EGT Probe question

[DVST8R]

If you can't afford to take the risk of having the EGT probe Pre- turbine, you can't afford to play. In which case you are stock, and you don't need a pyrometer.  :wink:

[jackbombay]

If you were really that concerned about egts, you'd have a probe at each cylinder exhaust port proximal to the exhaust valve.

  No you wouldn't, the weak link as far as high temps are concerned is the turbine, the pistons are oil cooled, they can take the heat, that's why the turbin inlet temp is the only thing that is of any importance.

What I am saying is, nobody, including yourself, has provided compelling evidence that there is not a functional and useful relationship between pre-and post-turbine temperature.

Uhhhhh,

At low boost levels there is little difference in pre and post turbine temps, at high boost levels there is a big difference, the one place you want/need it to be accurate is the place that it's least accurate.

  But hey, If you like gauges that have massive inaccuracy at the most critical moment thats cool too. HOw about a tach that read correctly between 1000 ans 2000 RPM, but at 5000 rpm it reads anywhere from 800 to 2000 RPM to low? But you neve know how much it's reading off, sound good? that's what you think is good for an EGT gauge...

[larry104]

If anyone is interested, I've done some calculations on turbine deltaTs and am in the process of checking them more thoroughly. Interestingly, I've run a few cases and get about 200 deg F, the same  number I've read elsewhere.

[larry104]

If you can't afford to take the risk of having the EGT probe Pre- turbine, you can't afford to play. In which case you are stock, and you don't need a pyrometer.  :wink:

uh uh...Giles modded pump, GTD nozzles, 2.25" free-flowing exhaust, manual boost controller (15 psi boost),  cold air inlet, thermostatically controlled, fan-cooled intercooler,  EGT :wink:

[larry104]

At low boost levels there is little difference in pre and post turbine temps, at high boost levels there is a big difference, the one place you want/need it to be accurate is the place that it's least accurate.

I hardly call that compelling. :roll:  Show me some actual numbers or thermodynamic equations that support your ideas.

[jackbombay]

At low boost levels there is little difference in pre and post turbine temps, at high boost levels there is a big difference, the one place you want/need it to be accurate is the place that it's least accurate.

I hardly call that compelling. :roll:  Show me some actual numbers or thermodynamic equations that support your ideas.

  TDIMeister ran all those calculations a while back on TDI club to illustrate that post turbine placement is fairly worthless, he knows more about this than all of us put together, he does this stuff for a living, I'll take his word on it over yours any day of the year.

  EDIT-  You are determined to put your probe post turbine, do it, you'll never convince us its better, and we'll never convince you that pre turbine is the way to go...

[larry104]

300% longer is actually equal to 3 times longer.

Yes, you are correct. Something can't be more than 100% shorter than another thing or it wouldn't change, exist, etc.

I'm not so interested in the calculations on paper, but rather actual pre and post turbo measurements.  

Me as well, which is why I'm asking the question, which incidently, nobody here has answered. However, I find it interesting that my calculations give deltaTs close to commonly cited values.

[HarryMann]

YUp,

Put it post turbine, rig up a temporary pre-turbine instrument, calibrate the post-turbine one against it for steady-state at various boost levels and rpm...

but the lag will always be there, and the boost/rpm correction read-off tables too!

Maybe a HUD unit would come in useful for their display  :roll:

[jackbombay]

The readings are actually 300% slower if you use an inline flux capacitor.

  But that's in 4th gear, in lower gears you have to multiply %300 by the gear ratio, in 1st it responds about %1000 slower, in 5th it actually responds a little quicker than %300.

[larry104]

At low boost levels there is little difference in pre and post turbine temps, at high boost levels there is a big difference, the one place you want/need it to be accurate is the place that it's least accurate.

  TDIMeister ran all those calculations a while back on TDI club to illustrate that post turbine placement is fairly worthless, he knows more about this than all of us put together, he does this stuff for a living, I'll take his word on it over yours any day of the year.

  EDIT-  You are determined to put your probe post turbine, do it, you'll never convince us its better, and we'll never convince you that pre turbine is the way to go...

jackbombay, I don't know you.  But I'm not trying to convince  anyone or put people down. I just have a tough time accepting something without solid proof. I'd simply like to see numbers to back up the claims.  :mrgreen:

[larry104]

Where did you get convinced that the probes fall apart prior to other significant engine damage?  What numbers convinced you of the viability of that scenario?

Andrew

Who said anything about other significant engine damage? Probes are often encased in stainless steel.  Stainless steel, depending on the grade, is prone to vibrational cracking, especially at elevated temperatures.  A friend and expert stainless steel fabricator steered me away from 304ss for my downpipe for that reason.  321ss is a better choice.  I honestly don't know which stainless my probe is made from, but I was involved in high-temperature engine seal development and saw Type K probes break when they were repeatedly exposed to temps of about 1,600 F and little vibration. The cases became brittle. We replaced quite a few.

Look at it this way: If I am flying a Space Shuttle, there are hundreds of critical-one components, those which failure of could compromise the ship and crew. If engineers decided a probe could be relocated to eliminate one of those failure modes, and still give them the information they desire, would they not do it? On the other hand, if the probe's location were absolutely essential to its proper operation, then they accept the risk. Though my 92 Jetta Ecodiesel is far from a Space Shuttle, the same principles apply. A failed $30 probe  could likely destroy an $800 turbocharger. Conversely, if I can get the info I need without the risk, I'll do it.

[saurkraut]

You are right.  No one has mounted two thermocouples on their engine and recorded these measurements.  I doubt anyone ever will, except Slave2School.  He clearly offered to do it for fifty bucks.  So there is a valide offer to produce the data you desire.

Equally, no one here will say its ok to put it in the down pipe and use it for engine monitoring.  Say what you want, rant on... your not going to get it here.  Also, I used to fly jets for the navy, worked for aerospace frims offering temperature measurement for miltary, commercial and space applications, and my probe is preturbo, and I'm fine with that

You could do a poll, and see how many are pre turbine, what the milage is, and what the mods are.  There is no evidence on this forum of probe failure in the pre turbo installation.  I've probably put 150K miles on my preturbo probe, I drive it like i stole it, and the probe is fine.

Somewhere on Freds page, TDImeister has done some calculations.  Search it out and see if its what you seek.

Or go post turbine, mentally subtract 200 or 300 degrees (your choice) from you gauge measurements and hope for the best.

[larry104]

You are right.  No one has mounted two thermocouples on their engine and recorded these measurements.  I doubt anyone ever will, except Slave2School.  He clearly offered to do it for fifty bucks.  So there is a valide offer to produce the data you desire.

Equally, no one here will say its ok to put it in the down pipe and use it for engine monitoring.  Say what you want, rant on... your not going to get it here.

Somewhere on Freds page, TDImeister has done some calculations.  Search it out and see if its what you seek.

Or go post turbine, mentally subtract 200 or 300 degrees (your choice) from you gauge measurements and hope for the best.

Ranting?Nope. I'm trying to have a meaningful discussion on a discussion board about a topic that, evidently, people have strong opinions on, but have little to no information on which to base those opinions. And no, I won't just"hope for the best." If it turns out I'm wrong, I'll relocate the probe pre turbine and accept the risk.

[HarryMann]

Larry,

Engineering is indeed a compromise, and a good engineer makes a good compromise... most things are in fact not black and white and grey areas do exist. This might well be one, but..

You could restrict yorself to 1600F or source a better probe.

This position thing would be much less important in an aircraft, as that's pretty much a steady state situation, with reference EGTs for diffeernt flight phases (e.g. max Take-off, climb high alt. cruise etc.), but even then I think they might and indeed do risk a proven installation pre-turbine.

It's just a case of getting ther MTBF of the probe another order of magnitude above the MTBO of the engine. The odd rogue probe might come along, but those specified for a given aircraft application would generally be from an approved source, even for light aviation.

In the case of an automobile, the grey area moves a long way towards white, as the outcome is rarely going to be life threatening, and using the true and fast reading egt gauge itself, most remaining darkness can effectively be ruled out. If detonation played a part, as it does with glow-plugs, I'd be more circumspect, but temps themselves that should be under control I'd dismiss unless on a test bench and going for it...

In the vanishingly small likelihood of a probe failure within the working range, one has to accept you are in the lap of the gods.

[larry104]

According to your info you saw some probes fail at 1700.  If you are running a VW diesel then your probe will never see 1700*F pre-turbine (the damage limit to probes that you propose) unless you don't care if your engine melts.   I suspect that 1700 is fairly low for probe failure, as most are rated significantly higher than that and most ratings for materials incorporate a safety factor  against liability.  If a probe fails at 1700 for a gauge that reads 1800 then the manufacturer of the sender would be liable for damages.  Most companies try to avoid litigation by incorporating a safety factor.

Andrew, I said and meant 1,600 F, not 1,700 F. The system was cold-junction compensated with an RTD.  Safety factors? Liability? I'm reporting facts.