Some tips regarding a dual exhaust
The exhaust system on my 1985 El Camino goes into one catalytic converter. If I separate the two exhaust pipes coming off the exhaust manifolds and install another catalytic converter and install full flow mufflers will I improve the performance of the engine? Will this impair passing the smog test for California?
In general, enabling an engine to breathe better by reducing back pressure by converting to a true dual exhaust system will enhance its performance.
There are, however, a lot of parameters that influence how or even whether increasing flow or reducing back pressure of an exhaust system will improve performance. Though it’s counter-intuitive, in some instances, “improving” the exhaust system’s flow will actually diminish engine performance.
Determining what the optimum exhaust configuration is begins with defining your objectives. Oftentimes, changes to the exhaust will simply move the torque and/or power curve up or down without significantly changing the peak numbers. So if you use the vehicle for towing, low-end torque may be most important, but if you use it for drag racing, peak power higher in the rpm range may be more desirable.
How, exactly, a different exhaust system will change your engine’s performance characteristics depends on a host of factors, including the cam profile, compression ratio, fuel and ignition curves, existing exhaust system configuration, etc. The only way to know for sure how a new exhaust will affect your car is to run the car on a chassis dyno both before and after the installation of the new system.
In my experience, the biggest benefit and often the only benefit people enjoy from changing to a “high-performance” exhaust system is that it makes them feel better.
Changing to a true dual exhaust with a catalytic converter for each side will probably make the tailpipe emissions cleaner than they were before, so from a purely functional perspective the change will not impair your ability to pass a California smog test. However, part of the emissions test in California and elsewhere includes a visual inspection of emissions related parts and systems to determine if any specified things have been changed, eliminated, disconnected, bypassed, etc. The best way to deal with this is to install a California Air Resources Board (CARB)-approved aftermarket exhaust system. A system that is not CARB approved will impair your ability to pass the test.