Overview

To tune our engines and get the most power out of them, we need to be able to monitor the exhaust gas temperature (EGT).  This is because to get more power, we would generally like to run leaner.  The problem is that running leaner means running hotter. We can find out how hot we are running by installing a gauge that displays the information and a probe that takes the measurement of the temperature.  Basically, what we need to know is how hot it gets in the cylinder when the gas is ignited.  This is important because if it gets too hot, a piston or part of the head could melt resulting in catastrophic (and expensive) engine damage.  Remember, aluminum has a relatively low melting point.

So the question is where to take the measurement?  We can either measure before or after the turbo.  This paper deals with measuring before the turbo. There are two candidates for measuring before the turbo; the exhaust manifold collector, and the hottest exhaust manifold runner.  For this discussion, it is asserted that the #1 runner is the hottest since it is at the end of the fuel rail and therefore the leanest.

The Time Transient Nature of Heat

This is an important concept but the best way to describe this is probably just to demonstrate it.  First get a butane lighter and a watch that displays seconds.  Ignite the lighter and hold your hand over it at some distance where you can just stand the heat for one second.  Now, alternate holding your hand over the lighter for one second and then moving it away for three seconds.  Notice that you feel the heat only when your hand is over the lighter and very quickly after you take your hand away (or move the lighter), the temperature drops quickly to the room temperature.  Repeat this a few times;  1 second on, 3 seconds off.

Now let's change it up.  Go 2 seconds on and 2 seconds off and repeat that for a while.  Notice that your hand feels overall warmer than it did in the first experiment.  How about 3 seconds on and 1 second off?  Yup, warmer still.  If you continued the progression, you'd have 4 seconds on and 0 seconds off and you'd quickly BBQ your hand :-)

Our Exhaust Model

The diagram below is a simplified model of the exhaust system of a typical turbocharged 4 cylinder engine (between the engine and the turbo).  It is important to keep in mind that there is not a steady stream of exhaust from each of the cylinders but rather it goes in pulses.  Each half revolution of the motor, one cylinder's exhaust side valve(s) opens and the waste exhaust gasses rush out into it's exhaust manifold runner. These exhaust pulses act and travel like waves in the ocean as they propagate down the runner. So in our model below, each exhaust pulse is represented in red.  Notice the sequential nature of the pulses in the runners (one period of heat, followed by three periods of no heat, just like in our demonstration above). 

Look at the two possible probe locations above.  It is clear that the one in the collector is hotter because it is bombarded with continuous pulses of heat whereas the one in the runner only gets a pulse 1/4th of the time.

Experimental Data

We conducted the experiment on Ryan Larsen's '92 Talon AWD.  Our baseline run had the EGT probe located in the collector and whenever the car was under load, the EGTs would easily climb past 1600^oF (at which point we let off not wanting to damage the engine).  After that, a new exhaust manifold was acquired and the EGT probe plumbed into the runner this time.  Additionally, Ryan upgraded to a SteveTek  High Flow Fuel Delivery System which has been consistently shown to drop EGT's about 50^oF.  Other than that, everything was kept constant (AFC programming, fuel octane, pump, injectors, etc.)

With the probe now  mounted in the  runner, we went out for a few runs and found that we could not induce the EGTs above 1410^oF!!!  Or in other words it ran at least 150^oF cooler than when the probe was mounted in the collector (accounting for 50^o decrease due to the new fuel deliver system).  It is probably a bit more as we could've driven the collector mounted probe temp easily above 1600^oF.

Conclusions

Mounting the EGT probe in the runner gives engine tuners a better idea of cylinder temps and is highly recommended.  When the probe is mounted in the collector, one could also have the problem of temperature averaging which has the potential to kill an engine.  For instance, assume you had three rich cylinders and one very lean cylinder.  In the collector, you would be getting one very hot pulse but three cooler ones and since the probe does not react instantaneously, the reading would be skewed to the three cooler cylinders thus leaving a dangerously lean condition undetected.

Copyright 2000, by SteveTek Inc. 
Please direct any and all correspondence regarding this document to engineering_staff@stevetek.com