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Duramax 6600 Discussion Forum for the Duramax 6600 Diesel Engine, including the LB7, LLY, LBZ, LMM & LML engine specific topics.

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Old 12-24-2003, 08:40 PM
64postcarnut 64postcarnut is offline
Join Date: Dec 2003
Location: Aurora,Co.
Posts: 27

I'm familiar with the hp loss ratio of 4%/1000ft as it applies to a gas engine.So,here in Denver I lose 20-24%.

Does this same rule apply to diesel engines?
\'04 GMC,White,EC,4x4,LB,D/A<br /><br />Dave Walters<br />Aurora,Co.<br />
Old 12-24-2003, 10:24 PM
FisHn2DMax FisHn2DMax is offline
Join Date: Nov 2002
Location: Utah
Posts: 250

Yes, there is altitude power loss in a diesel. However, based on my own personal experience, the altitude loss is not as pronounced in a Diesel? I live and drive in a high altitude mountain area in Utah. My diesel doesn't seem to lose much power compared to my gas vehicles in higher mountain passes? My theory as to why is due to how the Diesel burns, and the use of a turbo. Maybe someone here can explain the exact science as to why the Diesel doesn't seem to follow the gas altitude power loss rule of minus 4-6% HP per every 1000' rise in elevation? The diesel like all piston engines will have some power loss the higher in elevation.

Good Luck!
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Old 12-25-2003, 05:29 AM
Posts: n/a

The reason that the diesel loses less is the basic principle of the engines. The gasoline engine uses a fuel/air mixture, which is either generated volumetrically in a carburator or throttle body. When the number of air molecules per volume decreases at altitude, the computer is "fooled" by the equal volume. The diesel compresses a fixed volume of air and then injects fuel at full compression. The reduced air concentration has a lower effect at compression than at ambient conditions. The diesel is also more fuel-efficient, getting more BTUs per unit out of the fuel, which also aids in minimizing the effect. I suppose that the mass air flow sensor will make the diesel run a bit rich, but most diesels don't mind that, they just smoke a bit and rumble on! The air/fuel mixture in a gasoline engine is much more sensitive to losses when running rich or lean.
Old 12-25-2003, 06:21 AM
SoMnDMAX SoMnDMAX is offline
Join Date: Apr 2001
Location: Southern Minnesota
Posts: 1,202

A turbo diesel shouldn't lose much power, if any. As altitude increases, the air gets "thinner", like Tom said- less molecules per volume. This "thinner" air allows the turbo to spin FASTER, creating the same amount of manifold pressure as you would see at say, 800 ft. That is the beauty of a turbocharger.

Put a turbo on a gas engine, you can see the same results- namely snowmobiles. Folks turbocharge mountain sleds all the time to negate the effects that altitude have on tuning and power production. The turbo not only boosts power significantly, but the sled will not lose any power as it goes up the mountain side.
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Old 12-25-2003, 01:18 PM
McRat McRat is offline
Join Date: Aug 2003
Location: Corona CA
Posts: 38


Since there is less air being drawn in each cycle, the gas engine injects less fuel, and makes less power.

With a diesel, it still draws in less air at high altitude. But most diesels DON'T inject as much fuel as they can at WOT. There is actually "unused" air in the cylinder after it fires. This is how the chips work, they inject more fuel to take advantage of the "unused" air.

Then the turbo. It will ram in nearly the same amount air regardless of altitude. Yes it does lose some boost, but not as much as Naturally Aspirated motors do. Belt driven superchargers DO lose significant HP at altitude if they are not overdriven (wastegate design). Turbos generate boost by engine load, while regular supers are RPM dependant.
Tuner of fast "diesel" powered trucks,

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Old 12-25-2003, 03:50 PM
dmaxalliTech dmaxalliTech is offline
Join Date: Feb 2003
Location: Holland, MI
Posts: 697

The Dmax uses a Baro sensor that should be able to compensate somewhat for altitude changes.
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Old 12-26-2003, 07:29 AM
KevinK KevinK is offline
Join Date: Aug 2001
Location: Lafayette, CO, USA
Posts: 30

I asked the same question over on the TDR a couple of years ago and was told by one person that the loss is about 0.5% per 1000ft due to less oxygen (basically what was said above). I have noticed that as you gain elevation, you lose boost. While I hit 30lbs. around town, I only hit maybe 28 (or less) at higher elevations. The biggest affect I've noticed is higher EGT's. If you've got a chipped truck, the EGT's can quickly get to 1400 or 1500 on a hard pull. I can run only about 1/2 throttle up the passes on I-70 because of it (pulling a 10k trailer at 60-65).

Pulling roughly the same weight over the same passes, my stock diesel (at the time) would run cricles around my modified 454.

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Old 12-26-2003, 08:49 PM
BlueOx03 BlueOx03 is offline
Join Date: Jul 2003
Posts: 67

You could run into a slight if even noticeable loss of power if you drove strait from a low altitude to a high one without shutting down. Like Eric said the D-max uses a barometric sensor to detect and compensate for low barometric pressure. The barometric pressure is read at key on, before the engine is started via the boost sensor. As stated before though, it shouldn't be any big difference.
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