I thought i should spell out once and for all how the twin turbo's work, there's much misconception out there on the subject, as well as a bunch of myths...
Well, first off, as most of you know, both iterations of subaru's Twin Turbo (that is, the BD/BG Master-4's, and the BE/BH Phase-2's) are sequential setups. That is, one turbo does the first bit then the second turbo kicks in after a certain rev range. First myth: The first turbo is small, and spools early (true) and the second turbo (a bigger one) then takes over (false). What's actually happening in the crossover period is the primary turbo despools, then both of them spool up in parallel. Both turbo's are about the same size! (more on that in a sec) - "secondary" is ambiguous, it will mean the second turbo, and also the phase where both turbo's are going at once i.e; "I'm boosting on secondary" Which actually means you're boosting on both turbo's, or on the secondary phase.. I guess this leads to some people thinking secondary is actually one big turbo, it's basically not. The main difference between turbo's is the primary is reversed to sit on the left side of the engine (the passenger side in rice hand drive), and the secondary doesn't have a wastegate (except A-rev). And the turbo's are mostly the same size, or at least very similar. In the VF18/19's case, the compressors are identical, In some other configs the secondary is sliiiightly larger. If you want to get technical, there's actually a phase between primary and secondary, the exhaust valve opens slightly to start spooling the secondary turbo, before opening fully, partially despooling primary, then spooling both at once. After you drive a TT for a few months, you'll notice this as the primary running out of "puff" in the moments before VOD (valley of death) and then secondary boost.
For more technical info and docs, I have a new thread up here with a bunch of technical info https://sl-i.net/FORUM/showthread.ph...-twin-turbo-eh
I should also explain VoD for you too. VoD (Valley of Death) simply put, is the gap between primary and secondary. It's actually a great feeling when boosting to redline, it almost feels like switching on nos, you get a great thump in the back as you suddenly get all this extra power from nowhere. BUT, that's the only thing good about it. The non linear power curve often pisses people off - but that's not the worst part about VoD... When driving normally, VoD kicks in at about 4000 rpm... this is the speed most people like to change gears. If you change gear anywhere NEAR VoD, you get a massive great big gap where you have no boost whatsoever!!! For example, you're cruising up a hill, at, say... 10psi - Change gear at 4200 rpm or something, and - *wham* - NO power at all... and pretty much no boost until you get into secondary turbo. It's such a pain in the arse, i spent spent about $1500 (a bargain) lifting my engine, and converting the car to a single turbo! Yeah, VoD is no fun at all!
Now, er... BOV's. Myth: There is two BOV's/Bypass Valves (true), one for the primary turbo, one for the secondary (false!!!). There are two BOV's, yes. One is attached to the intercooler exactly like a single turbo subie. This BOV, if you think about it is obviously working for both turbos! Since the two turbo's meet up at the intercooler. This BOV, you can replace with an aftermarket one just like on a WRX, no fuss. The second BOV is actually for an entirely different purpose. It sits, confined, in a nest of pipes and vac lines, nicely hidden near the secondary turbo. Subaru must have done that intentionally, cause you don't want to mess with it, it's got something to do with the whole TT system, and nothing to do with actually "blowing off" air in the traditional sense :razz:. "The function of the supercharged pressure relief valve is relief of the pressurized air with the secondary turbocharger at the time of preparatory rotation. The purpose of this leakage is to provide smooth switching from single supercharging to twin supercharging"... So there ya go, the second BOV makes VoD smooth. That "preparatory rotation" is the middle phase i talked about where the exhaust valve opens a tad to start spooling the secondary turbo.
Aw heck... while i'm at it. If you want to know how it works, or what BBoD means.. well. SOLENOIDS! Lots of solenoids, and vac lines. I don't even understand it all myself. Basically, there's something like 24 different vacuum lines, about 6 solenoids, two boost control solenoids, and a differential pressure sensor, And the vacuum tank. The solenoids are all contained in the BBoD... "Black Box of DOOM" (Solenoid box). Here is a diagram.. Except for the actual wastegate solenoid, which is in the front left guard next to the vac tank of all places. And it all just magically WORKS... I think i saw someone say the exhaust valve actuator is operated electrically once.. just... NO. It's all vacuum and solenoids. So much vacuum, there's the aforementioned vacuum tank in the guard to store enough just to work the whole thing!
This is why i recommend to all in the US who are thinking about TT-ing their car, to stay the f*** away, and just go single turbo. Which works with a single wastegate actuator and a boost control solenoid. And the result is, well, a better drive! single turbo's KICK twin turbo arse.
That is all.
https://sl-i.net/FORUM/images/import...eYouKnow-1.jpg
