you would place them onto external surface of the exhaust pipes. As the meaningful location would be the upper portion of the exhaust, upstream of the catalyzer

you would place them onto external surface of the exhaust pipes. As the meaningful location would be the upper portion of the exhaust, upstream of the catalyzer, it would decrease the temperature of the exhaust coming to the catalyzer, so it may potentially affect it work when engine is at low-power.