Subaru Legacy Forums

Subaru Legacy Forums (
-   Turbo Powertrains (2.0GT, 2.5GT, 2.5XT) (
-   -   To those of you considering aftermarket forged pistons... (

Boostin 02-07-2010 08:00 AM

To those of you considering aftermarket forged pistons...
1 Attachment(s)
Some of you may have blown a motor (lost a ringland perhaps) or may blow one soon, and you may be thinking "while I'm in there, I might as well upgrade to forged pistons. I don't want something to break again in x miles."

Just keep in mind that there is always a tradeoff. Aftermarket forged pistons require looser clearance between the piston and the cylinder wall. There are a number of reasons for this, and we can debate all day, but it has to do with the material these pistons are made of and the way people tend to use their motors when they have these aftermarket pistons.

On factory pistons, the clearance between the piston and the cylinder wall is going to be between 4 and 12 ten-thousandths of an inch. On aftermarket forged pistons, clearance may be between 25 and 40 ten-thousandths of an inch, sometimes more. That is at least twice the clearance compared to OEM style pistons.

The extra clearance means a couple things. It gives the aftermarket pistons more room to expand in the cylinder, and this may be good for very intense driving and high horsepower setups. You don't want your pistons to seize. However, because the clearance is greater, your motor will have increased blowby and increased wear on the cylinder walls.

The increased wear occurs due to possible piston slap during startup. Piston slap is when the piston moves laterally against the cylinder wall due to the large clearance. It can be audible and annoying. As the motor warms up, thermal expansion will tighten the clearance and the piston slap should go away. Piston slap is not the end of the world but it doesn't help longevity.

Many of you deciding on pistons may have a relatively mild setup, like a small bolt on turbo upgrade or even just a stage 2 setup. Longevity may be of great importance to you. And in that case, aftermarket forged pistons are not a "no brainer" answer. It doesn't mean you shouldn't get them. It just means you should ask yourself what you are really using this car for and also ask yourself whether you can tolerate the drawbacks of aftermarket forged pistons.

I have also included an article written by Inline 4 (Honda Tuner) explaining the difference between forged pistons and cast:


All Motor - OEM or Forged Pistons??
The Final Lap - Mar. 2009 Newsletter ver. 1.0

To all of our patrons and supporters,

This month, I am going to touch on the differences between OEM and Forged pistons, and what is an appropriate application to use them in. Maybe it's because we constantly have a daily onslaught of technical questions or lately there has been a huge spike in engine rebuilds, but I've been sounding like a broken record lately, suggesting specific OEM pistons in popular B-series N/A (Naturally Aspirated) applications. Majority of our customers drive their vehicles daily. Therefore, reliability is always a factor to consider when building a street-engine. The single best way to obtain a healthy compression ratio, stay very reliable and make good "useable" horsepower, is to use OEM pistons in your build. Why not forged pistons, you ask?? They are lighter and stronger, so why can't we use them? The answer: They ARE NOT designed to be used in a daily driven application. Well, what are they designed to be used in Brandon? The answer: A RACE ENGINE. And what is done to a race engine after a race or after a certain number of races? They are checked from top to bottom and rebuilt if needed. Anyone who builds a street purpose motor has no intentions of tearing that puppy down every 10k just to see how things are doing in there.

Now to make sense of all this. The reliability factor between OEM and forged pistons is in the material and how it behaves. Forged pistons are made from two different alloys. 2618 aluminum alloy and 4032 aluminum alloy. The difference is that 4032 has more silicone built into the piston, allowing for LESS thermal expansion, and allowing you to run a tighter piston-to-wall clearance. What is piston-to-wall clearance? Piston-to-wall clearance is just what it says. It's the area between the piston and the cylinder wall. This clearance is built into the bore so the piston can expand correctly. 2618 aluminum alloy has less silicone and expands much more than 4032. However, 2618 has proved to be stronger and can take much more abuse (detonation). Under detonation, I have seen the "ring lands" become brittle and crack in 4032 alloy pistons. The disadvantage to having 2618 alloy is because of the loose piston-to-wall clearance that must be run. Typically, most manufacturers of 2618 alloy pistons require a wall clearance of .0030 to .0045in. (3 to 4 1/2 thousandths of an inch). When it is this loose, there will be a considerable amount of "piston slap". When you first start the engine and it is cold, you will hear the pistons rocking back and forth in the bore, giving off a "clackity clackity" sound. As the engine warms up, this sound will diminish or dissappear entirely, depending on the clearance.

What's so bad about that?? Well, imagine this event in a daily driven engine. The car is started on an average of 2-4 times a day. Multiply that by a number of days, then weeks, months, etc. What ends up happening is your blocks cylinder walls become "chaffed" and "out-of-round". Thus, leading to improper sealing and oil consumption. Equalling an oil drinking motor, with crappy compression numbers, decreased power and loss of performance. All of this, after you spent a few thousand on your engine build and it's only been about 20 thousand miles?? Race teams understand this and use the piston for what it is intended. To accept the abuse from the rigors of racing. The engine will be checked and if it needs new pistons and bearings after 3 races, then so be it....that's just how racing is. As a matter of fact, in some all motor drag race applications, I know of a team, who at the beginning of the season, will start off at a 84.5mm bore, but by the end, will be at 86mm bore. They purposely build the engine "loose" to get as much horsepower as possible, but this requires them to "correct" the chaffed cylinder and go larger on the bore on each rebuild. At the end of the season, the block is thrown away and a new build started on a fresh block for next season.

Ok, so i've thrown all this at you about forged pistons, now what about OEM pistons? Where do they stand in all of this? OEM pistons are used in many "autocross" and "roadracing" applications and obviously, should be used in your street builds. OEM pistons are used in roadracing over forged pistons, because of the effective sealing properties and reliability that they provide. As you may know, many B-series pistons are interchangeable between B-series blocks and you can achieve a great performing and reliable build with them. Remember, that not all of the build is in the bottom-end (block). You still have the cylinder head, camshafts and most of all, tuning. For those running a 81mm or 81.25mm bore, the Type-R series pistons are ideal to run. Depending on your block, you can run Japan spec Integra Type-R pistons or Civic Type-R pistons. What about 81.5mm or 82mm?? Well on an "un-sleeved" block, 82mm is the biggest I would suggest. We have our own "il4 performance" pistons that are cast by the same manufacturer that casts them for Honda. This company casts the US Spec Integra Type-R piston for us in 81.5mm, 82mm and 84mm, 84.5mm for B20 applications. OEM Honda is all you will ever need for your daily driven, all motor build or for your autocross, roadrace build.

Give us a call at 714-903-1898 for pricing and info on other services such as connecting rod service, crankshaft service and custom piston machining.

In the next newsletter, I will discuss parts researching, buying quality and not just the name.

Till next time...
Brandon Bacio
Director of Operations

Underdog 02-07-2010 08:40 AM

Good summary Boostin! :)

vduber 02-07-2010 09:48 AM

Good post I was talking about this with integroid the other day. This is why there are things like piston slap with built motors until its warmed up. It all comes with the territory of having of having a high hp application that requires rods/pistons, I believe it all depends on how good of a builder you have that produces the outcome of your motor.

rao 02-07-2010 10:19 AM

Nice summary, but anybody that actually has taken the time to educate themselves about this topic knows the facts. Most people don't care because we all know that FORGED PISTONS ARE BETTER AND MAKE YOU A BETTER PERSON :lol:

Legend 02-07-2010 10:47 AM

Good info, thanks.

JoeFromPA 02-07-2010 12:15 PM

Thanks Boostin. One thing I learned (after reading way more than I should have had to) is that for most of the ~250-350awhp daily driven crowd, a pair of well-made cast pistons with stock-like clearance would be an ideal solution. But all we've got is 6 different types of forged pistons, or stock. I think it was GTTuner who recently posted a pair of forged pistons with massive depth to the ringlands. Would that be possible with a cast solution?

And, secondly, are the stock pistons really that much of a problem or is it simply that the stock tune is very questionable & alot of tunes also have dangerous running conditions, leading to the illusion that the stock pistons are weak for both stock and tuned cars. Get me?


Boostin 02-07-2010 12:18 PM


Originally Posted by JoeFromPA (Post 2819453)
And, secondly, are the stock pistons really that much of a problem or is it simply that the stock tune is very questionable & alot of tunes also have dangerous running conditions, leading to the illusion that the stock pistons are weak for both stock and tuned cars. Get me?

The million dollar question. For the 05-06, I don't think the stock tune is questionable on a 100% stock car. But I can't say for sure if there is an inherent design flaw in the motor which leads to the ringland failures. Nor can I vouch for the quality of everybody else's tunes. But I do believe that the Subaru ECU is very forgiving and has an aggressive knock control system, IMO more aggressive than any other turbo car I've ever messed with. There are so many cars out there that if you make one mistake, you're dead.

Ridgeracer 02-07-2010 12:40 PM

Agreed on the knock control.....

FireB 02-07-2010 12:52 PM

nice write up.
I'm hoping I won't need it but this is definitely some information I would need just in case something happens.

rao 02-07-2010 12:53 PM

There is no question that Subaru's knock control system is excellent.

The interesting question is the one that is almost impossible to answer unless you can see the piston when it breaks - is the failure due to too much MaD PoWEr or due to knock, wit a turbo engine the answer is almost invariably knock.

John M 02-07-2010 01:35 PM

Anybody got shots of a high-mileage teardown of an engine w/ forged pistons?

I think a lot of the issue is purely academic. Yes, forged pistons require looser tolerances and will cause more wear. How much more wear is the sticking point. Does it mean forged gets "only" 100k miles between rebuilds, or does it mean the block is good for 150k when stock pistons wouldn't wear that much until 300k?

I'm happy with the tradeoff. It'll take me another decade to reach 100k miles and being able to do so reliably is worth the rebuild cost sometime in the year 2018-2020.

Boostin 02-07-2010 06:05 PM

It's too hard to make predictions of engine life, either the original engine or a rebuilt engine with aftermarket forged pistons and looser clearances. There are so many factors involved, so many possible clearances to use, so many things that can make an engine fail.

I think what I wanted to do here is to debunk the idea that aftermarket forged pistons are always 100% better with no tradeoffs.

NSFW 02-08-2010 12:26 AM


Originally Posted by JoeFromPA (Post 2819453)
And, secondly, are the stock pistons really that much of a problem or is it simply that the stock tune is very questionable & alot of tunes also have dangerous running conditions, leading to the illusion that the stock pistons are weak for both stock and tuned cars. Get me?

I get you, and I think you're on the right track.

On one hand, the stock pistons aren't great. In spite of our ECU's knock mitigation people still manage to kill stock pistons fairly often. Busted ring lands happen for other makes too but they seem to happen to Subarus more often. I think it was Boostin who mentioned that in other niches, people just listen for audible knock, and that approach works well enough. I think there's a reason that nobody tunes Subarus that way. Not successfully anyhow.

On the other hand, there are plenty of Subarus out there with crap tunes. I don't know exactly what models/years of factory tunes have this problem but I do know that it's been well documented in 07 STIs. They stay at 14.7 AFR way into boost, before switching to richer AFR. I don't think it's a coincidence that a lot of people at NASIOC believe that the pistons got weaker around the 07 model year, since the failure rate went up. However other people claim that there's no difference in the pistons, and I believe those people. We know the STI tunes changed for the worse, I don't see why people want to speculate that the pistons did too.

It's been seen in other models/years besides the 07 STI, I haven't paid close attention to which. I have not seen evidence that any LGT models are affected by that particular problem, though.

I tried to help a guy at NASIOC clean up what I thought was a simple problem with an 02 Cobb WRX tune, not too long ago, and eventually decided that the timing was just so messed up it would be better to start over from scratch. For starters, the car was pulling 2.3 g/rev and the timing table only extended to 2.1. I don't believe that all Cobb tunes are bad, since I've only seen one up close, but that one wasn't good.

I've also had random people send me logs enough times to know that there are people out there calling themselves tuners who have no business tuning.

I don't think there's any question that bad tunes - tunes that knock a lot - will kill pistons. So when people (usually at NASIOC) post up about their dead pistons, I like to ask what they were doing to monitor knock. In all but one case, the answer was some variation of "nothing, I just trust my tuner."

My guess is that if the tune knocks very little, the stock pistons will last a very long time. I'm hoping to prove that theory by running 400whp, logging a lot, knocking very little, and keeping my stock pistons for another 100k miles.

I'm also ready to buy a new motor though, just in case. :)

JoeFromPA 02-08-2010 08:03 AM

NSFW - I think enough guys on here have been running well-tuned cars at 350-400whp to show that stock pistons blowing at 270whp are more likely due to the tuning.

I've shown myself to be a worry-wart, and all I can say is that I find it extremely unlikely that stock pistons on a stock car in a good state of tune running 92 or 93 octane....well, I think they are fairly safe. I'm going to switch back to a OEM air filter (from AVO) to not push my luck.

And, to your point, the stock pistons + a reliable aftermarket turbo + an opened up downpipe (to help the compressor remain stable + a nice conservative and rich tune = a long-life, reliable LGT. In my mind at least.

Boostin 02-08-2010 08:45 AM

I'll be honest, the ringland thing is really a Subaru deal... not that it doesn't happen in other cars, but I NEVER hear any other car community talk about it. I never hear "Oh shit I lost a ringland on the LS1 at the strip." On Hondas people start freaking out about the stock sleeves (I had a friend drop a sleeve on his Integra), but you don't hear people with other types of aluminum blocks worrying about sleeves much.

On 7M Supras and twin turbo 3000GT's spun rod bearings are a major concern (I had rod knock on my old Supra), but you don't see people with Subarus freaking out about that much. On a DSM it's a timing belt jumping or crank walk in some cases which makes owners fret. On Rx-7s it's apex seals but it's almost never bearings or the crank. You don't need a built motor to make 500whp on that car as long as you can control heat and detonation. Every car has its strengths and weaknesses and then the internet forums can just amplify whatever concerns people have. And remember that many car communities have a "silent majority" of people who are happy with their car, don't have many problems, and don't feel the need to post on the internet about it.

All times are GMT -6. The time now is 11:09 AM.

vBulletin Optimisation provided by vB Optimise v2.6.3 (Lite) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.