CAST/FORGED/Hypertutetic???

[Pamela H.]

Looking for clarification on hypertutetic pistons...Was advised by someone in the know that Hypertutetic (sorry if the spelling is wrong) pistons were a step up from Forged pistons...Doesn't sound quite right to me..Can anyone provide the definitive information.
Thanks!

[Duke W.]

Re: CAST/FORGED/Hypertutetic???

Hypereutectic is a step up from conventional cast, not forged, pistons, but forged pistons have "issues", which is why they are rarely found in production cars, today.

These issues include a greater thermal expansion rate, which requires greater cold clearance that can cause more noise including noticeable cold piston slap. They are also said by some to have greater skirt wear rate, but I've never seen an OE engine that needed a rebuild due to skirt wear. One the plus side, forged pistons usually offer greater resistance to detonation damage.

Forged pistons are available in two alloys - 2618 and 4032. The former, which will take the greatest detonation abuse has a low silicone content and high thermal expansion rate, which requires skirt clearance of about .006" on a 4" bore. The 4032 alloy, which is what all OE forged pistons are made from has high silicone content that only requires about half the cold clearance and 4032 shows lower skirt wear rates than 2618, so the 4032 alloy is preferable for road engines. The 2618 alloy should only be used in dedicated racing engines or highly boosted road engines that are not expected to have OE longevity.

As a general rule, it's a good idea to replace vintage conventional cast pistons with modern hypereutectics, and there is a good assortment to choose from to achieve nearly every target compression ratio range for vintage engines, including raising the compression ratio of "regular fuel" engines to achieve better across the range torque/power and fuel economy at a very slight increase in fuel cost. Premium fuel now only costs five percent more than regular, but the additional 1.5-2 points compression that premium will support increases across the range torque, power, and fuel economy by 5-10 percent.

As a general rule I recommend OE replacement (4032 alloy) forged pistons for SHP/FI engines that originally had them in order to achieve the OE range compression ratios, but some choose to replace them with hypereutectics. This usually sacrifices several tenths of a point compression because hypers don't usually have as much dome volume as many OE forged pistons. For example, the 327 OE forged pistons have a dome volume of 5.3 cc, but the KB hypereutectic "replacement" for this piston is only 0.5 cc.

Hypers may also alter SHP/FI engine mechanical noise characteristics because of their tighter clearance, which virtually eliminates both cold and hot piston slap.

Duke