EGT Figures for Reference

[ISX]

Well I got to admit ISX is right stay in 4th gear 1:1 ratio set the cruise for 53 MPH (2K RPM's) towing the Jayco and made new record 13.1 MPG when I got home... That climbing a 6% grade with tight corners (25 MPH) in 3rd Gear. Towing a Travel Trailer (GCWR ~15,000) EGT: 600-800*F Ambient Temp: 85F Humidity: 12% Boost: 2-15psi Speed: 53mph RPM:2000 IAT Was locked at 140*F...

Right about what? All I can think of is I told you I go a lot faster when I pull the trailer so that I am never anywhere near lugging it Works really good for me. Maybe you noticed your EGT's never become a worry when you do that. When I do 70 (2000RPM) I can hold it at 70 up any hill and am lucky to see anything over 900. I would do as you but 2000RPM in 4th on mine is not exactly moving lol.

[Mopar1973Man]

Hmmm... Playing with ISX idea... I went into town today and cruised 55 MPH and took my time EGT's hovered about 500-550*F downhill to Riggins, ID. Now once I was done came back home the same road set my cruise for 55 again but instead of 5th gear I grab 4th gear for the grade back up. This put me at about 2,100 RPM's but EGT's hover at 550-600*F for the trip home. But typically same rate of speed in 5th gear would turn out 800-850*F for the climb home.

[ISX]

Hmmm... Playing with ISX idea... I went into town today and cruised 55 MPH and took my time EGT's hovered about 500-550*F downhill to Riggins, ID. Now once I was done came back home the same road set my cruise for 55 again but instead of 5th gear I grab 4th gear for the grade back up. This put me at about 2,100 RPM's but EGT's hover at 550-600*F for the trip home. But typically same rate of speed in 5th gear would turn out 800-850*F for the climb home.

Hook your TT on and do the same test... Without any real load the engine just ends up using more fuel to turn more RPM's since it isn't using any actual power to move the truck 55mph, just using fuel to turn the engine faster. The EGT's would be even higher but 4th gear has more leverage so it keeps them just a little over 5th gear's EGT's. The turnaround point comes when you hook on to a load.

When the truck is in 5th gear pulling 20k lbs (drastic to get the point across) up a 7% grade at 55mph, he is gonna be pulling maybe 1600RPM's, black smoking like crazy, melting pistons, everything, but the turbo will be maxed out at wastegate pressure. Exploring this deeper as to why his EGT's were high enough to melt the whole engine, we get to a simple formula that states that more air going into the engine means more air going out of the engine, which means more heat expelling out the exhaust. The other part of the formula states that fuel quantity does the opposite, raising the combustion temp as more is added. If we come up with a chart showing how much fuel it takes to move an unloaded truck a set speed at different RPM's, you will get a baseline. You will see engine turning horsepower is exponential, starting low and then getting steeper. Engine flow rate based on just cylinder displacement will increase linearly. Boost will be relatively low (remember we are talking constant RPM's, not acceleration). Because there is hardly any boost, the only way to remove heat is with RPM, but with no engine load (no TT), the engine isn't making much heat and therefore the effect is not seen much, if at all.

With this baseline, we can compare it with trailer towing and you will see something completely different. The engine turning HP (with TT and all) will look the same (though it will be higher numbers), the engine flow rate will be more because you are adding more fuel, more fuel means more heat which means more expansion and therefore more flow, why else would you get more power from more fuel. If you think of a steam engine it would be like putting a drop of water in or a cup of water, obviously the drop won't do anything but make you think someone let the air out of your tire. Because you get more flow, you get more boost, which loops around back to the intake and gives it more air to get hot and expand more and get even more flow until it all equalizes out. The boost will be much higher than baseline because of all the expanding air from the fuel combustion.

Now what does this mean for changing gears. The key here is engine flow.. The effect seen with an unloaded truck is reversed when loaded (though the principle is the same just not seen much, as I stated). In 5th gear the RPM's are low, so there isn't much heat removed solely from engine flow itself, the boost helps it out and allows that hot air to be shoved out. The fuel injected per power stroke is high. The high fuel and low engine flow can cause the black smoke if boost isn't there to make up for the loss in engine flow.

Stick it in 4th, now you have engine flow. The pistons are shoving all the hot air out, though boost isn't as much, it is overcompensated for by engine flow. The fuel injected per power stroke is lower, further helping the EGT's remain lower.

IF you were able to raise boost to the point of making up for the loss in engine flow, you could remain in 5th gear. Obviously as load increases, boost will need to increase, because you will be using more fuel so need more boost to compensate for....... Most of the issue is getting it to make boost during any situation/load. And so the VGT was born, the rest is history.

If I'm wrong let me know

[Mopar1973Man]

Make sense...