When the MR2’s 1.6-liter, 16valve, twin-cam EFI engine received its Roots-type supercharged and air/air intercooled engine (4A-GZE), horsepower increased by roughly 30 percent—from 112-hp at 6,600 rpm to 145-hp at 6,400 rpm. Torque was also increased almost 45 percent, from 97 lbs/ft at 4,800 rpm to 140 at 4,000 rpm. This enabled the MR2 to achieve 0 to 60 times in just under 7 seconds. It seemed that this injection of additional power was enough to gratify most MR2 enthusiasts, but then there are three who continually strive for more and more.
Although an improvement, the MR2s new breath of life was still not enough for the die-hard performance enthusiasts of HKS.
Based in Torrance, California, HKS systems are available in “Stages.’ There are “Stages to fit every performance requirement and budget. The HKS MR2 upgrade has three optional stages.
All HKS performance stages are well engineered to allow ease of installation, as well as solid, reliable performance, Reliability is always as important as higher performance, with the balance representing quality design and manufacturing.
This bolt-on exhaust system (HKS Supercharged (S/C) Exhaust System) replaces the entire system starting at the end of the manifold. Included is a new front pipe, a pre-silencer (non-catalyst replacement for converter), a new, specially designed muffler and all required gaskets and hardware. The muffler design is unique in that the flow requirements necessary for a substantial power gain exceed that of a normally aspirated MR2, thus eliminating the usefulness of the HKS Sport Exhaust System.
The muffler that HKS supplies for its MR2 Stage 2 turbo system was originally tried for Stage 1, but its excess flow caused loss of engine response and torque. The resulting muffler design offers more exhaust flow capacity than the Sport muffler, but a little less than the Turbo muffler to improve all aspects of the power band. Engine response and torque are dramatically improved, with peak horsepower increased by 12 to 17 hp. The power band is also broadened overall, thereby preparing the car for Stage 2.
The next step to increase the power and efficiency of the stock supercharger system is to increase the boost pressure slightly through a higher supercharger rotational speed. ‘lb accomplish this, 11KB manufactures a high quality replacement crankshaft pulley (HKS Crankshaft Overdrive Pulley) that is 10mm larger in diameter than the stock pulley. The pulley is a direct bolt-on replacement designed to be used with the stock drive belt and incorporates the proper ignition timing marks.
Although it is more common for performance product manufacturers to produce an underdrive pulley for the supercharger, HKS felt that the complexities of changing the pulley on the blower, due to the complex clutch mechanism, would make the design and use of one impractical. Hence, the overdrive crank pulley. In keeping true to HKS’s high quality standards, the pulley is manufactured much like the original one. It is comprised of a two piece (inner and outer) machined casting that is joined together by a vulcanized rubber vibration dampener. In other words, the HKS pulley, except for the size, is manufactured to virtually the same high standards as the original equipment pulley. Installation of the pulley will effectively increase the boost pressure 1 to 2 psi. A stock 8 psi, will now yield 10 psi, which will net about 170-horsepower.
The final phase in the upgrade is the installation of a pair of mild HKS camshafts (HKS Camshaft Set). According to HKS, the design objective of these cams encompasses several requirements. First, they had to be bolt-on. In other words, HKS wanted to make them as easy as possible to install and eliminate the need for any additional tuning after installation. The only thing required for installation are a few factory replacement parts like cam seals, valve cover gaskets, and of course, new valve clearance shims (all available from a Toyota dealer). Second, they had to be a mild grind that wouldn’t have a negative affect on the idle or the low-end driveability and response. Finally, since the power from the supercharger is so strong on the bottom-end, the cams had to provide more peak horsepower and a broader power band in the upper rpm ranges to give the car more top-end performance.
These objectives resulted in a 256’ intake and 256’ exhaust combination that definitely satisfies all of the aforementioned requirements. As can be seen on the accompanying power chart, the low-end power is unaffected by the cams. The top-end power, however, is greatly improved, resulting in faster acceleration times. The HKS camshafts are made to original equipment quality standards with new, high quality nickeliron billets, not regrinds.
At this point, the factory engine management system is still compensating properly to maintain a correct air/fuel ratio and using only about 88 percent of the injector duty cycle time under full load. In other words, no fuel system modifications are required on this car to safely obtain this increased horsepower.
During our testing of this MR2 it was immediately apparent that performance over the entire powerband was improved significantly over that of the stock supercharged version. When compared with the performance of the normally aspirated MR2—night and day.
These types of performance modifications require as little time as a weekend to accomplish with little or no headaches. The final result is a supercharged MR2 that is sure to get your feet off the ground singing “Oh what a