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-   -   Pistons (http://legacygt.com/forums/showthread.php?t=201104)

fahr_side 02-16-2013 03:33 AM

Agree. If you lower compression you may gain power potential at the top end. Retaining that beefy midrange however will likely require careful tweaks to cam phasing which will take quite some time on the dyno.
I'd have JE make you some custom pistons.

SurlyOldManMN 02-16-2013 08:38 AM

Quote:

Originally Posted by fahr_side (Post 4296657)
Agree. If you lower compression you may gain power potential at the top end. Retaining that beefy midrange however will likely require careful tweaks to cam phasing which will take quite some time on the dyno.
I'd have JE make you some custom pistons.

Oh? You're thinking less than 8.5:1?

frank_ster 02-16-2013 08:46 AM

i'm at like 7.2-1 :)

fahr_side 02-16-2013 08:53 AM

Quote:

Originally Posted by SurlyOldManMN (Post 4296779)
Oh? You're thinking less than 8.5:1?

No, in your position I'd probably order some forgies with the stock 9.5:1 and as frank_ster suggests, go a little gentler on timing.

SurlyOldManMN 02-16-2013 10:50 AM

Stock is 8.4:1. The common offerings are 9.5 vs 8.5.

RabidWombat 02-16-2013 11:29 AM

Its possible Subaru is messing with the phasing as an effort to push gas mileage up.

The Toyota hybrid engines run a 13:1 CR on 87 gas. The trick is that its not a real 13:1, since the compression stroke is actually shorter than the exhaust stroke due to the way the valves open.

I'm guessing Subaru is trying to eek a little more mileage out by trying the same tricks.

fahr_side 02-16-2013 09:26 PM

Quote:

Originally Posted by SurlyOldManMN (Post 4296916)
Stock is 8.4:1. The common offerings are 9.5 vs 8.5.

Oh boy, that's quite a shock to my long-held beliefs. :eek::redface: I'm sure I read somewhere when the '10 was launched that it had higher compression than before. Not sure now where I found that info, and I'll be looking for that source.
I'm working on EDM models, and it could be the early ones did indeed have higher compression, or the initial press releases or brochures were wrong. It certainly explains why I can run so much boost and quite a lot of timing on these engines without problems!

I'd still be prepared to shop custom pistons even if sticking to 8.4:1 CR, since Subaru were very likely to have changed the combustion chamber shape with the new heads. You want to be sure you get the squish area right to avoid problems.

Subaru has long used what we call a 'tree-hugger spike' in AVCS timing to dis-improve VE and allow more overlap in cruise areas, improving fuel consumption and emissions by retaining more of the spent exhaust gasses in the cylinders, as EGR did in older cars. Flattening that spike out improves driveability dramatically, but costs a little in fuel economy. I'm sure that be able to change the phasing on both cams allows Subaru to refine this strategy a great deal.

SurlyOldManMN 02-16-2013 09:32 PM

Awesome, thank you!

8.4:1 comes from the USDM owner's manual.

Are you thinking it's worth pulling the pistons and sending them in to JE?

I'll call them on Monday to see what they think. The more insight I can have going in to that conversation the better. Very interested in getting it right the first time. I seriously do not want to have to go through this process twice...

fahr_side 02-16-2013 10:08 PM

I'd say you should measure the deck height carefully, and.cc both the piston dish and combustion chambers. Send that info to JE and see what they think.

I'd very much appreciate it if you could also measure the base circle and lobe height on the cams, so we can get an idea how they differ from the GRB items.

tytek 02-16-2013 10:15 PM

How many miles do you drive a year Surly?

Forged pistons will require a little bit of care o make sure they run for a long time. Low expansion allows for tighter tolerance. Thus leads to less slap and slower ring and piston wear. Warming up the motor to operating temps is critical. So is running quality oil at lower OCI. It is normal to expect a much shorter life of the components, before you start eating oil, and will require new rings and possibly a hone.

I do 30k miles per year and would not want forged internals. I would have probably gone for a new stock block. But your goals may be different. Just my $0.02.

SurlyOldManMN 02-16-2013 10:39 PM

I'm ok with watching operating temps like a hawk and giving it ample time to warm before giving it any more throttle than I would on a sketchy clutch. Par for the course in the frozen north. Well understood that it's no longer a matter of preference if I go with forged, especially JE's made of 2618 (nod to bigjweb for sending me down that research path).

I don't play around with my intervals either. I shoot for 3k and never more than the factory prescribed 3750 on Rotella T6.

24k/year isn't unusual for mileage.

I swear I'm trying not to take the thread over with my specific scenario... I'm just trying to nail down all the right piston-related questions to ask when figuring out what to do with an unknown platform. :)

islandborn 02-16-2013 10:41 PM

THis is a good place to talk about them Surly! :) And if someone wants to ask about pistons they have a direct place to go and follow the conversation. Much love!

SurlyOldManMN 02-16-2013 10:43 PM

Quote:

Originally Posted by fahr_side (Post 4297617)
I'd say you should measure the deck height carefully, and.cc both the piston dish and combustion chambers. Send that info to JE and see what they think.

I'd very much appreciate it if you could also measure the base circle and lobe height on the cams, so we can get an idea how they differ from the GRB items.

I'll reach out to NF next week and see how soon I can get that info. We've had a rash of people upgrading, both forced and otherwise lately. They also just opened a new brick and mortar garage. They're backed up quite a bit.

Anything else that would be really useful to measure/photograph/otherwise-document?

fahr_side 02-16-2013 11:16 PM

Quote:

Originally Posted by SurlyOldManMN (Post 4297670)

Anything else that would be really useful to measure/photograph/otherwise-document?

I won't ask you to pull stuff apart that doesn't need to be, like the heads, but it would be nice if the valve diameters could be measured.
If NF has a degree wheel they could possibly find the cam durations and centerlines. When you look at the BR9 AVCS mapping vs. those on the GRB it's clear something is very different!

fahr_side 02-18-2013 09:35 AM

I think I know where the confusion comes from regarding the compression ratio.
A friend of mine was around when a new '11 shortblock was used to make a hybrid with EJ205 heads, and gave me the following data:

Bore: 99.5 mm
Stroke: 79 mm
HG Bore: 101 mm
HG Thickness: 0.6 mm
Piston Dish: 19 cc
Piston below Deck: 0.3 mm

None of the new dual AVCS LGT heads were around for measurent. The WRX and '07~'09 LGT heads measure 49cc and the piston dishes 24cc, which gives 8.6:1 compression.

If you assume the new LGT heads measure the same, the new 19cc dish pistons calculate out to 9.17:1 if the measurements are correct.

Interestingly, if you calculate the 19cc dish pistons with the 57cc W25 DAVCS heads from the '08~ STi, you get exactly 8.4:1 CR!

Has anyone looked at the casting marks on their heads?


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