Subaru Legacy Forums

Subaru Legacy Forums (http://legacygt.com/forums/index.php)
-   Fifth Generation Legacy (2010 - 2014 ) (http://legacygt.com/forums/forumdisplay.php?f=92)
-   -   Pistons (http://legacygt.com/forums/showthread.php?t=201104)

minuccims 02-06-2013 12:08 PM

When looking into this a few weeks back, forged pistons change the compression, but just slightly. If remembering correctly, if there is no damage to the engine the pistons and rings can be swapped out from underneath. IE, not an engine tear down. Labor is "not that bad".

GTTuner 02-06-2013 12:16 PM

Quote:

Originally Posted by minuccims (Post 4281284)
When looking into this a few weeks back, forged pistons change the compression, but just slightly. If remembering correctly, if there is no damage to the engine the pistons and rings can be swapped out from underneath. IE, not an engine tear down. Labor is "not that bad".

I'd like to see that done! :lol:

You would still need to pull the tranny to get the wrist pin c clips off of cyl's 3 and 4

minuccims 02-06-2013 12:20 PM

Haahaa! yes, very poor choice of "underneath". Thanks for clarifying.

fahr_side 02-09-2013 03:04 AM

Damn Surly, you didn't have to break a piston to prove us wrong eh!

Really sorry you were the first. Very unsure as to what options are open to you by way of replacements. I don't know of anyone making '10~ specific pistons.

Rutchard 02-09-2013 05:58 AM

Quote:

Originally Posted by fahr_side (Post 4286083)
Damn Surly, you didn't have to break a piston to prove us wrong eh!

Really sorry you were the first. Very unsure as to what options are open to you by way of replacements. I don't know of anyone making '10~ specific pistons.

I don't know if Surly's engine was a piston yet or not. They haven't torn it down yet, right? It's all spread out over a few threads so maybe I missed something along the way...

SurlyOldManMN 02-09-2013 08:30 AM

Quote:

Originally Posted by Rutchard (Post 4286124)
I don't know if Surly's engine was a piston yet or not. They haven't torn it down yet, right? It's all spread out over a few threads so maybe I missed something along the way...

Correct, we don't know yet. I will be keeping documentation here:

http://legacygt.com/forums/showthrea...ad-201225.html

We won't be looking at lgt-specific pistons since a bore is highly recommended due to probable damage. JE 99.75, 8.5:1 are a likely candidate. According to NF Performance those should give me 9.2-9.3 final compression:

http://www.jepistons.com/Products/296348.aspx

I plan on chatting with them Sunday during our Drive n' Dine to make triple sure that's what they want ordered. I'm tempted to wait until it's pulled apart so we can check for any design and/or damage surprises first.

fahr_side 02-09-2013 06:06 PM

Agree. I'd want to cc the heads and check deck height first. I haven't seen this data published anywhere yet.

SurlyOldManMN 02-09-2013 06:23 PM

Indeed, nothing out there that I've been able to find. Hopefully nobody else gets the "opportunity" to find out before I get a chance to document measurements.

SurlyOldManMN 02-09-2013 06:34 PM

May as well throw it out there standard 99.5 x 79.0 bore and stroke that you would expect with an EJ25. Probably goes without saying.

SurlyOldManMN 02-14-2013 01:48 AM

Quote:

Originally Posted by frank_ster (Post 4278901)
you can choose compression ratio : lower compression ratio if you plan to make more boost. higher compression ratio if you want better efficiency at cruise

and the sizes are for over bore . so if you were to just change out pistons you would choose the smallest one. the others are if you rebored the cylinders if there are scratches .

Just for a quick sanity check... higher compression comes with a reliability hit, correct?

fahr_side 02-14-2013 06:58 AM

Quote:

Originally Posted by SurlyOldManMN (Post 4293745)
Just for a quick sanity check... higher compression comes with a reliability hit, correct?

Given the same combustion chamber design / efficiency, you will run into knock faster as compression ratio increases. In theory that limits the useful boost you can run.
The fact that this new engine runs a CR a full point higher than the previous generation engine, yet can tolerate similar boost and timing, makes me wonder about a few things.

Maybe the cam timing (more overlap) in the midrange reduces dynamic compression a lot.

Maybe the ports are smaller than before, compromising VE and both increasing boost and allowing more timing to be run.

Maybe the combustion chamber design is much better than before.

Maybe there's a combination of the above factors.

Lots of maybe.

frank_ster 02-14-2013 07:40 AM

Quote:

Originally Posted by SurlyOldManMN (Post 4293745)
Just for a quick sanity check... higher compression comes with a reliability hit, correct?

well if you put it that way lower compression ratio would probably be tougher...
less chance of knock.


but BUT . i have issues with being to low of compression, where the compression ratio is so low the that knock doesn't happen when over advanced in timing. instead of what happens is the pistons and rings get to hot and you can have the top ring shatter due to the gap closing. and top of piston rubbing on cylinder walls! so the method of advancing timing until knock doesn't apply !

frank_ster 02-14-2013 07:46 AM

Quote:

Originally Posted by fahr_side (Post 4293853)
Given the same combustion chamber design / efficiency, you will run into knock faster as compression ratio increases. In theory that limits the useful boost you can run.
The fact that this new engine runs a CR a full point higher than the previous generation engine, yet can tolerate similar boost and timing, makes me wonder about a few things.

Maybe the cam timing (more overlap) in the midrange reduces dynamic compression a lot.

Maybe the ports are smaller than before, compromising VE and both increasing boost and allowing more timing to be run.

Maybe the combustion chamber design is much better than before.

Maybe there's a combination of the above factors.

Lots of maybe.

cam overlap can reduce dynamic compression,

yes all of those factors work together. yo should see the hayabusa engines turbo charged. they are 12-13 to 1 compression ratio running 25 lbs boost on pump gas!

SurlyOldManMN 02-14-2013 08:41 AM

Quote:

Originally Posted by frank_ster (Post 4293886)
well if you put it that way lower compression ratio would probably be tougher...
less chance of knock.


but BUT . i have issues with being to low of compression, where the compression ratio is so low the that knock doesn't happen when over advanced in timing. instead of what happens is the pistons and rings get to hot and you can have the top ring shatter due to the gap closing. and top of piston rubbing on cylinder walls! so the method of advancing timing until knock doesn't apply !

Interesting stuff... The preschool version I have so far is higher compression gives you generally better all around performance, but if/when something goes a little wrong, the ability to gracefully handle the event and keep it from going a lot wrong is severely hampered. Otherwise it would be a no-brainer to go HC all the time.

There's also a lot of FUD out there on what constitutes "high boost" for this application. The vast majority of opinions and articles I can get my hand on are wishy washy on this point. Mostly the conversation around FI and HC pistons centers lol-charging NA motors and we're talking single digit boost targets.

Assuming a conservative power goal that isn't going to require squeezing every last drop either way, does it make more sense to go HC and pull back the turbo or LC and let the FI get you the rest of the way? Which goes boom first?

frank_ster 02-14-2013 10:23 AM

go stock settings( compression ratio) put forged .. keep same ignition table have the car tuned on a dyno .. go conservative on ignition timing make up the power with a lil more boost.


All times are GMT -6. The time now is 07:21 AM.



vBulletin Optimisation provided by vB Optimise v2.6.3 (Lite) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
LegacyGT.com