Wednesday, September 15, 2010

6.5L GM Diesel Towing: Making the Grade

Can Your Diesel Make The Grade? A Five-Mile, Six Percent Mountain Grade in Washington State Puts Our Project 6.5L Suburban to the Test
Suburban Renewal walked you through the upgrade process for a stock 6.5L using a representative Suburban 4x4. In that article, we focused reporting on the overall reliability and performance improvements realized through the upgrades. While these are, of course, vital to the everyday use of your truck, we will now focus on the uphill, heavy towing performance gains realized as a result of the upgrades. For many of us, this is what matters the most and it is during these times of high stress that we want to be able to depend on our work trucks. We took the time to test the towing capabilities at both the near-stock configuration and after completing our regimen of upgrades. Looking at the results, you will see that, with the right modifications, the 6.5L can comfortably “make the grade” and perform as a work truck should.
Our evaluation included the following level-grade tests measured in both elapsed time and distance consumed:
  • 0-to-60 MPH Acceleration
  • 40-to-60 MPH Passing Performance
  • The third test measured overall speed capability on a steady uphill six percent grade.
During the tests we noted:
  • Engine Coolant Temperature
  • Exhaust Gas Temperature
  • Boost Pressure
  • Transmission Temperature 


In order to provide the most accurate “before-and-after” results, we ran all towing tests using a loaded 9,000-pound trailer with sandbags to round the weight off exactly. With 9,000 pounds hanging off the rear of our 7,600-pound Suburban (driver plus three well-fed passengers) the gross combined vehicle weight (GCVW) was 16,600 pounds. While this is certainly not the heaviest towing combination, it was plenty heavy enough for our testing purposes and, in our opinion, pushes the maximum practical GCVW for this vehicle.
We elected to perform our baseline testing after a few “reliability-enhancing” upgrades had been done in order to assure a solid factory-stock level of performance.
These upgrades included:
  • PMD Isolator System
  • Heavy Duty Lift Pump / Oil Pressure Switch Combo
  • Updated Electronic Filter Harness
The initial test drives with this load demonstrate, with certainty, that our stock 190 HP Suburban, is not up to the task. I would not want to do a lot of towing with this rig: it is short on grunt.


A problem that plagues most 6.5s is keeping them cool while towing. The 6.5 powered C-K vehicles produced from 1992 to 2000, especially those produced prior to 1997, are equipped with cooling systems barely adequate to maintain a practical limit on coolant temperature. This temperature must be kept at or below 210ºF when towing. When these cooling systems have suffered the normal accumulation of bugs, dirt, leaves and weed seed, the level of efficiency they originally enjoyed is degraded and overheat conditions are set. 
A word of caution is due here. Because GM did not provide a properly marked ECT gauge, 6.5L drivers should use the 210ºF mark on the temperature gauge as the towing-temperature limit. 
Even though it was not exactly hot on the day of these tests (84ºF), our Sub was trying to run toward the 210ºF – and this was on mostly level, local area roads.
When the cooling system is healthy, the normal, level roadway temperature indication ought to be two notches below the 210ºF mark on the gauge. In the case of our project Suburban, some cooling system work was needed before any serious performance evaluations could be done, so we headed to the shop to check things out.
We elected to correct this cooling system issue before attempting any towing tests. Heath Diesel’s mechanics installed the company’s Extra Heavy-Duty cooling system upgrade as detailed in Suburban Renewal. In the process of the installation, they also checked the radiator and air conditioner condenser for any plugging by bugs and dirt, a common problem. Sure enough, these two areas were pretty well loaded up, especially the air conditioner core, which was solidly packed with crud in the areas behind each of the oil coolers. Since the radiator showed signs of failure of the bonding between the coolant tubes and their fins, the guys felt that it was best to replace the radiator with a new GM-issue unit. This type of radiator failure can be found by sighting across the radiator surface. The fins will show raised or lumpy-looking areas. When you see this, it is time to replace the radiator. With the health of our cooling system restored, we headed back out to pick up where we had left off. We found that the coolant temp was now right where it should be, two notches below the 210ºF mark when traveling on level surfaces. Our confidence was renewed! Now we returned to our level road testing area to record the 0-to-60 acceleration and 40-to-60 passing performances.

Tests at Near Stock Configuration

The first acceleration tests were run with the Suburban in near stock condition. The exceptions to this have been noted above. With trailer in tow along with our complement of people on board, the GCVW was 16,600 pounds. Each test was run three times. Here are the averages of those runs:
  • The 0-to-60 MPH acceleration test required 39.2 seconds and consumed 2,800 feet of roadway
  • The 40-to-60 MPH passing test required 26.2 seconds and consumed 1,900 feet of roadway.
With the acceleration tests completed, we headed for the mountain grade and the uphill towing portion of our testing sequence.
I pulled over to the side of the roadway and stopped, with the rig pointed uphill. I shared with my passengers some detail about what to expect. We talked about the 1,100ºF exhaust gas temperature limit, 210ºF coolant temperature limit and that we needed to keep an eye on these reported temperatures as we moved uphill.
In order to make this as real-world as possible, we turned on both the front and rear air-conditioner units. Seat belts were checked and we began the uphill rollout. I explained to the guys that this is no drag race and that it is always wise, when towing, to gently roll into the throttle as the rig moves out in first gear.
In the graph above, the black represents the results of the near-stock Suburban while the green shows the improvements realized with the upgrades. The 0-to-60 and 40-to-60 acceleration runs were performed on level ground and were measured in both time and distance consumed; the top speed tests were performed on a persistent six percent uphill grade. All tests were performed pulling a 9,000 pound trailer. * The 50 MPH result was measured at full-throttle while the 68 MPH result was achieved at only 75% throttle!
I Thought I Could...
We moved out onto the roadway, as I pushed the pedal down toward the floor in first gear. While the Sub was running smoothly and making an effort, there was certainly no danger of whiplash. As we slowly gathered speed, our passengers began to understand that this grade was one that will separate those who can from those who cannot! At exactly one-and-a-half miles into our pull, we were up to 48 MPH and it did not seem that we were going to see much more speed than this. With the throttle to the mat, the exhaust temperature had climbed to 1,175ºF (measured pre-turbocharger), about 75ºF higher than recommended (I wanted to demonstrate factory power-level exhaust temperature) and the boost pressure was fluctuating between five and six PSI. The coolant temperature gauge had finally climbed up to and leveled out at one notch below the 210ºF mark, which is very good and to be expected when running the Extra Heavy Duty cooling system upgrade and a clean radiator.
At this point, the transmission had established itself in third gear and the torque converter clutch was not locked up due to the low speed. Torque converter-clutch lockup occurs at around 63 mph in this vehicle with its 3.73 axle ratio. When these trucks develop enough power to get the speed beyond the torque converter lock up point, conditions will improve dramatically. The transmission temperature will be reduced and the engine will be more fully applied to the load. It is always better to run above the lock up speed. At two-and-a-half miles into our run, we had leveled out at 53 MPH. It was fairly apparent that we could expect no more. 
We continued the pull at full throttle in third gear for another mile on our way to the top of the grade, never topping 53 MPH. At this point, I backed off the throttle a bit to see if we could maintain the 53 MPH, but found that any relaxation of the throttle resulted in a slow-down. I went back to full throttle and at the top of the hill and the end of our pull, we noted:
  • The exhaust gas temp stabilized at 1,175ºF (75ºF over recommended).
  • The factory vacuum-controlled boost pressure wavered between five and six PSI.
  • The coolant temperature had stabilized a notch below 210ºF.
  • The transmission temperature leveled out at 225ºF (warmer than we liked, but this was due to the fact that the lock up clutch was not activated at the lower speed).
  • Our speed peaked at 53 MPH. That was all we could get out of our stock Suburban.
We completed two more of these uphill pulls. Each turned out exactly like the first. Our Sub was running well and staying cool but was too underpowered to pull the 9,000-pound trailer up the six percent grade at a respectable speed.
We headed back to the shop for the power and performance upgrades featured in Suburban Renewal.
  • Super-Flow Four-Inch Exhaust System
  • AFE Air Filter Cartridge
  • Hi-Flow Exhaust Crossover Pipe
  • Turbo-Master Boost Controller
  • Max-E-Tork Computer PROM with HPT series programming
  • High-Output Fuel Injectors
Here again, the performance of the near-stock configuration is shown in the black shaded blocks and the upgraded configuration performance is shown in the green shaded areas. The upgraded Suburban used 75% throttle with 25% still available.

Tests with Upgrades

With the upgrades completed, we were excited about having another chance at the hill. After a little local driving to gain an impression of its revised characteristics, we were thoroughly impressed! Our project Suburban had been rejuvenated! It now felt rather zippy, even with the big load fastened behind.
At this point, it was time to resume testing with our acceleration runs. The upgraded Suburban produced these figures:
  • 0-to-60 MPH required 22 seconds and consumed 1,650 feet (originally required 39.2 seconds and consumed 2,800 feet).
  • 40-to-60 MPH passing test required 16 seconds and consumed 1,200 feet. (originally required 26.2 seconds and consumed 1,900 feet)
Recalling the results of our first round of testing with the Suburban in stock form, you can see that the 0-to-60 time was reduced by 17.2 seconds; that is a 44% quicker elapsed time! The distance consumed was reduced by 1,600 feet, a solid 43% reduction in distance required to get up to speed! Now, with great confidence, we headed for the hill to run the uphill-towing portion of the evaluation. This time, we anticipated a strong towing performance. We could tell by the feel of our project Sub that it was going to make us proud on the hill.
As we rolled away from a stop, there was no mistaking the change in engine torque. This time, we rolled out onto the uphill pull and into traffic with confidence, knowing that we now had power and, with it, could handle difficult highway towing situations. When you tow in the mountains, you are going to have to deal with slower moving traffic. We felt confident now that, if we were to find ourselves trapped behind a slow moving truck, we would be able to pull out, pass and get back up to speed again: our Sub felt that good.
Our revitalized Suburban was accelerating through 65 MPH when we passed the one-and-a-half mile point; and, it was still gaining speed on our trip up the grade. Because this was no drag race – and in order to help make driveline parts live a long and happy life – I used only about 75% throttle from the beginning of our run uphill. At the two-mile mark, we had accelerated to 68 MPH in third gear. At this point, I set the cruise control. After things had stabilized, we noted that the boost pressure was steady at 14 PSI, the exhaust gas temperature was 1,075ºF and the coolant temperature was perfect at one notch below the 210ºF mark. The Sub could have operated at this power level till the fuel tank went dry. My passengers were curious, so we decided to see just how fast we could run with this combo. I shoved the throttle to the floor. We could feel the power and the Sub moved on up to 71 MPH; however, as expected, the exhaust gas temperature also climbed and peaked at 1,200ºF, which was higher than we normally recommend. Clearly, with this tune-up, we were limited by exhaust temperature to a practical, uphill towing speed of 68 MPH. If we would have used the Standard output injectors instead of the high output versions that we installed, exhaust temperature would not have been an issue. With the standard injectors, our uphill speed would have been about the same at 68 MPH; however, we could have done that speed without regard for the exhaust gas temperature. Towing is less stressful / more fun when you do not have to worry about exhaust gas temperature, but then we would have given up some power and acceleration on the flats with the standard injectors. This is the give and take you will face when choosing which injectors to install. We will be glad to discuss it with you to help you come to a decision that is right for you.
As noted, the transmission temperature was lower on these higher speed runs because the torque converter clutch was locked up and doing exactly the job it had been designed to do.
We repeated this uphill run three additional times, all with the same results. Our Suburban had proven itself capable of performing in a high stress towing job. It seemed to be fully in charge of the work and able to get it done without an issue. I never cease to be impressed with how well the 6.5 can be made to perform. They are a great vehicle and they can be made to do a very impressive towing job.
In summary, everyone was thoroughly pleased with the transformation of the project Suburban. While it cannot be expected to compete with a modified DMax, this 6.5 did a very respectable job of pulling its heavy load up that long, steep grade. In fact, it did this job in a confidence-inspiring way. It was smooth and quiet and moved the 9,000-pound load with authority. One of the main contributors to the 6.5’s increase in power is the installation of the Max-E-Tork PROM / Turbo-Master combo.
Here is an interesting bit of info: at stock engine output (no performance upgrades), our Sub’s 6.5 was pulling at 2,300 RPM (53 MPH in third gear without the help of the lockup converter) while climbing up the hill. When full throttle at that RPM, our stock 190 HP engine would have been producing about 155 HP and 353 pounds-feet of torque at the flywheel. By way of comparison, the upgraded Sub would produce 208 HP and 475 pounds feet of torque at this same engine speed of 2300 RPM. This helps us to understand why the Sub climbed right on through the former maximum speed of 53 MPH and accelerated up to 68 MPH at only 75% throttle.
The Sub’s newfound towing-performance was directly connected to the stronger, much broader torque curve our 6.5 now produced.
In summary, the performance gains made in the project Suburban have substantially improved its ability to move a heavy load and indeed, to drag that load up long, steep grades with confidence. It does this now with an impressive composure: everyone was impressed! 

My Take on Black Smoke

During this evaluation, we demonstrated that unless the throttle is slapped to the mat at a stoplight, we were not able to make this 6.5’s exhaust smoke. More importantly, the exhaust produces no visible smoke, even when running hard uphill with a big load in tow. The project Sub’s exhaust ran cool and clear. While it seems that some people take delight in producing as much black smoke as they can, as often as they can, the upgrades that we outfitted the Suburban have just the opposite result in mind. Producing black smoke during an off-road competition is one thing, puffing this stuff into the air while running on public roadways is another. In our opinion, performance-enhancing products ought to create superior ‘drivability’, maximized combustion efficiency and the highest possible power output with the least possible black exhaust smoke. Toward that goal, we promote what we call ‘Total-Performance Engineering’. This phrase refers to the practice of supplying components that are engineered to provide maximum engine efficiency when operated as a package or system.
We believe that sooner or later, the “black-smoke vehicles” en masse will attract unwanted attention and result in regulation changes that affect whether or not – or to what degree – we are allowed modify our highway diesels. It is time to wake up and think straight: black exhaust smoke is a bad thing and it will ring the alarm in Washington DC, especially if someone feels they can use it for political advantage.


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  33. The towing test was done wrong. With the accelerator all the way down, the transmission will not upshift. You are basically running out of RPM in the motor. Let off the throttle a little, let the transmission shift, and you will get more speed and power out of the engine. I've hauled A LOT of weight with my 6.5L..... I found that flooring it at speed, just made the truck slow down when the trans down shifted.