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The Ultimate Guide to Crankshafts: Stock vs Performance Crankshafts

Engine modifications are done with one goal in mind – raising overall power. With so many parts, there are mild, moderate or extreme changes, that are done either on their own, or in combination to balance out horsepower and torque figures. Pairing parts is crucial as they are mutually dependent. Basic guidelines advise engine rebuilders to do things steadily and in stages. Stage one involves minor changes to bodywork and exhaust systems, stage two deals with fuelling and forced induction, and stage three delves deep into the engine innards.  

Once you’re in the engine, then things get serious. And costly. But since you come this far in tuning your car, you’ll know what you’re doing and what to expect. The largest performance gains are right here. Swapping out pistons and conrods, also means you’ll want to go for a higher-end crankshaft than the stock variant. Something that is better made, lighter, able to take the torture while also spinning faster. You’ll want to choose between a handful of performance crankshafts. When you’ve got this up and running, then you can call it a day. Your car is ready.

What is a Crankshaft? 

close-up of engine rebuild kit
Source: ger.co.nz

A crankshaft is the engine part tasked to deliver (all) the power from the combustion process acting on the pistons and connecting rods to the flywheel, transmission and wheels. It converts the linear movement of the piston and conrod combo into rotational force. Consider it the backbone of the engine, from which other parts stem out. It needs to guide the pistons up and down in the cylinder bores in a precise sequence, and thousands of times a minute.  

Different parts allow this to happen. A crankshaft consists of main bearing journals which form the axis of rotation; the crankpins or rod journals upon which the connecting rods act and are attached; crank arms which connect rod and main journals, counterweights to smoothen out forces during rotation; oil passageways to allow lubrication, and the whole assembly is connected to the flywheel with the flywheel flange. A complex feat of engineering, with numerous parts all needing to work in sync.  

Stock Crankshafts 

close-up of stock crankshaft
Source: autoguru.com.au

Crankshafts in mass-produced cars are geared more to durability, than high-end performance. As such they don’t need to be engineered to take the toll of higher RPMs, as well as the increased rotational forces that bend and twist a crankshaft as it moves. Materials here consist of cast iron, which is more than adequate for the intended use. However, if you’ve done all the modifications up to here, then it’s time for something better.  

Performance Crankshafts 

Better production processes and materials are the first difference between stock and performance crankshafts. The latter are made of forged alloys, particularly carbon or Chromoly steel, that reduces overall weight while significantly increasing strength. Forged cranks come in two variants – those with the crank throw forged in place all at once or twisted into position during the manufacturing process. A non-twisted forged crankshaft allows it to withstand higher stress levels at higher RPMs. Even stronger are billet crankshafts, literally carved out and machined to specific proportions from a single block of billet steel. The benefits of billet cranks are the high level of precision shaping, which also means increased higher strength and better balancing during rotation. While forged crankshafts offer acceptable gains for most of us, a one-off billet crank will put you on the podium.  

Machining a performance crankshaft is another factor that makes them so expensive. The ultimate goal here is reducing weight (where possible) without impeding on strength, as well as more efficient lubrication. Counterweights are the first parts to see weight loss, with pendulum cutting shaving off unwanted weight and rotational mass. Lightening holes in rod journals also shed weight. A performance crank will also have better lubrication provided with machined, gun-drilled holes in the main journals, to efficiently redirect oil into each of the bearings as the whole assembly spins. Then there’s the final touches in the overall finish. More care is taken to bring the journals to a ground and polished finish and within exact standards.  

close-up of setting up a perfromance crankshaft
Source: youtube.com/carsgarage

The last point is done with various forms of strength treatment. Heat treatment is applied to forged crankshafts to improve their durability and resistance to wear, by making them harder. Nitriding is the most common form of strengthening the crank. The crank is heated in a special furnace and is subjected to ammonia and nitrogen gas. The process hardens the steel alloys, by bonding the nitrides with the carbon, and this also reduces fatigue levels. A recent treatment is cryogenics. This freezes the crankshaft to hundreds of degrees below zero, and then brings it back to room temperature slowly and in controlled conditions. The goal here is to reduce stress and friction levels by realigning the metal grain as it warms to normal temperatures.  

With lighter and stronger cranks comes faster rotation. In order to bear the forces exerted by the crank, forged H or I beam connecting rods and forged pistons are chosen. This ensures that rods won’t bend or rupture, or pistons won’t seize or crack under the additional stresses. Pair your modifications with caution to get a revvy, but balanced engine.  

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