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Old 01-01-2013, 07:46 AM   #1
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Cat 3126/c7 exhaust temp

Running a CAT 3126, allison 6 speed in a Freightliner frame.

No engine information at the drivers seat except oil pressure and engine temp.

So, first question: what is the collective view of safe exhaust temp between the head and turbo inlet?

Next question: To those of you who have a pyrometer, where is the sensor located in the exhaust system?

Next question: Given the same load, same exterior temp, same road speed, same grade which of the following would produce the highest exhaust temp:

Sixth gear at say 2100 RPM

Fifth gear at say 1800 RPM

I know which would create the highest block temp but the question is exhaust temp.
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Old 01-01-2013, 10:31 PM   #2
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Glenn I would think 6th at 2100 would produce more heat because it is alot faster than 1800 in 5th so more speed more heat.
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Old 01-04-2013, 04:51 PM   #3
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Glenn
I installed my pyrometer after turbo since that was the easiest method and I was short of time.

As far as exhaust temp, shifting from 6 to 5 does make an almost immediate exhaust temperature drop. When towing a vehicle in the southern Oregon mountains I always drop to 5th and sometimes lower to keep rpms above 2000 RPM.

Quote: "Next question: Given the same load, same exterior temp, same road speed, same grade which of the following would produce the highest exhaust temp:" I ask did you reverse your rpms? "Sixth gear at say 2100 RPM Fifth gear at say 1800 RPM" Wouldn't it be 2100 in fifth and 1800 in sixth?

I do notice significant exhaust temp difference towing or not towing. Generally when towing I do not need to shift down to keep exhaust temps in the safe range.
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Old 01-06-2013, 01:28 PM   #4
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Yes, I have the RPM's reversed, sorry.

In any case it looks like having the Cat engine pulling a load results in higher exhaust temps than having the Cat engine working less hard and running at a higher RPM's, all other things equal.

Do not have a pyrometer as of yet and its low on the things to do/add. But when I do looks like 1500* is the high limit, and 1300* for the long haul is better.

Glenn
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Old 01-06-2013, 06:03 PM   #5
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Along with exhaust temp CAT engine temp can be a problem especially in the hot days of summer going up a grade and towing. Other threads on engine temp suggest keeping the CAT rpms at or above 2000 rpm to keep from over heating the engine. At 2000 rpm and above the fan seems to push enough air through the radiator & air cooler to keep engine coolent in the safe range. That rpm also helps keep the exhaust temp down.
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Old 02-28-2013, 11:22 AM   #6
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If you intend to install a pyrometer put it in the exhaust manifold right next to the mating surface where the turbo bolts on. 1350 degrees is safe for long pulls. 1400 for a few minutes is OK. TS Performance that makes the MP-8 has pulled a C-7 on their dyno for one hour at 1500 degrees with no issues.


The highest EGT you will see will be pulling hard with Econo mode engaged at 1400 rpm. It is not a good idea to use Econo mode pulling hills. Keep the RPM at or above 2000 when pulling hard.

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Old 03-08-2013, 04:31 AM   #7
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Be careful in comparing EGTs when it comes to maximum allowable values.

When the maximum EGT limits are quoted (eg 1300F or so), it usually refers to readings from pyrometer probes which are placed before the turbo, ie, in the exhaust manifold close to the engine.

If the probe is placed downstream behind the turbo, the measured exhaust temperatures are several hundred degrees cooler than they were in the manifold. Therefore, in this case, using the normally quoted values for engine operating temperatures limits are not valid.
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Old 03-08-2013, 09:31 AM   #8
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6 is right. Further post turbo sensor location is of minimal value as there is no defined temperature limits available for post-turbo sensor locations and any number you see is nothing more than a guess. The only reason for placing the sensor post turbo is its easy.

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Old 03-08-2013, 12:48 PM   #9
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The rule of thumb for post turbo sensors is 300 degrees difference from pre turbo
locations.
Another issue is a lot of these after market EGT kits do not have long enough
probes for install in the larger diesel exhaust pipes and give false temps, all low.
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Old 03-08-2013, 02:27 PM   #10
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I am going to get a pyrometer in the next little while. If they can't install it pre-turbo, is it worth getting it installed at all? And how far behind the turbo is still satisfactory?
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Old 03-08-2013, 02:37 PM   #11
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If you install it after the turbo get it as close as you can to the turbo.
Then remember the temp it will tell you will be about 300 degrees under true
exhaust temp.
Make sure the probe is long enough to reach half way thru the pipe, i.e. a 4 inch
exhaust pipe needs at least 2 inches or a little more so you get the best reading
you can.
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Old 03-08-2013, 03:52 PM   #12
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The right place is pre-turbo. It is a matter of drilling and tapping a 1/8 pipe tap hole in the exhaust manifold right next to the mating surface where the turbo bolts on. If you are going to the expense of installing a pyrometer you might as well do it right.

Rule of thumb is another way of saying guess.
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Old 03-08-2013, 04:17 PM   #13
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Even installing as Jim stays still is not accurate.
One could install like they do in airplanes and have probes installed at the exhaust
ports of each cylinder then one can see the temp of each cylinder.
And each cylinder will be different and the engine is operated paying attention to
the hottest cylinder.
The bottom line is the further one gets from the engine cylinder the lower the reading will be, the biggest difference is after the turbo.
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Old 03-08-2013, 06:34 PM   #14
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This thread has just ventured into the twilight zone.
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