Why is MPG lower in with LLY engine

[Dave]

From personal expeariance the average mpg on the LLY engine is lower the the LB7. Both my trucks have been 3500 Crew Cab duallys, yet the LLY gets about 2~3 mpg less under the same driving conditions. Reading the other posts on here is seems that is what others a expeariancing also.

Is it due to the Cat?

Is it due to the EGR?

Both?

Or is it just due to the different ECM programing?

[More Power]

More than likely, the slightly lower fuel economy for the LLY is due to a variety of reasons, with the biggest one being emissions.

A couple of looming prospects for all diesel engine manufacturers in the next few years is cooled EGR - where exhaust gases are routed through the intercooler, and using fuel to cool the exhaust/combustion flame temperature to control NOx. These emissions control methods are discussed in the Duramax SAE paper dated a few years ago as theoretical possibilities that would allow meeting the more demanding regulations in the years ahead.

Routing exhaust through the intercooler means that soot will also be passing through the IC. Using fuel to cool the exhaust/combustion temperatures could affect fuel economy. High pressure fuel systems make this a possibility - in that engineers could program the system to inject a tiny pulse of fuel during the exhaust stroke.

MP

[G. Gearloose]

Yickky...
Won't soot in the intercooler quickly compromise its cooling efficiency?

[More Power]

Luckily, the Duramax is relatively soot free, but it could accumulate over time. I don't like the idea either, but more demanding emissions control methods could mean the difference between having new on-road diesels and not.

Beginning with the LLY, GM began routing crankcase gases into the air intake system (sort of like the 6.5 does), where the LB7 crankcase was vented to atmosphere. Crankcase gases also contain some oily vapors, which coats the internals of the air intake system - including the IC. An oil coating can affect heat transfer by some small amount.

MP

[richp]

I also wonder if some of it could be injector design. When my injectors were replaced (98k miles, 2001 K2500) my fuel economy dropped -- in my case by a solid 2 mpg. Others have reported similar decreases when they had the new style injectors installed.

Just a thought......

Rich Phillips
Member #27

[FLEETMANSPH]

Hello everyone, also the LLY is a 1600 RPM Torque
engine. Thats why the six speed Allison is going to help the LLY fuel MPG. The LB7 is a 1800 RPM
engine.

[G. Gearloose]

Good chance they reflashed your ecm when they did the injectors, also, no?

[richp]

Yes.

[Kennedy]

I've had good mpg results with my LLY. As good or better than my LB7. I haven't been able to do a valid tow mpg comparison, but the LLY seems to be in there, possibly a little behind, but not by a lot. I got 12.6 pulling my tractor and trailer. LB7 may have done better. I'll have to run the same route again and see how it does...

[richp]

I'm one of those crazy compuslive types that tracks fuel consumption and vehicle cost. I log every tank of fuel, its cost, and the mileage, as well as all maintenance, insurance, and other costs. Then periodically I crank those numbers into a crude spreadsheet that gives me not only fuel economy on each tank, but also overall fuel economy and overall operating cost. I know -- I'm sick.

Results are that as soon as I had the new injectors installed, non-towing mileage dropped about 2 mpg (I'll be doing a little towing this coming week and will have a handle soon on any changes there). Could be the re-flash, could be injectors -- I don't know. But I do know that I'm disappointed.

Now because I know somebody is going to ask, I'll tell you that my overall operating costs for the first 95,000 miles were $0.53 per mile (that's including cost of truck, sprayed bedliner, topper, tires, insurance, fuel, oil, filters -- everything needed to keep the truck on the road). And the overall fuel economy was 13.51 mpg, about 60% of which was towing a 12,000 pound 32' fifth wheel trailer. Worst was 6.58 mpg towing all uphill fast against a headwind. Best was 21.25 unloaded, with a cap, and not in a hurry.

For what it's worth.

Rich Phillips
Member #27

[Quack_Addict]

MP,

Wouldn't injecting fuel into the cylinder during the exhaust stroke (to cool the exhaust temp) only add HC to the emissions?

My experience with gassers says that playing around with ignition timing may hold promise as well. Adding ignition advance increases power and fuel economy - at the cost of increased HC and NOx. Taking away advance (retarding) reduces HC and NOx but increases EGT's...

[Jim Brzozowski]

I always thought that Nox was created due to the high temperature of the combustion gasses. Seems to me the only way to reduce Nox is to lower the temperature of combustion. Once its present, I don't think lowering the exhaust gas temperature will remove it. Thats a job for the converter isn't it? If it were as simple as lowering the exhaust gas temperature before it exited the pipe, we'd all be running finned exhaust pipe to help pull off the heat before it got to the end of the pipe. I may not understand all I know about it but I'm not real sure who does anymore.

[DieselEnvy]

EGR cools the combustion gases by introducing a small amount of "supposedly inert (or nonreactive)" exhaust gas into the combustion chamber......not by cooling the exhaust gas itself.

Seems counterintuitive to add "hot" exhaust gas to "cool" a mixture.........but whadda ya gonna do?!

[OC_DMAX]

In recent years, the emission goals for Diesels has been to reduce soot and NOx. Unfortunately, the two require almost opposite approaches. So while you lower combustion chamber temps with the EGR, you raise soot because of non-optimal combustion temps (so they add a CAT). Bosch has a decent book out on Diesel Engine Management with several chapters dedicated to current and future emission requirements and what the system solutions might be.

[More Power]

QA, Injecting fuel during an exhaust stroke might not raise hydrocarbon emissions. Some of it will be recycled in the EGR gases and most of the rest will be burned in the cat. I also suspect that this strategy injects only a tiny amount of fuel. In any case, it could affect fuel economy by some amount unless efficiency improvements are made in other areas. I should add that using fuel to cool combustion temps is theoretical, and I have not heard whether GM will make this a reality in the Duramax now or ever.

For what it's worth, I've heard that the 2006 engine will get a bump in max boost pressure - from about 18-20 for the LB7/early LLY to 30 psi. This excess air is probably there to reduce soot and NOx.

MP

[tennykimble]

wow, u guys are getting really technical about 2 mpg!! u know u dont get something for nothing!! the lly has more hp and tq than the lb7 do u really expect it to consume the same or less amount of fuel WHILE producing more power ?!?! i am a diesel mechanic (for 10 years), i get customers all the time that want more power out of their engines, but they think twice when i tell them it will consume more fuel, im a mechanic NOT a magician!! their is NO magic horse power screw, that doesent increase fuel consumption!! gm did a fantastic job of increasing the power that much, and only loosing about 2 mpg ! but hey, u guys do make good points about mpg loss. no i do not own a d-max, im waiting to see the new 06' then i'll make my decision. :D :cool:

[madmatt]

often times when done in moderation and operated reasonably, a small increase in HP will usually increase fuel milage due to the fact the engine isn't operating @ it's "lug" (peak torque range) point as often. I belive I read in another post of yours you work for a Cat dealer??? Most all E models I've rerated from 475s to 550s usually picked up a couple mpgs and I've seen C-16s get just as good. The reason being that they weren't being operated in the 1250-1300 range, where they consume the most fuel during a pull, as often as before. As in most cases, I've found the operaters foot plays the biggest part in fuel consumption.

[gmcbman]

My LLY 05 is getting 3-4 mpg better than my 02 got on its best day. But the 02 never seemed right. 18.5-19.5mpg from new truck, never saw more than 14 from the 02

[ktmrfs]

Friend has a 03 LB7 CC/SB 4x4 I have 04.5 LLY cc/sb 4x4. We both tow identical 7x14 enclosed bike haulers to races. Guess what. our mileage is virtually identical towing and unloaded when we drive the same route together. 14 or so loaded, 19-20 unloaded.

[mdadgar]

Originally posted by ktmrfs:
Friend has a 03 LB7 CC/SB 4x4 I have 04.5 LLY cc/sb 4x4. We both tow identical 7x14 enclosed bike haulers to races. Guess what. our mileage is virtually identical towing and unloaded when we drive the same route together. 14 or so loaded, 19-20 unloaded. I'm baffled. How fast are you running unloaded? I get about 16mpg max in my '02 LB7 CC/SB 4x4 running at 70mph or so.

- Mark

[DarylB]

That sounds consistent with my 2002 EC 4x4. I've had a warn Winch/Transformer Mount on the front since new. Best was 19.2 mpg (about 2 or 3 tanks) with a consistent 16-17mpg driving 70-75mph.

[ 08-10-2005, 04:55 AM: Message edited by: DarylB ]

[Driveshaft]

I still get 19 to 20 with my 2001 EC LB 2wd at 70 mph empty on highway, 15 to 17 city, when I can keep my driving habits in control.

[JD Diesel]

I get about high 15s and low 16s with mine on the high way. JD tongue.gif

[ktmrfs]

Originally posted by StrangeEngine:
</font><blockquote>quote:</font><hr />Originally posted by ktmrfs:
Friend has a 03 LB7 CC/SB 4x4 I have 04.5 LLY cc/sb 4x4. We both tow identical 7x14 enclosed bike haulers to races. Guess what. our mileage is virtually identical towing and unloaded when we drive the same route together. 14 or so loaded, 19-20 unloaded. I'm baffled. How fast are you running unloaded? I get about 16mpg max in my '02 LB7 CC/SB 4x4 running at 70mph or so.

On two rounds trips from Portland Or, to Great Falls Mt, (1800 miles round trip) over lookout pass, rogers pass etc. cruis control set at 70 worst tank was 18.2 MPG, best was 19.8. Overall average for both trips was just over of 18.8. On every tank I fill it to the top of the filler neck so I get consistent fill for averages. Interestingly mileage to Spokane which is flat is not noticeably better than over the passes between Spokane and Great Falls. Bump it up to 75 and I will loose over 1 mpg.

- Mark </font>[/QUOTE]

[REDTRUCK05]

Originally posted by FLEETMANSPH:
Hello everyone, also the LLY is a 1600 RPM Torque
engine. Thats why the six speed Allison is going to help the LLY fuel MPG. The LB7 is a 1800 RPM
engine. Is this going to a 2007 change?

[REDTRUCK05]

Originally posted by JD Diesel:
I get about high 15s and low 16s with mine on the high way. JD tongue.gif Im with you... on the interstate going 70-75 unloaded i get 14-15. going 60 i get 19. But who travels at 60 anymore?

[Stage1]

A little more info, '03 3500 CC 2 wheel drive ( everything stock, except for Michelin XPS tires). Key factors for me are high speed and hills. On level grade at 60 to 65 it gets just under 20 MPG. At 55 MPH w/hills and 75 MPH on interstate 15 to 17 MPG. This is w/ good summer fuel. Also, my milage is improving as I get more miles, I am now approaching 20K, I use it mostly for towing.

FYI, lowest is about 9 MPG something, towing about 14K lb. trailer up W. VA mountains, 2nd gear at about 45MPH ( semi's are creeping up at 5 to 10 MPH).

[stock05hd]

Originally posted by gmcbman:
My LLY 05 is getting 3-4 mpg better than my 02 got on its best day. But the 02 never seemed right. 18.5-19.5mpg from new truck, never saw more than 14 from the 02 how do you such great mpg?? 18.5-19 i get 15-16 at best

[rob@rone.ca]

I find it depends a lot on whether you believe the aforementioned DIC, or whether you believe the averages you get when you fill the tank evenly several times, and compare that to actual mileage driven.

Under good circumstances, for level highway driving at about 60 mph or so, once warmed up the computer will show as high as 20 US mpg or more.

If you include warmups, idling, the inevitable starts, stops, hills (imagine packing four tons up a hill), headwinds, backing up!!, heavy loads, heavy foot, low or high speeds, etc, pick any three, and it can drop dramatically.

Compared to my old 93 6.5 TD 8600 GVW 4wd standard (averaged 19 empty), my new 3500 LLY srw 4wd allison 9900 gvw gives me about 2 mpg less in identical mixed driving for now before breakin.

I can easily attribute that to losses in the automatic, and the extra weight. If it improves at all after breakin, I'll be real happy.

[REDTRUCK05]

Originally posted by richp:
I'm one of those crazy compuslive types that tracks fuel consumption and vehicle cost. I log every tank of fuel, its cost, and the mileage, as well as all maintenance, insurance, and other costs. Then periodically I crank those numbers into a crude spreadsheet that gives me not only fuel economy on each tank, but also overall fuel economy and overall operating cost. I know -- I'm sick.

Results are that as soon as I had the new injectors installed, non-towing mileage dropped about 2 mpg (I'll be doing a little towing this coming week and will have a handle soon on any changes there). Could be the re-flash, could be injectors -- I don't know. But I do know that I'm disappointed.

Now because I know somebody is going to ask, I'll tell you that my overall operating costs for the first 95,000 miles were $0.53 per mile (that's including cost of truck, sprayed bedliner, topper, tires, insurance, fuel, oil, filters -- everything needed to keep the truck on the road). And the overall fuel economy was 13.51 mpg, about 60% of which was towing a 12,000 pound 32' fifth wheel trailer. Worst was 6.58 mpg towing all uphill fast against a headwind. Best was 21.25 unloaded, with a cap, and not in a hurry.

For what it's worth.

Rich Phillips
Member #27 What was your average speed?

[REDTRUCK05]

Originally posted by kenny:
wow, u guys are getting really technical about 2 mpg!! u know u dont get something for nothing!! the lly has more hp and tq than the lb7 do u really expect it to consume the same or less amount of fuel WHILE producing more power ?!?! i am a diesel mechanic (for 10 years), i get customers all the time that want more power out of their engines, but they think twice when i tell them it will consume more fuel, im a mechanic NOT a magician!! their is NO magic horse power screw, that doesent increase fuel consumption!! gm did a fantastic job of increasing the power that much, and only loosing about 2 mpg ! but hey, u guys do make good points about mpg loss. no i do not own a d-max, im waiting to see the new 06' then i'll make my decision. :D :cool: Good posting.. But as you say... "you do not own one". If everytime they do this we loose 2mpg what happens?