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What do you think?

No.

Why?

Twin Turbo's are a waste. And the EJ20TT is a waste.

A) The EJ20TT is just an EJ20. You could turn a USDM EJ20TT into a TT if you really wanted to and you'd do just have to do a bit of fabbing (like a custom downpipe setup, it's really not that hard actually).

B) Twin Turbo's have 1 advantage. Drivability. There is a sliding scale between drivability and performance. The higher towards the right you get, the further from the left. Twin Turbos (sequential, Parallel is a completely different beast) are GREAT for drivability as you can spool quickly and what not. They AREN'T great for power. In Fact, Twin Turbo's are anti-power, even in stock trim (the EJ20TT would make a good bit more power if it was making the same boost with a single turbo).

Why? Look at it from a single turbo standpoint. Crank the stock VF11 up to 15psi. It peters off after 4,000 RPM and only pushes 11pounds max. It's a good bit out of it's efficiency range, and you are making a good bit of heat. Stick a larger turbo in front of it. At 4000RPM, when the VF11 is running out of juice, the larger, say, 16G, is coming to life.

Now you've got a problem. Your 16G is pumping away, making obscene amounts of boost well within efficiency. But not really. While your 16G is making 18psi, you've got a significantly overtaxed VF11 doing 3 things. 1) Blocking the air. It's a big restriction, the air has to hit a superheated wall before getting to the big turbo. 2) HEATING the air, as it's well outside of it's efficiency range, all it's doing is heating up. 3) It's dying. Again, going that far out of the efficiency range is not good for the turbo's life span.

Run that same 18psi through JUST the 16G, and spool times are slower, but you make more power. And carrying power to redline, and no lag after a shift is key. The Vf11 won't be able to do this, a Twin turbo won't be able to do this efficiently, a single turbo is the best option.

Why do you think Supra guys swap from the stock Twin setup to a big single? You can really only make about 440whp on the stock twin turbo setup.

I wouldn't waste the time or money, but I WOULD consider a parallel twin turbo. Now THAT would be interesting...

BAC5.2 wrote:I wouldn't waste the time or money, but I WOULD consider a parallel twin turbo. Now THAT would be interesting...

Just what I was going to say...

it surprises me that the smaller turbo doesnt have a big fat flapper bypass valve; that would negate the big problem you presented.

Wait a minute -- You're not suggesting the compressors are plumbed in series, are you, Phil? Cuz they're not. That would be weird, because compressors operate at pressure ratios, not amounts of boost. Even "sequential" twin turbo systems have the compressors in parallel.

Can you describe that a bit Vikash? The way I've always thought it was 1 turbo feeding another...

I don't completely understand it either... I think Corky Bell wrote in his book Maximum Boost that a sequential twin turbo setup rivals the Space Shuttle in complexity.

But basically, in almost all twin-turbo setups the compressors are in physically parallel. Either or both can blow air into the engine. Like with MkIV Supras, you can actually just do some monkeying with the vacuum hoses and turn the sequential twin turbos into parallel twin turbos.

I think the turbines are also generally plumbed in parallel; exhaust gas goes through both turbos' hot sides.

I could be wrong about all this, but I'm pretty sure...

one turbo feeding another is actualy a good thing if the turbos are the right size for each other and if you can bypass the smaller one as soon as the bigger one has come to life.

as for the orignal question a usdm single turbo ej25t sti would be much better or a single turbo ej20 sti jdm. the usdm motor is not as turbo dependent because of its larger displacement and it also has more torq. there fore it goes thourgh trannys like nothing. if you want to build a drag car sti motors are best but if you want driveabilty and a car for autocross go for a twin turbo. a usdm sti will do mid 10's at sea level with just an fp green and suporting mods and mid high 11 at 5000 feet.

to be honest, im not sure if this is accurate at all, but i think this is how the two systems work.


The pressure valves just have to notice where the greater psi is comming from and switch to it.

The advantage of twin turboes would be the simplicity- no valves and whatnot at the expense of having to get small, high boost turboes.

The advantage of the sequential system is greater control over when the secondary turbo kicks in, (hell, it could be ecu controlled) and potentially more power and better low end.

I'd appreciate your imput

Vikash - Now that you mention it, it actually makes MORE sense that it happens this way (exhaust going through both hotsides).

I've got a link back at school for a TT setup on a Dodge 5.9L Diesel. I vaguely recall it, but I do think the hotside of the first turbo dumps into the hotside of the second with a 90 degree elbow kind of pipe. I'll find the link.

Maximum boost huh? I'll be sure to pick that one up. There's several books on my "Borders" list that I want to pick up.

And yes, i realize i am drawing TT systems on v4 engines.

Scott - When exhaust gas is split between the turbo's, they HAVE to be smaller (hot side) or they won't ever get enough gas to spool real amounts of boost. This limits them greatly, and is a reason why most TT guys will do a single turbo swap. It's also the reason why a parallel (well, sort of parallel, but one turbo for each bank) wouldn't really work on a small engine (like a Subaru engine). Just not enough exhaust gas to make reasonable power from.

you will have to excuse me, my brain doesnt come back from vacation until thursday.

I just said a bunch of stuff i knew was wrong. You're right, bac.

Wow, thanx for the quick responses... So to get on everyones nerves...

No worries, it sparked a good discussion!

blueslsti2004 wrote:Wow, thanx for the quick responses... So to get on everyones nerves...

you could have told us what you wanted to use it for. it would make the worth it question much easier to answer. good discussion none the less

azn2nr wrote:

blueslsti2004 wrote:Wow, thanx for the quick responses... Sorry to get on everyones nerves...

you could have told us what you wanted to use it for. it would make the worth it question much easier to answer. good discussion none the less

I suppose I could of... I'll tell you now, lol... I want to use the twin turbo setup for a daily driver as well as autocrossing (someday). I also want to be different and unique. I don't really want to follow the crowd. I only know of two other people that have a twin turbo setup here in the states. I'm not lookin for huge amount of hp RIGHT NOW, but somewhere along the lines of 300-400 hp with little turbo lag. COme to think of it... that's a lot considering right now I only have 120 . What do you guys think? I can get a engine for pretty cheap along with the tranny, and dash through my "connections".


-Jon-