Im wondering if...when purchasing your pistons...did you consider the shape of the dish? If you look at a cp vs cobb vs wiseco...they are all different. Does the shape have a 'huge' significance? i understand the quench and from the www.cobbtuning.com website, it seems that they have done some disigning to match our 2.5l cylinder heads. it was a good selling point for me and really made me think about the top of the pistons. I was just wanting your opinions and what you went with and why. thanks
POSTING MORE PISTON PICS.....RAN OUT OF ROOM IN LOWER THREAD...........

ARIAS pistons:
http://i23.photobucket.com/albums/b399/modaddict/arias.jpg

N/A or turbo the quench area does matter. Although standard dished pistons are Ok they really arent so much of a plus over the factory pistons other than the higher cylinder pressures and heat they can handle.

I've seen the CP pistons for a Subaru, anyone have pics of Cobb or Wiseco pistons?

The better the quench area of the piston matches up with the head, the better the combustion will be. Having proper quench really helps speed up combustion and reduce the tendancy to detonate as the quench area will greatly increase the amount of turbulence in the combustion chamber.

Ideally the quench area of the piston will match up with the head.

Here's what i could find off of the cobb, rallispec, crawford, axis, nasioc, and cosworth websites:
Notice the valve reliefs and the overall differences in pistons. the wiseco's have the raised part in the middle, similar to the cp's and turbo diesels that i have had experiences with in the past. What do you think each piston brand has for pro's and con's. which ones did you pic? maybe it wasnt any of these.?
COBB PISTONS...they start out as JE slugs...finished by cobb inhouse.
http://i23.photobucket.com/albums/b399/modaddict/cobbpistons.jpg
WISECO PISTONS
http://i23.photobucket.com/albums/b399/modaddict/wisecopistons.jpg
CRAWFORD PERFORMANCE designed CP pistons
http://i23.photobucket.com/albums/b399/modaddict/cppistons.jpg
AXIS POWER RACING PISTONS
-Notice how they look similar to the cobb's...yet they are made by CP.
http://i23.photobucket.com/albums/b399/modaddict/axispistons.jpg
STOCK VS. MAHLE PISTONS....taken from
http://forums.nasioc.com/forums/showthread.php?t=964995
http://i23.photobucket.com/albums/b399/modaddict/pistons3.jpg
COSWORTH PISTONS
http://i23.photobucket.com/albums/b399/modaddict/cosworthpistons2.jpg
STOCK STI PISTON
http://i23.photobucket.com/albums/b399/modaddict/stipiston.jpg
TECHWORKS: HIGH QUENCH PISTON...made by wiseco
http://i23.photobucket.com/albums/b399/modaddict/techworkspiston.jpg
here is a list from Crawford Performance, all pistons made by CP and have various dishes for various compression ratios/cylinder heads used. taken from:http://www.crawfordperformance.com/forum/viewtopic.php?p=15637#15637
http://i23.photobucket.com/albums/b399/modaddict/crawfordpistondishes.jpg
Ross forged STi piston...long search but i found it.....taken from: http://forums.nasioc.com/forums/showthread.php?t=489989&highlight=ross+piston+pics
http://i23.photobucket.com/albums/b399/modaddict/ross.jpg

Cobb claims a difference. I don't think anyone knows. If there is one it's not that big. Preserving the quench area and distance is more important to me.

Those are CP pistons "made for" Crawford Performance. The real CP pistons don't have the valve reliefs.

Cobb claims a difference. I don't think anyone knows. If there is one it's not that big. Preserving the quench area and distance is more important to me.

Those are CP pistons "made for" Crawford Performance. The real CP pistons don't have the valve reliefs.

Thanks for the info, but you got it backwards. The CP pistons come with the exact same dish as the OEM piston, where our custom order pistons have no valve reliefs. :)

On the piston dish design, we use four different dishes to go with the different motor combos. This is necessary to achieve the proper compression ratio while using the different cylinder heads and displacement of the short blocks available.

Quirt Crawford
Crawford Performance
http://www.crawfordperformance.com/images/pistons/various_pistons_04.jpg

I was looking at the picture were the piston sits on crawford cardboard. I certainly agree, the crawford dish is/looks the same as stock. Sounds like both of you (CP and Crawford Performance) sell pistons without the valve relief.

Cobb claims a difference. I don't think anyone knows. If there is one it's not that big. Preserving the quench area and distance is more important to me.

bboy.
could you elaborate on your last sentence. which piston do you think preserves the quench area the best?

Does anyone have a picture of a factory STi piston? i cannot find one that shows the top of the piston. all pics i found were from the broken ring
lands :lol:

thanks

Does anyone have a picture of a factory STi piston? i cannot find one that shows the top of the piston. all pics i found were from the broken ring
lands :lol:
thanks

There are several pictures here: http://www.airpowersystems.com.au/us_spec/subaru/sti/pistons/pistons.htm

There are several pictures here: http://www.airpowersystems.com.au/us_spec/subaru/sti/pistons/pistons.htm

i new those were there but when i went to www.airpowersystems.com the site said it was under construction.....thankyou for that link. ill post a pic up top.

Don't forget to include TechWorks' pistons in the comparison.

http://www.techworkseng.com/Products/Subaru_Forged_Pistons.htm

Coaster

Don't forget to include TechWorks' pistons in the comparison.

http://www.techworkseng.com/Products/Subaru_Forged_Pistons.htmThe first high quench piston mentioned in this thread?

http://www.techworkseng.com/images/products/pistonsti_small.jpg (http://www.techworkseng.com/Products/Subaru_Forged_Pistons/pistons_sti.html)

(click on pic for hi-res)

Don't forget to include TechWorks' pistons in the comparison.

http://www.techworkseng.com/Products/Subaru_Forged_Pistons.htm

Coaster
Do you have any experience with these pistons?
i havent heard of them before....

I've met the crown designer in Calgary. (I have rebuilt heads and full exhaust on my RS). The forgings are Wiseco. I've touched 'em, and seen them in motors, but never run them.

Coaster

I hadn't realized Quirt's offering had changed that much.
http://i23.photobucket.com/albums/b399/modaddict/cppistons.jpg


The one's I bought from Crawford about OCT '04, I think.
http://awd.chargeair.boxerwagon.gpatmac.com/New_Engine/new_piston/new2.jpg

I hadn't realized Quirt's offering had changed that much.
http://i23.photobucket.com/albums/b399/modaddict/cppistons.jpg


The one's I bought from Crawford about OCT '04, I think.
http://awd.chargeair.boxerwagon.gpatmac.com/New_Engine/new_piston/new2.jpg
the top picture i posted is a picture of an STi replacement piston. I beleive CP has redesigned their pistons 3 times. Hence look at www.crawfordperformance.com and see that under the pistons, it says 'revision 3'.

the bottom picture looks like it is an STi piston for use with WRX 2.0l heads. also seen on Crawfords website. all in all, very interesting.

is the bottom picture of that piston the one you bought for your WRX? or were they for an STi?

For a 257 block in my WRX. I was using my WRX heads at the time. Its been so long, I don't recall if I specified that they should bea certain dish in order to maintain low comp along with the WRX heads.

I'm now using Jap v7 small port STi heads.

For a 257 block in my WRX. I was using my WRX heads at the time. Its been so long, I don't recall if I specified that they should bea certain dish in order to maintain low comp along with the WRX heads.

I'm now using Jap v7 small port STi heads.
yeah thats why, the bottom picture is for a 257 block w/wrx 2.0 heads. comparing the two pistons side by side is like apples and oranges. makes sense now. I still think that CP has redesigned the pistons. ive heard people say on here that they are different now than when they first bought them many moons ago. I really appreciate your input.....

No problem.

Thanks for the info, but you got it backwards. The CP pistons come with the exact same dish as the OEM piston, where our custom order pistons have no valve reliefs. :)

On the piston dish design, we use four different dishes to go with the different motor combos. This is necessary to achieve the proper compression ratio while using the different cylinder heads and displacement of the short blocks available.

Quirt Crawford
Crawford Performance

jesus, don't get me started on your theories about how to fit pistons.

Let's not forget the trick Ross pistons designed by Magnus motorsports.

They have the most interesting crown shape of all, imo.

Sorry, no pics....search nmyeti 's 2.5L thread.

S.

Here's a link to some photos of our different piston dishes :)

http://www.crawfordperformance.com/forum/viewtopic.php?p=15637#15637

Quirt Crawford
Crawford Performance

jesus, don't get me started on your theories about how to fit pistons.
ohhh please :rolleyes:

what do you guys think....should this be a sticky for future piston buyers? such as Una's FAQ or in the top of this forum? or do you think not....

My vote would be yes. Or maybe a mod could just append it to the bottom of Ron's FAQ.

Could you elaborate about the design/application of each one and why the dish was designed that way?

http://www.crawfordperformance.com/images/pistons/various_pistons_10.jpg

Crawford it does not make sense why you have so many piston crown designs, between the subaru heads (asside from the NA stuff) There is basically 2 main chamber shape/desgin and volumes to deal with. WRX and the EJ257 chamber .. which customers get a standard dish and which get the reversed chamber piston? Is it that your lower builds use the standard dish.. because of a cost issue.
Could you explain?

Absolutely amazing! People flame and spread constant lies about Quirt and his “lack” of knowledge then want him to hand out information. Talk about biting the hand that feeds. :rolleyes:


Crawford it does not make sense why you have so many piston crown designs, between the subaru heads (asside from the NA stuff) There is basically 2 main chamber shape/desgin and volumes to deal with. WRX and the EJ257 chamber .. which customers get a standard dish and which get the reversed chamber piston? Is it that your lower builds use the standard dish.. because of a cost issue.
Could you explain?
Toyo - There is more to building a motor than just combustion chamber volume :D
FWIW, you would probably have better luck getting Quirt to answer your questions on his forum.

Chris

Absolutely amazing! People flame and spread constant lies about Quirt and his “lack” of knowledge then want him to hand out information. Talk about biting the hand that feeds. :rolleyes:All I want to know is what makes Crawford's pistons better than the rest? Should you expect me to buy their piston's or built engines because Quirt says it's the best? Why is it the best? After all, as the saying goes, an educated customer is the best customer!

Can someone find a true pic of a JE piston for a subaru. ive found some but i think they are generic pics. I have to go to work and havent found a good one yet....be back in 14 hrs!.....thanks......

if you went from a -6cc dish to a -12cc volume, how much would compression be lowered?

There are many many aspects to piston design.

Squish or Quench area determined by the shape of the dish in relation to the combustion chamber.

Compression ratio determined by size of dish, combustion chamber and engine size

Wrist pin location, determines compression height (up and down offset) and our subarus use a side to side offset as well.

Ring land thicknesses and material help determine the power capacity of the piston

Skirt length helps choose the attitude of the piston in the bore. A longer skirt keeps the piston squared up better and is less prone to slap (piston knock-not the same as detonation) while a shorter skirt offers less friction and less weight.

Oiling is also considered in piston design.

What I feel the consumer should know about ( this is my oppinion based on facts )
Quench area. Quench area is important because it causes turbulence in the combustion chamber when the fuel/air mix is squished between the flat part of the piston and the head. This turbulence helps to equalize temps across the piston and head surfaces as well as help with mixing. Equalizing temps on piston and head surfaces gives you the advantage of less hot spots in the combustion chamber that lead to detonation. We all know that detonation is the enemy. So any piston that utilizes this design is already a superior piston. These pistons are called High Quench pistons. High Quench pistons (like TWE) are more expensive because a left AND right piston has to be designed. This would not be required if subarus didn't use an offset wrist pin.... but they do, so we pay $$$. CP, Wiseco, JE, Cobb don't usually offer these type of pistons. However, close analysis of the dish will give you an idea of who has more quench than who. CP, Wiseco and JE pistons look a lot like the factory STi pistons with the sweeping curves and raised center section. They don't offer much squish. COBB has a more pronounced dish and offers slightly more squish. TWE and some ROSS pistons have an asymetrical dish that matches the combustion chamber very closely and offer lots of squish. This is awesome because you can run a higher compression ratio, get more torque and power on the same boost levels and be just as far from detonation. Or you can run the same CR and run a lot more boost and just be a giggling school girl everytime you push the loud pedal.

I personally would LOVE to run TWE pistons in one of my engines. However, everytime an engine has broke I was poor. I have built 3 engines with Wiseco pistons and 1 with JE. I am very pleased with the wiseco pistons, wasn't happy with the amount of noise the JE made. However, I believe that was a poor machine job by the shop.

I have STi pistons, Wiseco pistons, COBB pistons, JDM subaru 2L (9:1cr) , USDM subaru 2L, and Subaru 2.5L N/A pistons all here at my house, right now. If someone would like pictures I'll gladly post some after I get my camera charged up.

One look at the JDM 2L pistons or an EJ22T piston and you'll see that subaru has utilized an asymetrical dish on their pistons to maximize squish. That was back in the day of piston oil squirters and such. Since then they have moved to a symetrical dish to lower production costs and pump out more engines. This is probably also why they lowered the compression ratios. ROSS EJ22T pistons are almost a copy of the Subaru 22T pistons only they are made of forged aluminum and not cast. The skirts are a little shorter too (factory skirts are huge on that piston) , but really that was the only improvements to be made on the subaru piston.

Great info ImprezaRSX. That should be integrated into the Piston FAQ...

Awwww, he's just showing off.;)

Good stuff, Dominic.

this page has alot of info that I was too lazy to search for. So thanks for all the people who made my questions easy to find!

now I think it's time to closely compare TWE and CP pistons. I've for a while now, reverred (reveered? I don't remember. I'm tired.) all of TWE, Crawford, and Cobb for different reasons. TWE will do a fantastic valve job for less than $400 on two heads (5 angle intake, 4 angle exhaust) Crawford sells really nice internals at a decent price (and has specialty blocks), and Cobb makes some nice cams, and exhaust parts. (especially their n/a EL header!)

I don't think there is a comparison. TWE hands down. But that is my opinion based on quench area, dish design and volume, forging company (honestly partial to wiseco forgings and skirt design).

People I would trust to build an engine in order of my PERSONAL vibe
1. TWE- they tell you EVERYTHING, give you specs that they use, don't keep any secrets. They have INSANE tolerances (insane in a good way!) and a quality control that can't be beat
2. Myself- only because I trust myself. I won't keep any secrets. I don't own the proper tools to even come close to the degree of accuracy that TWE uses, but hey, at least I know what's going on in my motor
3. COBB, AXIS, Gruppe-S (supplied by axis?),
4. anyone else
5. Crawford Performance- Based on history, lack of a quality control system, keeping "secrets" etc.

Again, my oppinion.

I have a COBB built block here in my spare parts room. It seems ok. I thought the pistons seemed a little loose in the bores, but that can only really be properly verified by taking the engine apart and checking the pistons and bores at the right temperature etc.

I have never seen a TWE block. I have spent hours on the phone with them talking about the specifics and engineering aspects of their products. So no real experience personally, just a really really good vibe.

I have only seen crawford blocks from videos on the internet and watching Big Valley's car at Englishtown. Again no personal experience with the product. I have however, talked on the phone and via email to my EXTREME DISPLEASURE. Not pleased at all with the customer service I recieved.

Only heard good things about Axis, again no personal experience with them.

I've built 5 blocks (3 wiseco pistons, 1 JE, 1 Subaru factory) which by no means makes me a block assembling expert. I just wanted to give my hands on experience so you all could take my opinion for what it's worth.

Bravo sir!

One of the Weisco piston motors i helped him build, the machine shop went to the tightest tolerence reccomended by weisco and i have no slap and all of us, the shop included, thought that we would have some piston slap.

I have never seen TWE's pistons, by we have a set of CP's, stock STi's, Cobb's, WRX's and the Weisco's here at the house, and out of all those, i am most impressed with the weisco's dish design and overall construction.

just wanted to bump this up for more piston manufacturers that are not already listed.

ImprezaRSX, thankyou for your input and opinions. I would like pictures from the pistons that you have in your shop. please post.

i would like to hear from other people who have built engines and with what pistons they used.

i think this has turned into a great thread.....glad i started it.

Wiseco piston. Notice the very round dish, not much quench, but it does have very strong ring lands. Good for high hp strength but average detonation resistance. Comes with moly skirt coatings. avg $460 with rings and pins.
http://photos.rs-scooby.com/albums/TurboRS/wisecopiston.jpg

USDM STi piston. Also a sweeping, round dish. Made of crappier material. Comes with moly skirt coating like wiseco piston. Can find for about $150 ish online. This unused set is for sale.... :)
http://photos.rs-scooby.com/albums/TurboRS/STipiston.jpg

USDM WRX piston. More pronounced dish, better quench than previous pistons, however the dish is symetrical and heads are not. This is so subaru can use the same piston on either side of the engine. Less material at the edges of the dish by the rings. Not as strong.

http://photos.rs-scooby.com/albums/TurboRS/USDMwrxpiston.jpg


USDM N/A 2.5L Piston: Lots of suck. This piston only has 13k miles... owner used real crappy gas.

http://photos.rs-scooby.com/albums/TurboRS/NA25piston.jpg

COBB Piston. Notice the defined dish. Probably the most quench you can get on a symetrical dish piston. Uses same piston left and right. This one is ceramic coated, notice the pretty color. Strong piston. My second favorite design. Start at $570 a set + options
http://photos.rs-scooby.com/albums/TurboRS/cobbpiston.jpg

TWE piston. Highest quench aftermarket piston around. Notice the asymetrical dish and the thickness of the piston around the rings. Very strong, very good detonation resistance. My favorite piston. Also the most expensive of the bunch. Has a different piston for the right and the left side. Again this is due to subaru's offset wrist pin design. Start at $700 a set + options. I sit and stare at these pistons and dream.
http://photos.rs-scooby.com/albums/TurboRS/twepiston.jpg

jsut to add to what ImprezaRSX said, the Weisco's that are in my car now have more of a bump in the middle, sort of like the one's in Crawford's pic on the bottom right.

Very nice thread. I'll definately add it to my Piston FAQ. One thing I'll make a note of since there is a lot of discussion on quench. One cannot look at a piston and state something like "best quench". Quench has to do with careful piston design in relation to the clyinder head chamber.

If you put a gun to my head to recommend the "best" pistons, my vote would be for Cobb as they are the ONLY piston manufacturer to specifically state that their pistons were designed to match the shape of the cylinder head chamber. This would ensure the "best quench". No other manufacturer advertises that they have taken this critical step to "perfection" that Cobb has. They are symetrical as stated, but they are the closest thing to perfect that I can see on the market.

I'd have to disagree with you on that. TWE states that thier pistons are high quench also. If you look at the combustion chambers, you'll see that they are not symetrical. The intake has larger ports than the exhaust. The combustion chamber best matches that of the TWE pistons. I'd put COBB at no. 2. Now that is for 2.5L. If we bring the EJ22T into the mix, the ROSS pistons have the asymetrical dish that matches the combustion chamber also. COBB could improve the design, but the sacrifice would be higher pricing like the TWE pistons.

In this pic you can see the asymetrical design (on the horizontal axis) that matches the piston dish on the TWE pistons. This picture is taken at an angle (easy to see if you look IN the ports) making the exhaust look larger than they are, so the difference isn't as dramatic in the photo as it is in real life. This head is also a highly ported head with modified combustion chambers.

http://photos.rs-scooby.com/albums/TurboRS/cchamber.jpg

this pic is much better. This is the combustion chamber that the TWE piston I posted is supposed to match.
http://photos.rs-scooby.com/albums/TurboRS/cylhead.jpg

You can see very plainly how the twe piston would match that much better than the cobb.

From the TWE website: STi Turbo Our exclusive high-quench crown design for the STi:



Trench valve relief design

Increases low valve lift flow
Accommodates STi, WRX, EJ25 SOHC, and EJ25 DOHC heads
Increases combustion chamber boundary turbulence for less ignition lead and reduced detonation

Quench pad area is maximized for all head combinations

Dramatically reduces detonation
Allows higher boost and higher compression ratios
Creates a very rapid burn combustion chamber when combined with correct piston to head clearances

I love this thread.

if you went from a -6cc dish to a -12cc volume, how much would compression be lowered?

We would need more info to properly answer this question.
What size engine, what cc are the cc, what head gasket thickness, compression height, etc.

It would be less.... uh... 6-8 pts? :D:D ... dunno.

More great info - It looks like the Cobb pistons have an oversized quench area for the exhaust side comparing them to the head pics and the pics of the TWE pistons.

I would guess that is better than using smaller quench areas which match the intake side of the head and leave some of the exhaust quench area of the head unused.

It also looks like Cobb's quench area is larger in general than the TWE pistons which is probably why they need the valve relief cutouts.

You prefer TWE? Great. I prefer Cobb. This thread is about design and possibly preferences, no need to justify.

Your assessment that "twe piston would match that much better than the cobb" is 100% false though. There are more factors to piston performance than dish shape with respect to it's orientation to valves. While the theory that the TWE's pistons are a better match = better, the practical application of that theory + the other piston shape factors are something that could only be proven with a dyno, so no one knows the REAL answer. You are correct about the symnetrical issue though which is why I place the caveat in my post about Cobb's pistons not being perfect.

Preference is what this thread is about, not "this one is best" as no one will ever know which is "best".

If you put a gun to my head to recommend the "best" pistons, my vote would be for Cobb as they are the ONLY piston manufacturer to specifically state that their pistons were designed to match the shape of the cylinder head chamber.

Preference is what this thread is about, not "this one is best" as no one will ever know which is "best".


HUH?


Your assessment that "twe piston would match that much better than the cobb" is 100% false though

Quench pad area is maximized for all head combinations


Dramatically reduces detonation
Allows higher boost and higher compression ratios
This would ensure the "best quench". No other manufacturer advertises that they have taken this critical step to "perfection" that Cobb has. They are symetrical as stated, but they are the closest thing to perfect that I can see on the market.

huh?

Look man, it's ok to say you made a mistake. I do it all the time. Honestly, have you spent hours on the phone with the engineers of these pistons? I doubt it. I, however, have.

Let me remind everyone that I specify my opinion when it's opinion, and I am presenting fact when it's fact. If I missed an opportunity to specify an opinion at one point, I'm sorry.

This was a really good thread. Sorry.

I have an idea, let's ask COBB.
We'll ask them if there is ANY room for improvement. If getting a seperate cast piston for each side would be better to acheive a more optimal dish.

It is my OPINION that the TWE's are better, based on ENGINEERING FACTS AND THEORIES. I BELIEVE that COBB is a close second. I've provided my reasons, I'm not going to defend myself anymore. I'd like to see this thread get back on track. If anyone has any disputes with me, please PM me. We can settle things OFF the boards so I don't get banned for dogging on a moderators best friend again. ;)

I'm going to post up a bunch of articles for the purpose of increasing everyone's knowledge. Then they can make their own decisions and I won't get flamed for telling you what I prefer.

http://www.chevyhiperformance.com/techarticles/94138/

this is a good article on quench/squish as it relates to Piston to head clearance. The captions and pictures on the side talk a little more about piston dish shape, but the article again is on piston to head clearance.

Aritcle from popular hotrodding. http://popularhotrodding.com/tech/0605phr_pump_gas_secrets/index1.html

Quench It If You Can
The quench effect on engine efficiency has been well documented and researched since early in the last century. What is the quench effect, you ask? Simply put, it is designing in a close clearance between a substantial portion of the piston area and the bottom of the cylinder head when the piston is at top dead center. A closed-chamber head has a large flat area, the quench surface, over a substantial portion of the bore. It has been found that if the piston rises to within .050 inch or closer to the flat of the head, good things happen in the combustion process. The effects here are multifaceted. First, is the squish effect, wherein as the piston closes the gap in the quench portion of the head as it approaches TDC the combustible mix in this portion of the chamber is rapidly displaced, creating combustion-promoting turbulence, speeding the burn. In the compression process, the gasses in the chamber reach a very high temperature. As the propagating flame front expands, the pressure can get high enough to auto-ignite the end gas at the far side of the chamber. Since with a tight quench clearance, most of these end gasses are squeezed out near TDC, the chances of auto-ignition (detonation) are greatly reduced. The temperature of autoignition is approximately 1,375° F. Clearly, the cylinder head temperature is significantly cooler than the end gas temperature at or near autoignition levels. Due to the temperature differential, the thin layer of detonation-prone gasses at the extremities of the chamber are actually cooled by the proximity to the head, further diminishing the tendency to detonate. It is from this cooling effect that the term “quench” is derived.

An engine with an effective quench will be more detonation resistant, and it is typical for surprisingly substantial improvements in torque to result from the more efficient combustion. Most builders consider .040 inch or so to be an effective target for piston-to-quench-area clearance, a spec easily obtained with a closed-chamber head, a piston at zero deck, and a standard FelPro .039-inch compressed thickness gasket.

The chamber’s partner in the quench effort is the piston. To be effective, tighter piston-to-head clearance is better, but pushing it much closer than 0.040 inch becomes risky for a street application. Note the quench pad on this dished piston, a much-preferred setup as compared to a quenchless full dish.

I forgot to mention the downside of valve reliefs. They provide sharp points and sometimes thinner areas of material on the piston face. This causes hot spots in the cylinder and can lead to detonation also. So a good designed piston wouldn't even have valve reliefs. The dish would be deep enough to compensate for that already....


...

... like the TWE pistons. Unlike the COBB.

Wow...you should switch to decaf.

My opinion is that Cobb pistons are the best. I stated most of my reasons. Can I prove it in any way? No.
Your opinon is that TWE pistons are the best. You stated most of your reasons. Can you prove it in any way? No.

I can honestly say that I have spent more time discussing piston design, theory, and practical application in Subaru motors than anyone on this forum. I have spoken with both Trey Cobb and Jeremy Anderson (lead engine builder with Cobb Tuning) about their pistons at length. Their pistons are designed the way they are for maximum power. It is my guess that they might eek out more with a design similar to TWE's, but I'm pretty sure they already covered that as they are far smarter than you and I.

No need to get fussy either (unless I'm missinterpreting your written tone, which does happen). I'm a local for goodness sake! Ask Brian about me. :)

P.S. I have no problem with TWE pistons, I just like Cobb's stuff better.

Oh snap! I guess this is what you get when you put a couple of sailors in the same thread.:)

Sailors do have opinions! I think we are just too enthusiastic to share our perspective knowledge bases for our own good. :)

The piston top is the floor of the combustion chamber.

I RECOMMEND TO ANYONE ON THIS FORUM......
'POPULAR HOTRODDING' has 2 VERY good articals on 'pump gas secrets' (all about reducing detonation) and 'Bottom end basics' (all about building an engine.)

I really recomend getting this....for some, it will be basic....for others, it will be extremely helpful.

Lets keep this thread going......

I forgot to mention the downside of valve reliefs. They provide sharp points and sometimes thinner areas of material on the piston face. This causes hot spots in the cylinder and can lead to detonation also. So a good designed piston wouldn't even have valve reliefs. The dish would be deep enough to compensate for that already....


...

... like the TWE pistons. Unlike the COBB.


The opposite of this..the positives of valve reliefs= a tighter quench, if your piston isnt running a close enough tolerance to have the need for valve reliefs then your quench isnt the best it can be, or you englarge the crown design so that the top of the piston is larger than the area that would need the relief. Either way you will not be taking advantage of a tight quench area.

I agree with ImprezaRSX to an extent (Large Valve Reliefs can cause more trouble than a piston witout any reliefs at all) but if a piston is setup correctly with valve reliefs (still maintaining a certain crown thickness) and you make sure the relief edges are radiused I believe this is the way to go, and you could say this could be the best of both worlds keeping the tightest quench possible.

Note: I havent seen any turbo pistons on the market for the subarus with large/deep valve reliefs, although this would be more evident with an N/A piston.

very true.

The top Wiseco piston could use some radius work on the valve reliefs. The edges are very sharp. The COBB piston has smoother edges. COBB's newer pistons have even smoother edges on the valve reliefs.

You prefer TWE? Great. I prefer Cobb. This thread is about design and possibly preferences, no need to justify.

Uh??? :huh: I.E. "This thread is about personal opinions, not fact and science?" :confused:

My addition to the thread:

My old block:
EJ205 JDM WRX Type RA Version 5:
http://www.powerlabs.org/images/jdmblock.jpg
(post head gasket failure)

My new block:
EJ207 JDM STI Version 8:
http://www.powerlabs.org/images/P3310337.JPG
(30K miles, after professional cleaning)

I am told that the Type RA pistons are forged. The V8 STI ones are of course Hypereutectic cast, as we all know. I find it very interesting that Subaru changed the design and wonder if this was related to the manufacturing method (Cast Vs Forged) or if they saw some specific advantage in this new design. Could someone comment on this?

Hypereutectic pistons are cheaper to make than forged pistons. They are stronger than tradition cast pistons but weaker than forged pistons (traditionally). As far as what reason subaru went that route, I have no idea. I do know that they went through a lot of changes mid/late 90's that appear to be from a cost of production perspective. Loss of oil squirters, primarily open deck blocks, hyper cast pistons, cast rods, and non side specific pistons.

Are you sure that's a JDM block on top? That looks nearly identical to the USDM blocks and not like any of the JDM blocks that I've seen. Now, I have only seen 4 JDM blocks, and do not consider myself a professional on JDM blocks. But I thought that the JDM blocks had a higher compression ratio and late 90's had shaped dishes... that RA looks very US. On the other hand, that V8 block appears to be higher compression and has a shallow dish.

Also to note: Forged pistons (harder metal) are known to put more wear on the cylinders than cast/hyper piston, but like we know they can handle higher cylinder pressures and more heat.
This is one reason why I am a fan of piston sidewall coatings (this will give you a small barrier against wear).
For OEM power levels and heat ranges hyperneutectic pistons make more sense. Dramatically tighter piston to sidewall clearance, less room for blowby, less piston rock (so this makes for a quieter engine), hyper pistons are easier on the bore than a forged piston.

hyper pistons don't change size with heat as much as forged pistons.

So hyper pistons can have a tighter piston to wall clearance when cold. This also leads to a quieter engine also. Not just that, but it makes your acceptance criteria on a block less stringent. That means subaru can put a higher percentage of the manufactured shortblocks on the road, with less going back for remanufacture.

Exactly.

great thread as long as there is room for everyone's opinion.

well, long time bump as I've just finished up my piston and am waiting for them to come...
wondering what anyone else has come up with...


**sorry to see they banned dom (imprezarsx) as though he can be an ass at times he does know his stuff quite well and is an asset to tech discussions. I guess being a good tuner and having customers say so on here while not a "vendor" is bad joo joo. :(

It is in the nature of a discussion to argue. I don't know why people need to be banned for arguing over something. It is a discussion. We all share whatever knowledge we have and all feel to different degrees how true it is.

I happen to like what ImpressiveWRX had to say. I agree with it. TWE's pistons are unique in the market and have a great advantage over a lot of others out there.

I also like Cosworth and Cobb. What do folks think of the Cobb Vs Cosworth?

It is in the nature of a discussion to argue. I don't know why people need to be banned for arguing over something. It is a discussion. We all share whatever knowledge we have and all feel to different degrees how true it is.

I happen to like what ImpressiveWRX had to say. I agree with it. TWE's pistons are unique in the market and have a great advantage over a lot of others out there.

I also like Cosworth and Cobb. What do folks think of the Cobb Vs Cosworth?

ImprezaRSX was banned for vendoring without a vendor account. I have been warned already for the same thing so I make sure my customers don't make threads about me.

The Cobb Pistons are very good, and QUIET too. They have a combustion chamber matched dish for good quench without having to use thicker head gaskets like the TWEs which kills some of the quench advantage. The offset wrist pin keeps the pistons from slapping around, keeps them quiet which should allow the Knock Sensor to pick up on knock easier.

I have no experience with cosworth pistons, just TWE and Cobb and both of those are very forgiving. An error in mapping on a 07 STI had it running 40 degrees of timing at 17 psi with no knock on TWE pistons. :eek: Tim Bailey quickly fixed that problem but was also amazed it was taking that much timing with no knock on pump gas.

So you are saying the TWE's need to have a different head gasket to maintain the right clearence between head and block? Or that the TWE's only come is one style unlike the Cobb's which they can talor to you?

Nate

ImprezaRSX was banned for vendoring without a vendor account

thanks for clarifying that. his last post was within specs as far as temperment is concerned and i didn't get it, either.

yeah, dom has been tuning had a bunch of happy customers pushing his name on here, so he got the boot for it...
9.52:1 pistons
http://www.we-todd-did-racing.com/files/1029613_pykjs/DSC_0791%20small.JPG
http://www.we-todd-did-racing.com/files/1029615_odj86/DSC_0794%20small.JPG
8.58:1 pistons
http://www.we-todd-did-racing.com/files/1031973_cys3y/DSC_0844%20small.jpg

Here are the ones in my bottom end... JE pistons...
http://www.shvrdavid.com/piston.jpg

So you are saying the TWE's need to have a different head gasket to maintain the right clearence between head and block? Or that the TWE's only come is one style unlike the Cobb's which they can talor to you?

Nate

More like Valve to piston clearance. On the 2007 STI that the TWEs were installed in using OEM Gaskets, once the AVCS system activated during cam break in it bent all the intake valves on the pistons. After pulling the motor apart and replacing all the valves, a thicker head gasket was used and that fixed the issue (other than the motor kept blowing out gaskets, even with ARP studs).

IIRC the TWEs don't have valve reliefs cut into the pistons which required the use of a thicker gasket. Probably not an issue on non-avcs engines, who knows. After the head gasket issues the motor was rebuilt with a whole new block (to be sure there were no further issues) but using Cobb Pistons with OEM gaskets. I swear the engine is quieter than stock and we have had no issues at all so far.

I'm suprised twe didn't clay the piston and use an adjustable cam gear to find the limit of cam timing changes...really quite suprised based on there reputation.