High altitude performance question...

[1919a4]

Ran my 2002 CC D/A with Juice 3.5c through Northern Arizona at about 7K elevation, towing about 11K.

I have Isspro boost and pre-turbo pyro gauges, and watched them like a hawk.

Coupla things were interesting during the drive. First off, the performance drop with altitude was very significant. I was surprised, cos my thinking was that the turbo would act similar to that of a turbocharged airplane and keep the same power until one got to real thin air (like over 15,000 feet!). Not so- this one acted like an atmospheric motor and steadily lost power as we got higher.

The Duramax is prolly wastegated to ambient pressure, right? Meaning that the boost pressure you see is the boost pressure above ambient, not absolute. Is there a way to make the Duramax a better performer at altitude or is there a limitation such as the turbo size or manifold design? Would plugging the wastegate help?

If the wastegate was changed so that the boost pressure was kept at an absolute level independent of outside ambient pressure, would the in-cab boost gauge read higher boost as you went up in elevation?

Second, EGT's ran really high the higher up we went. I really had to get out of the throttle above 4,000 feet even if the engine speed was raised by kicking down a gear or two. Would additional boost help this condition? BTW, I was running the Juice at level 2 on T/H.

This has been bugging me and any input would be appreciated. By the way, the D/A runs like a champ with the new Allison installed and a factory updated reflash.

SImon

[FISHDMAX]

I live at 10,000 feet,and have towed 8,000 fifth wheel to the Yucatan and back. I notice very little change in performance between sea level and a 12,000 foot pass. My 2500HD is stock.

[ 10-04-2002: Message edited by: FISHDMAX ]</p>

[Colorado Kid]

Your boost guage reads relative pressure.

I suspect that your wastegate has never opened, so plugging the line would have little if any effect.

I don't have as much ability to observe it, but I know performance of my stock truck does not drop off like a normally aspirated gasser at high elevations. The boost lag gets longer, and more RPM are required to achieve boost, but she pulls like crazy even above 11,000 feet. I wouldn't know about EGT, but engine temp has never budged.

The Duramax measures absolute air pressure in the in the manifold (MAP) and mass of air flowing in the pipe entering the turbo (MAF). As the amient air pressure decreases these values will tend to decrease, and there is only a limited amount the turbo can do to increase them. Also as the ECM trims fuel delivery to prevent smoke it causes less exhaust flow available to drive the turbo...so there is a viscious cycle tending to reduce available air in the combustion chamber. I suspect that the Juice fuel curve is more closely tied to the available air than the stock curve (the sharp corner where the stock torque curve levels out leads me to believe that once there is "enough" air the fuel curve seperates from it stock, so that as elevation increases fueling remains constant until the avaiable air finally falls back through the "enough" point and fuel is then reduced. Perhaps with the the Juice the ECM keeps adding fuel above where the stocker stops, but has to trim it back out sooner as atmospheric density decreases.

Incidently, the only time my truck has been BELOW 4,000 feet was when it was in Montana for the Pull Off. I did enjoy the faster throttle response while I was there (it made it much easier to spin the tires :D ) but I didn't notice any increase in power.

[MountainMax]

My property is at 10,400' and I like to up from there. My truck is stock and performs great at high altitude. I notice better mileage in cooler temperatures. No doubt thin air and less oxygen will affect combustion some. But to me it has not been noticable. I've pulled a trailer uphill through some of the highest passes in Colo. Nothing has ever done it with the ease this truck has.

[dmatt]

I frequently drive from my home at 4,000 feet to the various passes in the area, ranging from 7,800
to 10,200. I have noticed no drop off in performance.

[1919a4]

I'm thinking I got a bunk unit (entire powertrain) from the get-go.

I had to replace (well, the dealer did it) the Allison 500 miles ago due to a failed clutchpack. While the truck was there, they re-flashed the ECM to whatever the new specs are. My truck was a 2/2002 build date.

Now, the truck gets noticeably better mileage, there's no clunks or shudders between shifts (which had been present at the delivery of the truck- thought it was normal for a heavy duty truck with a bus transmission...) and it's noticeably faster. The boost runs right up to 20 psi faster than before.

I haven't feel a significant urge to put the juice back on- truck feels "right spritely" without it, and it was a dog before. It's possible I had some "issues" with the engine as well.

SImon

[stillracing]

I live at 7600 ft. I get better performance at lower elevation. My truck acts just like your truck. I would like better performance at higher elevations as well.

[1919a4]

OK, so if the duramax is really going off ABSOLUTE pressure, the boost gauge should also be reading ABSOLUTE pressure(minus atmospheric). Therefore, the boost gauge should show an INCREASE in maximum boost as the truck's altitude increases.

IIRC, at sea level the absolute pressure is 14.7 PSI. If you go up in altitude, you lose 1 psi per 2000 feet or something like that. Therefore, at 10,000 feet, in order to have the same power, you'd need to be running 5 pounds more boost showing on your Isspro boost gauge, right?

SImon


SImon

[BadDog]

Hmmm, I recently towed an 8k trailer with my toy (rock buggy) plus about 1k in the bed from Phoenix, up through Flagstaff, and on to Moab Utah. My truck is totally stock (see sig) and I noticed very little power loss as altitude increased.

[Colorado Kid]

I'm more familar witht he turbo systems on piston engined airplanes...I'll try to explain how I know them to work, and some observed differences with the D-max.

The Continental TSIO-540 (that's Turbo-Supercharged, Injected, Opposed - 540 cubic inches) as used in Cessna 414 and other aircraft is a Turbocharged and Intercooled, fuel injected flat 6 that makes 325 HP at 2635 RPM at take-off power.

The S in the engine name means supercharged...some airplane engines are turbo-normalized...which means the turbo is not used to supercharge the engine at sea level, but is only used to make up for decreasing air density at altitude. (By the way, airplanes are operated at high altitudes, but our trucks are operated at high elevations, unless they leave the ground.)

Aircraft manifold pressure is conventionally reported in inches of mercury (") (like barometric pressure) and in absolute terms...so with the engine shut down at sea level the guage reports 30" (equivelent to 0 boost at sea level on your ispro).

The TSIO-540 is limited to 38" manifold pressure at take-off power. Roughly 1.25 total atmospheres of pressure. The D-max makes at least 20 psi of boost (14.7 psi = 29.92 "), and that's measured relative to ambient, so if it is at sealevel that's 2.35 atmosphere's...which is a whale of a lot of manifold pressure...would be more than 70" on an absolute manifold pressure guage. The point here is that, given that the two engines of roughly the same output will make roughly the same amount of exhaust, the turbo on the D-max is being asked for a lot more work the the one on the airplane.

The TSIO-540 has an absolute pressure wastegate controller. That meanes that if the pilot selects full throttle the wastegate controller opens or closes the wastegate as necessary to maintain 38", if possible. Up to about 15,000 feet it is possible, after that the wastegate is completely closed and insufficient exhaust flow exists to pressurise the ambient air from approximately 15" to 38" in the manifold.

The TSIO-540 also has a "pop-off" valve designed to open at 42" in the event the absolute pressure controller fails. The pop-off is simply a spring holding shut a valve that will be blow of its seat if the pressure exceeds the capacity of the spring to resist. (This also descibes the operating principle of the D-max's wastegate).

The D-max does not have a complicated wastegate controller. The gate is closed nearly all the time, and all ehaust flow is directed to the turbine. That's why boost pressure is directly coorelated to power...it is an indirect mesurement of the exhaust flow being produced. If you were able to get the D-max's simple mechanical wastegate to open you'd see a sudden decrease in manifold pressure when it happened, accompanied by a drop in power and/or a spike in EGT's. (See More Power's description of his run up the hill in Chris Bigley's truck, where he used to much rpm...in the pulloff 3 article). Since noine has descibed this happening in theier d-max, I don't think any of us has experianced the wastgate opening.

I believe the D-max ECM moderates fuel, perhaps with the intention of moderating boost levels, or perhaps simply to restrict maximum torque. I believe the wastegate gate is, like the pop-off on the TSIO-540, a fail-safe device. WHen someone hotrods the d-max it would be better if the wastegate opened than if the intercooler (for example) ruptured.

Because less exhaust flow is available and more compression is required to achieve the same manifold pressure...and the d-max is already using all the available exhaust flow at sea level to achieve the familiar boost numbers...it does not surpise me that observed boost drops with increasing elevation.

Having said that let me sate that the observed combination of increasing EGT and decreasing boost at higher elevations seems to indicate that the ECM is not pulling fuel just because of the somewhat reduced airflow...or as I siad in my first post, that the D-max fuels at a relatively constant rate once there is "enough" airflow...letting any excess air result in reduced EGT rather than more power.

This supports my "seat-o-pants-meter" observation that, while off boost power is reduced and time/rpm to build useful boost is increased at higher elevations...up to a point maximum power available does not seem to be reduced. If it is reduced it is certainly not to the same degree as occurs in a normally aspirated gas engine.

I used to drive school busses. Went on a trip with two busses, both 78 passenger Blue Bird forward control busses. One had a 534 cubic inch Ford gas V-8 with a carb, rated at 210 HP at sea level. The other had a 190 HP Cummins B series (12 Valve, 360 Cu in). At 5,000 feet, along the front range they were evenly matched. At the 11,000 foot mark on I-70 approaching the Eisenhower Tunnel the Ford was down to low gear, crawling along at 10-11 MPH. It was also buring about 1 gallon/mile at that point. The Cummins had climbed at 45 MPH in third and was just pushing 1000 degees (our emloyer's hyper-conservative limit) on the Pyro at the top. AS we started up the hill I was in front in the Ford, (didn't want to suck diesel fumes). AS my speed sunk below 40 MPH at about 8,000 feet the other driver went around...she waited in the parking area at the west portal...and had just about headed back to look for me when I finally made the summit.