I have been told that EGT should not exceed 1250°F.

However, I've seen various numbers throughout magazines about what they "safe" EGTs are.

I see that aluminum melts at 1220 °F...so I can see where 1250°F might be assumed.

However, the EGT probe only sees the exhaust cycles. But the piston experiences only the EGT for two of the four stroke cycle made in the combustion cycle so the high EGTs would be "felt" by the piston for an extremely short time - particularly as the RPMs increase.

Thoughts?

 

That will be a variable argued by many... why didn't you just start another oil war? Same result.

Many, many factors go into this. Tractor pullers often see 2000*. Other engines melt at 1300* so why?

Ok, so tractor pullers are usually using steel pistons... I know of some duramax's to hit 2000*...

Engine design and materials go a long way.

So our ubiquitous ISB... it has piston cooling jets of oil... lots of it. That oil is in turn cooled by the engine coolant. This raises the effective melt temp inside the engine.

Injection Timing, cylinder pressure, rpm and exhaust flow all play a role.

There really isn't a "set" answer you see. But for your year of truck, completely stock, 1400* is common. I wouldn't be comfortable with that sustained more than 20 seconds or so personally.

Water injection is your friend!

 

There was extensive testing done by Cummins on the early 5.9 engines to see "how hot was too hot" at the Cummins Midrange Engine Plant, Columbus, Ind. in the early 90's. I was told they ran the engines continually as hard as they could monitoring EGT's at both the manifold and post-turbo. This testing showed that the limit on "continual" heat was between 1200-1300 degrees (in manifold) and between 900-1000 post turbo. This goes along the lines on what you mentioned on the melting points and durability of the certain metals in the engine. In my opinion, I think it is pretty accurate but as soulezoo pointed out, there are alot of other "variables" like air temperature, humidity, air flow to the engine, timing etc. One thing is for sure, you won't EVER see me without a pyrometer gauge in any of my diesel trucks and I will always stick to the early "guide" that Cummins provided.

BTW,....as soulezoo also pointed out, water injection works very, very well to keep EGT's under control. I have used it for many years on my trucks.

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John_P

 

Cummins rated the early 5.9L (maybe all their engines?) at 1250F 100% duty cycle (continous).

You can go past 1250 but the duty cycle must be reduced.

 

...not looking to start a war, I just like hearing peoples thoughts so I can get a bigger picture of where these numbers might be coming from.

I didn't opt for the steel pistons when my motor was being rebuilt, but I do have upgraded oil jets and and MeOH injection. And who knows, I'm sure there are different aluminum alloys that have higher melting temperatures. It just hit me this morning that the piston is not exposed to 1250°F continually, so that's why thought I would ask.

I would also hazard a guess that once you start pushing over 1250, if one has a failing (leaky) injector, that's the piston in trouble - even though the driver might only see the combined temperature of all six cylinders on his gauge and believe all is ok.

 

Oh I know you weren't trying to start anything. I was being a little sarcastic expecting all the opinions to fly in with opposing information.

Starting with "well I saw xxxx degrees for yyy time and didn't hurt anything!"

We don't seem to have the "wars" like in years past. I used oil as that seemed to pop up every other week at one time.

And you are right, a leaky injector (usually a stuck injector) can torch the top of a piston in a way you describe; however, when it is that bad, it will ususally show on the pyrometer.

 

Let me add a little to what soulezoo has already said.

He and I have been here on DTR awhile and in addition to the "oil wars" he mentioned I can tell you there was some NASTY transmission wars too!
There were members BANNED over some of those! It has been pretty "mild" here on DTR the last couple of years, but occasionally we will have a problem on a thread.

Like soulezoo said,....sometimes the threads here get those "statements" without much to back it up.

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John_P

 

And that's why I brought up the question...I was wondering what the science was behind the number, because the number didn't make sense to me from a pure science standpoint.

All being said I would guess that EGT maximums have become convoluted by the online experiences. If everyone's cummins' had a temperature probe at each cylinder, we'd more accurately know the true limits. And I wouldn't doubt that there have been the odd improperly installed probes that have added to incorrect information being posted...

 

The way I just see it is to play it safe. Remember the tractor pullers are far from stock.... I don't mind a spike over 1250 for a split second but Whats the purpose of saying 1400-1600 degrees is safe and getting into that habit and wandering why problems occur later on. I live by the 1250 cause i want my truck to last for years.

 

And I totally agree - was just looking to see if there was a science basis behind the 1250, where the number came from...

 

Egts are irrelevant in my truck...it's seen 12-2000* and is doesn't care.

 

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Irrevelant huh?? You will see it is not "irrelevant" if you ever tear your engine down to rebuild it. Those high EGT's effect alot of parts even though you don't think they do.

 

I never get mine over 1150 pre-turbo even pulling my 5er at 15000lbs through the rockies in summer. How? Use that lever that sticks up through the floor. Primary tool to drive by is EGT.

 

I've been running mine 1400 or so the past 80k miles and no meltdown yet. She would go over 1500 as delivered stock!

 

Yeah. They run HOT in stock form. I was easily able to hit 1400+ stock. Chickened out at 1450 one time, but I can't imagine I should be able to hurt it in stock form. Got the hottest over 2500rpm. Once I put the Smarty Jr. on it's hard to get much over 1250, so I figure that's my new limit for myself. I imagine it all has to do with timing. Even though the EGTs are cooler now, that temperature didn't just disappear, it's in the cylinder now.