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#2: 04-15-2004, 04:56 PM
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Part 2: Turbos & Engine Tuning
A lot of people have wondered why not twin-turbo, or why not this setup or that. Here is my best definition of the systems Subaru has recently used and is more likely to.

There are 4 main types or turbochargers and layouts applicable to Subaru's:
-Single Scroll Turbochargers (most basic - used in all USDM applications)
-Twin Scroll Turbochargers (used in foreign 2.0L WRX STi & Legacy GT)
-Sequential Twin-Turbochargers (used in former late-model Legacy turbo models)
-Parallel Twin-Turbochargers (non used so far in a Subaru but mayb be with an H6 variety, best description is in the previous generation Audi S4 with 2.7TT)

Single-Scroll Turbocharger
The single scroll turbochargers are as noted above, the most basic in that they have a single exhaust volute. Most stock Subaru (produced by Mitsu or IHI) units do not run ball bearings (the preferential bearing type), which help reduce lag and increase theoretical longevity while being slightly more expensive. To have minimal amounts of lag with this type of turbocharger in a 2.0L application you will sacrifice top-end power for response, that being from a smaller turbocharger, and for higher output levels, low-end response will be sacrificed. There is little happy medium with this turbocharger and 2.0L displacements, big power or zero lag and ultimate response. The 2L displacement class can be fairly responsive with up to around 300chp running a VF28/29 sized turbocharger while the 2.5L variety has around up to 400chp while still being reasonably responsive on the low end. Our 2.5L pushes enough exhaust gasses to not need the following type of unit.

Twin-Scroll Turbocharger
The twin-scroll unit was devised to produce the benefits of a small turbocharger (near-zero lag, plenty of low-end torque), and allow for more power up top (though they don’t have quite the up-top power the mid to larger sized single-scroll units do.) Mitsu has been using this on their EVO’s for a number of years now IIRC and it has shown good results overall. This setup must be used with an equal-length header that is in a 4-2 configuration (not a 4-1 or 4-2-1) where one set of cylinders is directed to one volute, and likewise, the other set is directed to the 2nd volute. This way, there is a very even set of exhaust pulses on the turbine which gives it a much better opportunity to keep the turbo spooled with less volume of exhaust gasses. This setup has allowed Subaru to do away with the Sequential Twin-Turbo setup found in many BD/BG and BE/BH Legacy turbo models found abroad. It is mainly aimed at the 2.0L market as far as Subaru/Fuji is concerned though Litchfield Imports in the UK have produced a 2.5L STi with this type of turbocharger system producing a very responsive 350hp & 350lb-ft of torque. This is also more expensive than single-scroll units.

For more info regarding this type of turbocharger and how they work, you may want to visit Garrett’s website.

Sequential Twin-Turbochargers
These work just as it sounds, in sequence, one after the other. Used for several years in the BD/BG and BE/BH Legacy B4 RSK’s, RS, RS-B, GT, GT-B; this setup was alright, but not entirely effective as it had a large dead zone in the factory tuning between the first turbo cutting out and the second, larger turbo engaging. Additionally, it was overly complex and wasn’t the most cost effective solution. Thus the twin-scroll units have taken over.

Information can be found HERE for starters. More to come...

Parallel Twin-Turbochargers
These turbochargers work together in unison, one per bank of cylinders. Audi used this successfully with their 2.7L V6 in the Audi S4 during 2000-2002 as seen here:

It’s not the most effective setup for 4-cylinder Subaru’s considering the lack of exhause header piping, but considering the H6 would require even more piping, a parallel twin-turbo setup may work the best. I would say either using a setup similar to Audi’s small FMIC's (one per turbo) for a scoop-less H6 twin-turbo terror, or both ducted into a single larger top-mount would work all right. The B4 Asterope-like bumper would work better for the previous method. All in all, this isn’t really required unless you’re pushing more displacement and 6 or greater cylinders.

Conclusion: Based on the general info laid out above, for markets limited to 2.0L of displacement, twin-scroll units work rather well for a good mix of power and drivability, while those of us who can get the 2.5L of displacement have enough gas flow to effectively use single-scroll turbos which are the most effective in price and most widely available. The sequential system is likely to have seen it’s last days with Subaru, while we may see a parallel setup with a turbocharged H6, though it may be some time before we’ll actually see it.

Modern Subaru turbocharged engines do not require turbo timers (in stock configuration) as the cooling system is design to continue circulating coolant through the turbocharger post shut-down to preserve component life. Aftermarket applications may require a shut-down timer system.

Subaru has been adding some 'active' systems that in addition to the normal operating action of the engine are able to vary certain components to increase power and efficiency. Two of these systems, AVCS (Active Valve Control System) and AVLS (Active Valve Lift System) are very new to Subaru, with the latter being new this year and only available on the EZ30R as far as we know.

AVCS is a system that works with the cams (usually only on the intake side where the most benefits are shown) to adjust the duration of the intake valve opening, closing, and rates of change depending on what the computer determines according to sensor input. The AVCS system is actuated through oil pressure from the engine, which is sent through a computer controlled valve and then directs oil into an AVCS actuator to advance or retard cam timing in respect to the cam sprocket. The best thing about the system is that, the same heads can be used with or without AVCS activated. It’s just an auxiliary system in essence.

AVLS is another new technology to help improve volumetric efficiency. While AVCS can alter the cam duration and angle, either advancing or retarding the valve timing, AVLS is able to use two different profiles to alter the valve lift. The EZ30R is the first Subaru engine to use this technology. Instead of using a system like Honda’s VTEC with two cams, Subaru uses a double-profile single cam to control the same parameters. Much like the AVCS setup, AVLS actuates the switch between low and high lift lobes through an oil-actuated system. This system uses a double-tappet (the components which is in contact with both the valve and cam as shown below.) The switchable tappet switches between the low-duration and high duration generally in the 2,000 to 4,000 rpm range depending on ECU input (much lower than VTEC) allowing the EZ30R to breathe better at high rpm and improve low-rpm torque as well as emissions.


The main components of the Legacy exhaust are laid out above:
...although the downpipe and cat-back sections can be split into multiple sections as seen above.

Note on exhaust manifolds; the stock unequal length unit is considered rather good for most applications and is rather trouble-free. Generally for most it is only considered a necessary upgrade when going with a much higher-performance turbo setup or moving to a twin-scroll unit. So for most practical purposes, it's not worth the hassle of messing with.

Some of the things to look for in these components:
Up-Pipes come in a number of materials, with and without flex joints. There are some decent units out there with flex joints, but the consensus appears to be that if the tuner is able to hold tolerances tight enough, that a fully cast pipe is just as reliable and leak-free as one using a flex joint. Down the road you won't have potential issues with the flex joint itself also as miles add up.

Downpipe turbo outlet shape: Bellmouth is preferential, some say divorced is just as good. Some of the higher quality units used cast turbo outlets to better resist the effects of the high exhaust gas temps.

Material/size: 3" is the standard and preferred size for optimal flow. Stainless is the material of choice for it's metallurgical properties including corrosion resistance. Titanium is also available at a premium price though it does weigh less by a nice amount. Mandrel bends are also highly recommended over crush bends, which hurt flow ability.

Last edited by SUBE555; 08-29-2009 at 07:10 PM..
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