Thermocouples. FXL DATA has standardised on k type thermocouples. They have a red and yellow lead and use yellow connectors. These standards were established in 1911. The RED lead is NEGATIVE, the YELLOW is POSITIVE.

The FXL35 loggers (and FXL32, FXL25, FXL23) were designed to use open tip k type thermocouples totally isolated from earth.

The FXL22 (and MPE) was designed to use open tip k type thermocouples but are earthed at the logger. They must not be earthed at the probe end, or they simply wont produce a reading. The FXL22 (and MPE) thermocouples are positive earth, that is the yellow lead is earthed at the logger.

There is a huge difference in readings between open tip and closed tip thermocouples. I only supply open tip thermocouples and they read far faster and hotter than a closed tip thermocouple. Most people are under the misapprehension that what the thermocouple says is what the EGT is. But the thermocouple can only tell the averaged temperature. A closed tip probe may say the temperature is, say, 1150 degrees F, and an open tip probe may say 1450 degrees F. The actual temperature of the exhaust is well into the 3000 degreees F, but is only there for a brief period of time, so for 3/4 of the time there is no (significant) gas going past the probe, so it will come to an averaged temperature. As well as this, a closed tip probe will have a significant cooling effect from the exhaust pipe, because the sheath of the probe is clamped to the exhaust pipe.

Another myth is that thermocouples will tell you if you are rich or lean. THEY CANNOT. They can only tell temperature. The idea of rich or lean has come about over the years by tuners leaning out the fuel system, and getting a hotter reading in the exhaust temperatures. You still MUST tune by the plug readings.

I have seen situations where a customer has had the EGT go down compared to a previous pass, but show a hotter reading on the plug. This means that more power is being extracted from the cylinder, hence the indication of more heat in the plug, and less in the pipe. You still MUST tune by the plug readings.

You may now be wondering why have thermocouples if they cannot tell if you are rich or lean. Thermocouples will identify a misfiring cylinder instantly. If you had no thermocouples and you had a miss, how do you find which cylinder is missing? An example is a customer who had a C/MS car that was firing OK off the line in first then missed for 2 seconds, fired OK in second gear, then missed for 2 seconds, fired OK in third then missed all the way to the end of the track. The plugs could not identify the cylinder because the carby was set up rich on methanol and the cylinder was firing most of the time. The thermocouples identified #2 cylinder immediately and the owner threw out #2 spark plug and spark plug lead and it did not miss again. It cost the customer $1200 to do it, but the benefit is that you will get INSTANT IDENTIFICATION of the cylinder.

If you want to know if you are rich or lean, then you need oxygen sensors, but if you have 2 oxygen sensors and 8 cylinders, and you have a miss, then the oxygen sensor CANNOT tell you which cylinder is missing. That is why you need thermocouples. You will also get a lean reading from the oxygen sensor because there is excess oxygen in the exhaust gas because a cylinder is missing. Traps for the unwary.

The position of the thermocouple probe in the pipe is also important. The tip should be in the centre of the pipe and be 100mm from the head, if possible. It must be before any curve in the pipe as the temperature drops through the curve.

The open tip probes can easily change 200 degrees F per second. We have a chart taken from a big block Chev engine on methanol where on jet was blocked and cleared itself during the run and the temperature went up 825 degrees Farhenheit in 1.7 seconds.

I believe I have seen individual misfires on one cars EGT readings.

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24 January 2006