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The JE FSR piston is made out of tough 2618 low silicon alloy which is ductile and can withstand detonation and high cylinder pressures well. The 2618 alloy expands more than higher silicon alloy and hypereutectic pistons and requires greater piston to wall clearance but JE has some good skirt profiles that reduce noise. This is important for modern engines with knock sensors.
The semi-direct injection FA20 engine (it has both port and direct cylinder injectors) uses a very high stock compression ratio at 12.5:1. Since direct injection offers a lot of charge cooling and the cooling can improve volumetric efficiency, this is a good thing. However, 12.5:1 isn’t going to cut it with forced induction for very long- direct injection or not. To run this sort of compression even with good fuel, with forced induction the timing will have to be reduced considerably which has the cascading effect of increasing heat. None of this is very good for the engine’s internals.
We will be reducing the compression to a more FI friendly 10.5:1, which matches the FA20B engine found in turbo Subarus as well as turbo and direct injected VWs. We also used this compression ratio on Robert Kochis’ FR-S last year with really good power results. That engine is still running strong.
You might notice the JE piston’s unusual dome configuration. The dish you see is where the direct injection injector sprays. It is a pre-combustion chamber which allows the direct injector to create a stratified charge, putting a richer and easier to ignite mixture near the spark plug. The deeper than stock dish of the pre-chamber creates the lower 10.5:1 compression ratio.
The feature we like best on the FSR piston is its reduced skirt design. The FSR is a strut-type piston, meaning that it has very reduced skirts. These skirts are cut away to just small contact patches on the major and minor thrust surfaces, as opposed to a full-skirted piston that has a skirt that contacts the cylinder wall for 360 degrees.
A strut-type piston has the advantage of being lighter with less friction due to small contact areas. In the past, strut pistons were also known to be more of a short duration of use, race-type piston that wore quickly and was noisy on the street due to their limited contact area.
In the FSR piston, the major thrust side that sees most of the load has a cam-shaped profile to reduce noise while the much more lightly loaded minor thrust side of the piston has a different skirt profile for lower friction. The asymmetrical skirt profiles give the FSR piston longer life, less friction, and less noise. The differential major and minor profiles make these pistons exceptionally quiet for a 2618 alloy slipper skirt piston.
A straight wall pin is less likely to bow and grab the rod small end bushing and spin it like a super light taper wall tool steel pin can. We might use a taper wall pin in a high revving NA motor. On a high boost motor, we would prefer these thicker wall piston pins for the high loads imposed by the increased cylinder pressure.
The top compression ring is chrome plated for long wear while the second compression ring is ductile iron with a scraping Napier profile. Napier rings are good for reducing oil consumption and rapidly breaking in to give a good seal. The oil ring is narrow with low tension for friction reduction and has two scrapers with a separator rail.
