Graeme Fletcher
National Post

Friday, February 28, 2003

Acura's TSX is one of the cleaner-running automobiles to be sold in North America. To achieve this lofty standard, a lot of technology is shoehorned into its 2.4-litre, four-cylinder engine.

This motor is clean enough to meet the strict California Air Resources Board (CARB) Tier 2-Bin 5, Low Emission Vehicle-2 (LEV 2) standards. That little mouthful means a 75% reduction in nitrogen oxide (NOx) emissions when compared with the LEV 1 standards with which most engines currently comply. Better yet, it manages to accomplish this feat while churning out a commendable 85-horsepower-per-litre displacement, for a total of 200.

To achieve this level of cleanliness, the TSX incorporates Honda's variable-valve timing and lift electronic control (VTEC) system, but with a racy twist. As with other Honda engines, VTEC adjusts the lift (the amount the intake and exhaust valves are opened) and duration (the period of time they are open) of the valves to promote strong low-end torque as well as high-end horsepower.

Simply, the valvetrain incorporates three rocker arms for each pair of intake and exhaust valves. At low rpm, the valves are opened by the outer two rockers, which follow a mild cam profile designed to promote low-end torque. Above 6,000 rpm, the outer rockers are locked to the central arm, which follows a more aggressive cam profile. It is this higher lift and longer duration that delivers the top-end go.

The twist is that this version of VTEC differs from the norm in that it mimics the operation of that used in the NSX's engine. During low-rpm operation, both intake valves are opened together -- the more common VTEC design opens the first intake valve moments before the second. Opening both intake valves at the same time promotes better low-end power -- the other method typically places more of the emphasis on fuel economy.

The design also incorporates variable timing control (VTC), which puts the i in the 2.4's i-VTEC designation. While the VTEC system is doing its thing, altering the lift and duration, VTC continuously alters the intake cam phasing over a 30-degree window. Technically, this action alters the amount of valve overlap -- the time period the intake and exhaust valves are both open at the same time.

At idle, the intake cam's timing is retarded -- which reduces the amount of overlap -- bringing a much smoother idle and lower NOx emissions. At wide-open throttle or under heavy load conditions, the cam timing is fully advanced, which maximizes the overlap, promoting power and again cutting emissions by creating internal exhaust gas recirculation. Again, this lowers NOx by re-burning a small amount of exhaust gas during the next combustion cycle.

Driving the TSX proves all that complex work produces the promised results. The engine is smooth and flexible at low speeds, this in spite of its rather average torque production -- 166 pounds-feet at 4,500 rpm. Mat the gas and the engine begins to make hay in a hurry, delivering a rewarding turn of speed. However, things really come to a roaring boil when the tachometer sweeps past 6,000 rpm. As well as feeling the increased urgency as the cam profile is switched, the driver hears the change in the exhaust note. It changes from a somewhat buzzy (although not unpleasant) note to a throaty roar.

It is worth noting that the latter can only be enjoyed for a relatively short burst as the rev limiter kicks in at 7,500 rpm (this in spite of the marked 7,100-rpm redline). As short as the burst of speed is, it is nonetheless enjoyable. Fuel economy numbers of 11.2 litres per 100 kilometres in the city and 8.1 L/100 km on the highway put the icing on a very sweet cake.

An electronic throttle (a by-wire system) underscores the engine's technology quotient. It is a clever design that alters the relationship between the gas pedal and the throttle plate in the intake manifold according to the prevailing conditions. In stop-and-go traffic, the pedal response is slower than normal (a low gain in Honda-speak). This brings a smoother launch and less risk of wheelspin.

In the mid-range, the response rate speeds up so a smaller stab at the gas pedal brings a larger opening at the throttle. As a result, the tip-in is now much more aggressive and the car becomes more responsive. At the top end, the gain is maxed out to deliver full chat and better passing power when called upon. It really works very nicely.

The electronic throttle also works hand-in-hand with a couple of other components to further enhance the driving experience. Using the Sequential SportShift manumatic transmission cuts the shift time between first and second gears by a worthwhile 40%. Simply, the shift time drops from a sloppy 1.15 seconds to just 0.7 seconds.

It also cuts engine output when the vehicle stability assist (VSA is a dynamic stability control program that also oversees the traction control system) detects either understeer, oversteer or wheelspin. Essentially, when a skid or wheelspin is detected, the anti-lock brakes are applied to the appropriate wheel as the electronic throttle lifts out of the gas.

Pressed into some sharp canyon curves during the test, the TSX maintained a high degree of lateral stability. When the tires began to give up and slide into understeer, the VSA stepped in to right the minor wrong in a near-seamless manner. Being less intrusive than some other systems means the TSX is more forgiving at the limit than most, which is definitely a good thing.

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