Percolating gas into a hot manifold or any gas drip on a hot surface will set up a good stench in your garage that will last awhile. And it doesn’t take but a few drops on something really hot. Normal exhaust odor is not as intense or offensive to me but can take a couple of days to dissipate in my garage to where I can no longer smell it. I guess before catalytic converters we were so use to exhaust odors that we never gave it a second thought.