By David Reher, Reher-Morrison Racing Engines
“I can’t say categorically whether steel or aluminum connecting rods will make an engine perform better.”
Back in the early ’70s, when Buddy Morrison and I were building engines in the back room of Mansfield Auto Supply, choosing connecting rods was simple.
We used stock Chevy small-block rods, replaced the tiny 11/32-inch bolts, and hoped for the best. Later on, when we were building small-block Modified and Comp motors by the dozen, we graduated to aluminum rods. We figured these were real racing engines, and that’s what the fast guys used. Steel rods in racing engines? We thought those were just for roundy-round racers.
Of course, we also had a rotary telephone, a television with vacuum tubes, and we shipped parts by Greyhound bus because FedEx didn’t exist. Fast forward to 2015, and everything has changed – especially connecting rod technology.
Today the choices are nearly unlimited – and at times bewildering for racers. Materials include a variety of advanced steel, aluminum, and titanium alloys, and an alphabet soup of styles – A-beam, H-beam, I-beam, etc. Even production connecting rods have changed dramatically. Today many engines are assembled with powdered metal rods with fractured caps – the caps are literally broken off to create a textured mating surface. I’m thankful that Buddy and I didn’t have to rebuild those at Mansfield Auto Supply!
So what’s the right connecting rod for your racing engine? As always, it depends on the purpose, the power level, and the budget.
For the sake of simplicity, I’ll focus on steel and aluminum rods. We experimented with titanium connecting rods in our Pro Stock engines for a while, but the cost and complexity of titanium were drawbacks. Now Chevy LS1 engines are assembled with titanium rods at the factory, which shows just how radically technology has changed.
Aluminum rods have been the first choice in unrestricted classes for decades, from Top Fuel to the fast sportsman eliminators. Aluminum is lightweight, easily machined, and serves as a shock absorber to cushion loads on the crankshaft. Its downsides are also well documented – aluminum has a shorter fatigue life than steel and requires larger cross-sections to achieve comparable strength. If a customer wants to make 400 runs between engine rebuilds, I’ll steer him away from aluminum rods.
There was a time when I would have laughed at the notion of steel connecting rods in a Pro Stock engine, but the reality is that most Pro motors are now equipped with steel rods. These aren’t your father’s rods – they’re technically sophisticated, artfully machined, and command a premium price.
One of the advantages of steel rods is that the piston-to-head clearance can be minimized, which in turn raises the dynamic compression ratio. The engine builder’s goal is to maximize compression by using the smallest piston-to-head clearance that prevents hard contact between the components. The pistons are most likely to kiss the cylinder heads on the overlap stroke when the intake and exhaust valves are open and cylinder pressure is low. The unloaded rod is stretched to its maximum when the piston reverses direction at Top Dead Center. On the compression stroke, both valves are closed and cylinder pressure pushing against the piston compresses the assembly and inhibits rod stretch.
Aluminum rods typically require .010-inch more clearance than steel rods to accommodate their greater stretch under load and higher expansion rate when heated. This translates to an increase of about 1.5 cc in combustion volume at TDC (depending on cylinder diameter) on the compression stroke, with a corresponding reduction in dynamic compression ratio
Does this make a difference? It’s difficult to compare rods directly, but we did test steel vs. aluminum rods using two high-end, high-rpm engines. In both instances, we didn’t see a significant performance difference on the dyno or on the track. So I can’t say categorically that one or the other will make an engine perform better.
There are some practical considerations when choosing a connecting rod. Steel rods can become prohibitively heavy in large displacement, long-stroke engines. In a big-inch engine, steel rods can weigh more 900 grams, while comparable aluminum rods would be around 600 grams. That’s a significant difference in bobweight. The reality is that an engine with less rotating mass will accelerate faster, which tips the scale in favor of aluminum rods in these applications.
I’ve learned to appreciate the cushioning effects of aluminum rods. Pro Mod engines with nitrous oxide injection experience brutal spikes in cylinder pressure and are prone to detonation. This extreme cylinder pressure hammers the crankshaft and wrist pins unmercifully. Aluminum rods definitely extend crankshaft life and reduce distortion of the wrist pin bores in nitrous-injected engines.
I’ve also learned that wrist pins are not the place to save weight. Back in the Modified days, we thought that lightweight, thin-wall wrist pins were really neat. If they didn’t break, we kept making them thinner and lighter. Now I know that the wrist pins must be rigid to prevent damage to the connecting rod and piston. The wear and fretting in the pin bores that I once thought were caused by inadequate clearance and lubrication are actually due to distortion of the wrist pins. It takes horsepower to mess up rod bushings and pin bores, and that’s wasted power.
The benefits of rigid wrist pins are readily apparent in nitrous-injected Pro Mod motors. We’re now installing massive tool steel wrist pins that don’t distort under high loads, and consequently the rods’ little ends look terrific.
Some racers pine for the good old days. If that means going back to the stock connecting rods we used in the ’70s, count me out.