For as long as connecting rod manufacturers the world over have been using blends of aluminum to create lightweight, high performance rods, said pieces have lived under constant scrutiny regarding their long-term durability and usefulness. Even today, as technology has advanced light years beyond what the early aluminum rod manufacturers believed was possible, it continues. And while these concerns may have been warranted decades ago, those in the industry view them as nothing more than common myths these days. And they are myths they’d like to debunk, once and for all.
In order to zero in on the tall tales surrounding any discussion of aluminum rods and to help set the story straight, we placed a call to our good friend Brian Scollon at GRP Connecting Rods – one of the most respected and successful manufacturers of aluminum rods for the racing and high performance industry. Considering their experience in everything from Top Fuel Dragsters to muscle cars, suffice it to say, they know a thing or two about what an aluminum rod does and doesn’t do.
Over the years, only a couple of common myths regarding aluminum rods have stood the test of time, and while a “couple” may not seem like that big a deal of a deal, they strike right to the very core of the usefulness of aluminum for construction of such vital components.
Aluminum Rods Stretch, You Say?
From web forums to print magazine articles, virtually anywhere that aluminum rods are discussed, you’ll surely find the topic of rod stretch somewhere in the conversation. Or to be more specific, permanent stretching of the rods. It is the most commonly discussed “con” of choosing aluminum connecting rods for an engine, and according to GRP’s Brian Scollon, is also purely false in this day and age.
“Everyone out there is under the impression that aluminum rods permanently stretch, but this simply is not the case,” said Scollon. “If anything, we see that they actually compress. If there’s something going on, be it a hydraulic situation or just simply not enough rod for the power level, they can compress.” In his 17 years entrenched in the connecting rod business at GRP, Scollon adamantly states that not once has he seen an aluminum rods exhibit signs of permanent stretching.
The amount of time that you can get out of a rod is completely application-dependent, and people tend to group them all into one.
As every engine builder out there is aware, all connecting rods, regardless of their material or construction, stretch under higher RPM use, and aluminum is certainly no different. Tolerances for such stretch are accounted for and combustion chamber measurements are planned out accordingly depending not only on the material, but the process (billet, forged, cast). As a general rule of thumb, aluminum rods tend to stretch less than .010″ more than a steel rod. Thus, if you’re setting the deck of a Big Block for .050″ total piston to head, you’ll want to provide clearance for .060″ when utilizing aluminum rods. GRP actually tests such occurrences in-house by dropping their rods in boiling oil and measuring the growth.
This expansion that aluminum rods endure is attributed to the inherent thermal expansion of aluminum as a material more so than high RPM movement. “We’ve been hearing it for years, and have just never understood where that myth is coming from,” explains Scollon. “Some say that back in the early days with the old rods and some of the other original aluminum rods use to stretch, but they don’t do that anymore.”
Another common topic that many folks commonly misstate or underestimate is the overall life of aluminum connecting rods. Sure, on average, aluminum rods tend to carry a slightly shorter life expectancy than steel and other materials, but many out there would have you believe that you’ll be chucking a set of rods in your bracket racing engine every few passes like a nitro car. Scollon insists that this too is simply a misconception.
“The amount of time that you can get out of a rod is completely application-dependent, and people tend to group them all into one,” says Scollon. “A guy will call us to order rods for a bracket car and someone has told him he can only run an aluminum rod 25 passes before it needs to be replaced. That may be the case in a nitro or blown alcohol car, but in something milder, you can get hundreds and hundreds of passes out of them.”
What it essentially comes down to (and such is the case with all sorts of components), is selecting a rod that’s built for the combination. Even steel rods, while certainly carrying a longer life expectancy than aluminum, won’t last long if a proper part for the horsepower isn’t chosen.
In nitro engines, the compressive load placed upon the rods on the top end of the track is extreme to say the least, limiting their use to 8-10 passes at best. Blown alcohol and Pro Stock engines meanwhile, put a different form of extreme stress on the rods thanks to rotations at or above 10,000 RPM. For these racers, however, the performance benefits outweigh the cost and maintenance.
But for the weekend warrior bracket racers out there who would be most concerned about connecting rod life, Scollon explains that it’s nothing for a customer to put several hundred to a thousand-plus passes on a set.
Flexing Their Muscles
Think steel rods are stronger than aluminum? Aluminum rod manufacturers would beg to differ. “The topic of overall strength is also very application-dependent, but can we build an aluminum rod stronger than a stock steel rod? Absolutely,” Scollon exclaims.
Scollon continued, “If you take a look at the horsepower levels of Top Fuel, Pro Modified and anything of that nature that’s very high horsepower, it’s going to have aluminum rods in it. So it almost seems to be the other way around; that aluminum can take more than steel.” Calling aluminum stronger than steel, however, isn’t necessarily a fair assessment. As Scollon explains, aluminum isn’t stronger per se, but it endures the application better by acting as a shock absorber in powerful engines that need it. So you may be asking at this point how aluminum could be stronger in some cases, yet steel lasts considerably longer.
“If the power level of the application is actually taking the rod past it’s “fatigue life” or the power limit or RPM limit of the rod, it’s going to reduce it’s life, regardless of the material.”
If we step away from high horsepower combinations where aluminum is the de facto choice and focus on middle-of-the-road bracket racing engines, GRP and others in the industry believe aluminum has every bit the shelf life that steel does. But is aluminum a great choice for everything? “Absolutely not,” states Scollon.
It’s the aforementioned lighter rotating weight and shock absorbing tendency of aluminum that makes these rods a no-brainer for high horsepower, high RPM, boosted applications and the like. But in street engines, which are classified as low load applications where one might wish to get 100,000 miles or more from their engine, aluminum rods are considered an unnecessary risk of eventual failure over the course of time. Ever the salesman of their products, many aluminum rod manufacturers themselves will steer street car-inquiring customers in the direction of steel, even if it means losing a sale.
And so the debate rages on concerning the use of aluminum rods in a street engine. Scour the ‘net and you’ll find plenty of examples of mild street engines containing aluminum connecting rods with thousands upon thousands of hard, stop-and-go miles on them without fault, only further fueling the never-ending debate amongst enthusiasts and engine builders.
But for the vast majority of combinations specifically intended for racing use, and despite all of the tall tales and myths that you’ve heard, aluminum rods get the job done, and it’s our hope that this piece will in some way help further educate those sitting on the fence in their selection of steel or aluminum connecting rods in their race engine to make the proper decision for their needs and pocketbook.