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Old 12-25-2005, 05:40 PM   #26
BigJ04STi
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There's no way to weigh the actual weakness of the stock piston or the stock piston design...

For 1, we only only know which casting method they used and nothing else. We do not know any one the quality control info, on what poercentage of pistons would be expected to fail prematurely due to any number of external or internal related issues...

Also...

We have no control group of pistons or STi engines to compare 1 to another...no one uses the same fuel, lives in the exact environmental conditions (including weather, temp, humidity, etc), same elevation, etc...Also driving methods and distances can cause issues...care of the car can effect things also. Plus there's also tuning and other parts involved here...

We will never know a true number or percentage of failures in STi pistons...

This being said... Using better fuel, keeping tunes mild, not going for excessive boost amounts, or pushing the car at WOT 24/7, along with good car care should allow the pistons to last for their full life.

This all being said...just don't be stupid with your car...and you should be ok. Unless you got the set of pistons that were in the percentage group of premature failure due to manufacturing defects...in that case, there's nothing you can do...

Good write up...it's a solid place for people to begin.
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Old 12-26-2005, 10:43 AM   #27
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I love it.

Quote:
It is the primary silicon that gives the hypereutectic it’s thermal and wear characteristics. The primary silicon acts as small insulators keeping the heat in the combustion chamber and prevents heat transfer, thus allowing the rest of the piston to run cooler. Hypereutectic aluminum has 15% less thermal expansion than conventional piston alloys.
The 15% less thermal expansion figure really doesn't capture whats going on here. As you said "do the math" and 15% of 0.002 is not very much difference. With hypereutectic pistons the piston never heats up as much becuase the silicon particles reflect the heat. So it's a combination of 15% less expansion + NO expansion (pistion never "sees" the heat/temp). In other words compared to eutectic, the hypereutectic pistion stops expanding before the eutectic because it never gets as hot. It's the reflective property that contributes the most to the nearly 4X less clearance needed for hypereutectic.

When comparing 2816 to 4032 the reflective silicon grains are not present. It's only the expansion of the alloy you need to consider. The difference in 2618 and 4032 alloy expansion is trivial. If you can run 2618 CP pistons at 0.0025, then you could run the same pistons made of 4032 at 0.0022. That's not a big deal. The difference between 2618 and 4032 is hardness. It's the ability of the metal to resist deformation when pressure is applied. A metal is 'harder' than rubber, but rubber is more "forgiving" to sudden shock. Likewise, but with less disparity, 2618 is more forgiving of detonation than 4032. A rubber piston might completely absorb detonation shock, but is would not last very long oscillating in a cylinder.
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Old 12-26-2005, 04:26 PM   #28
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Quote:
Originally Posted by bboy
The 15% less thermal expansion figure really doesn't capture whats going on here. As you said "do the math" and 15% of 0.002 is not very much difference. With hypereutectic pistons the piston never heats up as much becuase the silicon particles reflect the heat. So it's a combination of 15% less expansion + NO expansion (pistion never "sees" the heat/temp). In other words compared to eutectic, the hypereutectic pistion stops expanding before the eutectic because it never gets as hot. It's the reflective property that contributes the most to the nearly 4X less clearance needed for hypereutectic.
That's interesting. I was curious about the reflectivity of silicon at thermal wavelengths, so I did some searching. It appears as though silicon is significantly LESS reflective than aluminum, especially at thermal wavelengths. Silicon is also much less reflective at visible wavelengths having an average visible reflectivity of around ~30% compared to aluminum's ~70% or so.

Here's a page showing Aluminum's thermal reflectivity at about 95%: http://www.mellesgriot.com/products/optics/oc_5_1.htm

Here's a page saying that Silicon is "nominally transparent" at thermal wavelengths: http://www.ligo.caltech.edu/docs/G/G...G050059-00.pdf

I think it's fair to say that silicon definitely does not reflect heat back into the combustion chamber. However, it may still be possible that an increased silicon content makes the aluminum piston more translucent to thermal energy, which would mean the thermal energy would go through the piston without heating the piston itself. Unfortunately, to make any significant difference, I would imagine the silicon content would have to be quite high, much higher than the 10-20% range in the near-eutectoid range. It would have to be very hypereutectic.

Anyway ... "thermally transparent pistons" ... the next rage at NASIOC.com.

-Adrian

Last edited by SaabTuner; 12-26-2005 at 04:36 PM.
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Old 12-26-2005, 07:09 PM   #29
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This post is totally pointless ... but I just found and even better grade of Beryllium, Instrument Grade I-220H. It's 30% stronger than S-200 Beryllium!!

Now if I could just win the lottery ...
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Old 12-29-2005, 12:07 AM   #30
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Why would you use beryllium as a piston material? Are you aware of the dangers associated with processing that material? There is more to a material than just the physical properties. Beryllium isn't used a whole lot as a primary material any more... (as an alloying element in things like copper, etc. it's still used)
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Old 12-29-2005, 12:17 AM   #31
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Here's some info off of the Subaru global site which talks about Subaru pistons, the coatings used, the skirt design, etc.

Quote:
The radical change in the piston of the Subaru engine began four years ago with the second generation Legacy, when the Subaru piston underwent a change in its shape and style. Its rebirth was heralded as a "piston revolution." Toward the twenty-first century, an engine with lower harmful emissions and reduced energy consumption has become highly sought after. Even Subaru's Horizontally- Opposed Engine, which has undergone continuous improvement over the last forty years required thorough re-examination of the smallest details.

The engine can be called the heart of a car and the piston is the most important part of an engine. The piston's job is tough and complex. It plays a role in compression, combustion, exhaust, and air intake through its up-down motion. For instance, a piston which executes 6000 revolutions in one minute and in the same time achieves speeds of about 90 km/h (approximately 50 mph) . This means, in a fraction of a second, the piston accelerates from zero to 90 km/h and then returns again to zero and this extremely vigorous motion reoccurs 6000 times a minute. A further example of the piston's brutal motion is a force of nearly 7 tons at the instant of combustion for it to generate 200 hp. The piston works tirelessly under an unrelenting motion. A mass-produced piston needs to be capable of undiminished durability and reliability even after 500, 000 kilometers and 30 years of use. Pistons were not parts that underwent revolutionary changes and mass produced pistons were designed based on conservative empirical engineering principles. Tatsumi Obayashi, in charge of Subaru Powerunit Research and Experiment Dept., Subaru Engineering Division at the time of this groundbreaking revolution in pistons, reflects; "The ideal piston is light and moves smoothly. This ideal is something we resolutely strive for."

Development of the piston took one and a half years. Experimentation was repeated many times to reach the conclusion that "a piston that moves up and down in a smooth rolling-like motion is ideal." The piston can be thought of as sliding up and down smoothly inside the cylinder. However, in reality the motion is frenetic. The piston hits the inside of the cylinder wall randomly in a number of places and this secures its up-down movement. Contact between the piston and the cylinder wall cannot be avoided. In view of this, can the violent motion of the piston be a smooth rolling-like motion?

Through experimentation, the engineers learned that when the piston is slightly barrel-shaped, the piston makes a smooth, rolling-like up and down motion, while friction, and vibration and noise are reduced.

On the other hand, in order to reduce weight, we focused on improving the piston skirt. In former technology, a long thick skirt was considered necessary to provide adequate rigidity for the piston and ensure a precise up and down motion. However, the engineers challenged this with the opposite concept of a shorter, thinner and softer skirt, which can change shape freely like a spring.

"We requested a short, thin, soft prototype skirt from the piston manufacturer. They wondered if we should even attempt such a thing. The concept departed from conventional wisdom to that extent," laughs Mr. Obayashi. In a break with the conventional wisdom, the world's shortest, thinnest and softest skirt is the most appropriate for "a piston that moves up and down with a smooth, rolling-like motion" When it came to accumulating the fundamental technology for this new piston, a range of improvements and refinements for details were made possible.

The compressed gas that builds up in the small spaces between the piston head above the compression rings and the inner wall of the cylinder is difficult to combust. This unburned gas is contaminated with exhausted gas and reduces fuel economy. To prevent this problem, the gap between the piston head and the inner wall of the cylinder was made smaller. For this reason, it was necessary to reduce the amount by which the piston pin is offset. To reduce the offset it was necessary to align the weight with an piston to adjust for the center of gravity.

The finishing touch was the coating process. To ensure that the grooves of the piston rings do not wear, they were treated with alumite. Since it is difficult to apply a coating only to the grooves of the piston ring, the entire piston head was treated with alumite. The skirt was coated with molybdenum to reduce frictional resistance. This results in a two-toned piston. The former piston was a mono-tone aluminum-gray.

The newly developed piston enhances fuel economy remarkably. Running fuel economy is improved 0.5 percent and idling fuel economy is improved 2.3 percent. Since fuel combusts well, fuel economy is improved, and at the same time, the exhaust gases are cleaner. This piston has been named the low fuel (LF) piston.

Currently, the LF piston has been adopted on the Legacy, Impreza and the minicar, Pleo (Japanese domestic model). Recently, the engines of European manufacturers and Formula 1 cars have adopted pistons with similar development concepts to the LF piston. These are not copies of the Subaru piston but the technology is similar due to the similar direction and aims in development. Subaru developed its piston technology a little faster than other companies.

The LF piston has a pleasing shape and a stylish two-tone color much like a precious metal. In the search for a technical rational for one part, a beautiful shape and color is certain to result. The Subaru piston provides evidence for the aesthetic nature of the technology.
They have a bunch of additional info on other parts of the boxer engine, too
http://www.subaru-global.com/about/parts/01.html
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Old 12-29-2005, 12:23 AM   #32
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Quote:
Originally Posted by SaabTuner
That's interesting. I was curious about the reflectivity of silicon at thermal wavelengths, so I did some searching. It appears as though silicon is significantly LESS reflective than aluminum, especially at thermal wavelengths. Silicon is also much less reflective at visible wavelengths having an average visible reflectivity of around ~30% compared to aluminum's ~70%
I haven't chased the data down myself, but one thing to consider is that you want to understand how the silicon alters the properties of the aluminum alloy which contains it - that is generally very, very different than looking at the properties of the base material (silicon) and trying to carry those properties over to the alloy. In other words, the properties of bulk silicon are almost irrelevant when considering the impact which silicon has on an aluminum alloy.
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Old 12-29-2005, 12:59 AM   #33
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Quote:
Originally Posted by DrD
Why would you use beryllium as a piston material? Are you aware of the dangers associated with processing that material? There is more to a material than just the physical properties. Beryllium isn't used a whole lot as a primary material any more... (as an alloying element in things like copper, etc. it's still used)
Yeah, the only pistons I've seen it used in used the Berylcast AlBe alloy or something similar. However, that was primarily to make it easier to form. If you spent the time to form it from a billet, you might be able to take its full advantageous properties, which are much greater than the AlBe MMC.

The reason for using it is its extremely high thermal conductivity (higher than Al, almost as high as Copper), high strength/weight ratio (almost as high, if not higher than, titanium) and high rigidity, which minimizes cyllinder-bore distortion under load. It's a very good combination of properties for a piston, but it's just not good enough to justify the huge cost for a 5 lb billet from which to cut each piston. We'd be talking $1,000 per piston here. Crazy expensive.

And, although I'm quite aware of the dangers, most of the new cases of Berylliosis are due to improper safety precautions. It should be treated like a toxic chemical and, instead, it is treated like a nuisance. When the dust collects around the machine from insufficient air venting people just vacuum (sometimes even sweep!) up the dust without even wearing a respirator or mask! People need to treat it like spray paint or some other toxic chemical. I think the problem is that people don't perceive it as a danger since they cannot smell it and since the symptoms don't show up for years in most cases. Even just wearing a breather/filter and NOT taking your work-clothes home, would probably prevent almost any case of illness. Fortunately, Berylliosis cases are dropping greatly and there haven't been any serious new sensitizations in years.

Now on to the next reply ...
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Old 12-29-2005, 01:08 AM   #34
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Quote:
Originally Posted by DrD
Here's some info off of the Subaru global site which talks about Subaru pistons, the coatings used, the skirt design, etc.
I actually read that quite a while ago, but I am skeptical of the benefits in terms of performance. I'm sure it's great for gas mileage and emissions, but in a performance engine, you do not want soft side-skirts. In fact, you want an extremely rigid piston to avoid distortion which sucks away power under high load through reducing the effective compression of the piston due to larger clearances around it.

This strive for rigidity is so great that Formula 1, in an effort to reduce costs for the less-well-funded teams, banned ANY material with a specific young's modulus over 40 GPa/(g/cc) because there were materials, like Beryllium/Aluminum alloys used by McLaren Mercedes, which were extremely expensive, but also extremely beneficial because of their higher Y.M.. In fact, they also banned its use in brakes because you can make the brake calipers drastically stiffer and more effective by using the expensive AlBe alloys.

The other HUGE benefit of AlBe alloys, and pure Beryllium, is the comparatively low coefficient of thermal expansion. That allows you to use a piston with less than half the cold-clearance and to operate the piston through a wider range of temperatures. That's definitely a potential benefit. It's a benefit shared with Carbon/Ceramic piston technology as well ... which is also banned in F1 along with Metal Matrix Composites anywhere in the engine.
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Old 12-29-2005, 01:18 AM   #35
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Ive been waiting for this FAQ good job!
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Old 12-29-2005, 01:21 AM   #36
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Quote:
Originally Posted by DrD
I haven't chased the data down myself, but one thing to consider is that you want to understand how the silicon alters the properties of the aluminum alloy which contains it - that is generally very, very different than looking at the properties of the base material (silicon) and trying to carry those properties over to the alloy. In other words, the properties of bulk silicon are almost irrelevant when considering the impact which silicon has on an aluminum alloy.
That's true. Properties of the elements added to an alloy do not define the properties of the alloy as a whole, but they do affect it.

As I know that you, of all people, are aware, there will still be extremely small, but seperate, crystals of aluminum and silicon forming the matrix. (unless it's an intermetallic alloy like Titanium Aluminide, where crystal structure is shared) Over extremely short distances, many of the properties of the elements themselves remain.

So I would assert that, were I to guess about the changes in the matrix's properties, the added silicon acts to "soften" the reflectivity of the otherwise opaque aluminum. I don't think it will make the aluminum "translucent" to heat. Rather instead that it would allow some of the infrared photons to penetrate deeper into the matrix (longer mean free path due to translucent silicon micro-crystals) and be absorbed by the aluminum and then dissipated into the cooling system as heat instead of being reflected back into the combustion process.

Absorbing, instead of reflecting, more thermal radiation might reduce the tendancy to detonate since thermal radiation plays a strong role in detonation.

But I still think it was just for the wear properties and because of the comparative cheapness. (High silicon aluminum alloys have a low casting reject rate, or so say Chrysler failure analysis engineers.)

-Adrian
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Old 02-23-2006, 08:35 PM   #37
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Beryllium is not that expensive or rare, speaking from my experience in molding.

We use this stuff in alloy form in every single one of our bottle molds (we have over 1K different molds about 10 cavities each). It is true that you can eventually develop Berylliosis from breathing constant Beryllium dust though, but I've never heard of anyone personally with the superior venting/closed environments of today's CNC machines (as opposed to an open Bridgeport mill)

There are several superior conductive materials that we use. Moldstar 150 being the highest and MoldMax Hi-Hard (BeCu).

I would think you'd want a non-conductive piston material though for turbo engines, to keep the heat in the exhaust, but what do I know? I am sure there is a point where high conductivity starts to hurt HP performance.
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Old 02-24-2006, 02:12 AM   #38
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Quote:
Originally Posted by wgknestrick
I would think you'd want a non-conductive piston material though for turbo engines, to keep the heat in the exhaust, but what do I know? I am sure there is a point where high conductivity starts to hurt HP performance.
A non-conductive material gets hotter and requires more bore clearance. It is also often said to exacerbate the propensity of an engine to detonate because it raises combustion temperatures.

Quote:
Beryllium is not that expensive or rare, speaking from my experience in molding.
Last time I checked, pure Beryllium was at around 300-500$ per pound. Consider then that it cannot be processed in a normal way as well. For instance, to be processed into a billet, it must be wrapped in a steal sheath before being pressed. AFAIK it also does not have the ductility to be forged.

Quote:
There are several superior conductive materials that we use. Moldstar 150 being the highest and MoldMax Hi-Hard (BeCu).
No offense, and I do appreciate your input, but I think you should have done more research first.

1) All but one of the alloys you've mentioned conduct less heat than pure, instrument grade, Beryllium.

2) Though MoldMax high-hard is twice as strong, it is almost five times heavier and conducts less than half as much heat. (104 W/m-K)

3) Moldstar 150, the most conductive of all the alloys relating to this, conducts 259 W/m-K. Instrument grade Beryllium conducts almost as much heat @ 216 W/m-K, is still almost five times lighter and almost as strong.

I think it's fair to say that, in light of Beryllium's low weight, high strength, high modulus of elasticity and high thermal conductivity, Beryllium is still the material of choice for pistons.

References...

MoldMax and Moldstar BeCu alloys:

http://www.matweb.com/search/Specifi...bassnum=NPER00
http://www.matweb.com/search/Specifi...bassnum=NPER01
http://www.matweb.com/search/Specifi...ssnum=NBUCOR31
http://www.matweb.com/search/Specifi...bassnum=NBBC17
http://www.matweb.com/search/Specifi...bassnum=NBBC19
http://www.matweb.com/search/Specifi...bassnum=NBBC18

Instrument grade Beryllium, again:

http://www.matweb.com/search/Specifi...bassnum=MBEI2H

-Adrian
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Old 07-30-2006, 04:25 PM   #39
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Quote:
Originally Posted by Phil Jr.
any data that says this "weakness" is in the pistons or the tune? Its hard to believe the pistons are weak past the 300whp mark when so many people have 400+whp daily drivers on stock internals...

How do I word this: I know for a fact that the majority of the time it is the piston when it comes to Hyper-U pistons. They physically cannot take the chamber pressures that are associated with high boost-high hp engines. They are good for what they are intended...but the line for not crossing is pretty clearly marked with them. You can almost tell at what point a hyper-u piston will fail (within batches...ie blown V-8 overhead cam engines)...

they are good in that they have more quality control than cast but bad in that they do not have the physical strength of forged.
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Old 08-15-2006, 11:02 PM   #40
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[quote=Unabomber][size=3]Piston FAQ[/si

For the record, Cobb Tuning’s pistons start life as JE Piston forgings that are wholly CNC machined by Cobb Tuning. This is a common practice though, as many “manufacturers” use other manufacturers’ forgings as a 2000 ton forge costs millions whereas a CNC machine may be within their capital reach. In fact, JE Piston themselves are rumored to use TRW forgings. Additionally, you may find XXX’s pistons are actually pistons from one of the above manufacturers made to XXX’s specifications. This list represents true manufacturers and not resellers. Also realize these are the companies that stock Subaru specific pistons. As long as you have the correct measurements, almost any aftermarket piston manufacturer can custom make pistons to your specifications.


TRW is no longer TRW FYI. In 2002 Northrop Grumman bought TRW and then sold TRW automotive and financial groups. They kept the space and technology group. Northrop didn't sell either group as a whole either it was pieced out (liquidated if you want). To my knowledge though the original plants are still manufacturing today under different ownership. I'm not sure but I don't think there are any TRW automotive parts anymore as the buyers of said assets are using a different name.

Jacob
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Old 09-25-2007, 06:45 PM   #41
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why 5-30 oil for breakin ??

I hear people using 10-30 all the time...
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Old 10-23-2007, 08:41 PM   #42
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I have a 04 sti and this is the second time I blow my stock block! and it is always the piston # 4. is it ok to just build one with only pistons and rods?
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Old 06-21-2008, 10:13 PM   #43
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Default Piston slap question

I have looked quite a bit over many diffrent threads and i can't really find the answer to my question. i have to say that this is a great write up by the way very useful thanks!! i just bought myself an 03 impreza wrx 2.0L turbo, i was wondering does piston slap effect this model or is it just the 2.5L motors. i have read in the other threads that apparently the piston slap problem that is occuring is in the bigger bored motors but i'm not sure. also is a slight valve tap normal?? sorry i know a little off the piston topic just figured i would throw it in. there is nothing done to the motor completly stock. thanks in advance to anyone who can answer.
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Old 08-06-2008, 10:57 PM   #44
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Quote:
Originally Posted by subypeeps03 View Post
I have looked quite a bit over many diffrent threads and i can't really find the answer to my question. i have to say that this is a great write up by the way very useful thanks!! i just bought myself an 03 impreza wrx 2.0L turbo, i was wondering does piston slap effect this model or is it just the 2.5L motors. i have read in the other threads that apparently the piston slap problem that is occuring is in the bigger bored motors but i'm not sure. also is a slight valve tap normal?? sorry i know a little off the piston topic just figured i would throw it in. there is nothing done to the motor completly stock. thanks in advance to anyone who can answer.
well let me just say piston slap that you can hear can be identified
by running the engine and if you motor has plug wires unplug the sparkplug wires from the coil (pack high ternimal)while listening to the suspected slap knock it ( the slap sound) should be noticeably quioter
when the plug wires disconected for a second .Be carful not to get shocked with the spark i sometimes loosen the wire first then carefully pull it loose the rest of the way or use a wood stick taped to the wire ..
if your motor has no wires just coils setting on the sprk plugs?
you might disconect the indevidual coil primary leads 1 at a time
....................................outherwise.... .........................................
alternatly rev the engine to say 2500 rpm the remove the sprak wire one at a time if your slapping is quiter when a certain wire is disconected there you may have a proublem (slap , rist pin,rod bearing?)...dont forget to snug wires up good after doin this test .... And .....as for the valves
tapping i might suggest an oil additive some will know it by the name CD2- oil treatment .. a detergent for desolving engine gum and varnish
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Old 08-07-2008, 07:47 PM   #45
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Smile thanks

thanks i will certainly use the oil additive becuase i now know that the wrx has a tap like sound because of being a boxer motor as for the piston slap i will def try that plug wire removal thanks again.
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Old 04-22-2009, 10:04 PM   #46
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I am at true awe at Unabombers informative posts...I feel like I know EVERYTHING after reading them. I've been to many automotive forums in my years, and none have as much information, so well organized, by ONE person.

The second best board MIGHT be VWvortex but they still don't have the amount of tech NASIOC has.
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Old 05-09-2009, 04:15 PM   #47
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Hello. I'd like your opinion about what kind of pistons to use.
I'm getting a new short block and i want only drop in pistons.
Untill now, i'd had race pistal pistons, but i'm using new short block because of big oil consumption.
The car is 450-460 hp, with amr-t50r turbo.
What about mahle pistons?
A friend of mine here in Greece, told me that in USA, mahle had very good resalts in subarus. Is that correct?
Thank you.
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Old 07-14-2009, 05:24 PM   #48
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2008 Black Miata

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I just talk to Customer Service at Mahle and was told the Subaru Pistons were 4032. Sucks because I have a set waiting to go in my 08 STI.
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Old 04-23-2010, 11:22 PM   #49
joe1204
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I have read through this whole post and am still wondering if someone had to give a rough figure where your pushing the stock rods and pistons what hp/boost range would that roughly be?
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Old 07-30-2010, 11:46 PM   #50
Jagular1785
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Join Date: Nov 2008
Chapter/Region: MWSOC
Location: Iowa City, IA
Vehicle:
The Mr.
Wizard

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I have an EJ22 phase I that I would like high compression pistons (11:1-11.5:1) for. Anyone ever really done this? Am I going to have to do custom pistons?
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