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I wouldn't be at all concerned about letting the truck reach it maximum recommended RPM during grade braking. This under power/grade braking RPM disparity has been part of the Duramax design since the original LB7 which had a dual marking set on the tach to show the powered and grade braking RPM limits however this set of markings was removed in later years because it created confusion with some customers who thought that the engine was going beyond "redline" under engine braking.

The rotating assembly and valve train components of these engines have no issue handling the higher rotational speed that occurs during grade braking. The relatively low RPM limit under power is simply because power falls off rapidly at higher RPM so there is no reason to allow the engine to run at higher RPM under power. It in no way implies that pieces are going to start flying out of the sides of the engine when RPM goes beyond 4K. The 6.6 is a relatively short stroke design which is equipped with a valve train that can easily handle a relatively high RPM.
 

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I just got.back from May long camping, ran into a situation where I had to floor it, (think it was in second gear), and the thing peeled out! I'm running 35" x 12.5 nittos, trailer is 12k. Way better then my tuned lml.

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Just got back from Apache Junction Arizona. We went from Michigan to Arizona and back around 5,500 miles. Every grade up and down was no problem pulling our 40' 16,000lb 5r. On the way to Arizona got 8.7 mpg but we were running in strong winds on the way back it was 10 mpg going 65mph both ways. This thing pulls like a freight train.
 

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[QUOTE

The rotating assembly and valve train components of these engines have no issue handling the higher rotational speed that occurs during grade braking. The relatively low RPM limit under power is simply because power falls off rapidly at higher RPM so there is no reason to allow the engine to run at higher RPM under power. It in no way implies that pieces are going to start flying out of the sides of the engine when RPM goes beyond 4K. The 6.6 is a relatively short stroke design which is equipped with a valve train that can easily handle a relatively high RPM .[/QUOTE]
====================================

Thank you VERY much for those comments. For those of us who are...well..."no longer spring chickens" this is great news. Some of us remember only too well the consequences of over-reving those longer-stroke motors of our younger days.
 

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IIRC, the earlier 6.6's had a lower 'powered max rpm',
of course it's managed by the ecm and tcm anyway, so the number is sort of academic.
They are smart enough not to hurt themselves.


6.6L L5P Duramax Turbo-Diesel Engine from GM Powertrain

6.6L L5P TECH SPECS

Type .................6.6L V-8 L5P Turbo-Diesel
Displacement.......403 ci
Compression Ratio.....16.0:1
Valve Configuration......Overhead
Valves Per Cylinder........4
Assembly Site..............Moraine, Ohio
Valve Lifters.................Mechanical roller
Firing Order..................1 - 2 - 7 - 8 - 4 - 5 - 6 - 3
Bore x Stroke................103 x 99 mm
Fuel System...................DI with high-pressure common rail
Fuel Type......................Ultra-low-sulfur diesel and B20 biodiesel
Maximum Powered Speed.....3450 rpm
Maximum Braking Speed...........4800 rpm

Emissions Controls....................Cooled Exhaust Gas Recirculation (EGR), Selective Catalytic Reduction (SCR), Diesel Particulate Filter (DPF), intake throttle
Horsepower.......................445 hp (322 kW) @ 2800 rpm*
Torque..............................910 lb-ft (1234 Nm) @ 1600 rpm*
Block................................Cast iron
Cylinder Head....................Cast aluminum
Intake Manifold..................Cast aluminum
Exhaust Manifold................Cast nodular iron with steel pipe extension
Main Bearing Caps..............Cast nodular iron
Crankshaft.........................Forged steel
Camshaft...........................Steel
Connecting Rods.................PM iron
Engine Oil Capacity.............10 qt./9.5L

* As tested in Chevrolet Silverado HD and GMC Sierra HD.
 

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replying to turbo fan and Fast , what was the outcome? I can shed a little input here with my 2017 3500 hd SRW high country . It is not a break in issue , nor is it a manual trans engagement issue. I pull a 3500 lb equipment trailer with a 9000 lb Cat track machine ( bumper pull) . I have the same hill everyday i take back to the shop. Currently have 26k miles on the truck . This thing ripped that hill everyday on the way home and seemed to shift down if needed prior to an occurance at about 22k. I was getting into the hill and all of a sudden a loss of power, almost like a limp mode and it decelerated up the hill topping at about 40mph. I unhooked at the shop and drove home with no issues? I wrote the occurance off as maybe it was doing a re- gen at the time of the hill climb? Now since then , mine is doing the same thing that FAST is stating. It holds 6th gear on the hills at 1700rpm and doesnt drop into 5th , ultimately topping the hill at 40mph. i have floored it to put it into 5th and sometimes it responds and sometimes it does not. i have noticed less grades it does well from 5th to 4th downshifts , but is not dropping to 5th from 6th on the big hill home. I am interested in the post on this thread relating it to a boost leak or air duct issue . I am curious of the outcomes from FAST and Turbo fan. i do not believe it is the # 4 injector as 1 person thought. In the past any of my trucks ( fords) with injectors out the truck would shake and idle rough.... my LLY has 386k with stock injectors i might add ( no one believes me tho ) I really want to keep this L5P , so far i have had low coolant light issue which is at the dealer now and seems to be an ez fix. i am not a fan of the crappy low beam head lights which also i am sure can be remedied.... but i cannot live with a sluggish hill climber that is being outdone by my 6.0 powerjoke ford f550 . I am hopefull it is a boost leak. i had the reflash of the ecm done by the dealer prior to driving it off the lot so i am ruling this out. i have never had the clunks or delayed shifts that some experience.any input here is appreciated !
 

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replying to turbo fan and Fast , what was the outcome? I can shed a little input here with my 2017 3500 hd SRW high country . It is not a break in issue , nor is it a manual trans engagement issue. I pull a 3500 lb equipment trailer with a 9000 lb Cat track machine ( bumper pull) . I have the same hill everyday i take back to the shop. Currently have 26k miles on the truck . This thing ripped that hill everyday on the way home and seemed to shift down if needed prior to an occurance at about 22k. I was getting into the hill and all of a sudden a loss of power, almost like a limp mode and it decelerated up the hill topping at about 40mph. I unhooked at the shop and drove home with no issues? I wrote the occurance off as maybe it was doing a re- gen at the time of the hill climb? Now since then , mine is doing the same thing that FAST is stating. It holds 6th gear on the hills at 1700rpm and doesnt drop into 5th , ultimately topping the hill at 40mph. i have floored it to put it into 5th and sometimes it responds and sometimes it does not. i have noticed less grades it does well from 5th to 4th downshifts , but is not dropping to 5th from 6th on the big hill home. I am interested in the post on this thread relating it to a boost leak or air duct issue . I am curious of the outcomes from FAST and Turbo fan. i do not believe it is the # 4 injector as 1 person thought. In the past any of my trucks ( fords) with injectors out the truck would shake and idle rough.... my LLY has 386k with stock injectors i might add ( no one believes me tho ) I really want to keep this L5P , so far i have had low coolant light issue which is at the dealer now and seems to be an ez fix. i am not a fan of the crappy low beam head lights which also i am sure can be remedied.... but i cannot live with a sluggish hill climber that is being outdone by my 6.0 powerjoke ford f550 . I am hopefull it is a boost leak. i had the reflash of the ecm done by the dealer prior to driving it off the lot so i am ruling this out. i have never had the clunks or delayed shifts that some experience.any input here is appreciated !
When was the 'drive it off the lot date'?

What's the dealer saying about the performance issue?

Sounds like you've got an issue that you can (and do) reproduce nearby at anytime. I'd think that if the dealer can't resolve that they'd be interested in getting GM's help, and that GM would be interested in figuring out why it changed.


Sometimes we hear of (or have) these issues and it is not practical to reproduce since it doesn't happen every time or it happens with an unusual load and/or a distant location.
 

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Discussion Starter #48
kid, my truck has gotten better with mileage. I also installed a Dr. Performance (similar to Stealth) Fuel module which has helped the overall performance. I still honestly feel that on certain occasions it hangs in gear. I've gone to manually shifting when going a certain route with large hills, and dropping down a gear manually works great. Like I said, sometimes it drops the gear, other times it does not. Overall my trailer is a big wind block (40 ft enclosed raised height Gooseneck race trailer), and weighs around 14-15,000 lbs, and I can usually keep it 65 mph and above on most hills.
 

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thanks redwinger, drove it off the lot december 2017 . the dealer said the same thing that it is hard to reproduce without the trailer and cat hooked up. we may have to take it for a ride because the hill to my shop and the dealership is only a few miles apart
 

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thanks FAST112 , that is helpful info to hear . I am going to check into the possible loose air duct or turbo duct just to make sure and i will let you know if i find anything. Glad you are happy with the truck !
 

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thanks redwinger, drove it off the lot december 2017 . the dealer said the same thing that it is hard to reproduce without the trailer and cat hooked up. we may have to take it for a ride because the hill to my shop and the dealership is only a few miles apart
That delivery date means that it will have had the recall update applied.
 

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I have a 2017 2500HD L5P with about 15,000 miles on it. It tows my 36' Montana 5th wheel like it's not even there. I previously had a 35' travel trailer and I was always fighting it to go in a straight line. Now with the 5er, those problems are over. Even though it has a net weight of 2500 lbs more than the TT, it still gets about the same fuel economy on the same route.

I am very happy with the towing performance of my L5P.
 

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I have a 2018 Denali HD 3500 SRW.

I pull a 30' toy hauler. Around 11k lbs.

So far I've pulled it all around Louisiana then from Louisiana to southern California. Some mountain passed along the way. Truck had plenty of power it seemed.

I averaged 7.6mpg, towing 10,000lb bumper pull, average speed was 57. I typically don't go over 70mph. Of course, max speed once in Cali, was 55mph.

Wondering why my fuel economy is lower than most of the other post in here..
 

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I am with FAST112, I have a 2018 ccsb srw completely stock, I just towed a 44ft gooseneck race trailer with extended height 10k empty and had 12k in the trailer. This was my first large tow and moved from Ogden UT, to Fairbanks Ak, roughly 3000 miles one way. I had no problem staying at 65 70, even most hills never went under 60. I had delayed downshift when going uphill but it usually took care of itself unless I went to manual and did it myself. Being my first duramax I have been more than happy with the choice to upgrade from a 3/4 ton gasser.
 

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I am with FAST112, I have a 2018 ccsb srw completely stock, I just towed a 44ft gooseneck race trailer with extended height 10k empty and had 12k in the trailer. This was my first large tow and moved from Ogden UT, to Fairbanks Ak, roughly 3000 miles one way. I had no problem staying at 65 70, even most hills never went under 60. I had delayed downshift when going uphill but it usually took care of itself unless I went to manual and did it myself. Being my first duramax I have been more than happy with the choice to upgrade from a 3/4 ton gasser.


Holy sh** that's a big trailer.
 

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I am with FAST112, I have a 2018 ccsb srw completely stock, I just towed a 44ft gooseneck race trailer with extended height 10k empty and had 12k in the trailer. This was my first large tow and moved from Ogden UT, to Fairbanks Ak, roughly 3000 miles one way. I had no problem staying at 65 70, even most hills never went under 60. I had delayed downshift when going uphill but it usually took care of itself unless I went to manual and did it myself. Being my first duramax I have been more than happy with the choice to upgrade from a 3/4 ton gasser.
I totally agree. I have a 44' enclosed goose neck dual car trailer and it's about 18.5k loaded. I've hauled the race cars all over the US and never had one complaint with my 2018 3500 SRW. I had 46k miles on my 2017 and 49k miles on my 2018, about 20k on each truck was towing the big trailer. I usually manually shift it before a long grade, but it'll usually maintain at least 65 MPH over anything I've encountered.

With that being said, I have noticed that both trucks got better with miles on them, especially after 25k or more.
 

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I have a 2018 Denali HD 3500 SRW.

I pull a 30' toy hauler. Around 11k lbs.

So far I've pulled it all around Louisiana then from Louisiana to southern California. Some mountain passed along the way. Truck had plenty of power it seemed.

I averaged 7.6mpg, towing 10,000lb bumper pull, average speed was 57. I typically don't go over 70mph. Of course, max speed once in Cali, was 55mph.

Wondering why my fuel economy is lower than most of the other post in here..
That sucks. I had 22K behind me and got a solid 9.8 9.9, no more no less.
 

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Obviously these trucks can pull without a problem, but how are they with actually holding the load? I guess that's the correct way to ask that haha

I know Fords are notorious about sagging in the back under heavy load. Ram I don't think is as bad, but also offers factory air bags to help level the load. Do the Chevy/GMC trucks sag much? Are air bags or more helper springs needed to help at least keep the truck level under load? Obviously it will depend on how heavy the load is, but you get the idea.
 

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Obviously these trucks can pull without a problem, but how are they with actually holding the load? I guess that's the correct way to ask that haha

I know Fords are notorious about sagging in the back under heavy load. Ram I don't think is as bad, but also offers factory air bags to help level the load. Do the Chevy/GMC trucks sag much? Are air bags or more helper springs needed to help at least keep the truck level under load? Obviously it will depend on how heavy the load is, but you get the idea.
I do not know the exact tongue weight, but I measured about 3 inch sag when fully loaded.
 
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