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.