Max fuel economy?

[More Power]

The 6.2L "C" series diesel engine was produced from 1982 through 1993. The recent listing of the "Fuel Miser" 6.5 engine by GM Powertrain is the most current rendition in design configuration(though lacking some emissions control devices).

Regarding scenario #3, needing to tow occasionally means the engine needs a turbo. If only light trailers and for infrequent towing duties, 3.73 gears, "C" cups, and a DB2-4911/6.5 injectors. If towing more or more often, 4.10 gears and possibly TD cups. Based on lots of discussions with lots of owners, there appears to be about a 2-mpg difference in cups alone.

If towing more than 5-7K, and doing it often, I'd then go to an 18:1 engine and all the other stuff necessary to keep things cool and make More Power, which is now beyond the scope of this thread.

MP

[grape]

if you are going for max fuel milage, with the least possible drag, who are you going to have align the truck.....Sears? I would suggest NOT letting anyone with a computer alignment machine touch the vehicle in question. Those things are the devil and assume the rear end is square to the centerline of the lower front susupension pickups. But this suggestion is only if you decide to get serious about this deal.

[CareyWeber]

Originally posted by grape:
if you are going for max fuel milage, with the least possible drag, who are you going to have align the truck.....Sears? I would suggest NOT letting anyone with a computer alignment machine touch the vehicle in question. Those things are the devil and assume the rear end is square to the centerline of the lower front susupension pickups. But this suggestion is only if you decide to get serious about this deal.

This seems like a worth while thing to do for best MPG.

Someone said once here that mirrors are a big factory in fuel mileage.

I think tire pressure is big deal.

Carey

[Stage1]

MP,
Thanks for the additional information on "C" cylinder heads, are these heads the same or different from the "J" engines of those years.

Have five CUCV Blazers w/engines which I will us to build my project Blazer. They all have low mileage 25 to 45K miles and the model years are 83' and 84'.
thanks,
Les

[dieseldummy]

The heads are the same, the only difference is the precups, which are interchangable...

[dieselhumvee]

I now get 13.5 MPG in my Hummer. With the price of diesel and my being a Hummer owner on a budget (talk about an Oxymoron), I would like to improve it's mileage. First off is to ditch some weight, I think I can get rid of 800 pounds by swapping the hardtop and steel doors with a military soft top and canvas doors. I now get 13.5 MPG so say the weight drop gets me 1 MPG, I would then be at 14.5. I like the idea of C precups and some head porting. I am confident that any power loss from switching to C precups from J precups would be cancelled out by the weight loss. So if I gain 2 MPG with the C precups, then I get to 16.5. My goal is 18 in a 6000 pound Hummer, right now it is 6800 with hartop.

I may experiment with the airflow tabs from http://www.airtab.com/ The hummer is atrocious when it comes to airflow, one of the worst aerodynamic designs ever, so I could pick up a little there.

Does anyone know if the diesel depot cam is worth any mileage improvement?

I am also thinking of more drastic weight reduction including removing the trailer hitch, factory airlift hooks and brackets, swapping the steel rims and runflats with aluminum wheels, sythetic winch line instead of steel, and swapping the heavy duty rear bumper with a light duty one. I think the additional mods will be worth 400 pounds of weight reduction, so I may get to around 5600-5800 depending on what I put back in like tools, etc. The 6800 is with spare tire and a few tools, but I will be adding more tools.

[jbell]

Forget ditching weight in attempts to get mileage. I have a similiar beast in terms of total weight. (85 3/4ton 'J' code suburban with 4.10 gears) With the tire size I had, I was right in the exact same ballpark you were in terms of final ratio. 15mpg was the best I ever got, and the motor was SCREAMING down the road. Swapped in a 700r4, and now get 20mpg at 75 mph. The best and only way for you to better your mileage is to get the rpm's out of the stratosphere. (That and a fastback, slanted soft top) The 700r4 did 2 things for me, 1 was a lockup torque converter, and the other was a .69 overdrive. Both contribute greatly to my reduced engine rpm's. I'm still in the mid 2000's driving down the road, but if you look at the fuel curve earlier in this thread, fuel mileage doesn't really go bad until after 2400.

My thought is to leave everything else alone, don't worry about cam or any of the other engine options, try to find some way of getting an OD trans with lockup, you'll easily hit 18. Not sure if you can get a 700r4 built up with a 32 spline output shaft or a spud adapter to mate up to your xfer without changing overall length. Call AdvanceAdapters.com or Novak.com -- they are the best in the business of making engine / trans / xfer cases mate up.

[jbell]

should have checked earlier, your np242 xfer can accept either a 32spline input or 27spline. If you are interested in a built up 700r4, here's the drill:

1. you'll need a new flexplate, the th400 and 700r4 flexplates are different.
2. you'll need the hard to find tvcable bracket that bolts to the back of the engine, and the tvcable extension rod that's easily found from gm.
3. you'll need a 89 or later 'K' case built up 700r4 with lockup converter and all the goodies (beast sunshell, etc...) I happen to know maddoglou makes a good transmission, not sure who else does.
4. you'll need the 83-91 suburban 700r4 diesel tv cable.
5. you'll need the 27spline female input for your np242 xfer. (check with advanceadapters to see if you need anything else)
6. you may need to make modifications to the crossmember location / mount, not sure on H1's. Overall length should be within 1/2" however.
7. You'll need your trans set to lock in 3&4 internally, run +12 to it through a brake pedal interrupter switch, ditch the vacuum modulator lines, and the kickdown switch from the TH400.
8. on mechanical speedo gears, there should be something that works for you, don't ask gm, just find a gear chart and get the speedo housing , gear that works out correctly with your 5.24's and 44" tires.

Your existing oil cooler lines will just bolt up -- no worries there.

On my application, my sub went from dog slow to much faster than average traffic. From so loud I couldn't stand it, to acceptable, and from 14-15 to 20mpg. In fact, I still have all of the TH400 stuff I pulled out -- anyone interested? TH400 spells bad mileage as far as I was concerned with the gear ratio that was present in my suburban.

My guess is that you can do all of this for about $1200 -- since you were talking cam, precups, and some other serious stuff -- it should be well within budget.

Good luck

[convert2diesel]

Jbell:

In regards to your observation of better fuel mileage at 2,100 RPMs, I ran across an older power curve published internally by GM for their service people. Sure enough, these engines have their peak torgue and lowest fuel consumption at 2,100 RPMs. Meaning you are probably correct if the gearing translates into 50 to 60 MPH.

If someone could instruct me on how to post directly into this site without first posting it elsewhere, I would be happy to scan this chart in.

Was talking to a guy the other day who does cold weather performance testing for John Deere and he said not to take the specific fuel consumption figures too seriously as these figures are taken at WOT. Very informative if you are designing stationary engine applications but not much use in automotive applications. To his knowledge, no-one does comprehensive flow testing at part throttle unless it is required for diagnostic reference, or to set up computer controls. Something to do with "base-lining" the tests.

Bill

[grape]

if somebody will do this little project I'm sure I can convince our engine builder to let us use his $3400 fuel flow meter on the test vehicle to take readings at a steady speed.

[convert2diesel]

Grape:

I feel the same test could be done using a chassis dyno and some graduated beakers/portable scales. That way we could determine the consumption using known perameters. The mechanical losses could be determined based upon load and the actual engine output could be compared to actual fuel useage.

I have no idea how the fuel delivery is effected at lower dome pressures and or IP RPMs. Another factor is where the advance piston is situated and how that effects the overall efficiency of the engine at light load on the mechanical pumps.

I had some idea I was going to go to southwest Texas for my holidays in April, and would gladly donate the bio-Buick for the test, but would prefer to use a fresh engine and fuel system to get a more accurate base line to work with.

Bill

[twaddle]

Hi Guys,
I've used 4x4 pick ups with 6.2's fitted with both manual and auto gearboxes, done several hundreds thousand miles mostly carrying tools and parts for work (I repair Cat Equipment throughout Scotland).

Several times a year I pull trailers with up to 4 or 5 tons on them. (yes well overloaded)

If I was going for economy and pulling power and running light use I would definately go for a 5 or 6 speed manual. 235/85/16 tyres , use a diff ratio that suits your average use.
The main thing is the manual gearbox, to get best economy you need to keep the engine working on its torque. I've always found that a good torquey diesel engine can be worked on the torque better with a manual because you can hold the gear ratio required whereas a auto box always has slippage when you don't want it or it changes down.

Just out of interest, at the moment I use a ex military K30 (M1008) pick up with a "J" spec 6.2, 3 speed th400(no lock up torque or O/D) and 4.56 diff ratios on 285/75/16 tyres.
On a good day it gets around 17mpg (average 15), that's to the US gallon, even with a 4 speed manual I would probably get over 19mpg. Now if I only had a O/D manual, better economy, quieter.
Thinking of fitting a 700R4 as they are getting pretty good value for money for a rebuilt unit and easy to fit.

Ah if I only had the time and money to build the dream trucks ........

Regards

Jim Twaddle
Biggar, Scotland.

[jbell]

convert2diesel:
I'd love a copy of the fuel curve you have. can you send it to my gmail account? (jbell.louisburg@gmail.com)

Yes I understand that fuel curves are typically WOT, and that fuel consumption is not exactly linear at a specific rpm under varying HP output.
(The lower the specific output, the more fuel/hp that's required as engine friction / pumping losses don't change) So when I did the calculations above, and came up with 60hp, I know it 's really a little less than that.

However, in defense of the fuel curve.....
It's scary on a stationary industrial applications using cummins motors how accurate the fuel curve is. (The farm pumps water, 24hours a day, 7 days a week during the summer and we know the HP required down to within a single HP, as we have had cummins diesel, and 3ph electric on the exact same well, and KwH will give you an EXACT HP draw.) I'm always within 10% and sometimes less than 5% different than the fuel curve, (depending on engine condition) on consumption, regardless of the fact that I'm pulling at about 60% vs. WOT. (That's using an water/air chiller to maintain constant charge air temp) I understand that fuel curve can't be trusted down in the 10% - maybe 30% range, but with a NA 6.2 only making a hair over 100hp @ 2000rpm, I think we are far enough up on the %of WOT, that the fuel curve is meaningful.

[More Power]

Fuel curves are very useful to compare engines in stationary applications, but the water gets murky once we try translating that to "miles per gallon".

Vehicle weight, vehicle rolling resistance, drag coefficient, elevation, engine tuning & overall health, driver skill & technique, and many other factors affect mpg, that are not necessarily reflective of a bmsfc chart when considering fuel economy in a motor vehicle.

MP

[jbell]

MP:
You are very correct in what determines fuel mileage -- HP requirements. What we do to our 6.2 engines should make very little difference in mileage. What we do that affects how much HP we require makes a HUGE difference in mileage. In the earlier post, I used a fuel curve to 'back in' to HP used -- I find it's useful to see if I'm even in the right ball park when comparing fuel consumption vs. estimated HP requirements.

60hp to move a suburban down the road at 60mph is not in the ball park, it should be down in the 30 to 40 hp at most. So what's that tell us? To me it says that the average suburban is wasting about 20 hp somewhere. We find out where, and we get better mileage.

I personally get 20mpg @75mph, why? With 4.10 gears on a 3/4 ton suburban -- my rpm's are too high, with the tall tires, it's too high off the ground, and it's an 85 with a 41 gallon tank, meaning, it weighs alot. I don't know what the right lane is, and letting people pass me, is, well, insulting..... (meaning I have a lot to learn to get max mpg on my driving skills.....)

(I did drop it to 18, running to western kansas over Christmas bucking a 30 knot headwind....)

Did the 2" lift blocks I put in the rear to get rid of suburban sag, help my aerodynamics? Did going to a 700r4 vs. a TH400 really lower my HP requirements that much? I'm running stock 155hp pump tuning,stock 'j' precups, the low pop 6.2 injectors, nothing special on the intake / exhaust, the 'inefficient' 14bolt rear axle -- yet I consistantly beat later model 6.2 / 6.5's by a huge percentage on mpg. why?

[More Power]

jbell, For whatever reason, your engine and drivetrain are above average in efficiency. No two engines and fuel systems are exactly the same, some are better than others to one degree or another.

And, I suspect your driving ability is above average when it comes to fuel economy. Some people are more intuitive than others when it comes to driving efficiently. Gearheads and those who perform their own maintenance generally do better because they have a sense about their vehicle that others might not have.

Fuel quality can also affect fuel economy.

MP

[moondoggie]

Good Day!

convert2diesel (or others): Was my 23 Dec post on-track or not? I don't mean to be a pest, but I was kinda interested to know if I'm out in left field on this or not.

TIA & Blessings!

Brian Johnson, #5044

[jbell]

moondoggie:
I think you are right on the money on the benefits of a 700r4. I personally had HUGE problems with them in the 80's (went through 6 of them behind a 2.8l gas -- biggest piece of @#$@#$ I'd ever seen.)

However, as with all things mechanical with time and effort, they get better. I know dieselhumvee may not like the idea of a 700r4 in his H1, but consider that a 4L65 is what is in the H2 that is similiar weight, and it lives behind whipple supercharged 6.0l gas motors with no problem. A 4L65 is nothing more than an electronic 700r4 with a couple (not all) of the aftermarket upgrades to support 380ft.lbs. That's up from 360ft.lbs in the corvette 700r4. An aftermarket 700r4 can actually withstand 500ft.lbs max with all of the goodies available.

It is the cheapest easiest upgrade any 6.2 with a TH400 can do. And one of the few upgrades that will actually pay for itself with fuel savings over its life. (I've already saved about $500 over 16,000 miles, and I got my 700r4 off of ebay for $125 from a guy with a rusted out 89 diesel burb, I added a couple hundred bucks to it for strength parts -- so I'm still ahead of the game) And as MP said in an earlier post in this thread -- "If I'd known the driving improvements it would make, I'd have done it years earlier."

In comparison, a TH400 is commonly rated at 450ft.lbs, so you can actually 'upgrade' to a higher torque handling capacity with an aftermarket 700r4.

Also, lockup vs. non lockup is night and day for mpg.

The 3.06 1st gear and .69 OD makes it the widest range OD 4 speed auto available. Above all, 6.2 diesels need gears to keep the rpm's happy.

As a completely irrelevant piece of information that burns my @#$#@$ -- the (it shouldn't have the name jeep on it) 2.8l diesel that is being put in the new jeep liberty makes more HP and Torque than our NA 6.2l........ Upgrade for the H1????

[Dan Wilson]

Hay guys I have done 2 62 car transplants. Both to real good on mpg. The first one was my 82 fleetwood caddy. It had a 5.7 diesel. I took it out and installed a J motor. I gets 21 around town and 29 on the intertstate. The second is a 81 gan prix that had a 5.7 diesel. Installed a 6.2 c motor and 700r4 with 3.08 gears. gets 27 around town and 35 on the interstate trips. I love both cars.
Dan

[More Power]

Click here for the all-time best fuel economy for a 6.5L N/A powered vehicle.

I couldn't bring myself to do that, but it does demonstrate what gearing, low vehicle weight and low drag can do.

MP