allows critical areas to be beefed up for added strength, and metal to be removed from less critical areas to reduce overall weight. CNC machining allows for precise metal removal. Thus, a well-designed rod can be both stronger and lighter than the stock rod it replaces.
With or without pin bushings is another decision to consider. A growing number of performance engine builders are opting to use rods that do not have bushings for the wrist pins. This leaves more metal around the wrist pin for added high rpm strength because the small end of the rod does not have to be drilled out to accept a bushing. To make this work, however, the pin hole must be highly polished to a smooth finish, and the wrist pin must be coated with Casadium or a similar hard coating to prevent it from galling. One of the tradeoffs of going this route is that it makes rebuilding the rod harder if the engine chews up a pin. There’s no bushing to replace so the small end has to be drilled out to accept a larger diameter pin, which means replacing the pistons, or drilling out the pin holes in the pistons to accept a larger (a probably heavier) pin.
Smaller and smaller diameter pins also seem to be more popular as a weight saving trick. One rod manufacturer said they are now making custom rods for pins as small as 0.787˝ in diameter.
Some rod manufacturers offer rods with a special oil-shedding coating to reduce windage at high rpm. Others say their rods don’t really need any coatings. But if you want some type of special coating, there are plenty of people who can apply almost anything you want.
Rod ratio is the length of a connecting rod (center to center) divided by the stroke of the crankshaft. The range in engines today may be from 1.5 to 2.1, but most performance engine builders are going with ratios in the 1.57 to 1.67 range. Some say that going with a rod ratio over 1.7 makes engine torque too “peaky.” Lower rod ratio numbers are typically associated with lower rpm torque motors (a 383 Chevy street motor with a stroker crank and a rod ratio of 1.52, for example), while higher rod ratio numbers tend to be high revving high horsepower motors (a 302 high revving Chevy with a rod ratio of 1.9).
Racers have long believed that longer rods provide better crankshaft geometry and allow the piston to dwell longer at top dead center on the compression stroke. This causes pressure to build a little longer in the combustion chamber before the piston starts to move down on its power stroke. The result is a little more power squeezed out of the air/fuel mixture, and a slightly flatter and broader torque curve.
But this thinking is changing. Breathing also contributes to how much power an engine makes. A longer rod that causes the piston to sit a few degrees longer at TDC on the compression stroke also does the same thing on the exhaust stroke and that may actually cost you some power.
The longer the piston sits at TDC on the exhaust stroke, the longer it takes to start moving down on the intake stroke to pull air and fuel into the combustion chamber. There are a lot of factors involved here, including the size and shape of the intake and exhaust valves and ports (which affect air velocity), how much overlap there is in the valve timing between the closing of the exhaust valve and the opening of the intake valve (which affects scavenging), and the design of the combustion chamber and the top of the piston (which affect airflow dynamics).
Top engine builders are always experimenting to find the ultimate combination that produces the most power and torque in the rpm range where they want it. There is no pat formula for rod ratios that work in every engine or every application. Many of today’s aftermarket performance heads flow tremendous amounts of air, so finding the right rod ratio for a given engine/head/cam combination is a trial-and-error process that separates the winners from the also-rans.
Off-The-Shelf or Custom-Made?
Many rod suppliers offer standard rod lengths for the most common engine applications, as well as rods in various incremental lengths (say 5.4˝ to 6.7˝) that are within the range of their forgings. Six-inch rods are one of the more popular lengths these days for small block Chevys, and 6.385˝, 6.535˝ and 6.700˝ for big block Chevys. These off-the-shelf rods are typically aimed at the engine builder who is not building a one-of-a-kind motor but a somewhat standardized performance motor. Off-the-shelf rods also tend to be competitively priced since many suppliers offer them.
Custom-made rods, on the other hand, are for engine builders who are doing something unique, different or special that requires one-of-a-kind parts made to their exact specifications. These rods are usually CNC machined from billet stock and cost a lot more than standard size rods because of the added time and effort it takes to produce them.
Most rod suppliers say they can usually make up a set of custom rods in a week or two, sometimes in a day or two if the rods are for a good customer. But most don’t want to do custom rods unless you order a minimum of several sets.
One application that is hot right now for custom rod manufacturers is diesels. Several rod manufacturers we interviewed said they can hardly keep up with the demand for custom rods for GM Duramax, Ford Powerstroke and Dodge Cummins diesel engines. Some of these rods can be rather pricey, costing up to $3,000 per set! But if that’s what it takes to handle the power, your customer will have to bite the bullet and come up with the cash.
One other very important thing to consider when choosing performance rods is the type of bolts that hold the cap in place. Standard bolts can stretch and fail at high rpms, so stronger is always better. Rod bolts of 8740 chrome-moly steel have a rated tensile strength of 200,000 psi. But many high rpm, high power racing engines today need ARP2000 bolts, or L19 or A625 alloy high strength bolts.
Watch out for counterfeit bolts that do not meet strength specifications due to low quality alloys or improper heat-treating. Unfortunately, it’s almost impossible to spot a counterfeit by appearance alone. You know you’ve got a bad one when it pulls apart and breaks.
Something else to watch out for is over stretching the rod bolts during engine assembly. When new rod bolts are installed and tightened down to fit bearings, using no lubricant or the wrong lubricant on the threads may damage the bolts. The next time the bolt is tightened down, the torque reading won’t be accurate and you may stretch it too far (which is a good reason to use a stretch gauge to measure rod bolt length).
So that’s all there is to it. Choosing the right connecting rods is easy once you know what you want, what works in a particular engine and what your customer can afford.
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