Part of the kit is a new reverse-flow baffled exhaust that mounts the muffler higher and h
XS uses the 2ZZ cast iron manifold from its Celica GTS turbo kit. Getting the manifold to mate with the 1ZZ cylinder head requires minor modifications to the cylinder head bolt pattern, and using bolts rather than studs. The manifold is made of heat-resistant, high sil-moly cast iron, a grade of ductile iron with a high silicone content for clean castings, with the addition of molybdenum and nickel for exceptional heat resistance. It's cast as a smooth, curving 4-1 design with good, flowing entrances into the turbine inlet, despite a compact design; far better than most cast or weld-el log manifolds.
Like any stock motor, the stock 1ZZ can only reliably take a half atmosphere (roughly 7psi) of boost pressure. To compensate, XS uses a smaller, lag-free IHI VF23 turbo for the kit. The compact VF23 is a popular aftermarket upgrade for mild tuned 2.0-liter WRXs because of its responsiveness when used in medium-displacement four-cylinder engines. Like most OE-style turbochargers, the VF23 has a ball-bearing, water-cooled center section for quick spooling and superior durability. Hsu sized a P20 exhaust housing for the turbo, which left us scratching our heads. We have no clue how big this housing is, since IHI doesn't use a typical industry standard of area ratio for their turbo housings (apparently P20 is IHI code for cross-sectional area of the turbine housing volute and is not useful for comparison to any other brand of turbos).
Our prototype kit uses a divorced discharge pipe at the turbine outlet to reduce backpress
Just like the production kit, our VF23 has an internal wastegate for boost pressure control and low inertia compressor and turbine wheels for fast response. The turbine discharge dumps into a 2.5-inch diameter exhaust silenced with a reverse-flow muffler positioned horizontally to make room for the intercooler. Although more restrictive, the reverse-flow muffler was deemed necessary to sufficiently quiet the MR2's extremely short mid-engine exhaust system. Unlike the production kit, XS fabricated a divorced discharge pipe for the wastegate that reintroduces the wastegate discharge flow back into the exhaust, several inches downstream of the turbo. This feature is usually good for at least 10 wheel-hp and reduces backpressure and lag by preventing the exhaust flow in the turbine outlet from slamming into the wastegate discharge. Our prototype turbo system's exhaust is made of coated mild steel, the production kit will be made of 304 stainless steel.
One of the worst things about turbocharging a mid-engined car is the difficulty in situating an effective intercooler. There is simply no good place to put a traditional air-to-air intercooler where it can get adequate airflow.
The expanding boost piping is short and elegant, part of the reason for the immediate turb
On the MR2, the turbo's pressurized air is directed to a huge tube-and-fin GT-R intercooler mounted under the car and the now horizontally-mounted muffler. This unusual mounting position keeps the intake track short for the best possible throttle response and sufficient flow. To direct airflow through the intercooler, a large scoop picks up underbody air and directs it up through the core and into the lower-pressure engine compartment. It's unorthodox, but XS's testing shows intake air temps of 105 degrees F on the street and 122 degrees F when being pounded at a track in the middle of the desert. XS claims this is as effective as a conventional front-mount system.
The intercooler piping diameter is kept small to minimize volume and to speed up the flow; the tubing starts off at 2.25 inches, steps up to 2.5 inches after the intercooler and finally flares to 2.75 inches at the throttle body. The prototype tubing pictured here is mild steel, but the production bits will be a lightweight and higher quality 0.040-inch thin wall stainless steel. The XS kit also uses high quality multi-ply silicone couplers and clamps to prevent blowoff.
XS relocates the air filter to the passenger side for simplicity's sake, but this cannot t
Since the 1ZZ motor has a turbo-unfriendly compression ratio of 10:1 and fragile pistons with the ring lands close to the crown (for emissions reduction), the boost pressure has been limited to 7.5psi. With such a low figure, XS has deemed the use of a blowoff or compressor bypass valve unnecessary. The IHI turbo's ball-bearing center section is exceedingly tough and can easily handle any momentary surge that a lack of blowoff valve may produce. The large-volume intercooler also acts as a big surge chamber to buffer spikes in boost pressure.
As a somewhat unusual step, XS has located the mass airflow meter (MAF) on the pressurized side after the turbo's compressor. The MR2's topography makes it difficult to mount the MAF far enough from the compressor inlet to avoid the turbulence and pulsation generated by the turbo. XS has found that this 'blow-through' configuration gives more accurate and stable metering, as there is less turbulence after the massive intercooler to throw off MAF readings. Although our SVT Focus had issues with this type of MAF configuration, XS mounted the sensor far enough away from the turbo to make it work, and it actually measures the transient flow more accurately.
By Mike Kojima
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