From Hot Rod Secret to Cool Legit Tip

Measuring strains on hot surfaces is a challenge. Besides the challenges of sensor installation to survive the temperature excursion, changes in surface temperature can cause thermal deformations not related to the desired load-induced strains. Foil Strain Sensors (Strain Gages/ Gauges) are uniquely capable of elevated-temperature measurements, because the sensing foil can be processed to work with many different materials and automatically cancel the undesirable thermal deformations.

Measuring strains developed in a vehicle exhaust system while the vehicle traverses a test course is one example of strain measurement at an elevated temperature. Compounding the measurement problem is the fact that near the vehicle engine, temperatures can easily exceed the upper temperature limit of the organic materials used to produce and install strain gage sensors (above 275°C).

So is there a simple method to overcome these challenges when measuring static and dynamic loading on a hot vehicle exhaust system?

The answer is yes! Vent the hot exhaust gases near the exhaust manifold by using a separate flexible exhaust pipe. If the vent system can be controlled, then by using a temperature sensor bonded to the exhaust, the vent can be opened and closed to provide varying temperatures during the testing.

Fun Fact

Before the creation of organized drag racing, early hot-rodders in southern California used this technique for more nefarious reasons. They learned that an engine with an open exhaust produced more horsepower than a muffled one, so they built cable-operated vents into their hot-rod exhausts so they could get maximum power for street racing, but be street-legal-quiet when the cops showed up.