Economy/MPG driven IDI build?

[dakotajeep]

I am curious if anyone besides agnem and david85 have built IDI's for good MPG drivers.

I am curious whether you could get a low 20's mpg IDI pickup. It would be fun to have pickup to drive for economy.....but I suppose that would be a diesel VW pickup fromt he 80's.....

In my mind I am thinking:

Fiberglass hood
3.07 gears
Possibly an aluminum flatbed (impractical?)
Light Weight bumpers?
Remove one fuel tank?
Stripping no essential peices from the vehicle.....

Just thinking out loud....sorta...

Thad

 

[91f2504x4]

A trip I recently made that was about 85-90% interstate, I averaged 18.9 mpg, I believe it would only get better if I had a smaller lighter tires and a little taller gear in the rearend, but being that I also pull a gooseneck a lot, I think I will just be happy with 18.9mpg. I think mine also might do a little better once I get it alligned and do a few other maintnence things, like replacing my steering box, that little bit of play that causes you to have to fight it from time to time can't be good for mileage.

 

[dakotajeep]

Those electric fans must be helping a bunch.....its something I didnt think of.

 

[towcat]

I tried the 3.07 idea for a while. It really sucked in the hills. in fact, I had to be deeper into the throttle and down a couple of gears to even maintain any speed. as a result, the mileage suffered intensely.

 

[91f2504x4]

Those electric fans must be helping a bunch.....its something I didnt think of.

The only time I have ever heard them even come on is if I leave the truck idling for a long time. My cooling system is all new so I guess it cools just fine without them, they are set to come on at 185F at the radiator, and I had let it idle in the drive one day for quite a while and they still weren't coming on, so I revved it up and down a few times and they kicked on for just a second and then went back off. But I have found that if you shut the truck off for a few minutes and allow the heat to soak into the radiator and then turn the key on, they will run for a couple seconds.

Hopefully once I get the A/C all hooked back up it will still be sufficient to cool everything.

 

[dyoung14]

i have gotten 20 mpg with my truck but thats also me just put putting around never gettin above 45-50 and not taking off fast and stuff like that, but if i "hot rod" it then my mpg tend to drop just a little bit

 

[DragRag]

I am curious if anyone besides agnem and david85 have built IDI's for good MPG drivers.

I am curious whether you could get a low 20's mpg IDI pickup. It would be fun to have pickup to drive for economy.....but I suppose that would be a diesel VW pickup fromt he 80's.....

In my mind I am thinking:

Fiberglass hood
3.07 gears
Possibly an aluminum flatbed (impractical?)
Light Weight bumpers?
Remove one fuel tank?
Stripping no essential peices from the vehicle.....

Just thinking out loud....sorta...

Thad

I would stick with 3.55 gears, they are high enough for the MPG's you want. Run 235/85/16 tires at max pressure, keep the speed below 60 mpg. No need to remove a fuel tank, it will not make a difference at all. The rest of the light weight stuff will not help that much either I don't think. Advance the timing andf bump the fuel pressure for better MPG. You will get better power to, but you will need to keep your foot out of it to see the returns. If you are running big tires now ditch them for a 1-2 mpg increase alone.

 

[LCAM-01XA]

Those electric fans must be helping a bunch.....its something I didnt think of.

Well, I'm sure they do help, however:

My truck used to heat up in traffic cause of shot fan clutch, so when I was up to speed the thing was essentially freewheeling better than when it was new, hence less drag on the engine - I used to get steady 18mpg then, with consistent 20 on long trips. Then once while I was in Chicago the darn thing almost overheated on me, so when I gt home I made metal braces and locked the fan to the water pump - and what do you know, fuel economy was still in the 18-20 mpg range. I now have a homemade CAI and even larger braces for the fan (old ones broke upon shutoff once, lol), and I'm now up to 20-21 mpg consistent

 

[dakotajeep]

I'm not really referring to my truck in particular. I was just thinking how cool it would be to have a runabout diesel truck that got good mpg.......

With the weight savings I was thinking more along the lines of trying to lose as much as possible to improve the power to weight ratio.......like agnem's F150 diesel which I think someone said weights around 3000lbs.

I know for me that if I went back to road tires I would gain back 1.5 mpg since thats what I lost going to mud tires. If I got 235's and filled them up to max pressure that would probably get me in the 18 mpg range.

The reason I brought it up is I saw a 87 2WD F250 for sale and thought that might be a cool idea.....to get good mileage from an old truck......

Thad

 

[david85]

Ground up type of deal eh?

For the electric fan question, I never had one on mine, but I did run my truck without any fan at all for most of this winter and probably will again. What I found is I had NO overheating problems at all, and for short trips in town, my MPGs gained 2-3 on average. Highway was something I wish I could have tested more, but didn't have much of that last winter with the slowdown in work. Best I could figure was a 1 MPG gain for freeway cruising. Most of the losses around town seem to be from the warmup time. Plugging the truck in might be an experiment for another day to see if that is indeed the case.

For the ultimate MPG IDI, I would consider 3.08s, but the difference from 3.55 might not be that big if you have an overdrive transmission. I'm at a loss to explain Towcat's experience with 3.07s though. I only notice a lag at the bottom end of 1st gear. Once moving I simply gear down and get the RPMs up as needed. For 70 MPH, 3rd gives me perfect hauling RPM.

I'm still not quite sure which engine would be better for pure MPGs though. The 6.9 has higher compression, but my understanding is the 7.3 has slightly better combustion chambers and a little more bottom end torque for twisting tall gearing.

You also need to consider what speed you will be spending most of your time in. If its all 70 MPH, then go as tall geared as possible. If its mostly city or urban, lower is better (4.10s are great around town).

I'd start off with a 2wd F250 regular cab and level the rear springs to lower the rear cross section of the truck. 5 speed manual would probably be the best bet for transmission, and a turbocharger might not be needed if the truck will spend most of its time empty. Just a good free flowing 3" exhaust. Stock sized highway ribbed tires at 80 PSI for all corners.

A truck like that could probably be around 5200lbs empty. A bed cover would probably help for highway miles and more than offset any losses in added weight.

If you are really thinking of going nuts with this as a MPG build, then consider a belly pan under the truck to smooth out the air flow on the highway.

I've also toyed with the idea of making a sheet metal plate cover to replace the grill on my truck. This would redirect the air flow around the front slightly. Not sure if doing this AND removing the fan would be safe, but for empty runs in the winter.....who knows.

 

[typ4]

I think davids combo with one of these torque cams would be the ticket.

 

[dakotajeep]

I think david has thought about this long and hard. Its more of a hypothetical for me...If I did have the money for the truck I would give it a whirl. If I run across an old IDI close to Milford I might buy it......it would have to be under $1K or the wife would bounce that idea out the door.....

Since I live in the Flint Hills currently I end up going up and down hills for 20 miles on my way to work and the lowest speed limit is 55 mph so tall would be better. Turbo would be a must!

Thad

 

[david85]

Truth be told, it would be hard to justify a build like this without a turbo if you are going through all that other trouble. If you are on the plains, then maybe, but any hills or wind and you will need all the help you can get at those low RPMs while anywhere above 65 MPH.

I'll never forget the first time I was able to chug over a 7% hill in overdrive. I had the canopy off and was completely empty with about 60% fuel load.

Pulled toward the higher speed zone at the bottom of the hill starting at 50 MPH and a mere 1200 RPM, it settled into 1400-1500 on the way up (about 60 MPH). Barely 2 PSI of boost and no smoke, EGTs held between 600 and 650. Just idled it all the way up. The engine was still fairly fresh then and still used small amounts of oil. Since then I have been able to even tow in overdrive if the conditions are mild enough (not on hills that steep).

I wish I had more chances to drive the truck empty because its a pleasure to drive on the highway. Just effortlessly holds speed. Other family members are caught off guard when they find the truck drifting up to 75 in a 50 zone...

 

[Alex S]

Could get an old GM 6.2 truck if its just a runabout and not used for serious work, they got about 25 mpg and i have herd people squeezing 30 out of the blazers. my brother has a 85 4x4 suburban fully loaded and he gets 25 highway (man it ****** me off)

 

[LCAM-01XA]

And speaking of 6.2 GMs, I found a '91 Suburban 4x4 with one of those and OD, and I'm really tempted to snatch it