Project GD Subaru STI: Building the Engine Part Two, Bottom End Bits With IAG Performance.

The IAG spec rods use ARP Custom Age 625 rod bolts which are quite a step up from the industry-standard ARP 2000 bolts. This is a nice touch and helps ensure that the engine is as bulletproof as can be.  The ultimate tensile strength of an ARP 2000 bolt is 200000 psi while a Custom Age 625 bolt is 260000 psi.  This is a nice bit of insurance if you are revving past 8000 rpm or if you misshift!

Our engine got the latest IAG spec JE FSR piston.  JE might make these pistons but they have been speced out by IAG and are pretty unique.  The pistons are forged from super tough low silicon 2618 alloy.  A very unique feature is that the IAG pistons are nickel-plated.  Nickle is a very slippery and tough surface and is great to protect the aluminum.  We have seen nickel-plated pistons in two strokes before but this is the first time that we have seen it used in an automotive application.  IAG says that the nickel reduces piston skirt wear and friction, reduces ring groove micro-welding under high boost, and provides some protection of the dome from heat and detonation.  It also seems to stay put better than ceramic thermal barrier coatings.

The IAG piston ring groove area has some unique features.  Since Subarus are known to break their ring lands, the top compression ring is placed pretty far down to strengthen the first ring land. The groove has 4 lateral gas ports to help improve ring seal but not so many as to increase ring wear much like a drag racing motor might have.  IAG forgoes the typical pressure equalization groove that many race pistons have to assure that the second ring land is also as strong as possible. Finally, note that the piston pin is up pretty high to accommodate the longer rod.

We have the latest IAG dome design on our pistons. The raised center section of the dome/dish is wider and flatter than previous designs and the dish is slightly deeper to maintain the compression ratio. This helps reduce the surface-to-volume ratio of the piston top and reduces the amount of heat that the piston absorbs.  IAG found that this helps from their road racing, long-duration use experience.  The nickel plating also helps protect the piston top from heat.

The piston skirts take advantage of JE’s thermal analysis and advanced skirt barrel profiles which allows tight piston to wall clearances for minimal wear and knocking even with 2618 forging’s greater coefficient of expansion over the stock pistons hypereutectic high silicon casting.  The stock piston can run very tight clearances due to the alloy’s low expansion but it is brittle and prone to crack which results in many of your typical Subaru engine failures.  The IAG pistons also feature a dry film lubricant skirt coating which further reduces friction and wear. The uncoated part at the bottom of the skirt is a pad for measuring the piston for calculating the piston to wall clearance.