How Stroking an Engine Works

How Stroker Kits Increase Horsepower

Installing a stroker kit in your high performance BMW engine will increase power output. You’ve heard others saying it time and time again and watch as stroker engines break world records. But what exactly are stroker kits, and how do they increase displacement? Follow along as we break down everything you need to know before installing your stroker kit:

Increasing Power

The most basic pursuit of every performance enthusiast? Make more power. And how can we do this? By extracting more energy from the compressed air / fuel mix our engines generate. We can make more power by increasing the volume of air / fuel inside of the combustion chamber.

Most solutions to increase the volume of air / fuel making its way inside the engine include using a power-adder, such as a larger turbocharger or supercharger that utilizes larger throttle bodies or valves, or, finally, by increasing displacement. There is a whole industry committed to developing new and innovative solutions in which we are gratefully a part of.

Increasing Displacement

An immediate solution for increased displacement is to bore the cylinders to a larger size and install larger pistons. However, this becomes difficult in BMW engines, where even slightly larger than factory bore will begin to breach water jackets. 

Stroking an engine is, and has been for decades, the shortest path to increasing displacement and power for BMW engines. Our kits increase engine displacement by lengthening the distance pistons travel within the cylinders, also known as piston stroke or piston throw. We are able to do this using pistons that maintain factory bore, or with cylinder sleeves, which line the block and seal the cylinder bore, allowing for overbore. 

Increasing piston stroke means that the piston travels deeper into the cylinder bore, allowing more air / fuel mix to enter the combustion chamber, meaning your engine will generate more power. While adding a larger turbo or bigger valves will also increase the power generated by the engine, a stroker kit offers the immediate benefit of increasing overall engine displacement, which can then be pushed further with intake modifications. 

Hence, while there is an entire industry dedicated to developing new solutions and power-adders, stroker kits remain one of the most tried and tested methods for generating more power. “There is no replacement for displacement.”

Developing A Stroker Crank

In the BMW Market, a few companies attempted to develop a billet crankshaft that could be used in stroker engines, but these couldn’t perform under pressure. While their billet cranks split in half, destroying engines, our tried, tested and proven 4340 billet cranks stood up to some of the highest horsepower BMW’s in the performance world. 

Today, we continue to develop new, updated crankshaft designs as BMW continues to release new engines, all standing up to the abuse, NOS, anti-lag, and high boost applications 5150 drivers subject them to.

Today, 5150's bully-grade crankshafts are machined from 4340 aerospace grade billet steel on Mazak 5-Axis Integrex 300-IV machines, some of the most technologically advanced CNC equipment available. Every 5150 crank that leaves our machines is quality checked to ensure that it is within the strict .0001" exacting tolerances we demand. 

All 5150 AutoSport Billet Stroker Crankshafts feature large radius journals that add rigidity for the high boost, high RPM, and of course high horsepower applications our customers demand. This is also where material selection becomes crucial; aerospace steel is the same material used to manufacture the landing equipment on aircraft, selected for its longevity and ability to perform under high impact, high stress applications. 

To ensure the best possible oiling and lubrication, the journals of each 5150 AutoSport Stroker Crankshaft utilize gun-drilled, CNC-ground oil holes, shaped like 'tear drops', are polished to a mirror finish, ensuring that each connecting rod bearing is properly oiled for lasting engine life. To ensure the life of the crankshaft itself, every 5150 Stroker Crank will undergo several heat treatments, including:

Stress Relieving: the crankshaft is heated to a below-critical temperature range. This 'relieves' the metal of any residual stresses that may have occurred during forming, straightening, rolling, or during machining.

Nitriding: during a multi-step heat treatment process, nitrogen is diffused into the surface of the crankshaft, effectively hardening the surface.

Shot Peening: the crankshaft is subjected to a cold working process where the surface is struck by 'shot,' or tiny round particles to harden the surface of the metal.

Magnaflux Inspecting: also known as magnetic particle inspection, this is a non-destructive testing process where a magnetic field is used for detecting cracks and flaws in the surface and subsurface of the crankshaft and confirm that it is ready for the intense conditions of a high performance engine.

Finally, to ensure every 5150 crankshaft has high RPM potential, each counterweight is carefully engineered for minimized windage through the crankcase. This means reduced oil resistance, meaning your crank will spin easier and faster through the rev range and extending the rev range.

Because 5150’s stroker crankshafts feature journals that are almost identical to OEM, engine builders can enjoy an easy installation process, with no modifications to the block necessary for installation. All 5150 Stroker Kits come pre-balanced and ready for installation, however we ask that V8 customers re-check, as piston, rod, and bearing weights can vary slightly between brands. 

Have more questions? Reach out to our sales team directly or request an engine build consultation.

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