LEGAMAX Premium CIVIC 1.5 Sedan 201731021 BH004 Code No. 31021 BH004 Overwhelming Low Exhaust Pressure 50 of Original Pressure 60 large diameter pipe and full straight structure reduce the exhaust pressure to 50% of the stock. This significant exhaust pressure reduction enables the turbine to run smoothly from low RPM range, which results in the engine output increase in an entire speed range. Also, this pressure reduction effect is enhanced as the tuning level becomes more advanced;