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Fuel consumption poser

Question : WOULD a bigger diameter exhaust pipe result in higher fuel consumption ? What if it comes stock standard from the factory ?

Cars that are more than 10 years of age tend to have higher fuel consumption, compared with new cars. Is this true ?

Answer :

1. Exhaust systems are designed to suit the characteristics of an engine and should not be replaced with anything different unless proper testing is done.

While it is commonly thought that the freer the flow of exhaust gases, the better, this is not particularly true because all systems need a certain amount of what is known as "back pressure" in the system in order that the engine runs efficiently.

What this means is that when the exhaust valve opens and the products of combustion rush out from the cylinder, this also creates a negative pressure in the cylinder that assists in drawing more air-fuel mixture into the cylinder as the intake valve is still open.

However, at a certain point, this must stop or else the fresh mixture will also go down the exhaust pipe, and so the exhaust system is tuned such that this extraction effect is limited by means of a "back pressure".

If one changes the silencer or the diameter of the pipes, the hole tuning of the system would go out and the system would develop "back pressure" at the wrong engine speeds, and in effect maybe give even poorer performance.

With regards to larger diameter pipes being fitted ex-factory, I can only assume that the manufacturers have done their homework before making the change and if your fuel consumption is high, it could be caused by something else.

2. Old cars having higher fuel consumption can be expected because as the engine gets worn, it becomes less efficient, compression pressures drop as well as valves start to leak and therefore the engine is not developing at its peak performance and so consumes more fuel for the same performance.


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