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Thread: Max fuel economy?

  1. #101
    convert2diesel Guest

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    Gearloose:

    It may sound strange but oddly enough, improving the flow, pressure and subsequently the spray pattern, means a more efficient burn. These engines were designed as a total compromise so any thing you do to improve the burn characteristics can only help to improve the fuel economy.

    Moondoggie:

    Maybe it would be easier to first convert to liters. Your girlie gallons only contain 3.8 liters while our macho gallons take up 4.546 liters.

    Either way the 80% rule is close enough.

    Bill

  2. #102
    Join Date
    Feb 2000
    Location
    Montana
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    11,382

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    Ratau,

    In a typical pickup or Suburban, I would gear the vehicle to produce 1800 rpm at 65-mph for best fuel economy. That usually works out to be 3.42 gears, .69 OD (700R4) and something like 30" tall tires. A lower/lighter/less drag vehicle may change the scenario somewhat, as would more weight, drag or higher speeds. If you're after economy, stay at or below 65.

    MP

  3. #103
    Join Date
    Jul 2004
    Location
    Albion, Illinois
    Posts
    115

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    I recently returned from FL and did some testing on the way. My truck is a 99 k3500 and I hooked up gages for boost pressure and back pressure. I normally pull a 4K hilo type camper. My mileage on this trip run about 14.5 to 15. On one tank I bumped the boost up about 3-4 psi to see what effect it had. Normally it runs about 5psi cruising at about 70mph. I actually lost about .5-1 mpg. What I found was that the back pressure rises significantly more than the boost. At 2100 rpm every lb of back pressure robs about 1 hp just from pumping losses. (power = Volumetric flow rate * pressure)have to use the right units

    I think there is some significant gains to be made by engine mods. My truck runs about 15-16 mm^3/ignition when in neutral and 2100 rpm. Try this if you have an electronic fuel injection system and can monitor the fuel rate. If you figure 63mm^3/ignintion gives 195 hp net then 63-16 gives this horspower. Then 16/(63-16)*195 gives how much horsepower it takes just to turn the engine over. This is 66 hp. I'm sure the fuel vs hp curve is not totally linear but it at least gives us an idea.

    I can also back figure the mileage empty of 20mpg at 2100 rpm gives 27mm/ignition which assuming 16 is taken to turn the engine over leaves 45 hp to drive the truck down the road at 70mph. I have done some test where I find the speed after coasting 1/4 mile on a flat road and windless day starting at 75mph. From this I can calculate an estimate of 50 hp to drive the truck down the road. This is good correlation for the crude methods I have at my disposal. Pulling my camper I have hp estimates in the 80-90 hp range.

    Anyway from these numbers it looks like there are major gains that could be made in the engine at least equal to that which can be gained by aerodynamics.

    I am anxious to see what improvement I have made with summer fuel. Last year going to FL I only got 12.5-13 mpg. Since then I have got rid of the duals and put a manual trans in. Considering when I got the truck I got 10.5-11.5 mpg I have come a long way.
    '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

  4. #104
    Join Date
    Jun 2003
    Location
    Pretoria South Africa
    Posts
    242

    Post

    Jdmetcalf57

    Would it possible for you to post the exhaust pressures you get at different boost levels, what turbo you use and the compressor housing A/R is?

    This info could add to Stray Cat
    Danie

    1982 Blazer ("Ratau" ROAR OFF THE BIG LION in African Tswana language.) 6.2L N/A, 700R4, 3.08 gears, 33" BF's. DSG timing gears, 12" Donaldson air filter, J intake and dual 2.5" exhaust. The rest stock.

  5. #105
    Join Date
    Jul 2004
    Location
    Albion, Illinois
    Posts
    115

    Post

    Won't be able to do it for some time. When I got back from FL I discovered antifreeze in my oil. The truck is at the dealer now and I should get new engine on warranty. I've heard rumors that the engines take a long time to get though.
    '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

  6. #106
    john8662 Guest

    Post

    I'm bumping this up to and keeping this post at the top for awhile now because of the increased need for better fuel economy. With fuel here at 2.36/gal I'm getting geared up for better economy and concentrating less on more rear wheel HP/TQ. Great ideas here!

  7. #107
    Join Date
    Apr 2004
    Posts
    133

    Post

    Yea, this is a good topic.

    Here's cummins take on the subject. Good info and puts things in perspective. What applies to tractor/trailers, applies to suburbans, and pickups, just on a smaller scale.
    http://www.cummins.com/na/pdf/en/pro..._Whitepape.pdf

    IMHO A front air dam, and a partial tonneau should be something we all try. I've had good success with both my toyota and my 6.2 burb.

    http://www.freepatentsonline.com/4573730.html

  8. #108
    Join Date
    Apr 2004
    Posts
    133

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    OK, I've got some mpg numbers from my 'redneck engineering' experiment on my toyota tacoma.
    It's a 95.5 model 4x4 sr5, on BFG AT 32x11.50r15's with the stock 3.4 v6, manual 5 speed and 4.10 gears.
    It currently has a little over 320,000 miles. It NEVER varies on fuel mileage, I drive 75mph average on a 90mile round trip commute daily.

    For the last 9 years I have ALWAYS gotten 18.5 mpg. (plus or minus 0.1mpg when commuting)

    I have averaged the following over multiple tanks, so this is for real, it's not a fluke.

    Here goes the modifications.

    1st. 6" front air dam, made out of 3/4 plywood, blunt, square, definately NOT aerodynamic.
    20.5 mpg
    2nd I added a 1/2 tonneau on the rear half of the box (like in gm's patent)
    20.5 mpg (Hmm.....)
    3rd, I put back in my low profile aluminum tool box, and left in the rear 1/2 tonneau. (1 foot open area between tool box and tonneau.)
    21.6 mpg. (I also noticed the wind noise is ALOT less inside the cab)

    total improvement 17%.

    Next is to make an actual 'real' aerodynamic front air dam, as the air dam appears to make the most difference.

    Goes to show that aero is the key to good mpg.

  9. #109
    Join Date
    Jul 2003
    Location
    N.Cal
    Posts
    0

    Lightbulb

    a cabbie was whining to me the otherday, about the cost of gas in the Crown Vic he rents. Why couldn't (wouldn't?) GM make taxi specials, w/cheap (NA -even cheaper) 6.5 in Caprice chassis? All the tooling is paid for. Seems like a perfect engine/car combo . . . for a cab for sure

    [ 05-16-2005, 11:59 AM: Message edited by: blazer84 ]

  10. #110
    Join Date
    Dec 2001
    Location
    Co Springs, CO, USA
    Posts
    232

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    Originally posted by jbell:
    It NEVER varies on fuel mileage, I drive 75mph average on a 90mile round trip commute daily.
    Not to be a wiseass, but I bet if you dropped it down to 55mph you would pick up another 2mpg.

    Pete
    6.2 NA, SM465, Dana 18, Saturn OD, Dana 60's all crammed into a 1970 Series IIa 88" Land Rover
    1998 K2500 Burb, relocated PSD, upgraded injectors, cold air intake.
    http://www.aloharovers.com

  11. #111
    K10ANDY Guest

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    i say more than 2 ! the 4x4 rags did a mpg article and if everyyone slowed to 55 it would be iirc 25% increase .

  12. #112
    Join Date
    Apr 2004
    Posts
    133

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    yea yea yea..... I know.

    The old racing saying "Speed costs, how fast do you want to go..." applies.

    However, in my case, I like to think of "Time is money" And I hate to waste either.... so 75+ it is..

    I've run several tanks full through, and still getting 21.5 average. Not bad for a vehicle that has seen over 1000 fill ups, and has as window sticker epa rating of 17-19. (When was the last time anyone else could exceed the epa mileage rating of a vehicle running over 75???)

  13. #113
    Join Date
    Jan 2005
    Location
    south eastern Nebraska
    Posts
    29

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    I've been following this thread and like what I have seen so far. I had the chance to do some checking this past weekend on a camping trip.

    the facts:
    total trip mileage : 93.5
    diesel used : 8.773 gal
    MPG : 10.6577
    head wind for about 30 miles : 10 MPH
    cross wind for the rest of the trip : 10 MPH
    average speed : 55
    truck used : 82 k1500 4x4, 6.2C with a Jcode intake, NP833a tranny, NP208 transfer case, 10 bolt 3.08 axles, 31x9.50x15 tires, stock 2 1/4" exhaust, stock everything except the J code intake,
    trailer : 2005 Keystone Outback 29fbhs, 31' fifth wheel, weight of 9500 lbs.

    I know that the trailer is well beyond GM's GCVWR for this setup but it is what I can afford at the moment.

    Jim
    \'82 1/2t 4x4 6.2L, I finally got the new J code manifold put on, NP833a tranny (3spdOD), 3.08 diffs

  14. #114
    Join Date
    Oct 2001
    Location
    NE CT
    Posts
    506

    Post

    Check out the graph , James.
    The fuel flow at 1600rpm is the same as 2800rpm, but the power almost doubles, and that means less foot on the pedal to make equivelent power. You could change gears with 3.73or 4.10 3/4 ton axles (they bolt right up), or go with a common mc465 tranny, with 1:1 top gear.

    appendum-
    Additionally, when you cruise on the high rpm side of the torque peak and you hit a hill, avail torque increases as you slow, instead of dropping away, this helps you maintain speed.

    [ 06-29-2005, 03:27 AM: Message edited by: G. Gearloose ]
    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?

  15. #115
    Tatraplan Guest

    Post

    I get an average of 19.6 mpg with a 150hp 6.2
    J-engine, 700R4, 4.10 rear & 9.50 R16.5-tires.

    Best so far is 20.3, but today I discovered a
    small fuel leak from one of the return hoses
    from the injectors. Replacing it might be an
    easy way to increase mpg a little.

    /Robert

  16. #116
    Join Date
    Feb 2000
    Location
    Montana
    Posts
    11,382

    Arrow

    Got a screaming deal on a new project vehicle over the weekend. Will be trailering it home today.

    This time next year, we should have most of the answers concerning what works for a vehicle (El pickup) to deliver max fuel economy using a 6.2/6.5.

    I'll post a pic or two and an outline for the project before long. [img]smile.gif[/img]

    Jim

  17. #117
    moondoggie Guest

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    Good Day!

    Thanks for making my day - can't wait to read EVERYTHING about this.

    Blessings!

  18. #118
    panchosteam Guest

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    Hi guys, i now have a 1986 silverado k 20 silverado suburban automatic transmision and 3.73 gears according to the glob box, took the family on a trip from Long Beach Ca. to perris, then from Perris to Sun City, back to perris then from there to the lake, according to the instrument panel it was 188 miles round trip ( just installed 235/85/16 DUNLOP ROAD ROVER A/T TIRES on saturday) filled up diesel tank before left, filled again on come back and the results are 16.22 gallons round trip for $2.89 gallon, is this any good? like 11.59 miles to a gallon?
    speed was between 70/75 most of the time. 1986 6.2 SILVERADO K20 SUBURBAN AUTOMATIC TRANSMISION AND 3.73 GEARS?

  19. #119
    Join Date
    Mar 2003
    Location
    Columbia, TN
    Posts
    186

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    quote:Originally posted by jbell:
    It NEVER varies on fuel mileage, I drive 75mph average on a 90mile round trip commute daily.

    Not to be a wiseass, but I bet if you dropped it down to 55mph you would pick up another 2mpg.

    Pete

    It's true.....in my TDI beetle, I dropped just 5mph from 75 to 70...went from avg. 47mpg back up to 50+mpg
    Haven't checked my 6.2 lately...may drive him this week.
    Greg

    07 Cadillac DTS NHP, 86 Chevy C10 6.2L Silverado SWB Apple Red--sold, 2001 White VW New Beetle GLS TDi 50mpg! 1995 VW GTI VR6--sold and greatly missed!
    "Try not!...Do!...or Do Not!...There is No Try!"--YODA

  20. #120
    moondoggie Guest

    Post

    Good Day!

    panchosteam: One tank of fuel tells you just about nothing about mpg. I don't say this to create any offense, but most folks are unaware that it takes a LOT more data than one tank to know anything about your mpg. If you want to know why I think this way, you might want to look at my gasohol mpg (click in colored text) post.

    The 84 Sub I used to own (3.73 gears, 700R4) would reliably return &gt; 20 mpg on the highway, but I always drive the posted, so I seldom drove even 65 mph. mpg on the 84 Sub, & the 82 pickup in my profile, seems to drop DRAMATICALLY over 60 or 65 mph.

    Blessings!

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