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 ?
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
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.