6V92TA high altitude performance
I drove my 1991 U300 with the 6V92TA DDEC controlled engine in the mountains around Santa Fe and Taos this fall and noticed a few things.
1. The turbocharger screams at high altitude. At low altitude I hardly ever hear it unless I have my head stuck out the window to back into a campsite or something. I presume the turbocharger achieves a much higher rpm in thin air than it does in dense air. The exhaust pressure on the turbine would be the same at any altitude but the density of the air on the impeller would change dramatically from sea level to 10,000 feet, which is probably what makes the turbo scream at high altitude.
2. There's a tiny puff of black smoke from the tailpipe for a moment when accelerating from a standing stop at about 8,000 feet and higher elevations. The puff of smoke goes away instantly as the turbocharger spins up. I hardly ever see a puff of smoke at lower elevations unless I really stomp on the accelerator from a standing stop.
3. There appears to be no loss of power at elevations up to 10,000 feet. I didn't get an opportunity to go any higher than that this time around. While climbing a steep grade from Española to Santa Fe a huge new tag axle bus with a Cummins and no toad got a full power run for it at the bottom of the hill and attempted to pass me on the uphill grade. I climbed the hill on cruise control at 60 mph without touching the accelerator, just like always. The much larger coach made it almost up beside me before he started falling back dramatically. He had fallen back about 1/8 mile or more by the time I cleared the top of the grade. I presume his Cummins was a 400-600 horsepower turbocharged model with a computer controlling it, because most of the new engines in big coaches fit that description. I realize the other coach probably weighs twice as much as mine, but he's supposedly got twice as much power. My 6V92TA with DDEC is set for 300 horsepower and is multistage turbo/supercharged, which basically means the air from the turbocharger then gets compressed a second time by the supercharger. The air gets cooled twice in the process also. After thinking about this for a long time I believe the DDEC computer will tell the 6V92TA to make 300 horsepower at any altitude, and the 6V92TA is easily capable of making 300+ horsepower, even at 10,000 feet. It's probably capable of making twice that power at sea level. I think what I was experiencing at high altitude was an engine that for the first time since I've owned it finally got to perform at the power level it was designed for, instead of the DDEC computer holding it back. Apparently the DDEC keeps adding more fuel until the engine makes 300 horsepower, no matter what the altitude. This makes me think the U300 with 6V92TA DDEC engine is the ultimate high altitude performer of motor coaches. The other engines of its vintage are non computerized and they're set for 300 horsepower at sea level. They would produce dramatically lower power at high altitude. The newer engines that are computerized are also set to produce their 400-600 horsepower at sea level. All those engines are probably maxed out at whatever horsepower they're set for at sea level. By the time they get up to 10,000 feet they're probably not able to produce anywhere near their rated power. The 6V92TA with DDEC is probably capable of producing twice as much power as its 300 horsepower setting at sea level, and that's probably why I experienced no power loss at 10,000 feet, because the DDEC computer doesn't care what altitude it's at, it just keeps making 300 horsepower. When I was at Stewart & Stevenson, the Detroit Diesel / Allison service center, they told me they can easily turn up the power on my engine just by plugging their hand held computer into the DDEC computer and telling it to make more horsepower. They said there's no need to change injectors, fuel pumps, turbocharger, or anything. I was considering turning it up to 450 because that's apparently what all the fire engines with 6V92TA engines are set for and we had a stationary 6V92TA where I worked that was set for 450 and it worked great. Based on the high altitude performance I experienced I don't know if it even makes sense to do that. The DDEC computer is gonna put the 6V92TA flat out on high altitude climbs, which is where the power is most needed. The rest of the time the engine is just barely working... Anyway it was interesting to experience the performance it seldom gets to use.
Have any of you folks noticed these things with your 6V92TA at high altitude? If you haven't actually driven a 6V92TA at high altitude please point out that you're speculating and not speaking from experience... but I still wanna hear from you 
Topic: 6V92TA high altitude performance