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2007 forester 2.5xt
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Discussion Starter · #1 ·
hi.need some advise.on 18g setup with 250bhp at the wheels on 18psi.was told by tuner that limiting factor in more aggresive tune is that the piston heads r the weakest link.is that true n how much more power can the pistons safely take?was recommending changing to forged mahle pistons n further engine work.
futhermore,wld changing to unequal length headers liberate additional ponies n a retune required?how much additional power to expect from headers if any, n wld b this power b at low/mid or high end?
 

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05 FXT
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You should be able to make a fair amount more given a bigger turbo and more boost w/ a stock bottom end.

From the swaps done previously there wouldn't be huge gains w/ headers; I'd say even less w/ the 18g you're on.
 

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Discussion Starter · #3 ·
need to know if pistons r ok for tune upto 300bhp at the wheels or can they take more b4 having to change to forged pistons.
 

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I should be in the neighborhood and beyond and on stock block with no issues. Checked compression recently and all 4 good across the board.
 

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subaru motors with stock pistons are kinda like rotaries: They're strong until they see detonation.

The pistons simply can't take detonation. If they do, they fail. Period. So the original question is a bit tough to answer. The stock pistons will take over 500 wheel horsepower, but could fail at less than 300 wheel fairly easily if the tune is bad/the car detonates.
 

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2017 VW Golf SportWagen 5MT
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If I were going for high power, I'd switch out the pistons. Many benefits, forged = ligher and stronger, plus you can go with a lower-compression pistons which will allow you to run more boost. Mahles have been used for many years in the aircooled worlds, good piston. I don't believe there are any truly-forged Mahle pistons though (the so-called "forged" are pressure-cast), but they are stronger than regular cast pistons.

Stan
 

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the Mahle is a very good piston. Stan is right though - it's really not a traditional forging. It has the needed strength in the ringland, however.

one other note about trying to decide to go with an upgraded piston--it's VERY hard to tell when the ringlands crack. The EJ's are a challenge to leakdown, and sometimes that doesn't even tell the whole story. The crazy configuration of the motor alone makes it worth upgrading the pistons if you can....unless you like pulling entire motors often.
 

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Any boosted motor doesn't like detonation, that's not just a Subaru thing. Same for cracked ringlands being hard to detect.
 

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2006 FXT
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How does bhp compare to whp? Is it 1:1?

I am putting down 306whp with my 18g on stock pistons. My limiting factor is the intercooler. Once I go to a front mount I expect to be able to increase boost to ~22psi and hopefully get ~340whp on that tune. That's as much as I would go without doing redoing the internals. I am not a tuner so I might have just lied to you.
 

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The ration is not 1:1 at all. I wish it worked that way! Depending on your driveline and trans you will loose power.
The best example I can give is that my 2002 WRX was rated at 220 hp at the motor. It made 165hp at the wheels stock.
 

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Like others have pointed out, stay away, far away from knock, and the stock pistons can withstand plenty of power. Well north of 500hp.

I simply don't believe those folks that say "I was only stg2 and my ring lands broke." They are only telling part of the story. It may be true, what they said, but the edited out the part of them doing donuts in the parking lot, bouncing off the rev limiter for 5 minutes. Or overheating on track days. Putting crap gas in the tank. Trying to run 25 psi at 6000 rpm on a TD04 13T. Running 25* at peak torque on pump gas. There always is a back story, and its usually not the pistons fault...

I always love when a spun rod bearing is found to, but there wasn't knock or they change their oil every 3k miles religiously.:shrug:

Sorry, end rant...
 

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The ration is not 1:1 at all. I wish it worked that way! Depending on your driveline and trans you will loose power.
The best example I can give is that my 2002 WRX was rated at 220 hp at the motor. It made 165hp at the wheels stock.
I understand the differences between engine hp and wheel hp. What I never understood was if bhp (brake hp) was equal to engine hp or wheel hp..
 

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At that level of HP you don't need a built motor. The caveat is that some kid can blow up a stock motor with enough stupidity and lack of maintenance as mentioned.

BHP is at the crank WHP is at the wheels. So it's approx. 20-25% drivetrain loss with AWD.
 

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I understand the differences between engine hp and wheel hp. What I never understood was if bhp (brake hp) was equal to engine hp or wheel hp..
brake = crank. As in measured on an engine dyno. Dyno loads the engine, as in a brake is applied...
 

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Any boosted motor doesn't like detonation, that's not just a Subaru thing. Same for cracked ringlands being hard to detect.
No motor, boosted or otherwise, likes detonation--it's destructive. I was talking about the detonation threshold of the stock piston; the Subaru turbo piston is far less comfortable with detonation than other o.e. turbo motors produced throughout the years.

and the diagnosis of the cracked ringlands is tougher in a Subaru motor due to its configuration. How easy can it be to leakdown a motor that makes spark plug changing a difficult task?
 

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^ gotcha, in that regard I won't disagree.

I'd say that boosted motors tend to be a little more destructive w/ dentonation as the cylinder pressures are that much higher.
 

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2007 forester 2.5xt
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Discussion Starter · #17 ·
fm wat u guys r saying,it's not the absolute bhp but the detonation.this detonation wld also b linked to boost pressure?wat is a safe level of boost seeing that i'm not on ewg?
 
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