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Also, infra red guns don't always read the temperature accurately. Emissivity depends on the surface state, the most accurate is a matt black one. I used one to set up a solar HWS and found that spraying a bit of matt black onto brass fittings made a huge difference to readings. They're better for tracking changes on one surface than measuring the difference between two different surfaces.
As Beeutey said.
Most of the slightly better ones(mine was still only $30 or so from ebay) have the ability to set the device for various surfaces.
The manual is fairly useless in describing it in clear English .. but my Chingrish is getting better with each new purchase .. so I worked it out.
I found at idle, after a run getting the exhaust up to temp, the IR gun is about 20 or so degrees lower, sometimes registering the same values.
From memory my 300tdi sits at about 150-180 at idle depending on ambient and stuff like that.
Can't remember if I've ever tried to get a reading post turbo tho.
The Thermoguard website shows that the difference in temps pre and post turbo on a 300 Tdi is about 200°C.
On a tangent, but related side note, I have my VDO coolant sender located in the brass bleed plug at the top of the thermostat housing.
It's numbers at idle correlate perfectly with the IR gun at the thermostat housing temperature(about 80-ish after a good warmup).
Head is generally at the same temps(eg. 80-85 on the whole) but some points on the head read about 90 or so on the IR gun.
The threaded hole in the front of the head down low near the thermostat housing, where many people bolt a head temp probe is always pretty much spot on as the VDO sender and thermostat housing tho.
Exactly,we use them on refrigeration equipment,and care has to be taken,or the readings are inaccurate.Quote:
Originally Posted by bee utey
The turbo gurus on the Patrol forum reckoned that there is an approx difference of 150C between pre and post turbo EGT. I personally never understood why one would put an EGT probe post turbo.
A true reading needs to be taken under full load and over a prolonged duration pre turbo.
I had my egt gauge pre turbo in the actual manifold. It would take more than a minute of load up a long gradient hill to get anywhere near 600c. It would literally come back down to 350-400 with 20 seconds; therefore taking a reading at idle is worthless. Airflow across the turbo under full load can be as variable as 200 degrees, not a typo; 200 degrees!.
150c seems to be a common figure. The temp difference between pre and post turbo does vary a lot over rpm, load, and boost levels. Greater boost means greater pressure drop over the turbine and a greater drop in temp.Quote:
Originally Posted by jbe
Pre turbo has got to be the way to go for a EGT probe. Readings are much closer to what is out of the engine, (going to melt pistons before the turbo) with post turbo EGTs peak temps take longer to develop. I have run pre and post EGT’s on a vehicle a the same time and it will surprise you how quickly pre turbo temps get up there compared to post turbo. The only advantage I can see with post turbo probe is for getting a better indication of turbo temp prior to shutdown
Cheers
Paul
In addition to all the variables mentioned above, consider that the actual EGT difference is a function of the mass of gas travelling through the turbine (hence throttle opening) and the amount of energy being extracted from this gas by the turbine (in turn a function of the boost pressure). This obviously changes all the time.
Then the relation between the temperature of the exhaust gas and the outside of the plumbing will depend on how rapidly the heat is removed from the outside of the pipes, which will largely be a function of airflow, but also depends on conduction of the heat.
My guess would be that it would be almost impossible to find any firm relation between the temperature difference of the input and output piping and the actual EGT!