Choosing the correct exhaust

Choosing the correct exhaust for gaining the most out of your vehicle is a rather simple process, yet one that most people do not follow directly. The old idea of bigger is better is certainly not correct, yet many still live by it. Depending on your vehicles air flow needs, pushing this limit can hinder over-all performance of the car.

The Diameter of the exhaust is your first choice. The smaller the displacement of the motor, the less actual exhaust it is creating, though this factor changes with forced induction vehicles. Putting a 3 inch exhaust on a 1.5 liter honda civic may seem cool, but the net gains from such a system would be horrible. As a general rule of thumb, motors with a displacement of under 2.5 liters should stay below 2.5" exhaust diameter, and larger systems are designed for higher displacement and forced induction motors.

The bends on the exhaust are important too. Mandrel bent exhaust maintains it's diameter throughout the bend unlike crush bent. In a crush bent exhaust system, every turn or swoop made under the vehicle bottle necks the exhaust, and furthermore the bends add turbulence which further hinder exhaust flow.

The options for catalytic converters also vary. On forced induction cars, generally no cat will offer the highest gains, however all obd-2 cars will set efficiency codes if this is removed, and in some states you will no longer pass emissions. If a cat must be used, a good high-flow cat will yield far better results then stock.

These all work into how much effective backpressure is on the motor. Increasing backpressure will generally give an increase in torque lower in the power-band, while reducing it shifts the power curve higher up. Going too high in the curve may yield a higher horsepower number, but overall if the car has lost too much mid-range and bottom end power with the change, the vehicle will be slower. Looking at the area under the curve in relation to what rpm's you would normally be at under normal circumstances is vital. In a drag car with a very loose converter that stays between 5k rpms and 7k rpm's the entire pass, losing torque in the 3k range is perfectly fine, but on a autocross car that will have to pull from lower rpm's frequently, it would have effectively slowed the car on the course.