The Basics: Aftermarket Piston Advantage

A high-quality set of aftermarket forged pistons not only improves an engine's reliability, but also provides an opportunity to improve performance. First, forged performance pistons are often selected in a bigger bore size to increase the displacement of the engine. As a rule of thumb, the percent increase in displacement will deliver an equal percent increase in torque across the entire powerband. Second, aftermarket performance pistons can be ordered in a higher compression ratio than the original piston. The higher compression ratio improves the thermal efficiency of the engine. This allows the engine to produce more power while reducing exhaust gas temperatures and improving fuel economy. Third, when combined with a quality set of piston rings, aftermarket forged pistons will generally deliver an improved ring seal. This improved ring seal means the pressure stays in the cylinder instead of escaping to the crankcase. As a result, the engine once again delivers even more power and runs morefuel efficiently. Fourth, a well-designed aftermarket performance piston will free up some additional horsepower by reducing the friction in the cylinder. Many times, the forged pistons will use thinner ring sets and have a skirt profile which dramatically reduces cylinder-to piston-contact. Some manufacturers take the technology even further and apply a dry-film lubricant to the piston skirts to minimize friction. Fifth, since the material and manufacturing process used to make a forged piston are superior to cast pistons, weight can often times be removed from the piston. A lighter-weight piston allows the engine to run more comfortably at higher RPMs, while making the engine more responsive. In addition to these basics, the shapes of the combustion chamber and piston dome may also be optimized for peak performance.

Getting in Shape: Combustion Chambers

Even if you are planning to use a set of off-the-shelf pistons, which provide a performance advantages over stock pistons, you may want to consider having the combustion chambers of your cylinder head CNC machined. When properly processed, the CNC chambers will reduce the variance in compression ratio across cylinders. In addition, it will also tend to equalize the burn rate and, when done properly, should eliminate any potential hot spots in the combustion chamber. These benefits allow the engine to run more smoothly as each cylinder is likely to produce equivalent power.

One of Japan's leading performance machine shops, Nagoya Precision or NAPREC, has been offering CNC machining service of combustion chambers for the past few years. We sent our Project RH9 GT-R's cylinder head to NAPREC in Japan to undergo the process. The process runs about $600 for a four cylinder head or $900 for a six-cylinder. Once the head is received at NAPREC, it's attached to the fixture that positions the head properly for the machining process. A multi-axis CNC mill then machines each combustion chamber to the same finished shape and volume. The end result is chambers that are within .2cc of each other, instead of a factory variance that may be up to 10x greater. The finish on the NAPREC?CNC?processed chambers was exceptional. We sent the cylinder head to MAHLE?Motorsports to digitize the chamber. This information allowed the piston dome shape to be optimized for the chamber. While the head was at Mahle, they commented on the excellent quality of the CNC chamber machining. In the past, we did send out a cylinder head to a CNC-porting company to have its chambers CNC machined. The finish was horrible. That company obviously spent little time in setting up the machine, programming and tooling to do the job right. The bottom line is that not all CNC chamber processings are of equal quality, so find the right company.

Shape Shifting

In addition to the basic "full-squish" style CNC machining of the RB26DETT cylinder head's pentroof chamber, NAPREC also offer two other chamber shapes. A "semi-circular" shape is also offered that removes the squish pad on the intake side of the chamber. A "full-circle" shape isญญญญญญ offered which removes the quench pads on both the intake and exhaust sides of the combustion chamber.

We opted for the "full-squish" processing. The "full-squish" design keeps the quench pads in place to promote squish. As mentioned above, the optional "semi-circular" shape removes the quench pads on the intake side of the chamber. This allows the intake valves to be un-shrouded for improved flow numbers while also reducing the likelihood of fuel puddling at high boost levels. The tradeoff is a slower rate of combustion that takes more time for the process to complete. The "full-circle" design goes a step further and removes both the intake and exhaust quench pads. As a result, the exhaust valves are also unshrouded for additional flow. However, the total time for complete combustion is also extended which means a slower rate of combustion. To compensate for slower rates, more ignition timing advance is generally required, which is actually actually a drawback. Unfortunately, many people mistakenly assume that an engine that can tolerate additional ignition timing is better. In fact, an engine that makes the most peak torque with the least amount of ignition timing is actually the most efficient.

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