There are many opinions on this matter, some choosing a 10-15% increase from factory size, while others going up to a 50% increase.
The best size is defined by providing the best power curve and throttle response within the limitations set by economy and law (for noise). Competition cars might be able to have a different exhaust size compared to a road car, due to the demand on the engine/car being different.
Furthermore ground clearance and weight is of an issue, where larger diameters decrease ground clearance and increase weight due to the large silencers. Therefore it is important to ensure that the performance increase outweighs the weight increase.
With the above factors in mind, the below table provides a base line of pipe sizes for turbo charged road cars. The provided size is to the back axle, where it can be decreased 0.25 inch if needed for clearance without any performance hit.
The below pipe sizes apply to road cars producing 120-150 hp per litre. For competition cars producing 200-250 hp per litre pipe sizes should be increased 0.5-1 inches.
|Engine Size (litres)||Exhaust size (inches)|
|2.5-3.0||3.5 or 2 x 2.5|
|3.5-4.0||4.0 or 2 x 2.75|
|5.0-5.7||2 x 3.0|