EGT pre-turbo vs. post-turbo -- a direct comparison

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encomiast

boosted kraut
Mar 31, 2005
192
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germany
hi all,
I got my new downpipe installed yesterday and had an additional EGT probe fitted in, so I can now monitor EGT in the exhaust manifold (runner of 6th cylinder) as well as EGT just behind the turbo outlet, along with wideband A/F, RPM, throttle position, ECU-Vf and boost.

to provide some fact about the ever re-occuring topic of measuring EGTs post-turbo, I went for a ride today and had my laptop log all the data for approximately one hour. even though it is common knowledge that measuring EGT post-turbo is rather pointless, I wanted to have some hard numbers.
well, as can be seen in the graphs below, post-turbo EGT roughly reflects the trend of the engine EGT, but it's far from being precise. on some occasions (especially on load changes), temperature after the turbo is even higher than in the exhaust manifold.

for clarity, only RPM + boost + pre-turbo EGT + post-turbo EGT are displayed:
graph #1
graph #2
graph #3
this is a stock CT26 @ ~13 psi of boost, no additional fuel management.

just thought I'd share :)
 
E

encomiast

boosted kraut
Mar 31, 2005
192
0
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germany
IJ. said:
It's interesting on graph #2 where at times there are opposing spikes with the preturbo going high and the afterturbo going low!
Click to expand...
ahm, I guess you mean the post-turbos going high and the pre-turbos going low...?
yes, that's intriguing me too.. quite interesting how the turbo is heating up the exhaust gases after boosting hard. poor ol' little CT26.... and I didn't even hit fuel cut :D
also, post-turbo temperatures are spiking higher than pre-turbo (810 vs. 780 °C).

BTW the downpipe is a "divorced and re-routed" one, so the EGT probe is actually only measuring the temperature of the exhaust gases going through the turbine.
another striking fact is that my pre-turbo EGTs are now higher than with the stock downpipe. you may remember my post in the "what is your EGT" thread; my pre-turbo EGT never exceeded 760° before, now with the DDP it's getting as high as 780°.
 
I

IJ.

Grumpy Old Man
Mar 30, 2005
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I come from a land down under
Econ: DOH yes that's exactly what I meant ;)
My "guess" on the egt's is our cars run pig rich above 75% throttle so this gives a cooler egt but maybe with the DDP it's flowing a bit more?

The post turbo EGT's being high could be from an afterburner effect when the unburnt fuel in the rich mix is hitting the air in the exhaust?
 
lagged

lagged

1991 1JZ
Mar 30, 2005
2,616
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new rochelle
maybe because of the added psi gained with the use of an aftermarket DP?

although i would think it should go down due to less restriction....
 
E

encomiast

boosted kraut
Mar 31, 2005
192
0
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germany
lagged said:
maybe because of the added psi gained with the use of an aftermarket DP?

although i would think it should go down due to less restriction....
Click to expand...
to be precise, I had a 3" turbo elbow and the stock downpipe with gutted precat before, boost was just below fuel cut.
after the DDP install, the boost went up by about 2-3 psi, so I adjusted my MBC to compensate for it.

I actually didn't expect the EGT to change at all... but I would have also thought that they would rather drop than rise. strange but actually unimportant for now :)
 
M

MKNTRAXX

Loves tire smoke
Mar 31, 2005
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Orange, va
I also think this is from fuel still burning in the exhaust causing the high post turbo readings.

how rich is the car running..
 
Dirgle

Dirgle

Conjurer of Boost
Mar 30, 2005
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Pauma Valley, CA
Could also be that under certain situations your other cylinders are running hotter than your #6, thus contributing to your higher post-turbo EGT's. Don't really know but it's a thought. But I'm glad somebody put up some hard evidence about this.
 
E

encomiast

boosted kraut
Mar 31, 2005
192
0
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germany
hm, interesting thoughts....

here's another graph, this time with A/F included:
click me
A/F during boosting once was between 9.4 and 10.2 (far right of the graph), that's just the stock ECU richening the mixture... but then again, the post-turbo spikes even occur when the mixture was almost at a perfect 14.7 for quite some time (about in the middle of the graph).
 
M

MKIIINA

Destroyer of Turbos
Mar 30, 2005
1,825
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Plano, TX
maybe it has to do with the construction of the Dp itself and the materials. The stock manifold is cast iron (correct me if im wrong) and the BIC DDP is made out of alumanized steel so maybe heat transfer somehow? Still you would think that coming right off #6 would be the hottest section....
 
Adjuster

Adjuster

Supramania Contributor
Apr 8, 2005
1,166
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Meridian, Idaho, United States
www.adjusteridaho.com
I think with the cost of good AF wideband meters around 300.00 these days, the EGT temp meter is less important than ever for tuning.

That being said, mine has always been after the turbo, in the elbow, and has worked fine to keep an eye on the EGT of the engine.

Your graphs are very interesting data, and I agree the "best" placement is pre turbo, but I'm not sure #6 is the best location as your only reading one cyc, when the others could be hotter. I think the best location is in the turbo housing, just before the exducer wheel where all the exhaust gas is combined prior to being forced over the wheel.

The other option would be 6 EGT probes, and using a computer to data log them all the time. (Too much information when your driving everyday however.) Also would only be useful if your ECU could trim fuel at each injector, and you had a WB sensor for each exhaust runner too. (more money in EGT and WB systems than the car is worth at that point.) LOL
 
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