The assembly of our 2.4L MIVEC engine is now complete. In our last installment, we focused on the cylinder head and induction system of our 4G64-based, E85-fueled engine. To recap the previous installment, we increased our cylinder head flow significantly with a custom port job, while installing Kelford 272-degree cams, Ferrea valve springs, an AMS F1i intake manifold, and a Mil-spec 65mm throttle body. With the engine reassembled, we dropped the new powerplant into our '06 Mitsubishi Lancer EVO IX and sought out the right turbo system to feed the beast.
Twin-Disc Clutch
While the engine was out, we knew an upgraded clutch would be necessary to handle the hefty increase in torque we were anticipating. Our solution was Exedy's Twin Disc HD clutch, arguably the most popular upgrade for high-horsepower, street-driven EVOs, delivering clutch pedal pressure that isn't rock-hard like many aftermarket single-plate units. Although the twin-disc clutch feels like the OEM clutch, it creates enough clamping force to handle over 535 lb-ft of torque. Cerametallic friction discs are utilized between its plates for superior resistance to slippage under harsh driving conditions. The entire clutch assembly mounts to the included lightweight chromoly flywheel, which helps to improve throttle response, while decreasing rotational mass by 15 pounds over our factory unit.
Twin-Scroll Turbo
When selecting a turbocharger for our 2.4L engine, we began looking for one that complements the rest of our engine package, while providing solid power numbers and good boost response. Since we planned on driving our EVO regularly on the street, a laggy turbo was not on the agenda. Installing a modified EVO IX turbo was tempting, but we found that it wouldn't produce enough airflow at higher rpm to support our needs. Geoff Raicer, of Full-Race, strongly recommended the BorgWarner S300SX 83-75 twin-scroll turbo: a journal-bearing, oil-cooled turbo that retails for only $999-significantly less than comparable ball-bearing options. Geoff had recently tested this turbo on a near-stock 4G63T 2.0L engine with very promising results, and now offers a complete bolt-on kit for the EVO.
The BW S300SX turbo is advertised to support 400-800 hp-more than sufficient for our power goals. At first, we were surprised at the unusually large size of the turbo compared to similarly rated versions on the market, but the overall quality was very nice, with a backing plate and center section made of rigid cast iron and a shaft twice the size of those found in competitors' turbos-exactly what we expected from a large OEM supplier. The T4-flanged, twin-scroll exhaust turbine housing we opted for measures 1.00 A/R, which is capable of handling 24-29 psi of boost; housings measuring 0.91 A/R (for up to 24 psi), and 1.10 A/R (30 psi or more) are available options. It's important to note that using the BorgWarner turbocharger on our particular engine/manifold setup required notching of the motor mount bracket to provide clearance for the larger-than-factory compressor outlet.
When mated with a properly designed exhaust manifold, the turbine's twin-scroll design routes exhaust gases from cylinders 1 and 4 through a separate passage within the turbine housing than cylinders 2 and 3. Due to the firing order of the 4G63T (1-3-4-2), the exhaust pulses alternate to create a higher velocity and undisturbed exhaust flow into the turbine wheel from each divide, while avoiding interruption by "non-complementary" exhaust gas pulses. This beneficial effect delivers improved exhaust gas scavenging, throttle response, and ultimately more power throughout the rpm range in comparison to conventional turbos. After passing through the turbine wheel, exhaust gases exit through a three-inch Full-Race downpipe via a custom V-band flange that is welded to the housing by Full-Race. This not only allows a smooth transition to the downpipe, but also allows the turbo to be replaced with virtually any T4-flanged Garrett 40R-series turbo in a hurry. Although we don't plan on swapping turbos down the road, it's always nice to know 750 hp is only a few minutes away!
The compressor wheel of the S300SX 83-75 turbo features a 60mm inducer, 83mm exducer, and what BorgWarner refers to as its "Extended Tip Technology": extended tips on their compressor wheels that are claimed to provide improved airflow by making smaller compressor wheels perform like much larger wheels. It may sound like the usual marketing hype, but many in the drag racing community have confirmed gains in horsepower and torque.
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