The Stock exhaust manifold was designed by the 4-2-1 type layout for a better performance for a lower engine speed torque. Because the first catalyzer is mounted at the exhaust manifold, exhaust pulsatory motion makes the exhaust gas to pull back to the pipe at middle RPM range. It causes a loss of output by behaving as an exhaust gas recirculation. The HKS exhaust manifold follows the same 4-2-1 type layout but making it without the catalyzer provides both more torque at middle RPM range and more peak power for high RPM. It’s very easy to drive and follow the acceleration well at middle RPM. And at a high RPM, it has much more power than the stock and feels clean of the NA's high RPM driving characteristic. With the optional Hi-Power SPEC-L Muffler and Front Pipe(no catalyzer) will have more power and fine NA sound.If you want to change the Exhaust Manifold(no catalyzer), the “Engine check” lamp will be on because of the first catalyzer will be eliminated, but you can avoid this by setting the included part. But if used with the Front Pipe(no catalyzer), engine check light will be on. We have checked the drive performance for fail-safe of engine check and found no problem. So depending on the situation, muffler + exhaust manifold or muffler + front pipe will be the options. Main material is SUS304 for better performance at circuit use. Adopted 4-2-1 layout design for better output from normal to high RPM range.