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Factory EGT sensor


awfulwaffle
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Heya LGT experts and disseminators of knowledge!

 

Getting a little curious about EGTs given I managed to smoke a piston recently. Ordered a single channel thermocouple signal conditioner and a TC with the intention of getting a probe into the manifold near one of the exhaust ports, and in the meantime I've been playing with the factory EGT sensor (since my car originally had a cat in the up-pipe). It's clear to me that, mounted that far downstream of the exhaust ports, that sensor reads on the low side due to heat transfer out of my unshielded manifold, and also has quite a bit of thermal latency/lag. On a third gear pull on my 93 map, I'm seeing temps climb continuously until redline, at a steady rate, and peak at only ~1200 degF.

 

I've confirmed that the front O2 heater is still active when the EGT reads 1200 degF so the temperature at the sensor isn't exceeding the ~1500 degF threshold for the heater to shut off, but that's about all I've got to make sure that 1200 at the factory EGT location doesn't mean much, much more at the exhaust port.

 

Curious, has anyone else watched the factory values? What did you see with your particular setup? I understand it'll vary based on tune and mod package, but would be interesting to hear the spread anyways. A bit of info on your setup and tune parameters would be great if available to share. Total timing at peak torque and redline, peak/taper boost, AFR, that kind of thing.

 

 

 

 

 

Thanks!

Edited by awfulwaffle
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Yeah, the tune I was running at the time of failure was done by Cryo. I touched base with him, and we concluded that the timing and AFR was safe. I also had lean boost (and other) warnings configured in MerpMod (CEL flash activation) which did not trigger at any point, indicating there wasn't some sudden failure that led things to get toasty.

 

Best Dave and I could figure was that there may have been some cumulative effect due to one of the lean spikes we saw during early tuning, when I was experiencing very nasty fuel pressure fluctuations with my topfeed conversion. It looks to me like the second ring closed the gap completely and loaded up into the ringlands hard, so I could see it being the case that once that happened, subsequent heat cycles could have fatigued the ringlands to failure. I'll post a picture later to illustrate what I saw.

 

I have been getting into tuning myself, which is part of my motivation for understanding EGT a little bit better. To me, that's critical information to have when dialing in fueling and timing, especially with E85 which likely won't register knock on our knock sensor. Someone like Dave has the experience to base his timing map on, since he knows what does and doesn't work from hundreds/thousands of tunes. I need data to make sure I'm not going to blow a hole in something.

Edited by awfulwaffle
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Photo of the ring gaps on the borked piston. Note the thickness increase adjacent to the gap in the 2nd ring, which to me seems to be an indicator that it butted and loaded into the wall when the cylinder got hot. Curious that the top ring doesn't show evidence of the same, despite the fact that it's supposed to be gapped tighter from the factory.

IMG_20201129_000855.thumb.jpg.fb262c08faae4bce53ada59d688d8b7d.jpg

Edited by awfulwaffle
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I don't know if I'm an expert with this stuff, but what I do have is some data. The graph posted is from the pull done in this video: https://www.youtube.com/watch?v=ZhFoi6puBUs

 

This is a very, very safe tune that is basically a failsafe tune that the ECU will revert to if a failsafe is tripped. You can see where the launch control is active how the EGT isn't very high which I always thought it would be with how retarded the timing is. -5 degrees to get combustion to happen later and spool the turbo, which it does. The pops are the wastegates opening.

 

One thing that I've always heard is that retarding timing will increase EGT, which I have found to be true, except in the instance of the launch control. While dialing in my knock sensors I backed the timing way off and saw 1500F at the top of a 3rd gear pull. This would have been much hotter if I had done a 1-4th gear pull. I'm going through a bunch of logs trying to find a good comparison with different timing to show the difference in EGT.

 

I have also heard that you can't just use EGT to tune timing. It was a video by HP Academy that explained why, but I can't remember the specifics.

124808871_dragylog.thumb.jpg.046f1997dd06e5619cec327e9e556896.jpg

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I guess to answer your original question, you can use the OE sensor, but you'll have to figure out what is a safe temp and what isn't. Some diesels have their EGT probe after the turbine. Just have to figure out how much temp is lost from the port to figure out what is going to be safe on the gauge. Those trucks have figured it out and have it marked on the gauge for you.

 

I think getting the aftermarket gauge installed and close to the port is going to be the best since then there's no guessing. Then you can use the gauge to see how much temp is lost going to the OE sensor.

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After thinking about this a bit more I think the biggest downside of the OE EGT is you don't know which cylinder is hottest. Even if you figure out the temp drop, is the hottest cylinder 50, 100, 150F degrees hotter than the average reading?
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Yeah that's definitely a downside. Still, having an average is better than nothing right? I'm curious how much harder it'd be to detect sudden hot shift at an average location if a single cylinder starts to lean out though...

 

Another thought occurred - I run an unshielded manifold. Given how low the OEM sensor already reads, makes me wonder how much more temperature drop you incur at speed as the airflow increases. Seems like the temperature drop might vary significantly with speed just because of how far downstream the sensor is.

 

 

Thanks for providing the plot and discussion, food for thought..... Would be great to see any more data you have.

 

I do have the one signal amp on the way, but it'd be simple enough to get 4 channels going. Scope creeeeeeeeeeep

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Yeah, having an average is better than nothing. I mean we tune entire engines based off of average AFR. Just have to keep in mind the limitations when you don't have per cylinder readings and err on the side of safety.

 

I would think that having the manifold in the air stream would probably affect the OE EGT reading, especially in the winter months. The more heat the manifold absorbs and sends off to the atmosphere isn't going to be picked up by the sensor. Makes sense to me.

 

Something you can do is have one EGT bung welded in each tube and then just swap them around until you find the hottest one. It probably will be #4, but I honestly can't say if the EJ25s propensity to break that cylinder will show up as a high EGT reading.

 

Here's a few more graphs of different pulls. Is there a way to upload a pic and then insert it in the actual post? It's such a hassle uploading to image sites and then copying the link. But I like doing that because I feel like I can make easier to follow posts with pictures.

 

Anyways, on all these pulls you need to add 2 psi to get the actual boost pressure. Haltech uses psig so atmospheric pressure at my elevation shows as -2.

 

First one is a 4 psi pull. One of the first pulls I did with the Haltech. This is with the lowest pressure springs in the wastegates. 12.5 AFR and getting up above 20 degrees timing. EGT maxes out around 1150-1200F. This just sort of show what things look like at really low power.

 

Next 2 pulls are both 8 psi, AFR around 11.7 at max RPM. The first one has a more natural timing curve. Timing low, 15, at peak torque and climbs, 19, as VE and torque fall off. EGT maxes between 1180-1240F. The second is just a flat 10 degrees. EGT maxes between 1260-1340F.

 

EGT is closely tied to power output. The more power, the higher they go. At 20 psi I don't think there's any way I could get EGTs down around 1100F. I mean, I probably could by causing misfires, but I'm talking a tune fairly close to optimal for any given boost level.

 

Sort of neat to see this kind of stuff.

289351212_4psi.thumb.jpg.3257c77852f24fd5c9ac2c3ec1badd6b.jpg

655559978_8psi.thumb.jpg.97b2e1169689166444eb68c66edbad8c.jpg

1440827983_8psi10deg.thumb.jpg.b3ade19001c46cf2d8cb05510ead26e2.jpg

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