eli5 – Why do old cars have more visible smoke coming out of their exhausts?

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Where is that smoke coming from? Does this means they consume more gasoline?

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Older engines are less efficient at burning gasoline. The smoke is partially burnt or even unburnt gasoline that is wasted.

Modern engines are better at ensuring a better fuel air ratio with more complete combustion making them more powerful and more efficient.

Old cars don’t have or have minimal environmental systems IE: catalytic converters, or modern efficient head designs. Another thing is they may have wear in the cylinders causing a gap between the piston and cylinder wall. This will cause the motor to burn oil as there is a larger then normal gap and the rigs can’t seal right.

But non of this is unique to old cars it is just more prevalent in them. Any car can smoke for one thing or another and different color smoke means different things.

Old Cars are shockingly good on gas. On my 1966 mustang it is not abnormal to average 17-20mpg. This is with a hopped up V8 and a 3 speed transmission. But granted I can get it down below 5mpg if I am romping on it and running light to light. Still even the full size cars like a LTD or Impala can get 12-15mpg across the entire tank.

Piston rings and other various seals wear over use and time and fluids that aren’t supposed to be in the combustion chamber sneak in. Blue/grey smoke is Oil. White/grey smoke is coolant. Black is fuel.

The engines are old and worn out. Oil leaks into the burney place. When oil burns, it makes a lot of smoke.