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Look at the underside of the JE pistons and you can see some machining marks just above the wrist pin. This was a special machining step to prevent contact with the high-mounted connecting rod. This design change had a few knock-on effects…
To maintain a safe minimum thickness on the piston crown, the dish in the top of the piston had to be made a little shallower. This reduces combustion chamber volume at Top Dead Center, which raises the compression ratio. Not a great plan if you're trying to run the most possible boost on crap 91 octane gas. To get the dish volume back, the lip around the top of the bowl was made narrower, as you can see in this picture. This is as narrow as you can go before you start reducing the strength of the bit overhanging the top compression ring, but it's just enough to get us back down to the laughably old-school 8.5:1 compression that keeps an SR20DET happy.
There's only one problem with this solution. The smaller lip reduces the effectiveness of the pathetic little quench pads in the SR20's head.
That bit with the red arrow pointing at it is the quench pad. When the piston approaches TDC, the air trapped between the piston and the quench pad gets squished out (that's why that's also referred to as squish area sometimes) like ketchup trapped between a table and your fist. This fast-moving squirt of fuel and air comes right as combustion starts and helps speed combustion up. And like I said earlier, faster combustion leaves less time for knock to happen, so we're all for it.
Since this squish motion is so critically important for knock resistance, we put a little bit of material back on that part of the piston to ensure we still had a full fist hitting that pack of ketchup. As a result, this extra material bumped our compression from 8.5:1 to 8.7:1. Close enough…
The end result of all this careful design work is almost a 100g weight savings.
And really, when you count the shorter wrist pin the JE piston is designed to accommodate, the total weight savings is over 120 grams per piston!
Take 120.8g per piston, plus 135.1g per rod, times 4 cylinders, and you have 1.02 kg, or 2.25 pounds less reciprocating weight. That will cause an incredible reduction in reciprocating loads!
