Pyrometer safe temps

[davidkerryedwards]

Check out this exchange about EGT's from a Mercedes diesel forum:

This is a totally coherent rambling of the apparent inaccuracy of the so-called
"safe EGT"
threshold that has been disputed and talked about on nearly every diesel and
performance
car oriented website and forum on the internet. This weekend a friend and I
removed the
pyrometer from my 2001 Powerstroke and installed it in his 1989 F-150 with a
turbocharged 300-6. This is an accurate and truthful collection of findings that
came from
this experiment. I feel that this totally debunks the "safe EGT" limit that we
have all heard
about. The magic number I'm referring to is 1240 degrees or thereabouts.

On the diesel engine, the pyrometer reading at idle hovered between 210 and 275
degrees
depending on electrical/AC load. The gasoline engine idles at nearly 640
degrees. Ok, no
big deal. The big change comes when cruising and when loaded. While cruising
down the
interstate at 75 MPH, the diesel consistently runs between 600 and 800 degrees.
This is no
load, level road surface. Under load, the temps can easily climb to 1200
degrees, which is
about 100 degrees hotter than I ever run it (since I've fallen into the 1240
degree myth).
On the other hand, the pyrometer is COMPLETELY USELESS in the gasoline engine
because
the needle is PEGGED past 1500 degrees AT ALL TIMES. When heavily loaded (8400
lb.
GCVW trailer, toys, and truck), the EGTs do fall to 1400 degrees due to the
11.5:1 air/fuel
ratio my friend has his tuning set at under WOT with 6 psi of boost. Just
judging by the
ease of "pegging" the gauge, it is safe to assume that an EGT of 1600 or 1650
degrees has
been reached. Keep in mind that as I write this, we are on an 80 mile trip and I
haven't
seen the the pyrometer gauge come "un-pegged" for at least 30 minutes.

Now it's time to talk about the physical differences/similarities between the
two engines
and why the EGT does not seem to matter.

First off, both engines use a specific fuel and air mixture to cause a
controlled expansion
of burned gasses by way of combustion. Obvious differences are that the diesel
engine
uses a precisely metered amount of fuel to control engine speed and power while
the
gasoline controls both air and fuel. Internet sources (taken with a ton of salt
obviously)
claim that the BTU of diesel fuel is somewhere between 170,000 and 180,000 while
gasoline is somewhere between 120,000 and 130,000 BTU. Air/fuel ratios of a
diesel
engine vary from close to 100:1 at an idle to anywhere lower than 20:1 depending
on
tuning. Gasoline engines on the flip side need to run between 11.5:1 and 16:1 to
operate.
Too rich and she misses, too lean and she misses. Either end of the spectrum
creates a
loss of power. It is interesting to note, however that the EGT of a gasoline
engine rises
during LEAN conditions and cooler during RICH conditions. This is completely
opposite of
our diesel friends. Under a steady cruise with no boost and an air/fuel ratio of
14.5-15.3:1
the EGT of the gasoline engine was NEVER less than 1500 degrees.

Obviously, both the diesel engine and the gasoline engine use ALUMINUM pistons.
This
fact is where the "magic" 1240 degrees comes from. Sure, the aluminum might just
become soft and pliable at a consistent 1240 degrees. However, the ENTIRE PISTON
IS NOT
1240 degrees. Not even the head (top) of the piston is a CONSTANT 1240 degrees
because
every other crankshaft revolution (every time the intake valve opens), you get
some nice
cool(er) air coming in. Let's not forget that aluminum is great at dissipating
heat and
transferring it to other places. The skirt (sides) of the piston are in contact
with a thin layer
of oil and then a water-cooled cylinder wall. The underside of the piston is
exposed to air
in the crankcase that is not 1240 degrees either. Add that to the fact that most
every
diesel engine uses piston cooling oil jets that spray oil at the underside of
the piston
head....again, not 1240 degrees.

The EGT is just that, Exhaust Gas Temperature. The temperature is not an
accurate
depiction of PISTON TEMPERATURE. I can easily understand that if temperature is
maintained too high for too long the heat cannot be dissipated fast enough.
However, how
much heat does it take and how long?

The 300 maintains a 1500+ degree for a long time. The gasoline piston is thinner
and
lighter than the diesel piston. The 300 DOES have piston cooling oil slingers
drilled into
the big end of the connecting rods, but nothing precisely aimed or really
pressurized. The
engine doesn't suffer any ill effects from running this temperature and like I
stated before,
I have seen this truck push 1500+ degree EGT for 30+ minutes straight.

Makes you think.

From now on, I'm not going to worry about my EGT.
__________________

Kerry
82 Fc 35
Denver


--- In WanderlodgeForum@yahoogroups.com, dspithaler@... wrote:
>
> Nice site Kerry Thanks I am always worried about that. My Truck Garage
> said it could melt down above 1200%.
> Don
> 89 SP 36'
> Butler, PA
>
> This site says 1300 degrees max:
>
> _http://flashoffroadhttp://flashoffrhttp:...://flahtt_
> (http://flashoffroad.com/Accessories/Gaug...st/EGT.htm)
>
> Kerry
> 82 Fc 35
> Denver
>
>
>
>
>
> [Non-text portions of this message have been removed]
>