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EGT Findings - Pre vs Post Turbo

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Old 05-19-2006, 01:03 AM
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EGT Findings - Pre vs Post Turbo

There has been debate and speculation over the safe limit for post turbo EGT applications before, so I decided that I would run some tests. I have probes in both pre and post turbo locations. This last weekend while on a road trip pulling a trailer, I made some EGT observations about the differences between the two.

Most of the time, the variance is around 200-250 degrees, occasionally bumping to around 300 degrees difference when accelerating while already at freeway speeds. Pre turbo EGTs stayed under 1200 during that time. This is right in line with what has been stated previously by others.

However, the noticeable difference came under hard acceleration from a stop or very low speeds. We went through several toll booths, so I had more than one chance to check this. On the acceleration runs I bumped the chip up to the 100 HP setting in order to get EGTs to climb fast, purely for testing these differences, as EGTs don't climb fast enough on lower settings to test what I wanted to. I don't run the chip on that high of a setting while towing.

What I was after was to see what post turbo read when the pre turbo probe showed 1300 degrees. What I found was a huge difference of around 400 degrees. On one acceleration run in particular, the pre turbo EGT hit just over 1300 degrees and the post turbo had not yet hit 900 degrees, but it was almost there. A couple other runs showed similar resulsts, pre turbo at 1300, post turbo right around 900.

So, the conclusion of what I found was that while the temperature difference is typically between 200-300 degrees, when you are under hard acceleration, which is where EGTs tend to climb quickly, the difference is greater. Based on these findings, I'd suggest that if you only have a post turbo probe, don't run it over 900 degrees, and to keep it a little under that to be safe, since pre turbo temps seem to be around 1300 degrees by the time the post turbo reads 900 degrees.
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Old 05-19-2006, 01:13 AM
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That's good info JT. I think it drives home the point of why I got a preturbo pyro. I think we can attribute that reaction time difference to various factors, but the major factor in my opinion is the "soak" that takes place in the turbo itself. I believe a lot of that heat is absorbed by the turbo before the post turbo sensor is able to tell you that the turbo is on the edge. That may be too late.

Besides, the preturbo probe lets me know a lot quicker when I'm about to toast my PMRs, or something in the cylinders.

Last edited by Tenn01PSD350; 05-19-2006 at 01:40 AM.
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Old 05-19-2006, 01:19 AM
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That's the other thing I forgot to point out. (My bad for starting another new thread late, when I'm already worn out )

The pre turbo probe is a lot more responsive to temperature changes than the post turbo probe is. The heat soak of the turbo probably has a lot to do with that as well.
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Old 05-19-2006, 01:26 AM
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Excellent information! Nice work.
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Old 05-19-2006, 08:16 AM
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Those are great findings, I installed a couple guages in my beast 3 weeks ago and that pyro is a real eye opener. It's surprising how fast the temp. rises.
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Old 05-19-2006, 08:27 AM
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Great info! When you drilled and tapped your exhaust manifold for the pre turbo, do you have to remove it, or how did you get the shavings out of the manifold?
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Old 05-19-2006, 08:28 AM
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Jtharvey that's some fun times there watching all those guages. Thanks for sharing.
.
I do have one point to debate on the heat soak of the turbo.
I don't think it soaks up all that heat, rather most of that heat is getting dissipated by the expansion of the exhaust gasses as they exit the turbo.
Air gets heated by compression enough to fire the diesel as it's injected, it will also cool off just as quickly when it gets expanded back out.
If the turbo was really absorbing 100% of all that heat energy it would quickly melt down.
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Old 05-19-2006, 08:40 AM
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Quote:
Originally Posted by yellow401
Great info! When you drilled and tapped your exhaust manifold for the pre turbo, do you have to remove it, or how did you get the shavings out of the manifold?
You don't have to remove it, I used a small magnet and a vacum cleaner to remove shavings. It also helps to spray wd-40 wile your drilling and tapping. I think it helped keep the shavings in one spot.
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Old 05-19-2006, 09:25 AM
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Kwik, you are right - much EGT reduction is due to expansion of gasses after they leave the turbine - but the turbo isn't a heat sink. Most of the temperature reduction is due to the energy it takes to drive the compressor.
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Old 05-19-2006, 10:31 AM
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Quote:
Originally Posted by yellow401
Great info! When you drilled and tapped your exhaust manifold for the pre turbo, do you have to remove it, or how did you get the shavings out of the manifold?
I did not remove it. I drilled very slowly, letting shavings fall out as best they could rather than into the manifold. Also, I stepped up drill bit sizes as I drilled rather than go at it with the big bit first. This way, if any shavings happen to get into the manifold and I can't get them out, they will be smaller. Once it was tapped, I used a small magnet on the end of a bendable probe to reach into the manifold and pick up any shavings. Lastly, I took a shop vac to the hold to try and suck out anything that might be left.


Quote:
Originally Posted by Kwikkordead
do have one point to debate on the heat soak of the turbo.
I don't think it soaks up all that heat, rather most of that heat is getting dissipated by the expansion of the exhaust gasses as they exit the turbo.
Air gets heated by compression enough to fire the diesel as it's injected, it will also cool off just as quickly when it gets expanded back out.
If the turbo was really absorbing 100% of all that heat energy it would quickly melt down.
Point taken, and agreed.
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Old 05-19-2006, 09:53 PM
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Quote:
Originally Posted by Kwikkordead
I do have one point to debate on the heat soak of the turbo.
I don't think it soaks up all that heat, rather most of that heat is getting dissipated by the expansion of the exhaust gasses as they exit the turbo.
Air gets heated by compression enough to fire the diesel as it's injected, it will also cool off just as quickly when it gets expanded back out.
If the turbo was really absorbing 100% of all that heat energy it would quickly melt down.
I like a debate.
I don't think the turbo soaks up ALL of the heat and don't think I implied that.
I do think that the turbo is the first place an obstruction is met on the exhaust side of things and this obstruction causes heat, as in your own words the air runs into something and cannot flow freely, then must compress or at a minimum heat further from the friction and collision with the exhaust turbine.
That extra time spent there trying to get by the turbo, and having more heated air pushing on it will translate to higher temps at the turbo housing which by it's nature will absorb and hold more heat due to it's mass as compared to up pipes and down pipes.
Gas laws state that a gas will not expand as it cools, but it will contract. True, if indeed the gas were still under pressure and allowed to expand leaving the cylinder (exhaust stroke through puny valve into bigger manifold and pipe), it would cool but I don't think this happens effectively as more hot gas is right behind it pushing it through the turbo. I believe this minimal expansion cooling would be offset at the turbo by increasing pressure there.
Once by the turbo, the gases will indeed cool due to expansion in the outlet and downpipe from both expansion as you've pointed out, and a lack of pressure/obstruction from there on. These observations seem to explain the differences that JT noted on his tests between the two probe locations especially under hard continuous acceleration.
So heat soak may not be the correct term, but something is definately going on.
Maybe we can convince JT to tap his exhast portion of the turbo housing and install another gauge in order to test this out. Better yet, one of those IR pointer gauges pointed at the different parts while a truck is on a dyno would answer volumes about where and what the heat is.

So I don't disagree with you, totally, I just think the actual temp is higher at the turbo housing than either the pre or post turbo probes are indicating with the pre turbo being closer. My .02 plus 2.75.
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Old 05-19-2006, 10:05 PM
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Awesome info Jt - you da MAN!!

- I think this info should be in a sticky at the top of the page - I don't know HOW many times I have posted, or read, about pre- VS post- readings, and what is "really" safe.... blah blah....

Last edited by CAT_man_963; 05-19-2006 at 10:07 PM.
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Old 05-19-2006, 11:49 PM
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I just put a link to this thread in the "Common Mods" sticky thread.

Link to it: https://www.ford-trucks.com/forums/s...32#post3684832

.
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Old 05-20-2006, 01:48 AM
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Thanks guys!! I appreciate the compliments and the posting of it in the common mods sticky. That thought never crossed my mind.

And, although I do have two turbos to play with at the time, I don't think I'll be tapping into any turbo housings anytime soon.
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Old 05-20-2006, 02:06 AM
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JT, there is an explanation as to why there is a 200 to 300 degree difference normally, but jumps to 400 degrees when you really push it.

The pressure and heat of the exhaust is what drives the turbine. The turbine spins the compressor, which compresses the incoming charge (yeah, yeah, elementary school, I know ).

Okay, under normal conditions, you aren't seeing a lot of boost - what, 5 to 12 psi or so? Maybe a little more? It doesn't take a lot of exhaust energy from the turbine to drive the compressor under these conditions.

Now you push HARD - your boost moves over 20, maybe over 25psi. That takes a LOT more energy out of the exhaust than normal driving. Since it takes so much more energy to drive the compressor, much more heat is pulled out of the exhaust.

That, of course, brings up a very important point which the post-turbo-pyros-are-just-as-accurate-just-at-a-lower-temperature crowd.

Namely, at the very time when you'll see the highest EGT is when you are under high boost conditions. And the high boost conditions drain MORE energy from the exhaust which leaves a bigger temperature gap between the pre- and post-turbo pyro locations. In other words, a post-turbo pyro is the most accurate when it doesn't matter, and the most innaccurate at the very worst time.

I'm so glad I took the trouble to put mine pre-turbo.
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