Sports exhaust design and performance gains.

"Blow that - exhausting stuff."

Blow - getting rid of the exhaust fumes is important and the less pressure there is in the exhaust system the higher the BHP. Polishing the inside of the exhaust will aid the fast exit of gases.

Under a 1.6l engine, TorqueCars recommend you stick with a maximum 1.5 to 2 inch pipe (the smaller the engine the smaller the bore size). Under 2.5l engines go with 2-2.5 inches and over this go with a 3 inch exhaust. This bore size applies throughout the length of the exhaust up to the back box.

The back box & exit pipe can be a little wider but this will not release more power, it will just give a deeper sound.).

Gases flow better from a large pipe to a small pipe if the join is cone shaped. Steps will cause turbulence and this can interfere with the airflow and affect performance.

The shape of the exhaust manifold (painted red in picture) is also critical as is the primary and secondary length. This is something best left to the experts to calculate – most manufacturers do a reasonable job. Aftermarket performance exhaust headers will obviously be better.

However you can improve a standard exhaust by smoothing out the inside with a grinding wheel on a drill. Alternatively buy a purpose built stainless steel unit for your car.

The best systems are called 4-2-1 which means they go from 4 pipes into 2 pipes and then into 1 in a 3 step process. Some  standard exhausts are 4-1 where all the pipes merge at one point lower down the manifold.

A larger diameter exhaust means that as the pressure needed to expel the air is lower the whole process will be quicker with a lower push needed. Cats suck power by slowing up the airflow. Catalysts are sadly a requirement on today's cars, there is a typical reduction of 1-4 bhp overall. 

If you find a high flow sports cat or use an off road only de cat pipe you will notice an improvement on most applications. Although this benefit will be very minimal on smaller engined cars.

Another option is to go for a 4-2-2 exhaust where you have twin exit pipes with 1 for 2 exhaust ports from the engine. 

Most sports silencers add a deep roar to the exhaust but this adds little to the overall experience other than the noise they make. One of the best exhausts I ever had was a standard one (from a larger engined car) with the rear silencer remover – the centre expansion section was larger to help muffle the sound but it really improved things a lot and sounded great.

TorqueCars recommend the "Cherry Bomb" for simplicity and a deep throaty sound but you will probably need to fabricate your own mounts. 

Just as induction length is critical so exhaust length is also vital. You should also aim to have the same exhaust length from each exhaust port.

Most standard exhausts are cast and have many imperfections on their internal surface. They often have ridges where surfaces are joined. Grinding the inside of the exhaust smooth will aid airflow and improve things.

While you are at it you could polish it to completely remove resistance. Performance exhausts are made of stainless steel which has a much smoother internal surface and is also much lighter.

Under bonnet temperature is one of your biggest performance killers see the induction and vents  articles for more suggestions. Wrapping the exhaust manifold with a heat resistant wrap can yield a dramatic reduction in temperature and will help the cat to reach operating temperature more quickly therefore prolonging its life and efficiency.

(Don't use cloth bandages as this becomes a fire hazard!) The application of a ceramic coating can substantially reduce the transmission of exhaust heat into the engine bay and will keep get the catalyst up to working temperature more quickly.

Why not join in the friendly banter in our members forum and discuss exhausts (and all aspects of car modification in detail).