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Thread: Max fuel economy for the 6.5TD

  1. #1
    Join Date
    Feb 2000
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    Montana
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    Arrow Max fuel economy for the 6.5TD

    We started a fuel economy thread over in the 6.2L forum that has become very interesting. What I'd like to do here in the 6.5TD forum is through cooperative effort suggest ways to build and run a 6.5L Turbo Diesel for maximum fuel economy.

    Scenario #1- You're willing to go inside the engine to make changes designed to improve fuel economy, and you're willing to perform some modifications to your vehicle. This scenario compromises other vehicle uses in ways to maximize fuel economy.

    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.

    Let's begin our discussion with scenario #1.....

    MP

  2. #2
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    Jan 2003
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    Auburn, CA
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    Timing gears would be a good start to eliminate the slop between programmed timing & actual.

    A well balanced fuel curve (chip/reflash) would seem logical, taking boost into account to trim fuel to the max that would take advantage of air pumped into IM. There's a point where adding fuel churns out more heat, but no more go-go. That probably dovetails w/req'mt for an intercooler to allow boost above 5-6psi. I'll defer to the chip programmers for additional commentary.

    Intake air path needs serious opening; consider a conical racing type air filter; may need to augment filtration on a gas unit to get the square inches but still have the filtration.

    Water injection is well worth consideration; gives both cooler intake air (so you could add boost) & added expansion potential in the cylinder, but uses no fuel. For fuel efficiency as an overriding objective, I'd say H2O injection is probably more productive than air-to-air intercooling. But you have to fill the water tank, & give up the space for it. Always a trade off.

    Torque converter lock up mod.

    Port matching. Port polishing for the meticulous.
    \'95 P30 Motorhome 16,500gvwr, 26,500mi<br />New Ipump @ 25k, boost, EGT, homemade remote FSD cooler

  3. #3
    DA BIG ONE Guest

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    I use my SUB for traveling, seldome tow and always on it around town. Just returned to Palm Beach from Tombstone average mpg was 26, this is with Heaths program and larger injectors.

    My gear change to 3.42:1 was the most productive for extra MPG. Because of the torque I hardly notice any change in low end performance.

    I am now considering going to AT 33.5" tires which would bring my rear ratio down to about 3.10:1 and just below max torque at highway speeds.

    After many mods, I am looking towards a 300 hp mech. injection pump, and stand alone transmission controller, I am thinking tweeking from here should yield slightly more mpg, I hope.

  4. #4
    moondoggie Guest

    Post

    Good Day!

    Someone mentioned elsewhere, probably in the 6.2 version of this topic, that prechamber cups might help mpg. Experts?
    1. Gears! My 95 pickup gained a pretty solid 2 mpg after I installed the Gear Vendors Aux. OD. You might want to look at an earlier post of mine. If the truck is 2WD (which by itself should help mpg), maybe it wouldn't be too far out of line for a max. mpg truck to go to a lighter 3.08 set of gears? Again, experts?
    2. Is it possible that factory wastegate programming might be best? In other words, does spinning the turbo waste power, as long as there's adequate combustion air getting to the engine? I'm referring to the way our trucks pull boost over 1800 rpm under certain other conditions, which is anathema to making more power, & maybe even bad for exhaust gas temps.</font>
    I can't WAIT to see where this goes.

    Blessings!

    Brian Johnson, #5044
    Last edited by More Power; 09-21-2007 at 16:30. Reason: dead link to old board

  5. #5
    David Brady Guest

    Post

    I have put a set of 2.73 gears in that does allow the lower rpms but I haven't adjusted the VSSB yet to track fuel use. It does allow me to comfortably keep up with traffic in the fast lane. I have wondered about the fuel useage between my old 6.2 and the 6.5. I think the reason that I don't see much better mpg is the increased power is easy to use and will require more fuel. My 6.2 ran at WOT for many miles but didn't produce power or use fuel. I just wish I had access to this site back then.

  6. #6
    Join Date
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    Arrow

    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.
    3- DSG - Phaser or gear drive timing set.
    4- If DS4, set TDC-Offset to -1.94. If DB2-4911, set timing up to 5 degrees BTDC.
    5- Hi-pop injectors.
    6- Free flowing exhaust - including port matched exhaust manifolds and 3-1/2" mandrel-bent exhaust system.
    7- Turbo set to produce 3-5 psi at a 65-mph cruise.
    8- A 4" lowered 2WD 1500 series truck/Sub with an under-bumper air dam, and riding on stock size radial rubber at max air pressure.
    9- Geared for 1800-rpm at 65-mph.

    25-mpg+

    MP

  7. #7
    catmandoo Guest

    Post

    mp my current truck is a 92 c1500 ext shortbox,it has a crate 6.5 n/a with 160,000 on it,i have touched nothing on this engine since i got it 2 yrs ago,with 60,000 on the engine.it also has the nv4500 5spd and 3:08 gears.the only major things i have done is add my banks turbo onto a factory turbo exhaust manifold and a mandrel downpipe with 3 inch all the way back,without muffler.i can run 25mpg all day long at 65,and thats where it wants to run but my boost is down around 1 1/2 lb,and as you mentioned running boost around 3-5 lbs at 65,thats what i'm after,i figure if i can get the boost up right off idle,it will increase my milage,even if i run at 55 the engine seems to sound like it's laboring yet it will pull up to 27mpg,and it's still running a boost 1/2 to 3/4 lb. i believe there is milage there to be gained by upping the boost in that 3-5 lb range but i don't think much over that 5 lbs at cruise is gonna show much more if any appreciable gain.i'm currently gathering parts to do a major change come spring,i want to drop 2 inchs in the front and 3-4 in the back,get rid of the elephant ear mirrors for some suburban units,maybe toss the rear bumper and put on a rollpan.my goal is 30mpg,and i think it is attainable.i may just get that thru the aero changes.but i would like to get my boost up first.

  8. #8
    Join Date
    Oct 2001
    Location
    NE CT
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    506

    Post

    Your lack of boost is just an indicater that your not using much HP to roll.

    Strap a sheet of plywood to your bumper and your boost will double or triple.
    1996 K1500 6.5, 1984 K5 6.2 Banks both \"Stock\" (tilting hand side-side like Sammy Davis in Cannonball Run)<br /><br />Got Boost?

  9. #9
    Join Date
    Feb 2001
    Location
    Fort Worth, Texas, US
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    945

    Post

    change the exhaust housing. call dave at majestic turbo in dallas and tell him what you have, and that you want a .81 A/R euro mount p trim exhaust housing.

  10. #10
    catmandoo Guest

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    plywood ,i like that,and thanks grape i'll give him a call.you wouldn't by chance have his number??

  11. #11
    Join Date
    Feb 2003
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    Mississippi
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    98 k2500 burb stock and I mean stock. 265/75/16 3.73 6 people 600 miles 55-75 mph was 16- 18 mpg used miliage off of lead car, bigger tires.
    1996 k2500 suburban vin f all stock 246000 miles. I pull a 7400lb 26 ft cabin cruiser.

  12. #12
    Join Date
    Nov 2004
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    33

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    Originally posted by More Power:
    Recipe for maximum fuel economy:
    Hey, MP (or anyone else)... I know there are several companies out there that build nice replacement 6.5's... are there any that do items 1-5 in one package? I imagine the heads would be the most unusual thing to include in a package...
    \'97 K3500 Quad Cab

  13. #13
    Join Date
    Jan 2002
    Location
    Fayetteville, NC, USA
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    MP,
    Can you elaborate on what you mean by "Geared for 1800-rpm at 65-mph"? At 65 mph I'm running 2000 rpm, what can be down to lower the rpm's?
    95 Suburban 6.5 TD; AFE Air Filter/Box; DSG Idler Gears; Remote Mounted FSD; High Output Water Pump; Autometer Lunar Guages - Boost, Trans Temp, EGT; GM-8 Turbo; JK Lighting Upgrade, Fluidamper, Max-E-Tork Eprom, Turbo-Master, HD Lift Pump

  14. #14
    Join Date
    Nov 2004
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    33

    Post

    Originally posted by gmenor:
    "Geared for 1800-rpm at 65-mph"?
    He means that if you are running at 65 mph, your engine is turning at 1800 RPM (the MPG sweet spot for our engines, I believe)

    Without going into the math (several posts here and elsewhere on the 'net) your tire size, differential ratio, transmission ratio (and auxiliary transmission ratio, if you spend the money for one) all affect engine speed at a given road speed.

    I run 2000 at 60 mph and would like to lower it (I assume that a) my tach is correct and b) my speedo is correct... have yet to verify both).

    - I'm already running 265/75's and have no desire to go to a larger tire.
    - I considered dropping my diff ratio from 4.10 to 3.73, but a) I'll lose some grunt when I start hauling and b) I'd have to do the front diff too and it would only drop it to 1850 (close, but I'm a perfectionist).
    - not gonna mess with the tranny until it goes out (if ever) so I'm stuck with the .75 (or .78, I can't remember) OD ratio.
    - planning to go with an auxiliary transmission (Gear Vendors or US Gear). IIRC, should drop my final engine speed closer to 1800 *and* I get to keep my low-end grunt.

    Hope that helps!
    -John
    \'97 K3500 Quad Cab

  15. #15
    Join Date
    Jun 2000
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    Arlington,MA U.S.
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    As far as motor mods , balancing, use of coatings on all moving parts to lower friction, windage tray with a scraper, coating the exhaust manifolds as well as the exhaust pipes, turbo blanket to trap the heat. I'm sure there are a few things I forgot that need to be mentioned but making the basic motor itself as efficient as possible can only help with the milage.
    I would also make a guess that a timing gear will create a slight bit more of friction compaered to a chain.
    My own best milage in my truck with cap on and with a weight of 6500lbs is 22.5mpg. This was over 1200 mile round trip
    1998 6.5 2500 4X4 dynatrac pro 60, ARB, 35"BFG's
    06 D Max Express 5"MBRP EFI live

  16. #16
    moondoggie Guest

    Post

    Good Day!

    More Power: Would it help or hinder this topic to report our mpg results, relating them to our equipment, etc.?

    Blessings!

    Brian Johnson, #5044

  17. #17
    Join Date
    May 2001
    Location
    Edmonton, Alberta, Canada
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    I think the friction of gears vs chain is meaningless when the majority of friction is in the piston and ring assemblies. Remember that at 2400 RPM they go up and down 40 times per second....times 8 cylinders! Brings me to a point. Since the rings have to scrape the oil off the cylinder to prevent oil consumption, a lighter viscosity oil is easier to scrape off and is said to increase fuel economy. That's probably why some of the new gas jobs are calling for a 5w-20. But for a high compression diesel I still think a higher viscosity should provide better protection...especially when under loads such as towing.
    ......just my .02 cents...... [img]smile.gif[/img]

  18. #18
    moondoggie Guest

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    Good Day!

    "But for a high compression diesel I still think a higher viscosity should provide better protection..." My very old & very weak understanding of oil viscosity tells me that the "W" number is the viscosity at 0

  19. #19
    Join Date
    Jul 2004
    Location
    Albion, Illinois
    Posts
    115

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    I recently added gages to my truck to read boost, back pressure, vacuum on the wastegate and fuel pressure. I just returned from a trip to the FL Keys and did some experimenting on the trip. One interesting thing surprised me. I normally pull about 4k with about 2k in the truck bed and drive 70 on the interstate. I am geared to run about 2050 at this speed. The back pressure is always higher than the boost by at least 2 psi. Stock boost settings gave level road readings of 3-4psi boost and 5-6 psi back pressure. I got on all tanks of fuel 14.5 to 15 mpg except the following.

    On one tank I adjusted my homemade boost fooler to get a about 5-6 psi boost which gave 8-10 psi back pressure. This tank I got 14 mpg. The worst on the trip.

    I also removed the vacuum hose from the waste gate to see how much back pressure I had just from the open exhaust and it was less than 1psi.

    I think a more efficient turbo would help mpg. My gages have both been calibrated at 5 psi so I know the readings are pretty accurate.

    One other thing the vacuum gage shows about 7" of vacuum under stock boost at my driving conditions. With more throttle or idle it goes up significantly.

    These mileage figures are the best I've ever got. On similar trips with the truck stock I got about 11 mpg.
    '99 GMC K3500 6.5TD Crew Cab dually now SRW, Auto, now 5spd Yeah no more sissy slushbox
    3.73 gears, 235/65 tires
    Pull about 80% of time, mostly 4K lb hilo type camper

  20. #20
    moondoggie Guest

    Post

    Good Day!

    "I got on all tanks of fuel 14.5 to 15 mpg except the following." "These mileage figures are the best I've ever got. On similar trips with the truck stock I got about 11 mpg." It's unclear to me what you changed that caused this major improvement in mpg. What did you do different to get 14.5 - 15 from when you were getting 11? I'd do just about anything within reason to achieve such an improvement. I'm not completely unhappy with my mpg unloaded, but only got 12 - 16 pulling our US Cargo SM625TA2 trailer (looks like the US Cargo SM727TA2 [Click in colored text] except it's 25' long instead of 27'), which isn't much of a load.

    Thanks & Blessings!

    Brian Johnson, #5044

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