Pre and Post turbo EGT's

[paccool]

I put this in the firsst gen section but thought it might be of interest to some of you other folks.



OK guys, I have always wondered about what people have said about the difference in the readings before and after the turbo. I had to se for myself. Was hard to believe that the readings could be that different in a span of about 8". I know about expanding gasses and all that but seemed hard to accept that kingd of difference. I originally had a direct reading (thermocouple)US Gear pyro located after the turbo and just last week installed a new ISSPRO electronic pyro in the ATS manifold (in front of the turbo) I put on last year. I went on a trip to Washington last Friday and returned Monday. Pulled a 5500 pound load up, came back with an empty trailer at 2250 pounds. Took a series of reading both directions and only took "stable state" readings. some were pulling a grade and some were on flat ground. Found some scary things and thought I'd pass them along.



The direct reading pyro is very slow to respond when compared to the new ISSPRO pyro. I could put the new pyro into the red and then cool back down before the old pyro hardly moved.



Also took readings with the transmission in OD going up a hill. Got the pre turbo temp up to 1200 degrees then shifted out of OD to get the RPM's up a little. Saw an immediate 150 degree drop in the EGT. This alone is a good reason to shift out of OD when towing up a hill!



Here are the readings I took pre and post turbo-at idle the readings were basically the same as the turbo wasn't working at all.



PRE POST TURBO

600 400

700 500

750 550

800 580

875 620

900 640

1000 700

1100 750

1150 800



As you can see, there is a 350 degree difference at the upper end of the scale. Based on this, I would try to keep the post readings at 900 or less if that was the only reading I had. I used to push it to 950 and a little higher when towing which put me up at 1300 degrees. Sure glad I put the new unit on as it is a much more realistic reading!

 

[fortunate1]

What was your boost during these readings?

 

[rbattelle]

Based on this, I would try to keep the post readings at 900 or less if that was the only reading I had.

That's why I have this pyro .



-Ryan

 

[paccool]

Ryan,

Thats what the new one is! Works great.



Fortunate1,

boost was different for all numbers shown. Some were flat ground, some were going up hill. Obviously the higher the temps, the higher the boost as well.



Stan

 

[Desperado]

Most of the time, on my set up, I see readings with in 100* on flat ground, up the hills I first see a 400* difference after a short time the pre tubo will come down and the post will come up to with in 250* of each other, but I can put the pre at 1500* at will and the post will only hit 1200* so in MHO if you are only going to run one pyro run it in the pre turbo side.

 

[Hohn]

You have to remember that the biggest difference between pre- and post- readings will take place during TRANSITIONS.



If you lightly run up through the gears, it might be 300°. Steady cruise, maybe 200°



I have personally seen over 500° difference in my truck during hard acceleration in our sorta high altitude (6K').



Pre turbo is much higher not really because it's closer to the engine. It's because it's under pressure, restricted by the turbo. Once the flow is past the turbo, the pressure drops dramatically.



An HX35 at 35psi can have 50psi of drive pressure. Think you have 50psi of back pressure in your post-turbo exhaust??



One thing that troubles me is this: Pyro response! If my engine can get to 1400° quickly, I wouldn't know it until my pyro SAYS it's reading 1400°. ]





So I ask this: how fast can a pyro using a K-type thermocouple respond??? Can it keep up with the engine, as far as ability to change temps? How fast can the CTD make EGTs get scary??



Just my ponderings...



jlh

 

[rbattelle]

Pre turbo is much higher not really because it's closer to the engine. It's because it's under pressure, restricted by the turbo.

Justin,



I must very respectfully disagree with you on this point. In my opinion, the temperature pre-turbo is higher specifically because it's combustion chamber discharge air close to the exit of the combustion chamber and it contains a lot of energy. Some proportion of that energy is transformed by the turbo into work, which causes a drop in temperature across the turbine (and a drop in pressure, of course).



Again, I say this very respectfully and I'm not just trying to say you're "wrong". I'm simply disagreeing. Respectfully.



-Ryan

P. S. - Did I mention I mean no disrespect?

 

[PC12Driver]

Our plane uses an air cycle machine (ACM) for it's air conditioning. It looks very similar to a turbocharger. Basically it takes air from the compressor section of the engine that's a couple of hundred degrees in temp and 150-200psi and runs it through the first side of the ACM which compresses the air charge even more, thus generating more heat. The air then passes through a small air/air cooler (exposed to whatever the outside ambient temp is), and then to the other side of the ACM where the relatively cooler and more dense air passes through a turbine wheel that drives the compressor side via a shaft (just like our turbos). After being expelled from the turbine the air is expanded and the pressure is reduced to about 5-10psi.



On a hot day (90F) with the engine at idle (not much air from the engine and no airflow across the air/air cooler) I can get about 45-50F degree air out of the vents. If I advance to high idle I can get the temps into the 30's. Nothing else changes except the pressure ratio from one side of the ACM to the other. If I get some airspeed and airflow over the A/A cooler I'll see a jet of "fog" coming out of the vents. At this point warm air from the hot side is being pumped into the cold side to keep things from freezing up. No freon, just a pressure drop.



So, I'm with Hohn on this one. I'd say that with more boost (i. e. higher drive pressure) the difference between pre and post would be greater due to the pressure drop.



Paccool, you can settle this: what pre/post temps do you see at idle (after a couple of minutes to let everything equalize)?



And of course, no disrespect to those with previously stated opposing views

 

[Hohn]

Justin,



I must very respectfully disagree with you on this point. In my opinion, the temperature pre-turbo is higher specifically because it's combustion chamber discharge air close to the exit of the combustion chamber and it contains a lot of energy. Some proportion of that energy is transformed by the turbo into work, which causes a drop in temperature across the turbine (and a drop in pressure, of course).



Again, I say this very respectfully and I'm not just trying to say you're "wrong". I'm simply disagreeing. Respectfully.



-Ryan

P. S. - Did I mention I mean no disrespect?

Ryan-- If I'm wrong, I'm wrong-- I can take it. No need for kid gloves and self-esteem precautions hehehe-- I appreciate the gesture, though.



I should probably clarify that while EGTs pre-turbo are hotter because they are closer to the chamber, I think that we commonly underestimate the effect of restriction that the turbo has. Again-- look at the cross-sectional area of the turbo's scrolls and compare that to the exhaust elbow-- Which is more restrictive? The 12cm housing is roughly equivalent to a pipe only 1. 56" in diameter. How does that compare to a 3. 5" exhaust elbow? Yeah-- the elbow flows over 4 times more!



I guess what I would say by way of illustration is consider a NA diesel with no turbo. Does anyone think we would still see a 300-400 degree temp difference in a matter of 10-12 inches of pipe length??



As the boost pressure comes up, the restriction posed by the turbo increases (it has to in order to extract the increased energy it requires to give more boost).



The difference between pre-turbo and post-turbo EGTs can DIRECTLY be correlated to the pressure differential between the exhaust manifold and the downpipe.



At least, that my opinion. Others much smarter than I am are sure to have different ideas.



Also meaning no disrespect



JLH

 

[rbattelle]

Justin,

I wholeheartedly agree with everything you said in the last post.



[BTW, these days I find it necessary to be very careful around here when disagreeing with someone... some people get very offended very easily!]



-Ryan

 

[NIsaacs]

Stan, this is some good info, however it is mostly relevant to your truck, with your mods,your guages with probes in your locations and your driving habits. If you have the time I would like to see the results of a similar test, with the guages switched, new one post and old one pre.



I would also like to say that the temp drop on the down shift, was more due to lower engine out put than an increase in RPM. If you kept your foot in it after the down shift, the temp would have stayed the same or increased.





"NICK"

 

[paccool]

Guys, I didn't post this to get a war started. Just thought it was some good basic data as I've heard there was about 300 degrees difference.



PC12driver - As I said in the post, at idle the temps were basically the same.



Nick - you are right about "keeping my foot in it". I was trying to keep a constant speed and since there are always questions about towing in or out of OD felt it sorta answered that question. (At least for a first gen)Had about 5500 pounds with the trailer and the small truck on it so it was a indicator. A 150 degree drop in EGT's is significant when towing up a long hill as far as transmission and engine temps go. Same hill and same load, higher RPMs and lower EGT and boost numbers.



Hohn - the readings were stable state as much as possible. I didn't take readings when accelerating, only when they had a chance to even out.



The direct reading is much slower than the new electronic one. Is it due to being behind the turbo? I don't know but I doubt it. I can switch the leads and retake temps but it will be a while. I developed a head gasket water leak on this last trip and need to get that fixed first. Didn't affect the engine, was just leaking a small amount of water to the outside edge of the head gasket.

 

[BGlidewell]

does'nt the turbo act as a large heat-sink too?