To this day, the S2000's performance envelope puts it in a class of few. To improve the dynamic performance without upsetting the superior balance and overall execution of the car would require careful thought. The stated goals were:
- Improved Acceleration
- Sharpened Handling
- Enhanced brake performance
- Improved man-machine interface.
To address these items, Honda's approach was to use both tried and true methods as well as technical innovation.
To improve acceleration, the old adage "there's no replacement for displacement" was applied. Through the use of a longer stroke (increased to 90.7mm from 84mm), the motor's displacement was increased from 1997cc (2.0L) to 2157 cc (2.2L), resulting in basically a 500rpm shift of the horsepower curve (to the left, on the rpm scale), meaning the new motor basically makes the the same power as the older F20C, simply 500rpms sooner at basically every point on the curve. Compression has been bumped up a tenth of a point to 11.1 (from 11.0:1) and the valve timing has been altered to further enhance mid-range performance. On paper, the numbers translate to 240hp at 7800 rpm (vs 240hp@8300 rpm) and 162 lb-ft @ 6500 rpm (vs 153 lb-ft @ 7500 rpm).
It might seem trivial to simply lengthen the stroke of a given motor, but there are several major design details to consider. Foremost in this case are the increased loads acting on the piston sides. In the 8000+ rpm neighborhood where the S2000 takes residence, these become quite significant, resulting in unstable piston movement. To mitigate this phenomenon, Honda has improved the accuracy of the cylinder machining process to an extremly high level. Borrowing a technique from race engine building programs, Honda used a dummy head honing technique to simulate the cylinder bore deformation that occurs when the cylinder head is bolted to the block (imagine the bore diameter enlarging slightly and with irregularity as the cylinder bore is compressed). In other words, the bores are honed while a dummy head is bolted to the block. This technique results in bore deformation that's reportedly less than 25% of the deformation on a block which has been bored using normal methods.
The enlarged displacement results in a 6-10% improvement in horsepower and torque from 1000 to 8000 rpms. For Honda this improvement alone wasn't enough. A tighter secondary gear reduction ratio (1.206 vs 1.16) is specified to provide a bit more snap off the line and to better match the characteristics of the new motor. This results in effectively 4% shorter ratios for 1st through 4th gears while the 5th ratio is effectively 1% shorter. 6th is effectively 2% longer, for a slightly more relaxed cruise speed. Due to the additional torque of the new 2.2L motor (and possibly due to real-world issues with the 2.0L drivetrain), the S2000's transmission features a redesigned clutch, clutch case, and differential.
To reduce the "impact torque" of a high rpm clutch dump, the new clutch has an additional orifice at the sleeve cylinder, which reduces the torque reaction acting on the drivetrain. The clutch case has incorporated additional rib reinforcements to reduce vibrations and noises generated by the drivetrain. The additional ribs have improved clutch case rigidity by approximately 10%. The differential has been beefed up considerably as well. By moving to an FCD material (from FC), the case rigidity was improved by 40% while the overall case strength was increased 20%. Gear fatigue resistance was also improved by 5% through a change in the shot peening method for the drive pinion gear.
Real World Results
What is the impact of these manifold improvements to the S2000's drivetrain? By Honda's own figures, the car offers substantially better acceleration in each gear, offset only slightly by the reduced top speed potential in each gear (in the older car, thanks to the slightly taller gearing and extra rev capacity, you're able to hold a gear a bit longer, resulting in better acceleration during that brief period where the '04 must upshift while the '00-'03 is able to remain in the lower gear). In time-to-speed terms, Honda claims the new car can accelerate from 20-50 mph in 4.5 seconds (using only 3rd gear), vs 5.1 seconds for the outgoing model. In the 15-45 mph (2nd gear only) sprint, the '04 runs 4.5 seconds vs 5.0 seconds for the outgoing model.