Max fuel economy for the 6.5TD

[jdmetcalf57]

When I bought the truck it was a k3500 drw auto trans stock exhaust with 4.10 rearend.

I have changed the following that has increased fuel economy.

Change the TDC learn to -1.8. about +.25

Replaced the exhaust from the down tube back with 3.5 in mandrel bent exhaust. about +.5 mpg

Replaced the rear end with 3.73 and increased tire dia. to give equivalent of 3.43. about +1mpg

Removed bug shield. about +.1-.25

Use fuel conditioner. about +.25

Change to single rear wheels. about +1

Replace the auto with a NV4500 manual. about +1

Use all synthetic oils except the engine. about +.25

I have experimented with higher boost and water injection but have not been able to see any mpg gains with these the way I drive.

The above mileage gains may not quite add up but over all I think they contribute in about these proportions. I am hoping this summer to hit 16mpg pulling my camper with summer fuel since I have hit 15 with winter fuel.

[coldtruk]

Hello,
Thought this might help with understanding the "oil Doughnut" information (on the cap of the container) according to chevron and its applications with diesels per oil type and usage, hope its helpfull.


http://www.chevron.ca/ProductsServic...orOilLabel.htm


Good day

coldtruk

[markrinker]

I would estimate my average mileage for either truck with trailer loaded or empty to be about 9-10 mpg. Empty without trailer (rarity) is about 10-11.

Have done lots of mods to #2 and experimented with tire sizes. 4.10s cost me some, but I never drive under 65 which is probably more of the issue.

[moondoggie]

Good Day!

jdmetcalf57: Thanks!

Blessings!

Brian Johnson, #5044

[johnny2can]

Originally posted by Mark Rinker:
but I never drive under 65 which is probably more of the issue.

Interesting you should say that... recently went on a trip and noticed a *HUGE* difference between 65 and 70... on the order of 3 MPG... was getting 13 in a hurry (>=70), but 16 when taking my time (<=65)... The difference, while noticeable, is not so drastic in my gasser 'burb, which lines up with what I've read on this site and others.

aerodynamics of a brick...

[moondoggie]

Good Day!

I too have noticed that my trucks seem to like 65 mph or less, for good mpg. At 70 mph, it seems like you can watch the gauge go down. Sure wish I could generate some hard data.

Aerodynamic drag increases as the square of speed, so an increase in speed of ≈ 7.7% (65 > 70 mph) causes an increase in aerodynamic drag of ≈ 16.0%. Is that enough to account for how much mpg drop we're seeing?

Blessings!

Brian Johnson, #5044

[jdmetcalf57]

The amount of mpg reduction from 65 to 70 will be highly dependent on what you are pulling. Aerodynamic drag, and rolling friction contribute to fuel consumption. The data I've seen at those speeds a semi has about equal rolling and aero drag.

There takes a major portion of fuel to turn the engine over. If you have a computer to monitor check how much fuel is being used when in neutral and 2000 rpm. I think my truck is around 16-18mm*3/fire. If you back figure based on average mpg when I am cruising I am using 36mm*3/fire pulling my typical load of my 4K camper.

As you can see from this just turning the motor over takes a substantial amount of fuel and Hp. From this fuel rate I have estimated this hp to be 50-60.

From this discussion you can see that aerodynamics is just a portion of fuel usage in my case probably around 40%

The cummins website has some interesting data on fuel economy and semis.

[moondoggie]

Good Day!

Scenario #2 (You use your 6.5TD for a variety of family and work related driving. From city traffic to long Interstate trips, you're looking for maximum fuel economy without compromising too much in vehicle versatility.) & Scenario #3 (You use your 6.5 to tow 5-10K trailers several times per year, so power and durability are equally important to fuel economy.) interest me most.

Back on 27 Dec 04, I asked, "...does spinning the turbo waste power, as long as there's adequate combustion air getting to the engine?" I was hoping our many experts might have some ideas here.

It appears from a couple years of reading what I could on the Page, lots of boost reduces EGT (assuming charge air cooling), presumably by moving more air through the cylinder than is necessary for complete combustion, allowing this extra air to cool the EGT into a safe area. If this is so, it makes for longer engine life.

Isn't it also possible then that, when operating at low loads (no trailer or such), that too much boost might waste power by spinning the turbo way faster than it needs to be spun? In other words, the load is light, so boosting beyond what's needed for complete combustion wastes power spinning the turbo?

If this is so, it wouldn't be impossible to build a controller that controlled the wastegate as a function of EGT. In other words, control the wastegate to hold EGT at 900 F (or whatever temp. was determined to produced best combustion for best mpg) while cruising.

If this is a really stupid idea, have a laugh at my expense. I know pretty close to nothing about these engines; the little I have learned has been here on the Page.

Blessings!

Brian Johnson

[BigDiesel]

Originally posted by More Power:
Recipe for maximum fuel economy:

1- Engine dynamically balanced and carefully assembled using stock CR pistons/gapless rings.
2- Cyl heads with 1982-1993 6.2L "C" series precups.
MP

Two questions here
1)I was under the impression that everyone was wanting 18:1 pistons, especially if turning up the boost.
2)Will using the latest heads (2002+)have that much less effect? What is the deal with 6.2 "C" series precup heads?
Dave

[damork]

I didn't see thermostat temperature mentioned. I've experienced the best economy when the engine is running hotter, with at least a 195F thermostat, with 21 mpg being my best with the temp gauge sitting at about 200F cruising at 65 mph (not towing).

[David Brady]

Has anyone been able to experiment with a set of Direct injection heads? Seems like that would help economy as well as power.

[johnny2can]

Originally posted by damork:
I didn't see thermostat temperature mentioned. I've experienced the best economy when the engine is running hotter, with at least a 195F thermostat, with 21 mpg being my best with the temp gauge sitting at about 200F cruising at 65 mph (not towing).

I've *read* that mileage goes up with higher coolant temps (ala Evans waterless coolant...) and last summer experienced it on my gasser 'burb. Stat failed at the beginning of long trip... Running at a consistent 235* to 240* increased my mileage by almost 2 mpg over similar trips in the past. I almost hated to replace the stat...

I've seen many posts recommending running 185* stats, but as long as you keep your critical temps in control (EGT, tranny, oil, PMD, etc.), and you run an additive to keep your IP lubed, what's wrong with running a higher temp on these machines?

Running a higher temp stat would even help those pulling heavy loads, I would think, because the higher temp differential (between radiator and air) would cause faster heat transfer allowing more heat to be rejected for a given ambient temp (again with the above assumptions).

My 6.5 stats are failing now (too cool) and so I plan to put in 195* or 200* (if I can find it)...

[kowsoc]

I've also experienced better fuel economy when the engine is warmer. In the winter I have the grill and bumper holes blocked completely to keep the under hood temperatures up. I am running a 195 stat but I believe there are 205's available.

[More Power]

Big Diesel, Since this thread is primarily concerned with fuel economy, I recommended the 6.2L "C" series cylinder head precups. These use a smaller port that the flame passes through between the precup and the piston, which tends to increase velocity, swirl and combustion efficiency. The 6.5L TD uses larger ports, which tend to help with power at higher manifold pressures. If I were building a "power" engine, I'd want the biggest TD precups.

Thermostats - In a pure economy engine, a higher t-stat rating might help.

In a 6.5TD that tows or is run hard, I personally would want cooler t-stats. This helps with transmission temperature, allows more cushion for engine coolant temperature and is kinder to the electronics (FSD). A case could also be made for cooler intake air (passing through the intake manifold) and cooler fuel, which might be a slight plus.

There's some good debate here about fuel economy. Keep it going! The collective experience of everyone adds to the goal of improving fuel economy.... [img]smile.gif[/img]

MP

[G. Gearloose]

Originally posted by johnny2can:
I would think, because the higher temp differential (between radiator and air) would cause faster heat transfer allowing more heat to be rejected for a given ambient temp (again with the above assumptions).
[/QB]

All thing being equal, I concur, but they are not... my observations are an 185 RS tstat opens wider at 190 degrees than a 195 AC or RS tstat opens at 210. Others have observed similar. Perhaps they open more at higher temps under pressure, but I suspect not.

[rjschoolcraft]

This is slightly off topic...

I've found that the cooling system on these engines is marginal at best. MP's comments are right on with my experience for anyone who works his 6.5TD hard, as I do. Read the 6.5TD Cooling Solutions That Work article. Logic and diesel theory argue for higher temp stats, but practical experience with this particular system says no...especially if you tow heavy in the Western United States.

There are those who will argue vehemently against me here on this subject. You draw your own conclusions.

For a purely mileage driven decision on a truck that doesn't tow...the 195's would probably be the best choice.

[Billman]

I currently have a 205* thermostat in my daily driver(gas). It has improved mileage 1.5 over a 195*. I check it religiously.

Although I don't have the Practical experience that you do Ron, I disagree. I also don't tow in the Western United States.

If you ever find that 205 thermostat kowsoc, please let us know. I have come up empty.

Without towing, I block my radiator for 7 months out of the year in order to achieve maximum fuel economy/(efficiency?)

[Marty Lau]

Thought I would put my .02 in here. I have gotten as high as 24.5mpg and see 20 mpg regular. My best was achieved cruising at 55mph across Norther AZ before the speed limit there was raised to 65. Aerodynaic drag is VERY important. If you add extra's to you rig like bug sheilds running boards, visors, bigger mirrors they will add drag and increase the need for power to over come the added drag, more power means more fuel used. As you increase speed aerodynamic drag increase by the square which increases need power by the square. In other words if double you speed from say 50 mph and it takes say 40 hp to travel at that speed and you double the speed you are traveling at to 100 mph it will take 4 times as much power or 160 hp not double. This is why bumping your speed up increases your fuel useage so much. So if if your in a mode trying to get max fuel ecconomy then slowing down from 80 mph to 65 will make a difference. This is also why GM spneds so much time and money in wind tunnels refining designs to reduce drag. Auto makers have no also designed new Pickup trucks so that the tailgate produces no extra drag so taking the tailgate of or down will not produce better mileage.

[jdmetcalf57]

16ga 4x4

Aerodynamic drag is not near all the drag however when only considering this drag. The power goes up by the cube of the speed. Or 2xs the speed needs 8xs the HP for just the aero portions. The aero drag force goes up with the square of the speed but since Power=speed x force. Hp is proportional to V^3.

However since you are moving faster and if you assume fuel usage is somewhat proportional to HP output. MPG for just the aero part goes down approximately as the sqare of speed.

From this you can see that as speed continues to increase aerodynamics becomes more of the dominating factor.

[Marty Lau]

jdmetcalf57;
You and I are on the sme page, I don't know the exact math just what I learned in my pilot training. The we are both making is that the faster you go the more the areo drag affects things that is part of the reason MPG drop off. The basics is to get more HP you burn more fuel pure and simple and the increase speed is not ofset by increased fuel burn. Just as any round trip with a wind will take more time and fuel than a no wind condition.