EGT: Tech closet by the janitor
01-20-2006, 04:12 PM
#1
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Join Date: Jun 2005
Posts: 996
EGT: Tech closet by the janitor
EGT or Exhaust Gas Temperature
The EGT comes in two parts the gauge and probe. The probe is a device that converts heat into an electric signal. This electric signal is converted to a value for display by the gauge. There are two gauges commonly associated with EGT. The EGT and the Air fuel ratio gauge. The AF ratio gauge reveal the same basic data but from a different prospective. A/F ratio (rich/lean)verses EGT temps. We will discuss EGT today.
Why is it critical to know the EGT.
When the fuel and air mixture is burnt it causes and explosion in the engines cylinders. The explosion is converted into work by the piston rod and crank. This is better known as horse power.
We all know that the higher the explosion the bigger the bang. The bigger the bang the faster we go. All of this equates to hp.
Where does EGT fit into this picture?
The exhaust gases are the results of the burnt fuel. The temperature of the exhaust gas is and indication of either a rich running engine or a lean running engine. Hot is lean cold is rich.
A lean running engine produces more power but does not last very long. The richer running engine will lasts longer but do not produce a much power as the leaner running engine.
The leaner you run the hotter the exhaust gasses are. The richer you run the lower the EGT can be.
Where to place the gauge and probe?
That one is easy. Where you can see it!
The probe is the question. The two places for the WRX/STi crowd is cylinder #3 or behind the turbo on the down pipe. Each has their merit.
Cylinder #3
There are two reasons.
Reason number 1: the exhaust port
It is the closer to the manifold compared the down pipe and will read the actual temps from the cylinder.
Why #3 and not 1,2 or 4
Of the 4 cylinders #3 is used because it is normally the hottest.
Why is 3 the hottest?
If you look at the engine’s heads in reference to the cylinder on the block, the intake manifold for cylinders #3 it does not line up with the intake. The intake port on the head is channel to match the cylinder. This channel or dog leg makes the air delivery uneven, causing #3 to run leaner than the rest of the cylinders. This is why most of the piston failures are on #3.
We cannot control the air delivery for each cylinder. It is one input from the turbo with 4 outputs one to each cylinder head.
Next is the down pipe.
The down pipe is used because of its location.
Because the probe is subject to so much heat it can burn up and slowly flake off and eventually break off into your turbo. If it does and the turbine is spinning @ 115000 rpm the impeller/turbo will be damaged. There are arguments to this theory but I have seen probes where the only thing left is the stem and no probe. So, I feel it can happen.
I still have a stock probe in my up-pipe but I have changed it out three times already (02WRX - I keep it for the warning/cel if something were to change) I do not let it get old enough to cause the problem anyway.
There are factors to take into consideration. If you are tuned to run lean and run hard then you might want to protect your turbo. If you are not running that aggressive then placing the probe in the exhaust port might not be a problem. It depends on how much time you are going to spend monitoring and checking out your engines operation and health. If you don’t plan on routine checks then you might be better off in the down pipe where it doesn’t matter. You just have to calculate the loss in temp from the exhaust manifold to the down pipe and know that your temps will inticate a slightly lower reading compared to one with a probe in the exhaust port.
The janitor.
The EGT comes in two parts the gauge and probe. The probe is a device that converts heat into an electric signal. This electric signal is converted to a value for display by the gauge. There are two gauges commonly associated with EGT. The EGT and the Air fuel ratio gauge. The AF ratio gauge reveal the same basic data but from a different prospective. A/F ratio (rich/lean)verses EGT temps. We will discuss EGT today.
Why is it critical to know the EGT.
When the fuel and air mixture is burnt it causes and explosion in the engines cylinders. The explosion is converted into work by the piston rod and crank. This is better known as horse power.
We all know that the higher the explosion the bigger the bang. The bigger the bang the faster we go. All of this equates to hp.
Where does EGT fit into this picture?
The exhaust gases are the results of the burnt fuel. The temperature of the exhaust gas is and indication of either a rich running engine or a lean running engine. Hot is lean cold is rich.
A lean running engine produces more power but does not last very long. The richer running engine will lasts longer but do not produce a much power as the leaner running engine.
The leaner you run the hotter the exhaust gasses are. The richer you run the lower the EGT can be.
Where to place the gauge and probe?
That one is easy. Where you can see it!
The probe is the question. The two places for the WRX/STi crowd is cylinder #3 or behind the turbo on the down pipe. Each has their merit.
Cylinder #3
There are two reasons.
Reason number 1: the exhaust port
It is the closer to the manifold compared the down pipe and will read the actual temps from the cylinder.
Why #3 and not 1,2 or 4
Of the 4 cylinders #3 is used because it is normally the hottest.
Why is 3 the hottest?
If you look at the engine’s heads in reference to the cylinder on the block, the intake manifold for cylinders #3 it does not line up with the intake. The intake port on the head is channel to match the cylinder. This channel or dog leg makes the air delivery uneven, causing #3 to run leaner than the rest of the cylinders. This is why most of the piston failures are on #3.
We cannot control the air delivery for each cylinder. It is one input from the turbo with 4 outputs one to each cylinder head.
Next is the down pipe.
The down pipe is used because of its location.
Because the probe is subject to so much heat it can burn up and slowly flake off and eventually break off into your turbo. If it does and the turbine is spinning @ 115000 rpm the impeller/turbo will be damaged. There are arguments to this theory but I have seen probes where the only thing left is the stem and no probe. So, I feel it can happen.
I still have a stock probe in my up-pipe but I have changed it out three times already (02WRX - I keep it for the warning/cel if something were to change) I do not let it get old enough to cause the problem anyway.
There are factors to take into consideration. If you are tuned to run lean and run hard then you might want to protect your turbo. If you are not running that aggressive then placing the probe in the exhaust port might not be a problem. It depends on how much time you are going to spend monitoring and checking out your engines operation and health. If you don’t plan on routine checks then you might be better off in the down pipe where it doesn’t matter. You just have to calculate the loss in temp from the exhaust manifold to the down pipe and know that your temps will inticate a slightly lower reading compared to one with a probe in the exhaust port.
The janitor.
Last edited by Ichinobu; 01-20-2006 at 04:16 PM.
01-20-2006, 04:44 PM
#9
Registered User
Join Date: Dec 2002
Location: Man who scatches ass should not bite fingernails
Posts: 83
Car Info: 2002 Primer-Chipped Silver WRX
Originally Posted by sigma pi
haha
and we are putting it after the turbo
we can run inj 3 to run longer and i dont want to tune for one cyl i want all of them checking in
and we are putting it after the turbo
we can run inj 3 to run longer and i dont want to tune for one cyl i want all of them checking in
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