It Takes the Ultimate Turbo to Feed the Ultimate SE-R
By Mike Kojima
[Put into HTML format by Mike Mager]
For the turbocharger, Rick Head of Turbonetics in Moorpark, Calif. mated a Garret T04E compressor wheel and housing to a lightweight, free-flowing T-31 turbine section. The TO4E compressor is one of Garret's newer designs. It is an incredible compressor, running close to 76 percent efficiency across most of the engine's operating range from a low of 10 psi to a high of 20 psi. This means charge air heating in minimal and the shaft power requirement of the compressor is low, making the turbo fairly lag free and allowing a free-flowing turbine wheel and exhaust housing to be used. This helps reduce power-robbing and heat-generating backpressure caused by restrictive turbine wheels. Ironically, the TO4E was originally designed for long-haul diesel truck use. The T-31 turbine wheel is huge, popular with the large-displacement Buick Grand National crowd. Conventional wisdom says a turbo as big as this should be unresponsive and soggy on a small-displacement four-cylinder due to all the inertial mass of the rotating parts. A few of Turbonetic's innovations, combined with Rick's skill in sizing turbo parts to their application, went a long way toward eliminating these undesirable characteristics.
Rick selected a 46-trim compressor due to the broad efficiency range of its map and its ability to pump enough air to reach the low-to-mid 400 hp range. A 0.50 A/R compressor housing was chosen to match the compressor wheel. A/R, or area ratio, is the way compressor housings and turbine housings are sized. The bigger the A/R numbers, the bigger the housing size. Typically a bigger A/R will produce better flow but induce more lag, partially because of a lower internal gas velocity that the bigger diameter creates. It's kind of like spraying a pinwheel with water from a garden hose. If you put your thumb on the end of the hose to make a concentrated spray, that spray can make the pinwheel spin up faster. But at the same time you are creating more backpressure inside of the hose by choking off its flow.
The T04E is much more efficient that the old-style T04B that many aftermarket turbo kits use when describing a T-4/T-3 hybrid. The T04E is a very big turbo and F-Max had to be quite creative in the mounting of the turbo considering the main goals of retaining all emissions and power accessories. On the turbine side, the 76-trim T-31 is the largest commonly available turbine from Garret's venerable T-3 family. It is a low-inertia design, also helping to reduce lag. Rick selected a 0.63 A/R turbine housing as a good compromise between exhaust flow and lag for our street/track application. Turbos are especially sensitive to A/R on the exhaust side so this was a critical choice in tailoring the engine's power band.
Finally, the turbo uses Turbonetics-exclusive ball bearing center section. This center section has a ceramic, angular contact, cartridge-type ball bearing to handle thrust and rotational loads as opposed to the traditional sleeve-type bearing that most typical turbochargers use. The ball bearing center section has far less viscous drag than the old-school sleeve bearings and can handle more thrust load than the OEM thrust washer bearings used on off-the-shelf Garret turbos. Thrust bearing wear is the Achilles heel of a performance turbocharger, causing most failures. The ball bearing can take up to 50 times more thrust load and usually has a life expectancy at least three times longer. The reduction in viscous drag and friction of the ball bearing center section can reduce turbo lag by 500-700 rpm in itself.
To ensure long life of the turbo in everyday street use, a water-cooled center section was used. This uses a cooling jacket of water circulating from the engine's coolant to help keep the turbo's bearings and internal oil passages from overheating and coking with burnt oil. This is a feature used in most OEM turbo systems but usually ignored in aftermarket add-on turbo kits.
Since the car was being built for dual-purpose street and track use, it was deemed important for the turbo to be efficient under a wide range of boost pressures. Usually turbos have a sweet spot of flow rate and pressure ratio where they are much more efficient than the rest of the operating range. The T04E is unique in that it is more efficient over a wider range of flow rates and boost pressures than most turbos. This means the turbo will work well from about 10 to 20 psi on this particular engine. This is partly because the T04E has a large compressor wheel. This large wheel does not need to spin very fast to produce boost. A T04E is happy humming along at 80,000 rpm as opposed to the 200,000 rpm that a smaller T-25 spins (the T-25 is found on the Mitsubishi Eclipse among others). The big, slow-spinning wheel does not thrash the air as much as the small, fast-spinning wheel, and thus does not heat the intake charge as much.
Just looking at the size of the turbo, the uninformed have been heard to say "that turbo is way too big for that engine, it's obviously too laggy to be street driven". We can gladly attest to the fact that all is not as it appears. The big turbo starts to make noticeable boost at only 2100 rpm. In fact, it spools up so readily that it takes some practice to keep the turbo out of boost in school zones and next to cars with light bars on the roof. The big T04E compressor wheel does take longer to spin up to speed than the tiny T-25, but it starts to make boost at a much lower shaft speed, negating some of the slower spin up issues.
Reprinted with Permission