Engine Tech, Building the Nissan VG30DETT part 1

Pistons

To withstand the stress of 20+ psi of turbo boost, we selected JWT’s forged pistons.  Forged pistons are stronger compared to the stock cast pistons due to the same grain refinement and flow characteristics described with the forged connecting rods.  JWT pistons are forged from high silicon aluminum alloy.  Although high silicon aluminum is not quite as tough and ductile as lower silicon alloys, it is more dimensionally stable over a wider range of temperatures.  Because of this, you can run much tighter cylinder to wall clearances with high silicon pistons.  This creates a quieter, longer lasting engine free of piston slap.  Ring life and seal is also improved with tighter clearances.  Road racing engines and high performance street engines are often assembled with high silicon pistons because long service life is important.  Low silicon pistons are best suited to short duration, extreme duty use events like drag racing, where life and noise are not important and can be sacrificed for toughness.  Typically a high silicon piston can run clearances as tight as 0.0025” where a low silicon piston runs from 0.0040”-0.0060”.  Even though a high silicon piston is slightly less ductile and tough than its low silicon brother it is still considerably stronger than any OEM type cast piston.

Forged JWT piston is 89mm and 8.5:1 compression.  We treated the dome with Swain gold coat and the skirt with Poly Moly.

The inherent strength of forged pistons will help them withstand the rigors of high boost without nearly as much worry about pinching ring lands or fracturing under the pounding of detonation.  This will help us gain reliability even with a high power output.  The JWT pistons are also about 40 grams lighter than the stock pistons, reducing reciprocating weight and strain on the connecting rods and crankshaft.

JWT pistons are also ground with a tapered barrel shape to the skirts with a slightly offset piston pin to reduce piston slap and the accompanying noise that is typical of racing forged pistons.  These machining tricks also cut down on cylinder wall, piston and ring wear.  These pistons are nearly as quiet as OEM cast pistons in terms of knocking or slapping noise.

The undersides of our piston received Swain’s heat rejecting coating.  We used a thickwall piston pin to prevent flex.

JWT pistons make use of genuine Nissan or other Japanese OEM type ring packages.  The OEM rings on a typical Japanese piston are low tension like a racing type ring for low friction but have several features such as chrome facings that wear much better than your typical moly faced or iron aftermarket ring that domestic engine builders favor.

The pistons feature a thick, straight wall tool steel piston pin for superior strength important when running boost pressure to prevent the pin from flexing and grabbing the small end rod bushing. The pins are also slightly shorter so there is no excess unnecessary metal tied up in the length of the piston pin.  The pins are retained with reliable double sprirolock retainers.  These spring steel retainers can withstand much more side load than the stock wire type retainers and are quite reliable, an important feature as you do not want your piston pin to drift loose into the cylinder wall at high rpm!

Our JWT piston in the bore during fitting.

We also had Swain Technologies coat our pistons to give them the final bit of protection.  We coated the domes with Swain’s extreme use multi layer gold coat.  Gold coat is unique because the base layers are thermo barrier but the middle layers are heat conductive.  This makes for a more even distribution of heat across the top of the dome reducing the chances of hot spots developing.  Gold coat is recommended for forced induction and really high compression applications.  The coating will protect our pistons and holding the heat in the chambers might just help power.

Swain also coated out skirts with their Poly Moly skirt coating.  Poly moly is also a unique engineered coating using tungsten disulfide, a tough but lubricious substance that also conducts heat.  Heat conduction is critical because pistons lose much of their heat through conduction through the skirts and many coating manufacturers don’t address this.  Poly Moly reduces friction and improves both piston and bore life.  It also helps forged pistons run a bit quieter.

Stay tuned!

Sources

Jim Wolf Technology

Cunningham Rods

Swain Technologies

Cryo Science