whats the difference - twin turbo & single turbo?

[theboyo19]

ive seen twin turbo engines (JDM) and i saw a jdm bd sedan that a guy has on the members section of this site. he had the twiin turbo engine and guys were asking him if he was gonna change to a single turbo setup
my question is whats the difference.. what are the benefits of each? im looking into doing an engine swap and want to find out alot before i go buying something that i cant upgrade/ wont have fun with

[StatGSR]

the differance is that a TT set up wont fit (easily) and that a single turbo will.....

[theboyo19]

is there a power difference?

[Wiscon_Mark]

is there a power difference?

Depends entirely on the setup.

The twin turbo that you're referring to is a sequential setup - a smaller turbo spins up to get good low power, and then a larger turbo spools up at higher RPMs to give it that kick.

Subaru never really perfected this setup, and there was rather a large gap between turbo spools, making the car laggy in the midrange.

Anyways, twin turbo is not possible on LHD cars because the turbo gets in the way of the power steering stuff, not to mention not worth it because a single turbo gets the job done just as well (if not better) than the twin turbo.

[Huffer]

The TT conversion can't really be done on the USDM car without going to RHD. In addition, the TT block is pretty weak - the bearings can go walkie with just a slight boost upgrade.

As for power - most of the TT cars made around 186kw (I forget the conversion). The "VOD" Valley of Death lag between the 1st and 2ndary turbines was pretty bad on some cars, but it depended on the individual cars. When the 2nd turbo kicked in, it kicked in pretty hard and fast. That was the appeal of the car. Small turbo for round town, get on the freeway and open it up.

The GT-B Legacy held a lap speedring record for about 7 years.

[sheepdog]

Long story short: For an engine swap, the twin turbo isn't worth the trouble. I know it sounds cool to say twin turbo, but just swap a single turbo motor in and save yourself no end of headaches.

[theboyo19]

sweet thanks

[Sarra]

Twin turbo setups (on 4 cylinder engines) use sequential turbos. One spools down low, then up higher it kicks out and the other one kicks in. You have a valley of death in the middle.

In JDM land, the steering rack is located on the opposite side, so when you swap a JDM TT engine into a USDM car, you have fitment issues since the turbos would occupy the same space that your USDM car's steering rack occupies.

Doing a TT swap would require you to replumb the turbo setup, or switch over to RHD. Good luck either way. :-)

[adw_terror]

yes, i can confirm that there is definately a VOD in the midrange powerband... i have a TT legacy and the first turbo kicks in pretty low, but around 4500 rpm there is a huge drop in power... i really dont even feel it pulling until about 5-5200 rpm again, and then its pretty powerful... i purchased a manual boost controller to try and help this, and while it helped a lot, there is still a pretty noticable void between the two turbos... im waiting until after my 2 yr inspection in september to fabricate a 4" downpipe, 3" exhaust setup, and thats supposed to help eliminate most of that lag
-Josh

[rallysam]

5000 rpm??? Jesus... that is a valley. I can spool the single sport sedan turbo below 2krpm. I dunno why the designers wouldn't just use a slightly smaller turbine for the secondary turbo. I mean, people can even make bigger power and find better compromises with only a single turbo, so they must've been able to do better with a twin.

[deadlydave]

rallysam, it does seem short-sighted of them. Guess they figured spirited drivers would keep the tach pegged. :twisted:

[MCarp22]

rallysam, it does seem short-sighted of them. Guess they figured spirited drivers would keep the tach pegged. :twisted:

Back in '93? or so when the cars were designed we didn't quite have the stuff that's available now 14 years later.

That said, don't those wind out to 7500? You shouldn't be in the midrange anyhow! ;)

[adw_terror]

well, im not too sure on this, but i believe there is some sort of valve or something that keeps exhaust gases from entering the second turbine until a certain rpm... my reason for thinking this is because when after that second turbo is initially spooled it keeps spooling even after you shift... for instance if i take off in second i will notice that VOD until the second turbo kicks in, but once i shift into third, even if the rpm's fall below 5k or if i let off the gas for a few seconds the car will still pull hard without a VOD
-Josh

[rougeben83]

well, im not too sure on this, but i believe there is some sort of valve or something that keeps exhaust gases from entering the second turbine until a certain rpm... my reason for thinking this is because when after that second turbo is initially spooled it keeps spooling even after you shift... for instance if i take off in second i will notice that VOD until the second turbo kicks in, but once i shift into third, even if the rpm's fall below 5k or if i let off the gas for a few seconds the car will still pull hard without a VOD
-Josh

Yeah, it's more of an ECU issue and not mechanical IIRC, as in that valve Josh is talking about is controlled by the computer. The tt setup in the Legacies actually used two turbos that were relatively the same size ( I think the smaller one is a vf22 and the other is a vf24? don't quote me on that). I remember reading that one of the first ways to address the transition problem is to reprogram the ECU along with the bigger exhaust (always good on turbo cars).

[Huffer]

I will just say that my NZ brothers that have run TT's (RSK, GTB) have found GREATLY REDUCE "VOD" when a decent grounding system was installed. This was actually dynoed and proven to reduce the lag considerably.

One other option is obviously to increase the size of primary turbine, or reduce the size of then 2ndary (with corresponding results).

Be careful removing the cats in the primary downpipe - many a GTB have lunched themselves by overboosting because of the removal of the catalytic converters. Remember, the ECU's in the earlier cars depended a lot on mechanical restrictions as much as electronic prowess to deliver power AND reliability.

[Mel Adjusted]

Fact of the matter is both turbo's are the same size. How the system works is primary turbo kicks in 1st., when it reaches a certian boost the exhaust valve on the exhaust pipe just before the secondary turbo opens and spools up. The time for it to spool up is called VOD (valley of death). There both very small ball bearing turbo designed for engines of 1,200cc size. Being the EJ20H is a 2,000cc engine the 2 turbo's combined are more than enough for the engine. For normal driving Subaru did a great job, but for hi-performance alot depends on the owner and tuner. They can be made to perform as will as a single turbo within there limits, and with a much better low end boost. Subaru abanded the TT do to emission problems.
As for the installation in USDM cars it's possible but the righthand turbo hits the break cylinder not the steering. Thats another myth posted by people that have never seen or used one of these engines.
A fact is it is much simpler to convert it to a single turbo set-up. The basic engine is the same as the earlier G's and later K's. I have a earlier one that is closed deck and 3 that are open deck.