Most of this is old news to most of you, but there may be someone on here that is not aware....so here goes.

A turbo boost gauge is in my opinion a very important tool. Just watch the boost at cruise, and realize anytime your boost goes up, more fuel goes in and mpg goes down. Hold the rpm where the engine tends to not need as much boost. I suppose that's what the old timer's call the "sweet spot," because it is. That's where the torque is. Yes, you can listen to the turbo, but by the time you hear it spooling up, you're already dumping fuel. I prefer the pressure, as it's more accurate. If nothing else, the gauge will determine the sweet spot on your engine at a specific weight on flat ground. I drive by the rpm and the boost gauge on the highway instead of speedometer. My last truck was 68 mph, and I could run it up to 72 with a light load and still maintain mileage. I did test my truck at 62 mph at the request of the fuel manager, and it got worse mileage at 1400rpm because of the loss of torque. I also tried the same thing with a Detroit 12.7, and it didn't like anything below 1650 for mileage, as the torque would just fall off. I ran it at the ISX rpm and lost .5 mpg. So, with the little knowledge and experimentation I've done, I think the ISX likes 1550 rpm's, and the Detroit 60 likes 1650 rpm's. Firebird, what you said about the charge air cooler is a big variable in all of this, because I've seen pressure gauges that read normal, but the pressure never made it to the engine, and the fuel mileage and power was off considerably. Money well spent for that check.

 

You can cite anecdotal evidence all you want and your experience may in fact be what you say in terms of mpg. However, torque ratings are not subjective nor arbitrary and nowhere will you find an ISX that is making peak torque anywhere near 1500+ rpms. The newer ISX's stay at peak torque to about 1400 rpms before dropping off and the older ones drop off even earlier, with the peak torque range generally between 1100-1400. At the rpms you're talking about you ARE into peak horsepower range, but that's a whole different animal. This information can be found on any torque map that Cummins puts out.

 

No-worries, I don't speak as an engineer, but as one that paid the fuel bill, and one that calculated the actual miles per gallon. I've also driven more than 10 trucks with this same engine, and even tried to run the rpm ranges for torque as you've cited, hoping there was something to it. 1100 rpm is laughable on an ISX 450 unless bobtailing, and wouldn't pull 30,000 pounds down the road. 1400 is lugging this engine under load and will cause the turbo to boost continually if heavily loaded. Maybe with 3.55 gears and slower speed this might be tolerable, but I was running at 70 mph out west with 3.25 gears. Anything higher than a 450 HP I have no experience with, but I would assume you could run a lower rpm because of more torque all things being equal. Whether you are talking about torque or horsepower, there is a correlation to the two when it comes to pulling weight down the highway and torque alone is only one way to measure the power. My personal feeling is that I'd rather have a higher horsepower engine that I could run at a lower rpm, because as you say the curve does fall off, and on an ISX the curve falls off sharply at 1600 rpms, and you can feel it backing up at slightly over 1550. Somewhere close to the edge is my preference, so I guess you can say I'm a horsepower man.........

 

I agree 1100 is way too low. I hear a lot of people that say the isx is a torque monster down low. I drive my dads from time to time. His truck is speced like mine. 435 hp. I got 3.73s and a 435 cat his is 435hp isx with 3.55s. But the trans in that truck the gears are very close. I do know his truck can easy go 1250 miles on the tanks. Running heavy across hills. I personally hate driving the isx. I think it's a gutless wonder but I hear different.

 

With 3.25s, you're geared to run fast and it will fall on it's face at low rpms. There's always a compromise and a truck that is geared to get decent mileage at higher speeds won't be able to efficiently use the torque that's available at the lower end. That same truck would get much better mileage running in the 12-1300 range but it would have to be geared properly which would have it cruising much slower. That truck would be terrible running 70+. The engine isn't the problem but it's only one part of the equation that includes drive-train as a whole and the way it's driven.