Potential turbos for 2.5TT?

[Canadian818]

Any Subaru experts on here wanna toss around ideas for a twin turbo setup? I understand there's no real benifits of running twins vs a quality single on such a small motor, but let's agree that for many there's a "cool" or "badass" factor associated with twin turbos. So for a moderate build, up to 350hp efficiently, what turbos would work with lag or top end loss?

[MST Engineer]

Just get a V8 JDM STI engine that has single turbo with a twin scroll exhaust and you get the best of both worlds.

[wallace18]

I would touch base with AJW. They are experts and give great advice and may help you with what you need. They have been a great help to me with my donor.

[Canadian818]

Should have known one of the first responses would be twin scroll. If it was going to be a single then yes, but I'd like to do it differently. I also have no interest in the jdm engines, I want a 2.5 which should help spool twins.

[Mechie3]

Just voicing an opinion (that goes against TT):

My 06 WRX with the stock td04 turbo (the small one, not the STI turbo) made 312bhp and fully spooled at 3100rpms (21psi). It had 18 psi around 2800 rpms or so. It spooled really quickly. A slightly bigger turbo would give you your 350 without the cost, complexity, and time required for twins.

If you wanted 500bhp it makes sense for twins as you will have a lot of lag. At 350, the lag is so minimal, the only thing you gain is the adoration of people who don't understand that 350hp on a Subie doesn't need twins.

There have been only a handful of TT builds in the Subaru world. Some way back in 02/03 when WRX's were new to the US.

[djtrav]

good luck, its a pain to do and not worth it at all. there might be one subaru in the world that has successfully accomplished this task... you will be paying and modding the crap out of your car trying to get it to work. cost vs benefit, sane vs crazy.

edit:

gave this some real thought, its very possible, but very costly, its all about physics and experience, you will pay for that labor though vs paying for trial and error.

found this on the interwebs for you.

http://img.photobucket.com/albums/v214/mattyk04/car%20meet/DSC_0428.jpg

[MST Engineer]

Should have known one of the first responses would be twin scroll. If it was going to be a single then yes, but I'd like to do it differently. I also have no interest in the jdm engines, I want a 2.5 which should help spool twins.
I agree with Mechie3 there is really no reason for a twin turbo with a 350hp target. If you wanted to aim for huge numbers with a big turbo then there may be a minor benifit but stock WRX and STI turbos already spool really quickly since they are relatively small especially with a good tune. If you want something different and are set on the 2.5 block then make a 2.5 hybrid with the 2.0 Ver8 twin scroll head so you have the torque of the 2.5 and the quick spool of the 2.0.

[BrandonDrums]

Well, I for one am going to be really interested what THIS 818 chassis being loaded pictured below for SEMA is all about. It's clearly a twin turbo. Weather it's a fabricated setup on a USDM EJ207 or EJ257 or one of the old twin turbo legacy 2.2tt's is yet to be seen.

Going back and looking at my old posts about turbo spool on a lightweight small displacement engine, I think there actually could be good merit to using a twin turbo setup on the 818 rather than a large single turbo. The smaller turbos would have more consistent spool on something like the 818 even though there's less displacement per turbo.

http://i996.photobucket.com/albums/af89/BrandondrumsWRX/818_twin_turbo.png

[AJW Performance]

Single turbo hands down.
Affordable, simple, reliable and yields impressive results depending on what your number goals and supporting mods are :)
I am speaking on behalf of Subaru platforms; and in no way comparing what is "best" from an open discussion with different variables (like other engines, etc)
My guess would be the rolling chassis has a twin turbo for 2 reasons;
- The goal was to have an engine installed for SEMA viewing (scooped this one up), giving visuals of how the motor mounts to the chassis, etc...
- shows it could be considered a true world car, since this engine choice is considered more common in other countries due to the fact that it was in production there.

[Mechie3]

Well, I for one am going to be really interested what THIS 818 chassis being loaded pictured below for SEMA is all about. It's clearly a twin turbo. Weather it's a fabricated setup on a USDM EJ207 or EJ257 or one of the old twin turbo legacy 2.2tt's is yet to be seen.


It's a legacy 2.0TT (not a 2.2tt, they don't exist). It's setup for show as it clearly isn't fully plumbed. It's for their chassis showoff.

[Canadian818]

I understand there's no real benifits of running twins vs a quality single on such a small motor

I was really trying to avoid you all wasting your time telling me what I already know. I don't mind the fab work, looking forward to it actually. I'm not sure how different it would be to tune, but I'd imagine the difference is slight. I'm hopeful that there's an OEM turbo solution, or at least something to start with. It would have to be smaller than a wrx turbo, maybe a KKK from an Audi or VW. Eventually I would like to run larger turbos, billet/BB, and 500+hp. However, this car will be a handful to get used to at 300-350hp so I'll build it in stages. Once a viable transmission is available, than i'll build up an engine and shoot for the moon. But for now, I'd still really like to run twins.

So back to the turbos, lol. Any suggestions?

[Turboguy]

So for a moderate build, up to 350hp efficiently, what turbos would work with lag or top end loss?

The stock turbo on the North American STi will easily support this.


A box stock 2.5L turbo with a performance intake & exhaust with a reprogrammed ECU will best 350HP easily. No new plumbing, and no new turbos needed :)

[Canadian818]

Going back and looking at my old posts about turbo spool on a lightweight small displacement engine, I think there actually could be good merit to using a twin turbo setup on the 818 rather than a large single turbo. The smaller turbos would have more consistent spool on something like the 818 even though there's less displacement per turbo.

http://i996.photobucket.com/albums/af89/BrandondrumsWRX/818_twin_turbo.png

Look at how neat and tidy that setup is! Now compare it to the twin turbo impreza above it...it's obvious why a twin setup is rare on 4 bangers. Also, imagine how short the turbo manifolds are, that should make for quick spooling. I would lose the Subaru sound that many love, however I plan to copy the crossover exhaust that all the 911 TT tuners use so it should still sound unique.

[Canadian818]

The stock turbo on the North American STi will easily support this.


A box stock 2.5L turbo with a performance intake & exhaust with a reprogrammed ECU will best 350HP easily. No new plumbing, and no new turbos needed :)

I'm banging my head against the wall here guys. Think outside the box, theorize with me.

[shim2]

I think the idea of Twin turbo 818 is great. The picture above is a JDM Legacy Twin sequential EJ20. I think the reason you don't see more TT Impreza's is because the compartment is so cramped with just a single turbo. I don't think that will be a huge problem with the 818 so TT, IMO would make more sense. Although you would lose the subi sound but as you said if/when you plan to do a crossover exhaust like the 911 guys do to keep the sound.

[Mechie3]

What TT impreza are you referring to? Tuning would require an aftermarket ecu.

Nm. Just saw the edit above. That was the wrx I was referring to as one of two TT subarus I'm aware of that wasn't using the OEM TT. That car was built before the 2.5l turbo motors came here if I recall correctly.

[Canadian818]

My plan was to use a Cobb.

Also, the 818TT chassis above is not a sequential twin turbo set up, that's different all together.

[Mechie3]

Cobb is not an ecu, it is a tool to reflash a stock ecu. The stock ecu to my knowledge has no means to control a second turbo, much less read sensor inputs from a second turbo setup.

The OEM legacy TT motor is sequential. Even Subaru says so. ;). Sequential means one turbo runs at low engine speeds and loads and at higher engine speeds it switches to either running a single large turbo or running two smaller turbos in parallel (how the legacy works).

http://www.veloce.pwp.blueyonder.co.uk/Twin%20Turbo%20Operation/ttopprt2.htm

[shim2]

Subaru NEVER made a parallel(identical turbos) twin turbo setup for Impreza, H6, Legacy ect ect. The only two turbo setup they made was sequential for the JDM Legacy. Anything/everything else is not OEM.

[Canadian818]

I'm not saying the legacy isn't sequential, I'm saying the engine on the 818TT above is not. A sequential turbo setup uses the turbos inline, together, usually a smaller turbo feeding a larger turbo. There's no connection between those turbos and there's two seperate inlets on the intercooler.

Also, I realize the Cobb is not an ECU, however I'm confident that it'll work just fine. The ECU doesn't controll the turbo(s), but rather a boost sensor which sends a signal to the wastegate(s). It's all going through one intercooler and one throttle body, and one intake manifold. It should be as simple as teeing the wastegate signal. (controlling the second turbo, I'm not implying that the entire setup is "simple")

[shim2]

I'm not saying the legacy isn't sequential, I'm saying the engine on the 818TT above is not. A sequential turbo setup uses the turbos inline, together, usually a smaller turbo feeding a larger turbo. There's no connection between those turbos and there's two seperate inlets on the intercooler.

Also, I realize the Cobb is not an ECU, however I'm confident that it'll work just fine. The ECU doesn't controll the turbo(s), but rather a boost sensor which sends a signal to the wastegate(s). It's all going through one intercooler and one throttle body, and one intake manifold. It should be as simple as teeing the wastegate signal. (controlling the second turbo, I'm not implying that the entire setup is "simple")

This is a image of a JDM EJ twin sequential turbo
http://img.auctiva.com/imgdata/1/5/0/9/8/9/7/webimg/583751803_o.jpg?nc=147

[Mechie3]

I'm not saying the legacy isn't sequential, I'm saying the engine on the 818TT above is not. )

The engine, on the 818TT, is the engine from the legacy. Theyre the same thing. Did you click that link I posted above? It clearly shows diagrams and explains the function and control of the turbos exhaust and the intake.


A sequential turbo setup uses the turbos inline, together, usually a smaller turbo feeding a larger turbo. )

Not quite. One turbo feeding another is generally referred to as compound charging, or staged (or staged sequential). Just plain sequential refers to a TT system in which the both turbos are not always operating.

http://en.wikipedia.org/wiki/Twin-turbo#Sequential_turbos


Sequential turbos refer to a set-up in which the motor utilizes one turbocharger for lower engine speeds, and a second or both turbochargers at higher engine speeds.

The Legacy motor can't be setup with just a cobb. They have valves controlling exhaust flow (all into one, then splitting it) and a valve controlling intake flow. In a true sequential setup, you can't just T everything together because at low engine speeds, only one turbo is operating.

EDIT:
Here's the exhaust routing for the Legacy TT.
http://www.syms.jp/products/images/le_ex-1.jpg

Here is a diagram showing the dual inlet intercooler from the subaru, again proving that the setup in the 818TT is the legacy sequential setup.
http://www.autozine.org/technical_school/engine/Turbo_Sequential.jpg

[Canadian818]

Wow, it's like we're in different worlds. I'm not interested in a sequential setup, so the legacy turbo shouldn't matter. Thanks to the pics I now see how the legacy runs sequential, and thank you for clearing up the difference between sequential and compound setups. However none of this helps me. I thought that on a kit car site people would be more open minded about doing things different than OEM and doing things different just for the sake of being different. I will pursue help elsewhere, and when the time comes I'll share what I've learned in a build thread.

[shim2]

I see nothing wrong with you planning to do a twin turbo setup. I think it's a great idea and I really don't think it would be all that difficult in the 818. All I was trying to get at is that the one in the go kart is the JDM sequential from a legacy.

[Mechie3]

Wow, it's like we're in different worlds.

Well....you are Canadian ;)

(just in jest)


. However none of this helps me. I thought that on a kit car site people would be more open minded about doing things different than OEM and doing things different just for the sake of being different. I will pursue help elsewhere, and when the time comes I'll share what I've learned in a build thread.

I'll be honest, I don't know much about building TT setups. I do know quite a bit about single turbo and the Legacy TT, so all I can do is give you background info. One reason I dont know much about TT Subaru setups is because no one does them, and there's reasons for that. Most people that set out to do it end up running out of money, or can never get it to run. Same reason why only a handful of people run NOS on an EJ motor. They all tend to blow up REALLY quickly. It's not that we're close minded as much as we've seen very similar build threads before where the outcome is always poor.

I do know (if I remember my numbers correctly) that the turbos on the TT Legacy are the VF32 and the VF33. VF turbos aren't the most efficient though, and are a bit outdated in their design.

If you do end up going this route, I woulnd't mount the turbos in the stock location (assuming the 818 chassis allows for them to be moved). Put them closer to the front of the motor. You will lose much less exhaust energy doing this. The last version of the Legacy GT did this.

http://car.watch.impress.co.jp/img/car/docs/169/682/lgc95.jpg

[JRach]

There's another reason the STI and WRX never had a twin turbo set up that ever caught on...
Firing order. Subarus fire 1-3-2-4, cylinders 1/3 are on the passenger side and 2/4 on the drivers side.
For a proper twin turbo set up you'd need a runner from one head to cross cross with a runner from the opposite head so the exhaust pulses are spaced evenly apart. Now, that you've criss crossed the runners so you have the exhaust pulses firing on opposite sides every time (example, run cyl 1 exhaust to the drivers side and cylinder 4 exhaust to the passenger side), you're going to have one really long runner paired up to one really short runner, so you'll need to also extend the short runners (cyl 2/3 using my example) to match the length of the long runners (cyl 1/4).

Without doing all of that you'll be losing any of the benefits to using a twin turbo set up vs a decent single turbo. Now think about all the weight and space all that plumbing and piping will take up, as well as the additional weight of a second turbo and plumbing from both to the intercooler, additional coolant/oil feed/return lines an it quickly loses any appeal!
The reason you've seen twin turbo H6 motors is because they already fire on opposite sides, so you can seperate the exhaust from each head to individual turbos without any additional work.

As for tuning, Open source! No need for a access port. Save the money and spend it elsewhere. :)

[JRach]

If you do end up going this route, I woulnd't mount the turbos in the stock location (assuming the 818 chassis allows for them to be moved). Put them closer to the front of the motor. You will lose much less exhaust energy doing this. The last version of the Legacy GT did this.

http://car.watch.impress.co.jp/img/car/docs/169/682/lgc95.jpg


Which is why both my STI swapped forester does use and my 818 will use a low mount twin scroll turbo manifold!

(As seen in my pic to the left)

[Canadian818]

I'd imagine it's never done for a few reasons, and I can think of a few reasons why it should be considered on a EJ motor when adequate room allows. Think about why most inline 6 builds run a single turbo while flat 6 or V6/V8 for that matter run twins. I plan to mount them roughly where they are on the legacy TT posted above, however there will be two seperate exhaust manifolds. I can't imagine why a Cobb accessport couldn't run that setup. Both turbos will be always running equally.

And yes, I'm Canadian...but I'm still in the same messed up world as you. Pretty much the same language too EH!

[Mechie3]

If both run equally in a parallel setup, you probably could just T everything together. Not sure of the minutae of doing that and overboosting here/there.

Yes, unfortunately we do life in the same messed up world. The unfortunate part, of course, being the messed up world, not that we're all in it. :)

[JRach]

But V6 and V8 motors fire back and forth between opposite heads with every combustion.
Subarus fire twice one the passenger head and then twice on the drivers side head. That's why running two seperate manifolds (one from each head to each turbo) doesn't work (removes any of the benefits gained by running two smaller turbos).

[Canadian818]

Didn't realized that about the firing order. Hmmm, gonna have to read up on how much that will effect spooling. Not to worried about equal length manifolds, that's what makes the Subaru sound, lol.

[Canadian818]

Been thinking about the firing order problem, at it seems to me I have a few options. I could stick with my original plan of running a turbo off each bank seperatly and hope that spool doesn't suffer from "pulse/pulse/pause/pause." I could fab two seperate manifolds that take a port from each bank (not caring about equal length) and get an even pulse/pause/pulse/pause. Or I could run a manifold like pictured above for the legacy TT that has the turbos linked through the manifold but run them constant instead of sequential.

I wonder if the IHI GTR OEM turbos would work, very efficient design.

[shim2]

Well from looking at this diagram at "high speed" each bank is feeding it's own turbo so I don't see why it wouldn't work at lower rpm. Maybe it would have a bit more lag but at high rpm it clearly works. BTW, I did some reading and the second turbo starts boosting around 4000-4500 rpm. Unequal length piping on the 1,3 bank could help with the fire fire, pause, pause issue.
http://www.autozine.org/technical_school/engine/Turbo_Sequential.jpg

[Turboguy]

I'm banging my head against the wall here guys. Think outside the box, theorize with me.


Actually that's exactly what it strikes me as - an exercise in banging one's head against the wall.

You seem hell bent on re-inventing the wheel, which is likely why people aren't breaking down the door to help. I'm not sure about others, but I perceive it to be a colossal waste of time.



Dude- box stock with minimal mods a 2.5L STi motor will do everything you've asked for and more. I just cannot see the reason for complicating it -- especially given you're not achieving anything special.


If your goal is a 1-off build, rarity or "thinking outside the box" how about shoe-horning in a Porsche flat 6? Or perhaps Subaru's new H6 and then throw a turbo on it a la Cobb Tuning?

[NonProfit]

I'm banging my head against the wall here guys. Think outside the box, theorize with me.

To expand on what Turboguy said, sometimes customization makes sense, sometimes it doesn't.

You're only limited by time, money and ability. That's not the case for the major manufactures. Every Subaru is mass marketed. There are some drawbacks to that. The car they sell in Anchorage, Alaska is the same car they sell in Key West, Florida. These cars need to perform well and remain durable in many different environments for many different driving styles. These cars need to appeal to many different types of buyers and must remain profitable. However, when you build your own, much of that is no longer an issue. That's where a kit excels, by allowing the builder to customize it to their use case.

But what no builder has is the R&D budget to research the customization of performance parts. Truthfully, twin turbos have been implemented before. And yea, twin is always cooler than one. (And quad would sound even better still!) But if the idea worked well, it would already be implemented by the major auto manufactures if the idea did does not conflict with the issues of mass marketability (cost, durability, etc.)

If you're wanting to try some crazy one-off idea, that's great! If you're looking for help to do it, that's wonderful. And if after seeking council you still want to go for it. Have at it, that's what DIY autos are all about! But it's really not too cool to ask for help and then get frustrated when the community universally agrees it's a bad idea.

[Xusia]

But it's really not too cool to ask for help and then get frustrated when the community universally agrees it's a bad idea.

Well stated. I understand wanting to do something unique to make it your own. It just seems like this is the most difficult thing that could be chosen - and with no real benefit other than being able to say "I did it."

The H6 should fit physically, and has a better firing order. Why not twin turbo that? And if/when you figure out how to mate it to a manual, I'll be able to put an H6 in mine! Non-turbo, of course... LOL

[Canadian818]

I get it guys, and I'm sorry for getting frustrated with you all when your infact trying to help me by steering me clear. However after all that, I still want to do it. I dont consider it on the same scale as fitting a Porsche or Subaru H6 in there. Nor should the cost be much more. IMHO, if I can get a twin turbo setup to perform equal to a single at my given modest HP goal than I'll be happier than if I followed the pack and stuck with the tried and true. I'm not looking for an advantage, just to achieve similar performance and be unique. I can't help but smile thinking about popping the hood and seeing symmetry, two of everything! lol. Now imagine the sound of two unequal length headers, and a porsch-esk cross over exhaust without cats...drool.

The only extra cost should be sourcing the turbos, and I'll have one to sell. I'll fab stainless exhaust manifolds for cheaper than any aftermarket headers, and I'll save weight to offset the second turbo by using titanium for the exhaust. I'm really excited about working with titanium, I spend most of my work day welding high pressure stainless piping, (I also have mild steel and aluminum pressure welding tickets.) So pluming the piping doesn't bother me. The only intimidating part to me is tuning, but I'll just have to pay for that.

For those interested, I've found some turbo ideas. I downloaded the Garrett app and punched in some basic numbers and a goal of 400hp and it recommended two different turbos. A GT1548, and a GT2052. If I raised peek power levels to 7k, it recommends a GT2869R which is ball bearing. That could be a potential stage 2 build when we have tranny options. However for now I'll focus on the GT15 and GT20. With some searching I found a site that lists most known OEM turbos, and it turns out pretty much every 4cyl turbo diesel from Germany uses turbos between a GT15-GT22, most of which being GT17's. So I might be able to find a pair of used turbos in decent condition and start there. Once it's up and running I'll have a better idea of what size turbo will be ideal.

[NonProfit]

Post a vid when you're done; you might just make one cool car!

[Samiam1017]

I think it's a very doable thing and truely custom mod not just a easy bolt on. I hope you succeed. I maybe looking into this too. Can you post the link to that site that list the factory turbos. Thx

[Canadian818]

http://www.dieselevante.it/turbochargers.asp?idmarca=97&idcat=2

I've come across some very interesting stuff, and now I'm really excited for this build. I'm gonna leave it alone for now, and once I have a kit ordered I'll start collecting parts. Once I have a build thread started you'll see why I'm excited. If there's interest, and of course if I'm successful, I'd be willing to make and sell the parts for those not so savy with a welder.

[Turboguy]

Now imagine the sound of two unequal length headers, and a porsch-esk cross over exhaust without cats...drool.


If sound is something that really gets you going, I'd recommend taking a Porsche flat-6 car out for a spin and wringing it out a bit. It may have you taking your project in an entirely different direction before your chassis kit even hits your driveway :)

[flynntuna]

Porsche air cooled flat-6 made glorious noises when wrung out. Subaru's can be almost as nice with the right exaust. No fart can!

[Xusia]

No fart can!

+1

(this part in parenthesis is only to meet the minimum post length requirement!)

[Canadian818]

A 996tt will be in my near future, and I can't wait. But I want to run Targa Newfoundland, and I'd rather roll a 818 in a ditch than Porsche. So 818 comes first.

[Canadian818]

I wonder what a Subaru would sound like with a sport bike muffler, lol. Seen it on a S2000, but they scream at ya anyway.

[Xusia]

Interesting concept. At first I didn't think that would work very well because of the differences in displacement, but I didn't initially take into account RPM. My motorcycle exhaust is built to handle the flow of 1 liter of displacement running at 15k RPMs. Theoretically, a 2.0L engine running at 7.5k RPMs should have similar flow, perhaps a bit more because of the boost. Maybe it would work on a 2.0L engine, but I think it would be too restrictive for the 2.5L.

Slip ons are cheap. I might try it!

[Evan78]

Interesting concept. At first I didn't think that would work very well because of the differences in displacement, but I didn't initially take into account RPM. My motorcycle exhaust is built to handle the flow of 1 liter of displacement running at 15k RPMs. Theoretically, a 2.0L engine running at 7.5k RPMs should have similar flow, perhaps a bit more because of the boost. Maybe it would work on a 2.0L engine, but I think it would be too restrictive for the 2.5L.

Slip ons are cheap. I might try it!I would expect peak HP to be the most convenient and relevant comparison.

[Xusia]

I really don't think that's the case. Engine efficiency varies and therefore the relationship of airflow to HP also varies between engines. Mufflers are designed to work with specific air flow characteristics regardless - and independent - of the amount of HP being generated.

[Evan78]

I agree that the muffler is designer for specific air flow characteristics, but what I'm saying is that my guess is that those characteristics have a greater correlation with the amount of power being produced than with the displacement and rpm of the engine.

As an example, you could have the same engine running the same max rpm, making 225 hp or 450 hp by using different turbos. They clearly have different exhaust characteristics even though rpm and displacement remain constant.

[Xusia]

What I was referring to when I said "...perhaps a bit more because of the boost" was the air flow; meaning there would be more air flow than a stock 2.0L motor because of the turbo. It's my opinion - and only that - the difference in flow between a N/A 2.0L and a stock 2.0L turbo (WRX) is not significant in terms of using a motorcycle exhaust. I think the design of the exhaust I have in mind will inherently support the additional air flow.

Taking turbos out of it for a moment - because they of course DO affect air flow - and without going down the proverbially rabbit hole and into the pedantic, air flow should be fairly well correlated to the displacement and RPM of an engine. This is because an engine is essentially a pump and displacement and RPM are the variables that determine flow for that kind of "pump." HP variations between engines of the same (or similar) displacement are generally the result of design characteristics (bore & stroke, compression, cam profiles, timing, etc.) which for the most part do not affect the amount of air pumped through the engine at a given RPM (again, without getting into the pedantic).

[Evan78]

Yes, I'm aware that engines are pumps. My simple thinking is that power is made by burning fuel and the air to fuel ratio across engines of all sizes is fairly consistent, so the most readily available indicator would be horsepower. It is a simple way to take the turbo into account. Just thought I'd throw it out there that if I were mixing and matching exhausts, I'd use that measurement before I'd use displacement and rpm.

[Turboguy]

A 996tt will be in my near future, and I can't wait. But I want to run Targa Newfoundland, and I'd rather roll a 818 in a ditch than Porsche. So 818 comes first.



What I meant, was that after sampling the sounds of a wrung out Porsche flat-6 you might opt to built a flat-6 818 - like perhaps one based on Subaru's new H-6 or the one they stuffed into the SVX.


Their air-cooled and water-cooled flat 6's sound remarkably similar, both to each other and Subie's flat 6's.

[KP1]

I would say forget the twin turbo set up. I have it in my JDM 2000MY Legacy. The Legacy is now semi-common here in the UK and all the owners I have seen on the forum complain about the set up. There is a distinct gap in the power delivery at approx 3000 to 3500 rpm, this is the change over period where the 1st turbo is slowing down and the 2nd turbo spooling up and if you change gears during this period you are left floundering in no mans land with no boost and no power as the ecu tries to work out what is going on and what it should do (this seems to take forever especially when you have just pulled out to overtake and have a huge shape bearing down on you in the rear view mirror at a rate of knots).
Most owners adapt their driving style so that they change gears either below 3k rpm or above 4k rpm to avoid this problem BUT it has a short ratio gearbox which can make this a real pain in town driving. Also this hammers your fuel economy on the motorway (freeway) as 4000 rpm in 5th gear is only 55mph. Cruising at (our limit of) 70mph you're doing approx 5500 rpm where the 2nd turbo is boosting along nicely, not good at all if you are stuck on the freeway for several more hours to go.

[Evan78]

Wow, that is really short. I can't imagine cruising along at 5500 rpm for very long.

[Canadian818]

I would say forget the twin turbo set up. I have it in my JDM 2000MY Legacy. The Legacy is now semi-common here in the UK and all the owners I have seen on the forum complain about the set up. There is a distinct gap in the power delivery at approx 3000 to 3500 rpm, this is the change over period where the 1st turbo is slowing down and the 2nd turbo spooling up and if you change gears during this period you are left floundering in no mans land with no boost and no power as the ecu tries to work out what is going on and what it should do (this seems to take forever especially when you have just pulled out to overtake and have a huge shape bearing down on you in the rear view mirror at a rate of knots).
Most owners adapt their driving style so that they change gears either below 3k rpm or above 4k rpm to avoid this problem BUT it has a short ratio gearbox which can make this a real pain in town driving. Also this hammers your fuel economy on the motorway (freeway) as 4000 rpm in 5th gear is only 55mph. Cruising at (our limit of) 70mph you're doing approx 5500 rpm where the 2nd turbo is boosting along nicely, not good at all if you are stuck on the freeway for several more hours to go.

This is exactly why I have no interest in running turbos sequentially. How fast does the legacy spool up on the first turbo? Please tell me it's before 3500rpm, that's ridiculous for such a small turbo.

[Mechie3]

The stock TD04 on my 06 hit 18psi around 2800 rpms and 21 psi at 3100 rpms. If a sequential turbo setup can't beat that it's pretty piss poor.