dcowley
Texas Panhandle
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Jaseds wrote:Best place is after turbo,reason being if the pyrometer probe should break off ,say good bye to your turbo. Only difference will be the reading showing couple hundred degrees less whenplaced inthe exhaust pipe right behind turbo.The older engines used to have them at the turbo but most all semis are after nowadays if equipped.
Jased is correct. In larger diesel engines burned probes haved ruined many exhaust fins on a turbo.
He is also correct to know that reading the 'cold side' (exhaust gas after the turbo) is approximately 200 degrees cooler than when in the exhaust manifold. Cummins, CAT and Mack figured this out in the 70's as more pyrometer thermocouplers failed and went shooting through the exhaust fins of the turbo which also is the most expensive part because it is made on the turbo shaft. Most intake fins are replaceable wheels but exhaust fin damage is always fatal.
If it was mine she'd be on the cold side and read accordingly.
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JustLabs
Washington State
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Jaseds wrote:Best place is after turbo,reason being if the pyrometer probe should break off ,say good bye to your turbo. Only difference will be the reading showing couple hundred degrees less whenplaced inthe exhaust pipe right behind turbo.The older engines used to have them at the turbo but most all semis are after nowadays if equipped.
Thats the same opinion that Cummins has.
They also claim that unless you have 6 probes that each read the temps on a cylinder,you're not getting any meaningful info.
When I install my probe,its going in the elbow directly behind the turbo. My Pacbrake is mounted there and has a hole thats already drilled and tapped for a probe.
2007 Dodge Ram 3500 SRW,4x4,Cummins 5.9, 6spd.
2007 Keystone Cougar 289BHS Fifth Wheel.
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LarryJM
NoVa
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bigeking wrote:Could someone tell me which reading is the most helpful----pre-turbo or after turbo.
The only logical place for a pyrometer if you're going to install one is pre-turbo. Post is meaningless since that temp can vary up to 600 deg from the temp of the exhaust gases leaving the cylinders and at least in the 7.3L Ford/Navistar engines what you're concerned with is the pistons and valves and Navistar says to keep the exhaust gas temps from the cylinders at or below 1250 deg. Almost any performance mod on the 7.3L engines can drive the pre turbo temps in excess of 1250 deg. I have yet to see one documented case of a pre turbo probe failing and causing turbo damage. I have been running both a pre and post turbo for 7 years and can attest that those that believe you can put a fixed temp between the two are living a false dream. I have personally at times seen a 450 deg temp difference between the two. Because of the location of my oversized downpipe and the Van configuration my DP is wrapped and I use the post turbo to determine the safe temp to shut off the engine when towing since it takes up to 3 min to get that temp down in the order of 350 deg or less so I don't coke the oil in the turbo bearings.
Larry
2001 standard box 7.3L E-350 PSD Van with 4.10 rear and 2007 Holiday Rambler Aluma-Lite 8306S Been RV'ing since 1974. TRAILER MODS
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dcowley
Texas Panhandle
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Quote:I have yet to see one documented case of a pre turbo probe failing and causing turbo damage.
I don't know about documentation but over the years I have personally replaced in excess of a dozen turbo's due to thermocoupler probe failure destroying the exhaust turbine. These were all on Class 8 truck engines and very common in the earlier days until they all figured it out and placed the thermocoupler behind the turbo (cold side).
Maybe these little pickup engines don't run the miles and therefore don't fatigue the probe. All the larger diesel engine manufactuers place the thermocoupler probe after the turbo, there must be a reason.
Why take the chance? Learning to read the cold side will allow for the same results which is; Not to damage an overfueled engine.
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RVnRobin
Pacific Northwest
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My probe is mounted in the cast part of the housing immediately post turbo as per the instructions that came with the pyrometer. I just looked at the intake to the turbo and the only place that could be used to mount the probe is immediately in front of the turbo. It seems to me that the temperature difference would be minimal since the post turbo position is only 6-8" down line from the pre turbo position. "Purists" may want the most accurate readings, with multiple probes mounted at the cylinder heads. As an indicator of when to shift down, the post turbo position works just fine for me.
RVnRobin, DW, and the cat.
The cat thinks she is the boss!
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downtheroad
Tumbling Waters, Washington
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wrote:Best place is after turbo,reason being if the pyrometer probe should break off ,say good bye to your turbo. Only difference will be the reading showing couple hundred degrees less whenplaced inthe exhaust pipe right behind turbo.The older engines used to have them at the turbo but most all semis are after nowadays if equipped.
Urban legend..
Post turbo placement is very inaccurate and much slower to respond.
If you want an accurate (not a guess within a couple of 100 degrees)and a quick response - mount the probe pre-turbo otherwise your gauge is really not all that useful.
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chuck4788
West of the Middle
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RVnRobin wrote:My probe is mounted in the cast part of the housing immediately post turbo as per the instructions that came with the pyrometer. I just looked at the intake to the turbo and the only place that could be used to mount the probe is immediately in front of the turbo. It seems to me that the temperature difference would be minimal since the post turbo position is only 6-8" down line from the pre turbo position. "Purists" may want the most accurate readings, with multiple probes mounted at the cylinder heads. As an indicator of when to shift down, the post turbo position works just fine for me.
6-8 inches of pipe would not change the temperature enough to notice BUT the turbo is a different story. The turbo compresses the intake air depending on engine speed and the amount of fuel used, the intake pressure boost could be zero to 30psi. The turbine in the turbo converts heat energy in the exhaust gas to mechanical energy to drive the compressor section. So the turbo will lower the exhaust gas temperature anywhere from zero to 350F depending on the amount of boost added to the intake air. As other said the max pre turbo EGT is 1250F to protect pistons and valves. Post turbo EGT will vary engine to engine so you need the specs for your engine to know what is the acceptable post EGT value. I have the spec sheets for some of the Cummins engines that I've owned and the post turbo EGT are all about 900F, plus or minus a few degrees. Only one model listed pre and post turbo EGTs (at 1230/890, a 340 degree difference).
Chuck
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LarryJM
NoVa
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RVnRobin wrote:My probe is mounted in the cast part of the housing immediately post turbo as per the instructions that came with the pyrometer. I just looked at the intake to the turbo and the only place that could be used to mount the probe is immediately in front of the turbo. It seems to me that the temperature difference would be minimal since the post turbo position is only 6-8" down line from the pre turbo position. "Purists" may want the most accurate readings, with multiple probes mounted at the cylinder heads. As an indicator of when to shift down, the post turbo position works just fine for me.
The proper place for a pre turbo probe is not just before the turbo, but as close to the cylinders as possible. On the 7.3L some put it in the DP just above the exhaust manifold with a clamp on probe and others drill and tap the driver's side manifold and put a screw in type there. I don't know much about the big rigs but what I've posted and recommended is applicable to all the LD diesel trucks especially those that have "performance enhancements" installed which again a lot of the big rigs don't have since they already have the power and gearing to do the work from the git go. IIRC the big locomotive engines have a pyro on each cylinder, but after you install a pyro on just one bank you can get a good baseline under varying conditions what the temps should be and at least you're have covered as far as individual cylinders go at least on the V-8 diesels. With me running both I think I would be able to tell even if the none pre turbo monitored bank went seriously out of whack because of the higher than normal unloaded cruising temps between pre and post turbo which for me is around 100 to 150 deg on the flat interstate.
Larry
* This post was
edited 05/22/08 04:49pm by LarryJM *
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RVnRobin
Pacific Northwest
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Thanks chuck4788 and LarryJM this is good information.
Some questions.
First, this is the first time I have heard of turbo physics described as using exhaust heat transferred to an increase in air flow in the intake. Once when I opened up a turbo it looked like two fans with different pitchs in the blades and a different number of blades. What was described to me several years ago is that the "air flow" of the exhaust side crams a greater volume of air through the intake side. So, is it the proxmity of the "fresh air" side of the turbo next to the "exhaust" side that is causing it to cool so much?
Second, what pre and post turbo temperatures should I expect on my 1994 2500 Dodge with Cummins?
Third question, will a probe clamped ontside the manifold show temperatures lower than the screw in probes due to heat loss through the metal or due to air flow around the clamp probe?
Fourth, I thought I was doing what is needed to monitor systems, should I make some changes?
BTW I am very happy with this pickup pulling a 10k lb TT. Plenty of power, unloaded mileage 18-20, loaded mileage 11-14, no computer. Just want to do what is best.
Thanks
* This post was
edited 05/22/08 08:33pm by RVnRobin *
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LarryJM
NoVa
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RVnRobin wrote:Thanks chuck4788 and LarryJM this is good information.
Some questions.
First, this is the first time I have heard of turbo physics described as using exhaust heat transferred to an increase in air flow in the intake. Once when I opened up a turbo it looked like two fans with different pitchs in the blades and a different number of blades. What was described to me several years ago is that the "air flow" of the exhaust side crams a greater volume of air through the intake side. So, is it the proxmity of the "fresh air" side of the turbo next to the "exhaust" side that is causing it to cool so much?
Turbo charged diesel engines are high compression engines like 15 to 1 IIRC and force air into the combustion chamber under pressure (i.e. via boost). There are basically two parts to a diesel turbo the "work" side and the "compression" side which are on a common shaft. The exhaust gases are sent thru the "work" side and that drives the "compression side" that takes the intake air and compress it which is then sent to the cylinders via the "aftercooler" or sometimes referred to as the "intercoolor". Thus the exhaust work side of the turbo is converting the heat in the exhaust gases into work to drive the compression side of the turbo. This is a simplistic description and since I'm not a turbo nerd my terminology might not be completely correct.
Second, what pre and post turbo temperatures should I expect on my 1994 2500 Dodge with Cummins?
I have absolutely no idea or the Cummins engines and you'd be better served to check out one of their forums for the real answer. I only know about the 7.3L Ford/Navistar engine, but would bet more that just a small amount that the answer would be similiar to what is applicable to the 7.3L engine and that is pre should be limited to around 1250 and post is useless except for shutdown temps.
Third question, will a probe clamped ontside the manifold show temperatures lower than the screw in probes due to heat loss through the metal or due to air flow around the clamp probe?
No, not IMO ... the probe tip need to be in the center of the actual exhaust gases. I have never heard of anyone trying to measure EGTs by simply clamping a probe to the outside of the pipe/turbo/manifold.
Fourth, I thought I was doing what is needed to monitor systems, should I make some changes?
Again you need to go to a Cummins specific site like TDR, etc. for the correct information, but unless you have added any "power enhancements" to the engine other than intake/exhaust mods monitoring EGTs might not be necessary. Even on the stock 7.3L Fords a biggie is to install a real tranny temp gauge on the autos since heat will kill them and things can get out of whack and the stock gauge is just a gloriied idiot light that tells you either you're O.K. or that you have just burnt up your tranny.
BTW I am very happy with this pickup pulling a 10k lb TT. Plenty of power, unloaded mileage 18-20, loaded mileage 11-14, no computer. Just want to do what is best.
Thanks
Larry
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