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Idiots guide to converting your legacy to parallel


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Found something broken, converted to parallel until I can replace said part (secondary bypass valve thing) and thought I\'d put this up, the internet makes it seem like such a hard job - it isn\'t. Changing oil takes longer.

First off - this is the *Free* way to do this, takes you 20 minutes, and is 100% reversible in the same amount of time (you don\'t even need to cut any hoses, nothing). It will not be as good as a dedicated setup (with parallel headers etc) and not as good again as a single setup - however there\'s a lot to be said for cost. All this is doing is short-cutting the TT system to lock it into the same arrangement as is normal above 4500, so no you will do the car no damage and have nothing to lose.

Secondly - if I\'ve gotten things wrong, please tell me. The reason I wrote this up is when googling for the diagram smurff put up years ago (which is down the bottom) I just found link after link of people either asking how to do it, know-it-alls going on about "I think it\'d be terrible, just convert to single turbo" without trying it, or people thinking it was a horrendous job of re-wiring, replumbing and confusion. You can make up your own mind about how it drives, some love it, some think it\'s a pig - I put some examples at the end of how mine drives.

So first thing: Loosen off the heat shield on the secondary turbo. You don\'t need to remove it outright - just take off the two bolts on top and the one down close to the chassis rail. There\'s a bastard of one tucked underneath, but once you remove those 3 you can tilt it right back.

Yay now you have access to the exhaust valve actuator rod - notice how it\'s threaded? Slightly off topic you can noticably change the behavior of the car by lengthening and shortening this rod, bit of trial and error but you can improve VOD and secondary spool a bit, bit of a balancing act.


ANYWAY - unscrew this right out. It\'s a reverse thread, and the locking nuts can get pretty stuck on there too. Keep winding until the little hexagonal rod falls off, or hopefully you grabbed it before it fell and got stuck in the steering rack.

Now it\'s pretty tight, but try and wire the bottom half of the rod down, basically rod down = valve open. It holds itself down anyway but may as well keep it there. IF you were doing this permanently, you would either remove the valve, or make a new up-pipe. The piping guide at the bottom suggests tricking the car into permanently opening it with pressure - I just took the short cut.

Now put the heat shield back on.

There is a vacuum hose going to this valve - this is the one which opens and closes the secondary turbo off from the intercooler outlet. It\'s held closed by vacuum... So pull the hose off, its spring loaded to stick open. Done. In theory you now have the TT system stuck open. (Oh yeah, block the hose off)


Last thing - you need to block the secondary dump valve OR tee it into your other BOV, it\'s the relief valve the TT system uses to pre-spool the secondary (the hose which runs from secondary turbo, to a plastic arm, then a black thing which is just a BOV but not used as one... sort of). Mine has a massive bolt in the hose (cause I\'m redneck bro) and this is temporary, what you are MEANT to do is to tee it into the main BOV line. I would have, but this thing being snapped is the reason I did this in the first place so uh.. yeah.

In theory you can now go for a drive, and then watch the check engine light come on after the first few times you floor it.

Reason being - the ECU in these things has a MAP sensor with the sole job of watching for boost to be on just primary, then on both to make sure the TT system is functioning correctly. One side of it goes after the intercooler valve and is watching primary boost, the other is in the plastic arm to the secondary relief valve thing.

Luckily here\'s something Subaru prepared earlier - the hose which goes to the TOP of the exhaust valve actuator conveniently has nothing coming through it until about 4 grand when it starts to get boost through it. You unplug this - and instead join it to the hose going to the map sensor. Pro photo mid-work below - if you\'ve replaced your hoses with pretty ones, the hose you use is the back one of the two running up the firewall. Blank off the hose which ran from the plastic arm bit, it has no use now. The hose to the top of the ECV is number 8, and the one to the MAP sensor is number 22. FYI the one to the bottom of the ECV gets vacuum only as a way of being able to half-open the valve.


Now that is literally it - there is no messing around with wiring, if you want to undo this (and to be honest it\'s quite likely you will) you just reverse what has been done. Just don\'t burn your hands.

So - how does it drive - whats the benefits and why would you do this?

I don\'t know if my car is a prime example but here\'s what I have found anyway - it\'s laggy and the long legs the legacy got makes things worse. Best comparison I can give is it\'s similar to an 1.8L GSR with a TD05, pretty doughy down low but worth it once it wakes up. Basically feels n/a below 3500.

5th gear @ 3k (100kph ish) gets you about 0.5-0.6 bar boost and a corresponding lack of speed. Dropping to fourth at the same speed (about 4 grand?) it will hit about 1 bar in around a second - in comparison with it TT it would spool the primary, hit VOD in an instant and then start to spool. From a standing start or dropping back a gear on the move it (parallel) canes the TT (sequential) setup as it just wakes both up, however say goodbye to getting a kick from booting it in second on a roll like you used to. I haven\'t driven something with one of the z/s sequential controllers on - so can\'t comment on how it compares to that.

The sound is drastically changed - it has a hint of the normal burble at idle but to be fair it sounds like ass until it starts to get some boost into it. Not much different to tuned length headers or a non turbo suby I guess - once it does get some boost on board I love it, sounds like nothing else.

IF I was to carry on with this I\'d be looking to get some proper headers and downpipes for it and try to improve the spool up, mine still has factory downpipes including primary cat which won\'t be helping matters.

You *could* look to use a V5/6 ecu as per a normal single conversion - and that\'d make sense to do - but isn\'t quite so reversable and involves spending $$ (I understand there is a difference in CR 8.5 / 9.0 between STi and B4?). If you could get the spool up improved a bit it\'d be a pretty sweet way to do things. I have seen it mentioned that some remove (and block off) the joiner pipe which connects left and right banks - the problem there is that you lose boost control to the 2ndry turbo - at the moment when the single WG on the primary vents, it\'s dumping gas from the whole manifold hence why it works just fine. I get absolutely 0 spiking or flickering on boost with the way things are now. In terms of outright speed - pass? 0-100 times are a bit hard to compare, 0-400m won\'t happen with this car - I timed it at under 3.5 sec 80-120k so it\'s got pace enough for me at least.

To do this to a BG/BD it is effectively the same - not sure if the hoses are numbered the same, but where they run to and from is.

I\'ve tried and tried to get it to throw a CEL at me but so far so good, things seem happy (touch wood).

Here\'s the diagrams as well - only thing I do different is not doing anything with the ECV hoses. Writing this post up took me 3 times as long but I know some find a diagram way way harder to follow than just a photo.

Anyway - have fun :)


Edited by Joker
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It won\'t be any faster - it\'s still the same turbos, running the same boost, just rather than one coming on then VOD and re-spooling it\'s just like one turbo which is far too large

Just "different" - I\'d do it if I was taking it to the drags or a trackday or something for sure, went back to normal last night and helloooo low down power again... and hello VOD again you suck

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 Marky said:

It won\'t be any faster - it\'s still the same turbos, running the same boost, just rather than one coming on then VOD and re-spooling it\'s just like one turbo which is far too large

Just "different" - I\'d do it if I was taking it to the drags or a trackday or something for sure, went back to normal last night and helloooo low down power again... and hello VOD again you suck

Ended up going back to normal again as well. Your right, track drags, etc it would be great, but around town you really miss the low down lol

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  • 3 weeks later...
  • 1 month later...

Tried it again, short version is it sucks the big one and I\'d steer people away from it haha

Thought it might have improved matters with downpipes done and bigger intercooler on, it does have a little bit more go off boost than before but still, it sucks.

Quick video to show it (ignore the audio, makes the car sound like it\'s rattling to death?) - was just a quick fang to 100. Uphill. At 3/4 throttle. In 45deg weather. On 91 octane. Or something.


Have timed it under 5 sec 0-100 in parallel so it can move when provoked, youtube just makes everything look slow haha.

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Uhhhhmmmmm yes you\'re right the hoses are slightly different on BG to BH sorry

You don\'t tee it into it as such - it just goes direct into it. Idea is that this way the MAP sensor gets a pressure signal on one side, then you use the pressure out of hose 8 to simulate the secondary coming awake (if that makes sense?)

Basically using the cars own system to fool itself into thinking it is still running TT sequentially, albiet with a super poked primary

You\'re in chch right? I did it to my old BG for a while as well, just can\'t remember exactly which hoses do what without looking at it, but the way it all works is identical.

TL;DR the diagram explains it anyway haha

also - found I\'d forgotten to seal up one hose yesterday when putting it back to sequential, had a leak on boost (nice one d!ckhead) so explains why it felt worse than last time lag-wise

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yeah i thought as much was going on .

so if it join it to my hose 8 there will be nothing goint to the 2nd waste gate and just use the primary one to conrtoll boost

if i t it into it it will still see boost at the right time but will still controll the 2nd wastegate . might try both .

going to see how it goes . its a shitty old auto with 250k on it and it might be better with slow then fast on the power band than a horrid mix of boost and vod lame shift points .

and coz i like fiddling

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No no I think 8 is the wrong number hose for a BG, the one you want is the one going to the top of the ECV, it gets positive pressure at 4kish to open the valve normally, so instead when fed straight to hose 22 it now just gives positive pressure to the map sensor and simulates the secondary waking up. The code 22 CEL you get is when that sensor doesn\'t see what its\' supposed to.

The second wastegate is literally almost never ever used on them anyway, my understanding is it\'s a last resort the ECU has or something, so just tee the second w/g feed into the the hose currently going to the primary W/G, have them work simultaneously.

P.s you\'re going to hate it, but who cares it\'s free, reversible and does give lols when you floor it from a standstill

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oh yep i was thinking bout it and had come to that conclusion pretty much

i just unhooked the end of the actuator from the ecv arm . car went as lame as fuck . actually made no boost untill 5 k . looked under the hood and the arm had re attached itself to theactuator rod . ffs lol

now its all good . i actually like it how it is now . maybe the auto and the small turbos is ok together

it starts making boost at like 2.5-3k . not much because foot flat = kick down in the box so its hard to measure . but yeah it is like na spec from idle to 2.5 then reasonable pull from there on upwards . suits me better it was before .

only hitting just over .5 bar . apparently 9 psi is stock for the auto. tempted to put a boost tap in 23 so i can get 11 or something

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  • 7 months later...

way i set mine up which is Completely different to this threads way.

i see 11-12 psi then shortly after i see a constant 14psi till i change gear , kinda feel a small kick aswell once both are online starting at like 4100 Rpm Upwards for fullish boost

Only reason it jumps is that its using factory Boost Controllers, Ecu Thinks It needs to do that Due to the Whole VOD And ECU Supposedly thinks its the secondary coming online which its already spooling,Coz its in Paro Mode Already

Hope that helps, i always type things ...Strange

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