TECH5 is a regular feature where EngineLabs asks industry leaders five technical questions. This week’s guest is Glen Schierholt, field engineer and tech support for United Engine & Machine — the manufacturers of Icon, KB Performance and Silv-o-lite pistons.
EngineLabs: The Icon piston line boasts “second-land accumulator grooves” and “top land mini groove technology.” Explain these two features and detail the benefits of each.
Glen Schierholt: The 2nd land accumulator groove provides an area for escaping combustion pressures and crankcase pressures that get past the rings to gather and dissipate. Without this feature the passing compression or crankcase losses could otherwise flutter the rings, removing stable ring-to-cylinder-wall seal. Mini grooves in the top land were designed to trade off heat from the piston crown to the cylinder wall, if the crown was overheated. The top land has multiple grooves machined into the land-to-cylinder-wall face. These small lands are .010 in-face width for less surface contact area if the piston crown were overheated, they then touch the cylinder wall to exchange heat and hopefully save the piston from these damaging temperatures. They are also beneficial at disturbing violent detonation waves that could damage the top ring or the supporting second land; these mini grooves disturb the waves with the qualities of the irregular face surface and close proximity to the cylinder wall.
EngineLabs: Explain the difference between compression ratio and effective compression ratio.
Schierholt: Compression ratio, often referred to as static compression ratio, is the head chamber cc volume, head gasket bore, top ring location on the piston, cc volume of the piston crown with valve reliefs, added together for a given volume. The ratio is arrived from the size of the available volume of the piston at top dead center to the total volume present with the piston at bottom dead center of travel, as if the cylinder were sealed at the bottom of the stroke, no open valves. Effective compression ratio is the available cylinder filling pressure the camshaft allows upon the closing of the intake valve after BDC. The later the intake closes after BDC the more bleed off occurs to the available compression pressure. Thus the operating compression ratio or effective ratio will change drastically depending on camshaft design. The more radical the cam the higher static compression you could start with and be able to run 92 octane pump fuel on the street.
EngineLabs: What are the factors that engine builders need to consider when choosing between 390 hypereutectic (KB line) and 2618 (Icon line) piston materials?
Schierholt: 390 Hypereutectic alloys are more suited for street performance or mild racing applications. The cast alloy will always have a limitation for severe duty applications due to grain alignment and having a rigid or more brittle alloy matrix. There are pro and con for hyper materials. The high silicon content of 390 alloy (KB at 16%) gives the material excellent wear characteristics on the skirt to cylinder wall and ring to ring land wear. The hyper material expands 15% less than other cast alloys, so it can be ran at a much tighter fit to the bore allowing better ring seal and quieter piston operation. Hypereutectic material reflects heat and will produce more power per given application, generally a 2% power increase. Keep compression ratios low when running pump fuels with iron heads. The hyper piston is not absorbing most of the combustion temperature and reflects it back into the combustion process and head. In return the head heat sinks which can lead to detonation tendencies. Usually for iron heads, 9.8:1 compression or slightly over 10:1 with a radical cam that closes the intake later. Aluminum heads may support upwards of 10.8:1 with the correct cam selection. 2618 alloy forged pistons also have their pro and con on application selections. The 2618 material is a forging, pressed with a die (500 ton) in a containment can; this shapes the piston slug in a rough form for machining. 2618 has less than .8% of silicon content which makes the alloy very ductile and malleable. The alloy is great for racing and extreme endurance situations. Machines easily, and will take impacts without breaking up. Now for the street the 2618 alloy is not the best choice if looking for a quiet running, high-mileage piston. The alloy absorbs heat and expands much more than its cast cousin. 2618 requires much more skirt clearance for the higher expansion of the alloy. The alloy is quite soft compared to the 390 cast alloy and will not have the long wear characteristics as the hyper. For all out competition and high horsepower street engines, 2618 alloy would definitely be your best choice and can support higher compression ratios on pump fuels compared to the cast hyper alloys.
EngineLabs: What are the top three mistakes engine builders make when selecting pistons or installing them?
Schierholt: #1, Wrong application selections in alloy for street, strip or racing. #2, Too high compression for pump fuel. #3, Improper ring gap with a hyper alloy.
EngineLabs: Explain your strategy behind “quench dome” technology.
Schierholt: The quench dome actually has several benefits. The quench area usually is at the same height a flat top piston would have. This gives us the quench benefits with a dish. The top then steps down to allow the dish area to be taken up mostly by the cylinder wall. With the step dish design the flame front is pushed or driven by the flat quench area to the other side of the combustion chamber to quicken the burn rate and promote more of a tumble to better atomize the chamber mixture. The step design also decreases the piston surface area that would be required for a dish. Less surface area means less heat and a lighter piston. This also allows the cylinder to become part of the dish displacement and share the heat absorption that otherwise the piston would have to handle on its own.