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Thread: Twin Turbo Myths

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    Dirt Tech / Vendor Reuben's Avatar
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    Twin Turbo Myths

    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 . "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.




    Last edited by Reuben; 06-23-2020 at 08:27 AM.
    Single turbo conversion, "How to" can be found here
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    Great post rubes, definately cleared up a couple stuff.
    That is the str8 forward explination.

    Found this out a while ago.
    https://sl-i.net/FORUM/viewtopic.php?t=5662

    I honestly Love TT's i plan on doing a WRX circuit car and i am higly contemplating the TT setup with a few mods, all around power no need to wait on the turbo to spool.

    Obvious reason why this might sound like a dumb idea is that blasted VOD, A Ecutek guy back in jamaica tuned his out plus he has a TD04 WRX on the secondary side and its running fine, untuned.
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    Very informative. Stickied...
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    Dirt Tech / Vendor Reuben's Avatar
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    Quote Originally Posted by subba
    Great post rubes, definately cleared up a couple stuff.
    That is the str8 forward explination.

    Found this out a while ago.
    https://sl-i.net/FORUM/viewtopic.php?t=5662

    I honestly Love TT's i plan on doing a WRX circuit car and i am higly contemplating the TT setup with a few mods, all around power no need to wait on the turbo to spool.

    Obvious reason why this might sound like a dumb idea is that blasted VOD, A Ecutek guy back in jamaica tuned his out plus he has a TD04 WRX on the secondary side and its running fine, untuned.
    They certainly have their merits, despite my cynicism being a bit rough at times.

    I'm not sure on the specifics of the turbo's, but as far as i know, they have different flanges to the normal IHI's. If you can find out anything about that from your tuner, please post it up! It would be nice to know what you can do with them. As for a simple upgrade, the VF25/27 combo is reportedly the best (biggest)

    And i wonder what replacing the secondary turbo would do to vod... the VF19 is roughly the size of a VF10 i think, which is pretty damn small (smallest not TT VF series turbo). dropping a VF24 or TD04 on there might make things go vroom vroom on secondary, my guess is delaying the crossover point with a sequential conrtoller or ECU mod will be vital to managing VoD though.

    And from that link you posted:

    The Turbochargers
    Despite being called twins, the B4's turbochargers are not identical. The primary turbocharger is an IHI VF33 unit, which uses a 46.5/35.4mm 9-blade turbine wheel and a 47.0mm/35.4mm 6 + 6 blade compressor. At idle, the turbo spins at around 20,000 rpm and it can go on to a maximum speed of 190,000 rpm. It has a 17mm diameter wastegate opening to bypass excess exhaust gas. The secondary turbocharger is an IHI VF32. On the exhaust side it uses a 46.5/35.4mm 9-blade turbine wheel, teamed with a 52.5/36.6mm 10-blade compressor wheel. It's rated at 180,000 rpm. Both the primary and secondary turbochargers use a floating metal centre bearing - not ball bearings.
    There lies the difference. As i said with the VF18/19 (BG GTB turbo's) they were the same. Not the case evidently with the BE/BH turbo's. Also note, the VF32/33 combination was for the aussie spec twin turbo's. The JDM's got the VF25/27, which are larger. (yeah, makes sense ay... just like the VF8 being bigger than the 10, and the 22 being HUGE compared to the small VF24 :P)
    Single turbo conversion, "How to" can be found here
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    Re: Twin Turbo Myths

    Guy i know is running a vf25 primary with a T4 as the secondary, few other mods, hes running over 300kw at the wheels and slightly under 10second quater mile, is fricken nuts aye. He also has sti 6speed in it with 2L bored out to 2.2L. Fulll tuned ecu to get rid of that lag in the middle as much as poss... id way rather this than any single turbo any day...

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    Re: Twin Turbo Myths

    im glad i found this it give me info on the motor im dropping in my 1995 sti. rueben i may need your help with this motor swap so dont lose you internet anytime soon!
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    Re: Twin Turbo Myths

    Quote Originally Posted by whogoesthere
    Guy i know is running a vf25 primary with a T4 as the secondary, few other mods, hes running over 300kw at the wheels and slightly under 10second quater mile, is fricken nuts aye. He also has sti 6speed in it with 2L bored out to 2.2L. Fulll tuned ecu to get rid of that lag in the middle as much as poss... id way rather this than any single turbo any day...
    +1 we are the few who has faith in the TTsetup
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    Re: Twin Turbo Myths

    Do u guys know where I can find a ej20r wiring diagram and vaccum? I got a 99 legacy postal wagon that im doing a ej20r thanks

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    Re: Twin Turbo Myths

    hey yall im living in NZ CHCH this my first post thought i should let ya know that dont know why but mhmm oh well

    anyways iv got a 2001 rsk-b4 twin turbo manual its my pride and joy im first owner in NZ
    straight out of japan. i drove heaps before i found this one and there was something else about this car.... i asked had it been worked at all the dealer said not that he knows of part from the earthing kit and a nice heavy duty clutch. i though mmhmm. it s**ts on my WRX. well
    little did i know., i found out about 3 weeks after buying it has the VF23-VF24 turbo set up why im not sure maybe some little japer wanted to do something different he sure did....
    iv takin it done the strip it does a 12.1sec flat 0-400m running aproxx 10pound iv since installed a boost controler and wound it up a little bit to 16pound. im going to dyno it sometime soon get some tunning done.

    i wouldnt go back to a sti unless it was a ver 11 or something close to that....

    just a quick Q how do you upload photos?? onto this??

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    Re: Twin Turbo Myths

    Quote Originally Posted by rsk-b4_own3r
    hey yall im living in NZ CHCH this my first post thought i should let ya know that dont know why but mhmm oh well

    anyways iv got a 2001 rsk-b4 twin turbo manual its my pride and joy im first owner in NZ
    straight out of japan. i drove heaps before i found this one and there was something else about this car.... i asked had it been worked at all the dealer said not that he knows of part from the earthing kit and a nice heavy duty clutch. i though mmhmm. it s**ts on my WRX. well
    little did i know., i found out about 3 weeks after buying it has the VF23-VF24 turbo set up why im not sure maybe some little japer wanted to do something different he sure did....
    iv takin it done the strip it does a 12.1sec flat 0-400m running aproxx 10pound iv since installed a boost controler and wound it up a little bit to 16pound. im going to dyno it sometime soon get some tunning done.

    i wouldnt go back to a sti unless it was a ver 11 or something close to that....

    just a quick Q how do you upload photos?? onto this??
    any update on this?

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    Re: Twin Turbo Myths

    My D rev RSK is a good combo of response, torque band and shorter cross over. Ive owned a BG5A, BG5C and now BE5D. i personally think smaller turbos with a higher C/R make for a more usefull engine in day to day use, where i was gunning to get a BE5A/B with big turbos and shorter gearing im happy with the D.

    On a side note, a VF10 is on decent big larger than a VF19, u can pull about 270-280 hp (flywheel) out of a VF10 when leaned on. id be surprised if you could touch 200 with a VF19 (remember that none of the TT combos make more than say 340 hp).

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    Re: Twin Turbo Myths

    My 2001 GTB TT someone has put a VF13 on it ! ???

    Maybe they wanted faster spool for day to day driving or maybe the last one blew and they went the cheap repair?

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    Re: Twin Turbo Myths

    So the "rev d" B4 is the one to get?? is that still the BE series? im in the market for one(desparately) and i want to get a good basis car before i start messing with it

    and Ecutek works with the BE right?

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    Re: Twin Turbo Myths

    Stumbled across this old thread.

    I don't see why you can't just get rid of the sequential turbos and run an actual straight TT full time with stock td04 turbos. If you dont want to merge your harness to a 02-04 WRX ECU to tune with opensource than using a PP6 or Emanage Ultimate should be sufficient with using factory harness.

    Reason why I say this is because the Supra and Rx7 guys do it with there sequential turbos. They just run both all the time the difference is their turbos are the same size so all they have to do is connect wastegates and get rid of the parts associated with the sequential setup and tune..

    I thought about doing a full non sequential td04 TT setup on my wrx swap using the jdm manifold, uppipes and downpipes.. Should be a fast spooling 325+awhp easily.

    *cliffs
    Buy the TT swap, Replace with stock wrx td04 turbos(which are cheap, rebuild-able, bolt ons), T or Y fit the the wastegates and bov together, upgrade injectors, get rid of the sequential system and tune using your stock ecu with a piggyback or merged wrx harness. It's really that simple.

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    Re: Twin Turbo Myths

    -fast spooling
    -Twin TD04
    -Parallel

    hahahahahahahaha

    the 2L engine in the subies simply does not flow enough exhaust gas to support a parallel twin-turbo setup
    even with the tiny twin turbo's that are already on there
    let alone using comparitively large TD04's..

    a supra runs a 3L motor
    and the RX7's have similar "displacement" too

    and adding to how little you know about the subie TT setup.
    both turbo's are very very similar in size,
    just a larger exhaust housing on the secondary

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    Re: Twin Turbo Myths

    If you want to parallel a TT subaru, you're mad. But here's how you'd do it... :P

    For a start, the secondary turbo's up-pipe is very restrictive, with the exhaust valve and everything f***ing things up. First step would to custom make some up-pipes for the turbo's. As for the turbo's, i'd go with VF18/19's off a BG GTB, since these are actually the same size as each other (not all TT combo's are!!!), sharing compressor and exhaust blades. Next up would be custom downpipes, but i suppose anything aftermarket would do. But mainly you want to ditch the cat in the primary downpipe, since there is none in the secondary DP. Next might be the intakes, you could probably leave stock, or you could to a twin intake setup. Oh, and remove the intercooler valve.

    As for management. Simple. Just the same as a single turbo conversion, chuck a single turbo ECU in, or aftermarket. Remove the BBOD and associated vac line mess in the engine bay. Use external wastegates, since only the VF18 has a wastegate, and not the 19, and go with an electronic boost controller to manage them.

    Done and dusted, you now have an overly complicated TT system to achieve exactly the same results as a single turbo setup. At least it's a bit more original that a single conversion!

    My bet is it'd cost the same to do, slightly more headache inducing due to custom up-pipes and such, and it leaves you with no room for swapping on bigger turbo's down the line. Power curves and such would be almost identical to single turbo.
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    Re: Twin Turbo Myths

    Hey Reuben,
    I have a 97 GT-B and I am changing blocks exactly as you did, except I am staying twin turbo. My question is what are type of pistons are the stock ej20r ones? Are they forged like the sti pistons and do you know what they are most similar too if I were to replace them?

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    Re: Twin Turbo Myths

    There are mixed reports floating around, some say that the EJ20R's have forged pistons, some say they are "hyper-eutectic" and others say they are just the same as any other non-forged pistons. the belief that they're forged come from the bad piston slap that most EJ20R's have when cold (the same as aftermarket forgies) but the EJ20R's just have very poor tolerances and you get piston slap. but they have good heads, low compression and just love boost.
    My Ej20R has only about 50,000km on it and slaps like all hell when cold.
    but it pulls like a train when you push 18PSi through the VF18/19 combo..!

    for most of our purposes (running less than 20PSi) the stock pistons are fine if you have a good tune and run good quality fuel to keep detonation away

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    Re: Twin Turbo Myths

    hey man thanks. I lost the big end so I said to hell with it and I am buying a new closed deck block and am gonna throw some boost at it. Closed deck ej20g, acl race bearings, stock rods, but I was wondering about the pistons and head gasket. Does anyone know where I can find some more info? also does anyone know if the ej20r pistons are interchangable with other pistons?

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    Re: Twin Turbo Myths

    Chuck some JE's in there, they are cheap enough. My current build is an EJ20G CD block, random 2L turbo crank, Genuine bearings, Scat rods, JE pistons, Genuine HG, and ARP studs.

    On the bearings, ACL race bearings are not as good as subaru factory bearings. I repeat, they are not as good!!! They are just cheaper!
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    Re: Twin Turbo Myths

    Nice read, was logging one today and rather stupefied by the extent of the VOD. Car is a 2002 JDM auto B4 with ecuid A4SHC01H. Please see the attached and I'd really appreciate your take(s) on it. My experience leans more on the STis and Forester Turbo all running single turbo setups. Thanks

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    Re: Twin Turbo Myths

    ?
    An ugly rusty Subaru is better then a shiny new Honda :smt002

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    Re: Twin Turbo Myths

    Quote Originally Posted by JunkerXL
    ?
    Yeah, looks like the forum doesn't allow me to post attachments; tried adding one like 3 times

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    Re: Twin Turbo Myths

    ahh lame. well at least thats one mystery solved.



    An ugly rusty Subaru is better then a shiny new Honda :smt002

  25. #25
    t3h ub3r m3mber 02_Legacy's Avatar
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    Re: Twin Turbo Myths

    Quote Originally Posted by ckibue
    Yeah, looks like the forum doesn't allow me to post attachments; tried adding one like 3 times
    Upload it to a file hosting site such as photobucket, then link to it here using the Img code.
    02 Legacy GT BE 5MT now with boost!! SOLD!
    04 RSTI
    98 Legacy Outback

  26. #26
    Needs to update their Email!
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    Re: Twin Turbo Myths

    Hey guy's this my first post stumbled across this web sit last night and thought that i must sign up any ways I have a 95 rs bd5 legy and was wondering if i could upgrade my twins at all i want to keep it twin turbo its all pretty much standard apart from the exhaust and what else would you suggest doing to the engine it is a ej20h i love the car so much and wanting to rebuild the engine due to lossing compression in the number four cylinder it has 35psi in it all the others had 160 psi any help would be great also is there any 1 that knows of any computer chips for my model i want to put some yellow top injectors in her to but was told she wont run right if i dont do something with the computer link here in new zealand cant do me a computer unless i can get a external controller for my twin turbo set up
    [color=#FF0040][size=1]A winner never quit's and a Quitter never wins[/size][/color]

  27. #27
    Needs to update their Email!
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    Re: Twin Turbo Myths

    Quote Originally Posted by whogoesthere
    Guy i know is running a vf25 primary with a T4 as the secondary, few other mods, hes running over 300kw at the wheels and slightly under 10second quater mile, is fricken nuts aye. He also has sti 6speed in it with 2L bored out to 2.2L. Fulll tuned ecu to get rid of that lag in the middle as much as poss... id way rather this than any single turbo any day...
    hey mate do you think that you could get in contact with him to find out where he got this done how and what mods i need to do to get this set up in my legacy
    [color=#FF0040][size=1]A winner never quit's and a Quitter never wins[/size][/color]

  28. #28
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    Re: Twin Turbo Myths

    hi i have a vf20 on the left and a vf14 on the right and i have no low down boost is that cause of the vf20 as the primary?"

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