What engine tuning parameters help to lower emissions?

The most significant reduction in emissions is when fuel control is running closed loop, near stoichiometric (chemically ideal) air/fuel ratio, in conjunction with a three-way catalyst. This technology came into the mainstream approximately 1982 for U.S. cars. Catalyst technology has made some dramatic jumps along the way. Get a catalyst that is for a '98 or newer application. The biggest thing is to use an oxygen sensor and a catalyst and run closed loop as soon after start as the sensor is working, on the order of 10 to 20 seconds. For normal driving this will provide good fuel economy and drivability. "Closed loop" operation with an oxygen sensor is an option on several aftermarket ECUs.

When you drive the car at wide open throttle, like during a drag race, you will probably care more about absolute best power than emissions. Also, under sustained pedal-to-the-floor driving, the exhaust valves and seats and catalyst will exceed their design limits for temperature unless your run rich. So under these conditions you run the optimum Air/Fuel ratio for torque (approx 12.5 to 13:1) or even richer if needed for thermal reasons. So if you care about emissions, don't drive like that.

For fuel economy and consequently the lowest emissions per mile, use optimum spark timing for best torque. This "sweet spot" is normally found on a dynamometer, especially for part-throttle loads. Under high loads and hot air temperatures, it may not be possible to run at optimum spark timing without knock that damages the engine. So this is a limitation. Running a higher octane fuel, especially during hot weather, helps avoid knock and can enable the engine to run closer to optimum.

The next big reduction in emissions in the late '90s was achieved by cleaner starts and getting the catalyst up to operating temperature quicker. Strategies to heat up the catalyst faster involved retarding the spark timing for the first minute or so of operation after a cold start. The amount of retard is limited by how much can be tolerated by the driver in terms of his perception that the engine is sluggish, hesitates or stumbles. Cleaner starts were achieved by optimizing the air/fuel ratio and spark timing during the start, with laboratory emissions and combustion measurement equipment. This is not something that you could do at home except for one thing: during the start, never run richer than 10% excess fuel (A/F ratio = 13) at temperatures above 20 F.

Trust me u need all the horsepower and torque for speed and acceleration. They are never enougth.

Take your car to a garage and have it tunned by a professional.