As The Crank Turns

The crankshaft and reciprocating assemblies inside our engines are often taken for granted, and that's not all bad. We assume that all will be well within the walls of our engine block, and since all of these parts are sealed away from sight, they stay out of mind. While the majority of street machiners rely on factory parts balanced by professionals and probably enhanced with an aftermarket balancer, there are those of us who need more.

For this group, aftermarket crankshafts provide the additional durability required in more extreme circumstances, be they obscene torque numbers, shrieking rpm redlines, or insane compression ratios. The need for extreme crankshafts forced crank manufacturers to study their craft intensely, and they learned many things that can benefit the street-bound enthusiast.

The weight of the crankshaft, rods, and pistons is a major factor in the rpm-producing potential of the powerplant. The lighter this reciprocating mass is, the easier it is to move and rev. This frees up rpm potential by lessening the parasitic weight the engine would normally have to overcome. Naturally, this weight increases dramatically with rpm, so the final goal is a lightweight grouping of rods, pistons, and crankshaft that are still durable enough to deal with the rigors of power production within the target range of the engine's design.

Since these parts are grouped as an inevitable assembly, they must be balanced as such. The weight of the connecting rods and the pistons is offset by the counterweights on the crankshaft, and fine racing engines keep this weight within .1 gram. Recent developments in production methods, materials, and machining have opened doors toward providing street enthusiasts with affordable reciprocating assemblies with durability and balance levels normally reserved for racing's elite. We wanted to share some of these fresh ideas with our readers, so we paid a visit to SCAT Enterprises in Redondo Beach, California.

SCAT has recently introduced their line of rotating assemblies to include big-block Chevy and big-block Ford products, and their line of small-block Chevy parts has been expanded to include their new $1,195 "Superlight" crankshaft that weighs in at a mere 44 pounds. Naturally, when SCAT sells a complete reciprocating assembly, it is pre-balanced to SCAT'S high standards, and requires no further balancing. All of this is good news, but we wanted to know more.

The balancing act going on inside your engine is changing constantly, depending upon whether you're accelerating, idling, or decelerating, and whether or not your car is in gear, out of gear, manually-shifted or automatic. Your crankshaft sees different types of stresses in all of these situations, and this is influenced further by the specific equipment under your hood. The type and weight of your harmonic balancer, the amount of accessories added to the front of your crankshaft snout to be driven by its motion, and the rpm range you regularly spend time at all influence the balancing of the crankshaft in given situations.

The crank is designed to be twisted, and is constantly dealing with the stress of twisting from the moment the engine fires. The abundance of load is on the rear of the crankshaft as long as the vehicle is in gear, of course. Once the clutch is let out or the shifter is placed in "Drive," the crankshaft sees twisting force at the rear of the engine as resistance to its movement. Adding various drives (like power steering, air conditioning, or a belt-driven supercharger) to the crank snout also adds load, but to the very front of the crankshaft. These loads on either end cannot help but add to the twisting forces working against the crank, and as rpm and power levels increase, so do these stresses.

The balance of a crankshaft and it's reciprocating assembly do their best to cancel out these harmful stresses, and it stands to reason that a lighter assembly would see less change, since the centrifugal forces acting upon it as it spins are lessened. The lighter spinning mass also carries the benefit of quicker revving and crisper throttle response, since there is less weight to move.

So, we've established the benefits of lighter reciprocating mass, but how can this help a "normal" guy running stock stuff in his Camaro? Well, if that Camaro is powered by a big-block with external weights bolted to his damper and welded to his flywheel to achieve a good balance, maybe we can help. If the Camaro is powered by a small-block that's not based on a 400ci crank, it's internally balanced, and that's better.

Why? For starters, you just read about the stresses placed on the front and rear of a crankshaft and how they add to the twisting force the crankshaft sees. Certainly, these forces are lessened when the weight is used to balance the crank, but the input is still coming from the very front and rear of the crank, and we're better served when that input is centralized. In addition to the non-400 small-block Chevy, big-block Fords (like the 429/460) are also internally balanced. Other popular externally-balanced engines include the small-block Ford (289-302-351W), and others.

The latest GM small-block, the LS1, and its predecessor, the LT1, are using a mix of both ideas. The front of the crankshaft is neutral, but weights are still added to the flywheel or flexplate to provide final balancing. When choosing a crankshaft to build upon, don't make any final decisions or purchases until you're sure about the rest of your reciprocating assembly. If you spend the extra cash for a lightweight crankshaft, then realize that your heavy H-beam rods and thick-deck nitrous pistons will force you to drop an additional $300 to your machinist to balance the assembly, you're not getting it right. Machinists add heavy Mallory metal to the counterweights of a crank to achieve the correct balance. This is what we're trying to avoid, and if you plan to run lightweight goodies ahead of time, you can reap the benefits and save the additional machining costs. A little research is all that's required.

All stroker cranks and serious aftermarket units are internally balanced. The manufacturers have done their homework and can recommend a good rod-and-piston combination that will keep you out of the Mallory metal and be strong enough for your chosen application. You want to get the lightest crankshaft that can support your target horsepower range, and that shouldn't be hard for a street-bound car. For instance, the SCAT "Superlight" crank we eluded to earlier has a 1,700-gram bob weight, and can be teamed with their 4340 forged I-beam rods (that weigh 600-grams each) and SRP Ultralight pistons to easily support 600 hp. This flyweight combination would rev quite freely, and a very lightweight balancer (like those offered by ATI or BHJ) is all that would be required to control harmonics. This is only one example, but if you're looking to step up to a more serious combination, we'd recommend touching base with a crank manufacturer and getting their recommendation based on your overall package and performance goals.