View Full Version : Pickin' a piston

01-15-2008, 08:01 AM
I actually wrote this for ChrisDs new site, but it should clear some things up between our own piston choices, or at least enlighten a bit :hehe: First some definitions to know.

First a few definitions.
Cast: Molted alloy is put into a mold and machined when cooled. Cheap to make compared to a forging.

Forged: A slug of alloy is forced into shape by pressure, much stronger and has a grain structure, more dense and conductive.

Hypo/Eutectic/Hyper: All this means is the %Si of the alloy. Eutectic
is the point of saturation of silicon into the alloy which is about 12.5%, anything above that has free silicon in this piston. Silicon imparts several things to the pistons.

NOTE: If using hypereucectic pistons, a T5 or T6 heat treatment is a MUST for high performance applications, to releave stress. T5 is more durable over time than T6, but a T6 treatment imparts more strength for the first ~100 hours of operation.

#1 It's harder than the metal alloy, while this reduces scuffing, it
also makes the piston more brittle. More silicon will increase
hardness and wear resistance, but will decrease detonation and
preignition resistance. Proper tuning is key.

#2 It expands much less than the metal alloy, so more silicon = less
thermal expansion. You can run tighter bore clearances, which reduces
cold start wear, and emissions.

#3 It acts as an insulator, the piston crown will absorb less heat
with a higher silicon content.

Now to explain some of the popular piston alloy choices:

SAE 332/F-132: Contains ~8.5-10.5% Si (these are the alloys for most production oem cast pistons)

4032: Contains ~11% Si may be higher or lower depending on manufacturer, but will be around 11%. Stock-like expansion in most cases.

2618: Contains ~2% Si may be higher lower depending on manufacturer. Will expand greatly, not recommended for engines that will see many cold starts.

Coatings: Things applied to the piston after final machining. May be added to the skirt for anti-scuffing lubrication, the crown for a thermal barrier, the underside of the piston to help shed oil. Adding a thermal barrier to the crown is not advised unless you are purpose tuning the rest of the engine to suit, otherwise preignition and detonation may result from increased charge heat.

So you can have a cast or forged piston, and it can be hypoeutectic, eutectic, or hypereuctectic.

That covers the most common definitions. Now onto some of the things to consider before buying.

#1 !!!MOST IMPORTANT!!! The piston controls the mixture motion as the piston rises, this, combined with the cylinder head shaping and intake charge velocity, will determine how your mixture burns when ignited. I will not cover how the piston crown surface should be shaped exactly, as that varies per application, but it's important to know that unless you are changing the head(s) configuration, the piston crown surface should be matched to stock most of the time. OEM engineers know what they are doing and shaped them as such on purpose, for a good reason. Ask the piston manufacturer about what to expect if in doubt.

#2 Weight. Run the lightest piston you can get away with for your durability goals. Always. Your connecting rods will thank you, your gudgeon pins will thank you, and your connecting rod bolts will certainly thank you. A lighter piston will increase the service life of all of the above components and the rpm potential as inertia decreases with decreased mass.

#3 Pressed vs. Floating gudgeon pins. For high rpm use or racing, get the floating pins, if you aren't going to see the high side of the engine too often, pressed is fine. Floating pins will tolerate more piston heat before seizure and thus are suited to such use. High performance street, unless it's a monster, pressed will be fine.

#4 Dimensions -This is the big one-

First the piston crown and first ring land. The thickness of the crown down to the top of the first piston ring, will determine how strong the piston is for the given alloy, and also, part of the emissions of the engine. The futher down the ring is, the stronger the piston, but all the volume above that ring will have hydrocarbons in it, less efficient per revolution, more emissions. Putting the top ring up as high as you can get it without melting the piston crown for efficiency is a popular trick amongst manufacturers nowadays. Unless you need the strength, you should do the same.

Second, the ring size. Measured in mm, it is the thickness of the piston ring. Low thickness will lend to much less internal friction, which is good, but it wil also be more prone to losing contact with the bore under stress, and the smaller contact patch will exchange less heat between the block and piston. The choice is this is engine goal dependant, so will not be covered, too many variables. Bore finish and minimal distortion are very important if running thinner rings to maintain contact. The piston grooves much be as flat as possible to maximize sealing and conductance.

Third and last, the skirt. This is the side of the piston from the bottom of the oil control ring down. The longer the piston skirt, the less rocking motion the piston will have when in motion, especially when cold. A longer skirt will add life to the piston, but increases friction and weight.

That covers the basic things to look for. Many manufacturers also have added performance and durability additions to their pistons, such as a machined grooved between the first and second ring grooves, or gas ports to improve ring seal at high rpm. Be sure to properly research each of these benefits to make sure it's what you want and will use, gas ports for example, will not avail you under 7,000rpm, so know what you are buying before you buy it.

edit: forgot to mention, once you get your pistons, make sure alll the edges on the crown are smooth. Some manufacturers do this for you, if not, emery cloth will do.

The following are the alloys used in our 3SGTE pistons (as of 1/14/08)

Mahle: 4032
Arias: 4032
Ross: 2618
CP: 2618
JE: 2618
Brian Crower: 2618
Wiseco: 2618

01-15-2008, 10:28 AM
Good stuffs WOT. Stickied in the NA forum.

01-15-2008, 09:34 PM
very good.