Anyhow, a piston is the primary source of reciprocating weight in an engine so the heavier it is, the more force the connecting rods, bearings, oil film, and crankshaft have to deal with. If a piston is accelerating at over 2000g’s (2000x the force of gravity), you can bet the piston’s weight plays a HUGE role on the longevity of all the other components. What this means is that forces that the rods, bearings, and crank have to deal with become exponentially higher the heavier a piston is. Needless to say, the lighter the piston is, the better it is so long as it’s strong enough for the intended application.
With this being said, I weighed the stock VR38DETT piston at 580 grams with the wristpin. This is one of the heaviest production pistons I have ever encountered (keep in mind I’ve never extensively worked with big American pushrod engines). In a stock engine at stock power and RPM levels this is acceptable. With big heavy piston, big heavy connecting rod, and plenty of rod bearing oil clearance (the VR38’s factory rod bearing oil clearance spec is the largest OE spec I’ve ever seen in my career), and healthy oil pressure, a big heavy piston can work at 480bhp @ 7000 rpm. But with people now making 700+bhp @ 8000 rpm with larger turbos, intercoolers, bolt ons and tuning/ecu work, the situation becomes extremely marginal for anything beyond a dyno pull or a 1/4 mile pass. Just because Joe Blow Racing made 700+hp on a stock bottom end doesn’t make it right either. 750+hp WILL end in catastrophe with a stock engine. Understand that just because somebody made 750+hp on the dyno, it does not mean that he is not running the engine beyond it’s design parameters. Even if a stock engine did not self destruct immediately, the high mass of the stock piston and rod will likely cause premature wear and failure of other components such as cylinder walls, wrist pin, rod/rod bushing, rod bearing, crank, etc.
This isn’t a scale we use for engine components, but it is still +/-1% accurate. At 580g, this piston and pin are ultra heavy. However, I’m sure Nissan had to keep the stock piston cast to keep the cost down. Remember: the R35 GT-R = $85k+ for a car that performs like a $130k Porsche. There’s no doubt the R35 GT-R is the absolute best bang for the buck on the new car market today, but Porsche does not cheap out and use heavy cast pistons from the factory either.
Here’s the newly released Cosworth VR38 piston and pin weighing in at a relatively light weight of 516 grams. This piston can easily handle 150+hp/cylinder which would be 900hp for the set.
Fundamentally the factory and Cosworth pistons are different designs. This is a VQ35 piston on the left, but the forging is similar to the VR38 forging. The Cosworth piston is a semi-slipper skirt piston which provides greater strength to support the skirt and the pin bosses with a smaller skirt area to reduce friction and leverage on the skirt. The pin is also significantly shorter on this type of piston design. With a shorter pin there is less leverage and therefore less stress on the pin bosses should the pin flex. One of the nice features about Cosworth pistons are the bespoke forgings. Pockets can be added to the forging to reduce weight further (see right outside of the pin bosses) where strength is not affected. Cosworth wrist pins are also made of EN24B tool steel. Your typical shelf piston wrist pin is usually made of some low buck steel alloy. Each Cosworth piston comes with polished tops, offset pin bore, tool steel pins, semi-slipper skirt forging, anti-detonation bands, internal milling, precision ground ring grooves, extrude hone deburring, and Xylan skirt coating. The factory Nissan piston on the right is an old school “jam pot” piston. Outside of the oil cooling chamber and hard anodized top ring groove, it is basically a heavy duty (with emphasis on the word heavy) econobox piston. A forged piston with oil cooling chamber is extremely expensive to manufacture at low quantities. The Cosworth piston is designed so that the oil cooling chamber is not required, but if you are planning on racing at the 24 hours of LeMans or Daytona, have the budget or are feeling really generous, feel free to give us a call and we’ll design a piston for you similar to some of our racing pistons.