Everything You Need to Know About Engine Pistons
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Performance, outright speed, and acceleration rimes can all be improved with the right choice of engine upgrades. While dozens of different parts can add a few more horsepower to the mix, going for the engine internals is where you’ll see the best results. Pistons, conrods, and cranks are where the magic happens. Swapping these out for something with more bite can completely transform an otherwise lethargic engine.
What are Pistons?
A central role in delivering power from the combustion cycle down to the wheels is allotted to engine pistons. These are the heart of any engine, They bear the immense heat, pressure, and mechanical stress generated through the combustion of fuel and air thousands of times each minute. And garner that energy down through connecting rods, bearings, and crankpins to turn the wheels. How engine pistons perform when making the journey up and down the cylinder bore, and how long they last depend on a mix of variables; from production processes and the choice of materials, as well as the difference in designs and geometries that cater to changes in engine speeds and forces coming down on pistons.
Parts makers have perfected the balance between weight, performance, and durability for passenger cars. There’s more scope though for variation in performance applications, even in cars that are ripe for tuning upgrades.
Parts of A Piston
Pistons are more complex than at first sight. They consist of distinct parts, each with its own role. Central to collecting the power from the combustion cycle is the piston head. This is comprised of the uppermost part of the piston crown, and the hollowed section underneath called the piston skirt. Crowns can assume different shapes, based on fuel or engine types and compression ratios among other factors. They can be either flat-top crowns, beneficial for improving both combustion efficiency and maintaining high compression ratios (mostly in naturally aspirated engines), dished or bowl piston crowns used to reduce compression ratios in boosted engines, or domed crowns that reduce the combustion chamber volume and hence increase compression ratios.
Sealing the piston within the cylinder bore is the task of the piston skirt. This is the (mainly) cylindrical part that elongates downward from the crown and has indentations or ring lands that house the piston rings. These expand and contract as temperatures change. Rings also dissipate heat away from the piston and towards the cylinder walls and engine block. The top and intermediate rings are mostly responsible for maintaining compression (or sealing), while the bottom oil ring wicks away excess oil as the piston travels through the cylinder bore.
The skirt also has a circular cutout or the piston bore which houses the piston pin (also called a gudgeon or wrist pin). This forms the pivoting point that connects the pistons to the connecting rods. Pins are often hollowed designs and kept in place with piston pin clips or locks.
Why Materials and Production Process Matter
Pistons can be cast, hypereutectic or forged. And all engine pistons today are made from aluminium alloys. As well as a varying percentage of different materials (mostly silicon). Cast pistons are made by pouring molten aluminium into a precision mould, and left to cool. The resulting piston ‘blank’ needs minimal machining. Another advantage is excellent wear resistance, especially in the ring lands and skirts, meaning optimal sealing and quiet operation. One area where cast pistons fail is their low ductility due to changes in temperatures and pressures, and the mechanical stresses at higher engine speeds. Cast pistons are fine for stock engines without any modifications, but for better performance, look elsewhere.
Hypereutectic pistons are made by the same casting process but have a higher silicon content, roughly 15 per cent. This improves hardness and produces a piston that is more resistant to wear and has better sealing. They’re often offered as a stock cost-effective option for higher-performance engines such as Subaru’s EJ205 and EJ255 in the WRX.
If you’re after ultimate performance, go with forged pistons. These are made by forcing a hot aluminium ingot into a forging die under extreme pressure. The resulting piston blank has a higher tensile strength that changes grain structure. The final product though comes to life through extensive machining and this raises costs. Forged pistons come in two types, 4032 and 2618, depending on the silicon content. 2618 forged pistons with roughly 2 per cent silicon have the highest ductility, meaning they can expand at a higher rate and withstand fracturing under heavier engine loads. One side effect of this, however, is piston slap at lower engine speeds (and temperatures).
Choosing Performance Pistons
Your choice of aftermarket engine pistons will be based on the type of engine (and vehicle), and how it will be used. Cast pistons are meant to last the lifetime of the stock engine, though stressed pistons have been known to bend, crack, melt, show irregular surface wear, or even snap. Rings can deform and will cause a loss of compression and hence power. More significant damage is with pistons seizing in the cylinder bore.
The strength and durability of forged pistons make them the go-to choice for tuning favourites, such as the aforementioned WRX, or STi from Subaru, the Toyota Supra, the Nissan Skyline, all Mitsibsuhi Evo models, and some local heroes, like the Barra XR6 in the Falcon and Nissan’s RB30 in the Commodore.
CP-Carrillo, JE Pistons and Mahle are some of the brands that produce durable, high-end forged pistons. California-based CP-Carillo made its name in motorsports, specifically the NASCAR series, providing both pistons and connecting rods. Anyone tuning a Skyline will be well served with the company’s CP Pistons RB25DET. And tuners with an early-noughties Subie in their sights have the option of CP Pistons EJ20 WRX / EJ205.
Piston-to-bore clearance is one metric buyers need to be aware of. Depending on the current condition of the cylinder block, some machining to bores may be needed to observe the recommended clearances.