Why does car exhaust smell anyway?



If modern cars burn 98%-99% of their input fuel, why do they smell the same as an old car when their catalytic converters are removed?

A 2007 Corvette with its catalytic converters removed smelled much the same as a 1967 Camaro with no emissions controls and a decade old engine rebuild.

Shouldn’t the Corvette smell as clean as a CNG powered truck? That truck didn’t have catalysts installed either, but it didn’t smell hardly at all.


Is this a joke? The only reason modern cars have clean exhaust is that they have catalytic converters. Remove this component and we are lost in the sixties again.


I’ve got one for you: What is the purpose of the catalytic converter? I’ve made it easier for you. Click on the following web address, and learn about catalytic converters: http://en.wikipedia.org/wiki/Catalytic_converter


The smell is hydrogen sulfide (H2S). There is a small amount of Sulfur in gasoline and, when burned, can form H2S and sulfur dioxide (SO2), depending on weather the mixture is rich or lean. The cat will convert the H2S to SO2, but you removed the cat:) The H2S is formed when the mixture is rich, so if you tune your cars to run slightly rich for more power, or accelerate hard you will notice the smell more.

Some new cars still smell like this, especially the little 4 cylinders when they are trying to go up a big hill.

As for the natural gas diesel; Natural gas is mostly Methane after it?s been processed, with very small amounts of H2S.


A 2007 'Vette with no converters…HHHmmmmm

Gasoline is not CNG or LPG. It’s nasty stuff. Please! Put the converters back on!


But have you heard that 6.0l breathing through straight pipes and nice mufflers? Wubba wubba. :slight_smile:


I’m sure it’s a beautiful thing:) I run dual exhaust on my 350, but I had to install dual cats to get a sticker.

If you remove the cat don’t you have to install “dummy” O2 sensors in place of the after cat sensors? I’ve always wondered how well that works.


I don’t thing a handful of muscle car enthusiasts with no cats is going to make any difference compared to the millions of cars on the road. Most of these cars are not daily drivers anyway.


With respect, there’s far more technology in a modern car to clean the exhaust than simply the cat converter. The combustion is actually far more complete.

Fuel injection creates a finer spray then carburators, because it’s pushing fuel through the orafice at a high pressure rather than the low pressure differential that draws fuel into the venturi of a carburator. Finer spray is smaller droplets, which burn more thoroughly in the limied time available in the combustion process. Multiport injection gets the spray right to the intake port, reducing the coalescing that takes place during travel through the intake manifold and further ensuring complete combustion.

Engines run at higher temperatures, further supporting total combustion. T-stats in the '60s were typically 165, 20 degrees lower than they are now.

Control of fuel metering is done by running signals from numerous engine demand sensors (manifold absolute pressure, mass airflow, temp, engine speed, and a few I’m forgetting) through a computer program to determine need and meter fuel accordingly. That’s much more accurate than simply using the vacuum in a venturi. Then it’s all monitored by the upstream oxygen sensor and further tweaked.

Engines burn cleaner today even without the cat converters.

The reason a new modern engine smells the same without the cat converter is simply that we can smell even an extremely small number of gasoline molecules in the air, and we can’t detect differences in the PPM amount unless they’re large.


A million muscle cars with no cats would not add as much pollution as a few diesel dumptrucks. The NOx and unburned hydrocarbons that pour out of those with those big black clouds of carbon is absolutely appalling.

Granted, a properly maintained diesel engine in good shape with modern systems can run a lot cleaner that the long line of carbon belching beat up trucks I see on route 3A avoiding the tolls every day, although they still won’t meet gas engine emissions requirements, but new clean running trucks are not the norm…they’re the exception.

Okay, I’ve had my monthly rant. I’m happy now.


It should work perfectly well. The second set of sensors are for monitoring the cats only. The upstream pair is for engine management.