1997 Mazda MX-5 Miata: JE Pistons Forged Side Relief Slugs In The House
February 28, 2012
Key to Project Miata's long-rod BP engine is its pistons. Clearly, no stock-replacement pistons would be suitable for use with the 7mm-longer-than-stock Mil-Spec connecting rods, since the pistons would be shoved too far up the bore. Top dead center indeed; emphasis on dead.
Remember, we're not changing the stroke. Stroke is dictated by the crankshaft throws, not rod length. We could have rods three miles long and the stroke wouldn't change one iota. Anyway, our stock crank means stock stroke.
Accommodating longer rods in our long-rod BP (which we delved into here) is accomplished with what you see above -- JE Pistons' asymmetric Forged Side Relief (FSR) pistons. These slugs have a lot of cool details we'll explore across the jump.
FSR forgings are clean slate designs that JE Pistons specifically tailored for high-output boosted engines, which works out well since that's what we're building. JE engineers tell us that the geometry of the crown and the struts and skirts are all intended to provide strength where need it while removing material where you dont.
The side reliefs in question refer to the sides that have been pared away. Slipper-skirt style pistons, in other words. Po-tay-to, po-tah-to. Theres a twist here the skirts on these particular FSR pistons are asymmetric, meaning one skirt is larger than the other (see orange vs green arrows above).
Theres a good reason for this. As a piston travels up and down the bore, it is alternately loaded against opposite sides of the cylinder. In the Mazda BPs case, the piston leans against the bore's exhaust side on the way up, and on the intake side on the downstroke.
The severity of this lean, known as side or thrust loading, is heavily influenced by cylinder pressure, which is most extreme (by a long shot) on the combustion stroke. This lopsided loading means we dont need nearly as much skirt on the exhaust-facing side of the piston as we do on the intake-facing side (corrections in bold; I reversed these originally). As in all aspects of life, less skirt is better -- here, less skirt means less mass, and less loss due to friction.
Again, accomodating longer rods meant moving the wrist pin further up the piston. Here the pin bore is as high as it can go. Intersecting the oil control ring groove may appear odd, but it's a standard practice even on some high-performance OEM engines (the GM LS7 comes to mind). You can think of the pin bore here simply as an extra-large oil drainback hole. An oil ring support rail bridges the gap formed by the pin bore.
The FSR pistons are of 2618 aluminum, which offers higher strength and fatigue resistance than 4032 aluminum. Boosted applications see higher piston loading and temperatures than normally aspirated ones, making 2618 the material of choice for turbo and supercharged engines.
However, 2618's higher rate of thermal expansion requires a bit more piston-to-wall clearance when cold. Large clearances here can promote noise, but JE Pistons' boffins are confident they've licked the noise issue with their 2618 FSR pistons by controlling the pistons' deflection characteristics and incorporating up to 0.030" wrist pin offset. In fact, we're told that making 2618 suitable for street use in this manner was one of the design objectives of the FSR forgings.
In our case, the FSR piston weighs 276 grams (300.5 grams with rings and support rail), compared to 336.6 grams for the stock BP VVT piston with rings. So despite being made from a denser alloy, and forged rather than cast, and 1 mm larger diameter than the stock pistons, with a much beefier crown than stock, plus those fillets and gussets, our JE pistons wound up lighter than the stock pistons.
We also found that the JE Pistons all weigh within a few tenths of a gram of each other, while the stockers varied by nearly 10 grams (!). Matching reciprocating masses on a per-cylinder basis makes for a smoother-running engine.
You can order pistons from JE Pistons with a huge variety of options like undercrown milling, ceramic coated crowns, hard-anodized ring grooves, low-friction diamond-like coatings on the wrist pins, the list goes on. In this case we opted only for the "Tuff Skirt" coating, the matte black stuff you see here. It provides a low-friction surface at the skirt-to-bore interface.
Our pistons are 84 mm diameter, which is 1 mm larger than stock. The resulting additional 44 cc displacement brings us up to a total of 1884 cc. Using rounding technology, we've got a 1.9-liter BP.
Compression ratio is 9:1 with these pistons, though this number may change slightly once we account for actual deck height (this varies from block to block) and combustion chamber volumes (same deal). The flat-top geometry of these pistons just worked out that way once the valve reliefs were sized to accommodate +1mm valves at +1mm lift relative to stock.
There's another piston-related topic on which I'll share some geeky thoughts. It'll be in a followup post since this one's running long.
--Jason Kavanagh, Engineering Editor