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since my engine is running great now, what boost can i (SAFELY) go to? i want to run a dual stage boost controller, basically stock boost for driving around, more for offroad and for a bit of fun every now and then.

this is what my engine has (or will have very very shortly):

stock engine
stock turbo
stock IM
liberty waic
plumb back bov
pod air filter
2.5" turbo back exhaust
custom exhaust manifold (1.75" mandrel from the exhaust ports, 2" from the merge to the turbo)

i run shell E10 (so about 94-95RON apparently), with the ignition advanced about 2 degrees, and a larger flowing fuel pump. good compression on all cylinders (all within 11psi of each other).

Well i have....

EA82T with 4 plug ECU
Liberty WAIC
TF035 Turbo (GT Forester)
Factory airbox/panel filter
2.25" turbo back exhaust
Modified turbo headers, (2" from the T join up to the turbo)

and ive been running 10psi for about a year now with no ill effects.... But....

I assume that you are running a 3 plug ecu?

Im not sure how a 3 plug ecu compensates for extra air because it uses a flap style air flow meter which isnt accurate at high air flows. This is why the 3 plug ecu ignores the afm at wide open throttle or boost and relies solely on the pre programed fuel maps.
Many people have run more than factory in the past and it has worked fine, but this may be because the factory tune is overly (safe) rich.

The 4 plug measures air throughout all load conditions because it uses a hotwire air flow meter which has a much more linear response and thus can add more fuel as airflow is increased.

If you want to do it properly, (and you will gain much more power from it) run an aftermarket ecu.

yes, 3 plug.

i think it would be very very stange to ignore the vaf at wot, even on the most simplistic ecu. i did read that somewhere else on here, and to be honest i just ignored it because it sounded just so strange.

i figured 10-12psi would be fine for the every now and then use (it's not as if i'll use that every single day) simply because of the free flowing exhaust and the extremely low compression ratio

I think the reason Subaru could ignore the flap air flow meter is that the volumetric efficiency of the engine is always constant (unless its modified) so they knew that at *what-ever* revs and if it was more than half throttle, it would be making boost and they could predict what fuel it needed.

Its only when you start changing the VE of the engine that you run into problems because the engine starts consuming more air than the ecu allowed for.

The factory tune is very rich to reduce combustion temps caused by bad head design and lack of intercooling.

With your intercooler and free flowing exhaust, you might be able to run more boost, but unless you have a wideband exhaust gas analyser, your in the dark for air fuel ratios.

what role (if any) does the narrow band oxygen sensor have in the 3 plug ecu?

The o2 sensor in the 3 plug ecu operates in closed loop (using feedback from the o2 sensor to fine tune the air fuel mixture) any time the engine is at operating temp and is idling, cruising or anywhere in between. The ecu goes into open loop as soon as you de-accelerate or push the throttle past half way.

How much boost can I run , If I had 2c for every time I heard that one asked I could buy a cheap car ...

If you want to be 100% safe with factory reliability and longevity run the std boost pressure .

The second you increase boost pressure over std the turbine inlet pressure (exhaust manifold pressure if you like) rises out of proportion to the inlet air pressure .
Manufacturers don't intend the engine to cope with the extra thermal loading because in std trim the engine never has it .
when the exhaust gas temperature rises it raises the temperature of everything it touches meaning the piston crowns/exhaust valves/exhaust ports/header/turbine housing/turbine .

If you raise boost pressure by a small amount its unlikely to go bang but you are reaching into the safety margins the designers built into the whole engine package . The safety margins are greater in EJ turbo engines because Subaru , and all the others , learned through experience that things like piston oil squirters/intercoolers/oil coolers/more modern turbos all increase the reliability of the engine .

In std form EA82T's are not exactly bullet proof when it comes to head gasket sealing/head castings cracking/turbine housings cracking .
Gannons idea of a TF035 or even the basic Rex TD04L is a better basis I think than pushing the long in the tooth outdated std IHI turbocharger .

As for the 3 plug system its honestly not a problem if its vane AFM is fully open before the rev ceiling is reached .
It IS a very simple thing to programme a std computer to run a std engine on two dimensional mapping high in its rev range -two dimensional meaning an RPM and a TPS signal .
High up in the rev range you're generally past the engines torque peak (point of highest volumetric efficiency) and VE drops off as the red line is approached . As long as the engine management can maintain a reasonably rich (was usually ~ 10:1 in 80's era turbo engines) the engine was safe and reliable . It was deliberately rich to allow for things like injectors getting a little grungy and dated thinking was that extra fuel through an engine helped cool the inlet charge .

It's a different story when using higher non std boost because the computer won't know about the extra air going in (AFM fully open) and the blanket safe 10:1 AFR will start to lean out - how much depending on how much extra air goes in .
Regardless of if its a 3 or 4 plug system the extra air will still result in higher EGTs , the hot-wire AFM may still measure the additional air but these systems also run mapped ignition timing .
It's a common practise for late systems to go into panic mode if they register more air than they are programmed to see , the dreaded RR (rich and retard) can easily spoil the fun at this point .

If you don't want to spend fortunes a good exhaust and maybe a better header should make quite a difference , after that a more modern turbocharger of about the same physical dimensions as the std one could be made to work well .

Unproven as yet but fingers crossed the later NA MPFI cams could make a difference - once the std header is improved/replaced .

Decent electronic boost controllers are good at keeping the wastegate shut until just before the std boost setting is reached , the std actuator will start to move at ~ 2/3 of its rated pressure so the boost rise is gentle - on purpose . The manufacturers don't want a sudden sharp torque rise particularly in an often front wheel drive breed of car , understeering off the road on wet corners is very possible in the hands of the unsuspecting broad stream motoring public . Many Cordia and front drive Laser turbos drivers found that one out the hard way .
Trust me , RWD and AWD cars a much more turbo friendly particularly AWD .

Just my thoughts , A .

it's definately a question you hear all the time - how much boost? i know exactly what i can do on my mazda's engine (know it inside out), but an engine that's 14 years older than it is a different story. i just figured someone would have played around and found an answer that's relatively reliable.

and yeah i'll get an EBC so the wastegate opens as late as possible

i think at the moment i'll get everything sorted and installed, then i'll borrow a mate's wideband and see what the air fuel ratio is doing, then can come up with an informed decision...and post results ofcourse.