[OBPSTI]

Greetings,

Everyone is talking about how Forged Pistons are the way to go and I agree it's definately stronger than stock, but yet what about the weak Ringland? What I'm desperately trying to research is the Piston/Ringland science and balance of power.

It seems that most of the stock motor fails are related to the Ringlands breaking and not so many problems with the pistons themselves grenading.

The most obvious question to me is why not make the Ringlands stronger in stock form?

Do Forged Pistons come with stronger Ringlands?

What is the science within a motor with Forged Pistons vs. Stock(Cast) that is allowing the Motor with Forged Pistons to make greater power levels yet is not as damaging to the Ringlands?

Looking forward to hearing any feedback.

[BBBsti LTD]

Just to help you clarify, the ringlands are part of the piston, the ringland is the surrounding area where the pistons rings are. If you buy forged pistons you will be getting forged ringlands. If the ringlands are forged, they will resist more heat, pressure and detonation, but that DOES NOT make your engine undestructible.

You can search in NASIOC FAQ about pistons, pretty good post.

I going to let the rest fot the most knowledge people here.

[USNSTI]

Yeah as above, the ringland is just the part of the pistons where the rings go, not a seperate part. If the piston is stronger, the ringland is going to be stronger.

[MartinSTi05]

This should give you a better understanding of the situation.

The stock pistons are an exercise in compromise as is everything in life. The engineers needed a piston that would provide reliability (at 300 WHP), have good emissions characteristics (low coefficient of thermal expansion to help with blowby on startup), be reliable in the long term, and be cost effective to manufacture.
The engineers ended up going with a casting process that is much more cost effective than forging and allows use of low CTE high silica alloys. This yields a very dimensionally stable piston that will not be noisy on cold start (think OEM customers complaining about noise) and wear at a slow rate due to the high silica content (notice they even low firction coated the side skirt on the OEM unit). The dark side of these choices though is a piston that is not very ductile. This means that any type of repetitive flexing the piston sees is more likely to casue a crack IE ringland.
Keep in mind though the attributes mentioned above wonderfully meet all goals and generally lead to a long and happy piston life within stock operating parameters. But inevitably there are always some who are not satisfied with stock operating parameters This has lead to many cases of (Ringland Itis). Mostly what people are talking about is a ring land cracking or entirely breaking off the piston.

An example of a piston with a broken 2 and 3 ringland.

A forged unit. Notice the increased thickness of the ring lands.

In an effort to make more power and pick up more high school age girls many have attempted to raise the horsepower output of the stock (or aftermarket configuration). This means higher cylinder pressures and temperature. The higher temperature weakens the piston alloy, and the elevated cylinder pressure leads to higher applied load on the piston. The Subaru engineers designed a safety factor into the piston and for the most part a good tune will not exceed the mechanical strength of the piston. But, due to a myriad of reasons, tune, fuel delivery IAT etc sometimes detonation can occur. The combination of high cylinder temperature and pressure heats and flexes the ring land until it gives up and breaks. Now the ring is unsupported, flutters and generally makes life not worth living, or worse breaks completely free and gets to know the combustion chamber.
Enter forged pistons. Generally there are two alloys offered 2618 and 4032. 4032 is somehwat more ductile than the stock alloy, weaker and has a slightly higher CTE. 2618 is also slightly weaker than the stock alloy, but much more ductlile with a much higher CTE. Aside from differing alloys forged pistons also benefit from different constructions (IE thicker ring lands) and the fact that the forging process itself lends to a better grain flow and metalurgy within the piston than a casting process.
2618 is the most common alloy choice for turbo/nitrous/supercharger applications with high cylinder pressure, temperature, loading and probability of detonation. The reason is that 2618 can more easily flex and absorb shocks than either 4032 or the stock cast alloy. All well and good, but there still is a compromise. All of the piston alloys mentioned in this post will expand at a higher rate than the iron cylinder liner. This means there must be clearance on a cold motor, if there was not as the piston grew at a higher rate than the block it would seize. Also the piston has a much lower thermal mass than the block meaning that for every watt it absorbs it will raise in temperature more than the block (further excasserbating the clearancing issue). 2618 has a farily high CTE (higher than the OEM Piston alloy) meaning that if set to stock piston to wall clearances you would be walking home after they wamred up and seized to the bore. Generally 2618 alloy P/W clearances are set around .0025"-.0035" depending on the thermal load the application imposes on the piston. 2618 is also a softer alloy than the OEM piston due to it's lower silica content. There are three main disadvantages to these pistons.
1. They will wear faster becasue 2618 is softer and there is more clearance at startup.
2. they will make noise at startup until the piston reaches operating temp.
3. ring seal on a piston is genrally not quite as good as OEM due to the larger P/W clearances (However the additional boost ETC you are allowed to run will easily offset these small losses).
Keep in mind that every sytem on a stock STi is desinged to be reliable at stock power levels; not just the pistons. Sure you may put a turboback on you car adressing the restrictive stock sytems, you may have even put in a larger fuel pump and injectors. But, have you considered whether or not the rails and lines can supply adequate fuel to EVERY cylinder, are you sure the wastegate on the turbo can still adequately control boost? Answer no to any one of those questions and you just bought a motor. I believe many of these ringland failures can be traced back to another sytem on the car that was not fully thought out, it helps to step back and look at the big picture (or pay someone who can properly visualize it to do it for you).
Hope this helps, Jordan
Here is a link to a good article highlighting the difference between 4032 and 2618 alloy.
http://www.moparmusclemagazine.com/t...son/index.html

[Flycaster]

Nice write-up. I've thought of piston lands as that portion of the piston under, or in between the rings; iow, what you call "piston land # 1" is what I've always called part of the crown. Whatever, I think you've done a really nice job of explaining this to the OP...and I also like the musclecar mag link too. Well done. Incidentally, in the top picture there also appears to be a head on fracture in what you referred to as "# 1", probably caused by ring wobble after the land below gave way...that thing got hammered.

OPBSTi - the "science" you're looking for is more art than it is metallurgy. It all boils down to how good your systems are, how well you are tuned, and ultimately, common sense. By that, I mean if you try and force unreasonable power out of any engine, it will fail. Power and reliability do not occupy the same space - they are pretty much inversely proportional.

[MartinSTi05]

Originally Posted by Flycaster

Nice write-up. I've thought of piston lands as that portion of the piston under, or in between the rings; iow, what you call "piston land # 1" is what I've always called part of the crown. Whatever, I think you've done a really nice job of explaining this to the OP...and I also like the musclecar mag link too. Well done. Incidentally, in the top picture there also appears to be a head on fracture in what you referred to as "# 1", probably caused by ring wobble after the land below gave way...that thing got hammered.

OPBSTi - the "science" you're looking for is more art than it is metallurgy. It all boils down to how good your systems are, how well you are tuned, and ultimately, common sense. By that, I mean if you try and force unreasonable power out of any engine, it will fail. Power and reliability do not occupy the same space - they are pretty much inversely proportional.

Thanks, Flycaster!
Simple differences in nomenclature.
Flycaster, you forget the third variable, cost;-) Generally you are only allowed to work with two at a time though
Also, I like how many people think the "old school hot rodders" and muscle car guys have nothing to offer on subjects in involving imports and turbos. In the end listening to and reading about what these guys know and have done lends valuable insight into our own situations. They have already figured out most of our problems for us, they have just bought twice as many pistons figuring it all out;-)

I agree with Flycaster in his systemic approach. As I mentioned in my earlier post just because you broke a ring land on a stock piston doesn't mean the piston is junk, you may just as well break a forged piston if it is the only thing you changed since blowing up the stock motor. There have been many on the forums running 400WHP or more for some time on a stock block/pistons carnage free. I am willing to bet those lucky few know a thing or two about proper fuel delivery and tuning though.

A piston stock or otherwise almost never dies from outright cylinder pressure (too much horsepower) under non detonation operating conditions. What kills pistons is almost always too much heat, detonation related cylinder pressure spike, or interference such as dropped valve etc.

[OBPSTI]

Thanks everyone for sharing such excellent information!

[Flycaster]

Originally Posted by MartinSTi05

...you forget the third variable, cost...

Unh, unh, not me. I remember it every time I get in the car and twist the wick in this little monster.

[Damon]

good thread... i learned something new today.

[stimikey]

All the above + think pro tune by someone who knows what they are doing to get you safe, reliable power increases.