Tech Talk - EGT Response Time

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Read about how to attain quicker reaction times from your exhaust gas thermocouples with DYNO-MAX 2000.

"Reprinted from the DYNOmite Spring - 2004 Newsletter article."

Dyno Tech Talk is a compilation or copyrighted material, however, relevant web sites and forums are welcome to link to the Dyno Tech Talk index page.

How can I speed up my EGTs response time?

Quicker EGT response time... can be simulated using advanced features in DYNO-MAX 2000 “Pro’s” Formula Editor. An example user editable formula named “Extra Fast EGT #1” is included since Version 9.32 =

Exhaust Temp #1 + (d/dt ( 0.500 {Exhaust Temp #1} * 0.800 )

Understand that all thermocouple elements take time to register temperature changes. This response lag is referred to as the thermocouple’s “Time Constant” (seconds to register 63.2% of a temperature change).

Typical heavy-duty (sheathed) probes can take several seconds to reach the exhaust’s actual temperature. Even exposed junction probes - using very fine (and fragile) wire - still have Time Constants measured in tenths of a second. In other words, thermocouples suitable for daily dynamometer testing fall far short of providing instantaneous response.

Fortunately, there is a way to compensate for this latency. When the medium- in this case exhaust gases - and their average velocity are relatively consistent, it becomes practical to extrapolate the true exhaust temperature from a thermocouple’s rate of change.

DYNO-MAX’s “Derivative” formula feature can easily resolve this Delta Temperature and then multiply it by a scaling constant* - returning the current temperature lag deviation. We add this correction value to the normal EGT reading - for a close estimate of the true instantaneous temperature.

*The “scaling constant” can be experimentally determined by recording EGT readings - while rapidly opening, holding, and then rapidly closing the throttle (under load). Graph and compare both the standard EGT #1 and the Extra Fast EGT #1formulas. Then edit the Extra Fast EGT #1 formula’s scaling constant (default is 0.800) until the Extra Fast EGT #1 response is close to instantaneous (without overshooting the steady state EGT #1 temps).