Did you see that article about gasser EGT's? The needle stays pegged above 1500°F. all the time.......from any engine speed above idle.

What's up with that?

Nope, I missed it. Have an an URL?

BTW - since MB turbodiesels have oil-cooled pistons, I would strongly encourage using a good synthetic oil like Mobil-1, Amsoil, or Red Line, especially if you're pusing power beyond stock levels.

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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.

Thanks Lance........I couldn't find it.

Very interesting piece. So, how to explain that picture of the melted piston on the EGT thread from a few months ago?? Does the fact that gasoline explodes and diesel burns make any difference in the effect of the final temperature on the pistons?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I think there is a failure in that line of reasoning. We should still worry about egt's. The problem is that the pressures in a diesel engine are far higher than the pressures in a gasoline engine. The EGT is measured after the gases in the compression chamber have already expanded and cooled down. The temperatures in the combustion chamber of a diesel engine are going to be significantly higher than the temperatures in a gasoline engine even though the temperatures of the gases in the exhaust manifold are the same simply as a consequence of the higher pressures.
Perhaps there is a physicist aboard the forum who could extrapolate backwards from the EGT's to the combustion chamber temperature differences based on the different pressures in gas and diesel engines.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

or just tell us what temp the piston melts and work back from there.

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Jim

1220 degrees according to what I have read. (the melting point of aluminum).

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

sooo - my friend with the souped up Ford PSD that is doing burn outs with the egt reaching 1200 dF should not expect 300k miles out of his truck engine.

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Jim

Don't forget that the pistons are an alloy, not pure aluminium.

I had thought about that. Anyone have any idea of the melting point of alloy pistons?
The Banks website puts the EGT limits at 1300-1400 degrees.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

So what's the most (read safest) PSI I should be tuning my A/R T3 for?
I'm gonna do a gasket change soon and I'm gonna have my huffer on a bench for a day or two...

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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A Holset Turbocharger won't do a hell of alot. if you want off the line boost, you'd be better to put a smaller turbo in addition to the stock one.
Besides, if you ever fully spooled up a Holset on a 617 it'd make 30PSI in a heartbeat

KABOOM!

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

I have a 1990 dodge 2.5l Turbo engine that's toast. the Turbo still works I'm going to toy with the idea of using that in addition to my T3

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

He is making an actual jet engine out of a old turbo as a project.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine