Be warned, this will be a long post. (This is a copy from a post I made over on OBN but wanted some feedback from the guys here too, hope nobody minds)

Since I'm tired from a long week and have nothing better to do on a blustery saturday afternoon, my mind is thinking impure thoughts again.....

Here's the story of my 1986 F150.

Its been in the family since the late 1990s - longer than any vehicle we ever owned - and handed down to me from my Dad when I turned 16. It has some partial restoration in it (box still needs replacing), sits fairly low to the ground in stock form and rides almost like a cadillac. This is the truck I learned to drive on and it was used as a spare when my 6.9 was being overhauled. For the most part it just sits in the yard waiting to be put to work again. Breaks my heart to see it like that.

This was ford's first truly modern half ton pickup with port fuel injected 5.0L V8 and automatic 4 speed overdrive with locked converter in 3rd and 4th gear. It still has the AOD transmission and factory 3.55 gears but the engine was replaced with a 302 propane carby setup before my dad bought it (not sure of the exact year). Maybe its the original engine and was simply rebuilt to the current LPG roller cam setup, I'm not really sure.

Best MPG this thing ever managed for me was 13.5 many years ago. Average is 10-12, which seems pretty typical for 302s (LPG is usually a minimum of 15% lower MPGs than gasoline assuming ideal tune). Not particularly powerful or economical, but in fairness the simplicity of the truck has made it extremely cheap to maintain and has in fact been more reliable than my diesel over the years I've had it - and that's saying something.

So what could I do with it?

For a little while I was toying with the idea of adapting a 5cyl MB diesel engine but the lesson from my truck and the evidence offered by the Nightmoose project* leads me to believe that while light, import diesels are nice, they might not be all that economical after all when you see what a 1000lb engine can deliver in terms of MPGs. So now I'm giving some thought to dropping either a 6.9 or 7.3 diesel into the F150. Not sure if turbocharging is worth while since this truck is normally a mere 4500lbs empty in stock form (right now its closer to 5000 or more with the propane bottle and canopy). The lower stance also means smaller wind resistance and with a bed cover it should scoot along just fine even if towing the flatbed trailer we use for the business. The idea here is for maximum MPGs even if some power is sacrificed and I'm not yet convinced that for running at light load a turbocharger makes a significant difference in economy.

Transmission options

AOD...

I love the stock AOD transmission but the gear spread is horrible even for a relatively high reving 302. Anything short of WOT and it falls on its face between 2nd and 3rd; as for 3rd to 4th - thats a 1000 RPM gap at medium throttle.....so even if it could be built up and adapted to a diesel IDI, I'm thinking it would be a very poor match. Too bad, because this is an all mechanical (not even a Vacuum Modulator) non computer shifted transmission.

ZF5...

I do like driving stick shift but my one experience with the ZF5 didn't impress me. The clutch was just too heavy for my taste and makes me think the truck would not be very comfortable to drive in stop and go traffic. As it is right now the F150 is easily the smoothest, and quietest vehicle in the family and I'd like to keep it that way even if it is used occasionally for work. One nice thing about it, is how low the tail gate is to the ground. So much easier to load and unload material or tools. Something that seems to have been lost with even most newer compact pickups having a high stance purely for fashion.

A single mass flywheel would reduce that problem but its still a fairly heavy clutch. I am also starting to question the notion that a manual transmission is superior to an automatic that is equipped with a taller overdrive and locking torque converter. If we compare the ~30 MPG results of the 3000lb Nightmoose with the ~25 MPG results of my taller, longer, 6000lb truck it makes me wonder if the E4OD really is such a fuel sucker.

4 speed stick...

The T18/19 transmission doesn't have overdrive and if used with tall gearing to compensate (which would be easy with a ford 8.8 rear end) then gear spread and uphill starts would become a problem. Adding an under or overdrive to bridge the gaps would address this but also add more parastatic losses and more weight to the truck. Over all, I don't think it would be a good match for what I'm after.

E4OD...

Well really, why should I mess with a good thing? Yeah, I know, this transmission doesn't have happy memories for many members here but my experience seems to be an exception. This gives me a taller overdrive ratio than all options but the AOD and decent gear spread thanks to the torque converter (compared to a T19 and some 2.5:1 diff gears, for example). It would also be the easiest to adapt into the truck since its already automatic. Since it would be installed in a fairly light, and overpowered vehicle, it should hold up even better than my current rig.

This time it would stay as a strictly 2wd setup though. Eliminating the transfer case would get rid of some extra weight and parastatic losses. This automatically adds over $500 for the baumann TCS system but since it works so well I feel its worth it.

Thoughts on gearing and final drive ratio

Right now, I'm running my F250 with a final drive ratio of about 2.18:1 in overdrive. For empty, it will easily burble its way over 5% hills anywhere over 55 MPH in top gear without smoke or high EGTs. As I documented in the "moose pump review" thread, it can also deliver consistent economy between 24 and 25 MPG when empty at that speed. Same conditions but towing a trailer, and it drops to the high 18s. Engine RPM was about 1200.

However, I later tested towing MPGs in overdrive at 70 MPH average and the results were enlightening. 2 tests; one in overdrive, one in 3rd. Both yielded 15 MPG. So getting low engine RPM isn't automatically going to deliver higher MPGs. The trick here is to get over all engine loading low at the same time as RPM otherwise it simply results in a bog down situation even if the engine isn't rolling visible smoke. Higher speed = higher engine loading and the RPM simply didn't line up perfectly for the fuel delivery/efficiency curve.

But, since the F150 will be a minimum of 1000lbs lighter and has a lower stance, it might be able to tolerate the tall gearing at higher speeds a little better. So the question is, what gearing to I aim for with something like this? Since this is a half ton rear end and I will not be converting to anything heavier, the gearing options are quite wide and 3.08s are not the tallest option available. The ford 9" rear end can supposedly take a bit more abuse in terms of torque and horsepower but it can't carry as much weight so I'm not sure if swapping to a 9" is worth while.

Since this I wouldn't put a transfer case in the truck, I would be weary of installing gears that are too tall. After all, how can it be still called a truck if it can't tow a trailer?

For tires I would run the stock size of 235/75/15 but am thinking of using load range E rated tires to get the higher pressure and lower rolling resistance....highway ribbed.

Currently the truck is sitting on mags. Not sure if going back to the stock set rims would make a noticeable difference.

Now for the engine

6.9...

Proven reliable, safer bet when buying since cavitation isn't a big factor with cylinder walls that are over 1/4" thick
That makes rebuilding or modifying one a much safer investment.
Higher compression (except early years),

7.3...

Available with serpentine belt (92 - up) which is a little more efficient, and I'm wondering if the newer prechamber design is better for economy (looking for feedback about this!!).
Stronger head bolts,
More low end torque
Lighter flywheel/flexplate (after 1992).
Better rocker arms.
Lighter block due to thinner cylinder walls

To turbo or not to turbo.....hmmmmmm
From what I've seen so far, there is little evidence that adding a turbo really makes a difference when running empty and since that my first goal is fuel economy, a turbo might not go in.

Torque cam**. Unfortunately, I still haven't seen much evidence on this in terms of economy results. Whats worse for me, is that tall geared trucks like mine are extremely rare so there is really no way to know for sure. At the very least however, it would be a cheaper alternative to a turbocharger. I wonder if 190-200 Hp would be unreasonable for a non turbo 7.3? That would be more than enough considering how much of that would be low end grunt.

Not sure if I would go through the trouble of a complete rebuild but it would be nice. Maybe have everything balanced this time too

For exhaust, simple clean flowing, constant diameter 3" main pipe and 2.25" collector plumbing should do nicely. A strait flowing muffler would have to be used since there is no turbo but it would be nice if I could find one that doesn't drone

Fuel system

6.9 style fuel filter no matter what engine is used with a separate clear bowl water separator.
Dual tanks. The truck still has most of the stuff for this anyway and an empty fuel tank won't add much weight. Besides, if the truck delivers a consistent 25 MPG, that works out to a 1000 mile range. How cool would that be?

Ideally I would run a baby moose pump with an economy tune (if it exists) and moose misters. The regular moose pump is more than capable of delivering good fuel economy at light power settings but without a big turbo, it would be a waste. However I'm not sure of the economy difference between the mutli-port injectors or regular code BBs is enough to be noticeable. Money no object the choice would be obvious though.

Some other considerations for the build

Since I would still like to work this truck a little, I would put a hydraulic brake booster in it since the stock F150 brakes of this era aren't good for much over bone dry empty. Replacing of all the brake lines would probably be a good idea at the same time. Can anyone give me a reason not to adapt the hydroboost to an F150?
I also might consider adding one extra leaf to the rear stack.

To accommodate the cooling package I would even consider building a new rad support from scratch out of aluminum to fit whatever aftermarket high output rad is available to keep the diesel cool. This might also save a little weight and it would be fun to build. Thinking about electric fans but so far, I haven't been able to really see for sure if its worth the cost in terms of MPG results. I could always just run without the fan anyway on some days

Color matched bed cover!

The general idea here is to have a nice riding, comfortable truck that delivers fuel economy better than any other truck in its size range while still having enough grunt and braking power for some towing. It should not look like a ricer with a lowrider stance and covered in plastic fairings and spun aluminum wheel covers! It still needs to be a truck.

I have my eye on 30 MPG @ 55 MPH empty
Probably will never happen but like I said, I'm not good for much else today than some day dreaming.

Any thoughts?

*nightmoose is a short box regular cab 2wd F150 that was converted to diesel using a 7.3 N/A and ZF5 speed with 3.55 gears

**Custom grind on a stock camshaft core that is said to increase low end torque without sacrificing high end power. Still wating on dyno results from some OBN members that are installing or running them.

 

WOW, that was long! and believe it or not, I actually read it.

Manuals always give better MPG than automatics. Get used to the tough clutch on the ZF5 or pay the pump.

Staying with Navistar power would be cool, but I doubt you would ever hit 30 mpg. Maybe low to mid 20's with a 6.9/T19/3.07 gears. A 4BT Cummins would give you a better shot at 30 MPG in my opinion.

For the record, my '02 crewcab Superduty gets a consistent 21 MPG at 65 mph with 3.73 gears and an automatic.

 

Sounds like you been doing some thinking. I personally like the diesel conversion idea myself over any gasser. For the trans maybe consider the gasser version of the t18-19 and use a Ranger torque splitter in front of it. These units would give you the option of useing up to 8 forward gears and the final overdrive is 27%. I plan to put one of these in front of mine when funds allow. They wiegh around 150lbs but are a simple mechanical unit.

 

The thing is that Nightmoose truck already gets around 30MPG highway (I think they said it was at 60 MPH) and it is a 5 speed. Supposedly its only 3000lbs empty and my automatic propelled truck is taller and twice the weight for only a ~5 MPG penalty at similar speeds.

I documented consistent 24-25 MPG (@55) with my truck this summer during consecutive fill ups. Lowest was 24.4, highest was 24.8 so there doesn't seem to be that big a difference and I feel most of the gain the smaller truck has is due to the lighter weight and smaller frontal cross section. My F150 will be heavier than theirs but will have a nearly identical frontal cross section. All of my MPG numbers are averaged for both directions on the same route within the same day to eliminate potential errors from prevailing winds.

So far I haven't seen anyone with a diesel the same size and weight as mine match that in manual or automatic.

 

If you are determined to hit 30 mpg, you should consider a 4BT. My 93 F150 get 24 mpg in mixed city/highway driving and 29 mpg on highway trips. I haven't done anything yet in an effort to improve mileage. I would expect that advancing the timing from stock and going from 3.55 to 3.08 gears will give me an additional 1-2 mpg.

Here is my build thread on 4btswaps.com

93 F150 with 4BTA

 

I have the T19 and gear vendor overdrive. The hydraulic clutch on my set-up makes it BY FAR the sweetest shifting manual transmission vehicle I have EVER driven... My wife even likes to drive it, and she hates manual transmissions! The gear vendor OD unit gives me 22% over in 2nd, 3rd and 4th. Makes the difference between 15 mpg and 17 mpg on mine, but it is a much heavier truck (high sitting F350 4X4 Crew Cab). Don't think you will get 30 mpg with a 6.9 in your truck though, no matter how hard you try. Smooth and quiet? Ha! The IDI's are a petroleum avalanche under the hood. You won't get that... but if you drive the 7.3 you already know this. Just plan on a good stereo to drown out the noise on long road trips. Think about the OD if you still want the truck to have some grunt off the bottom but highway gearing up top, i.e. if you want to work it and get mpg I wouldn't do it all with axle gearing.

 

you'll never get back the savings of $ invested and maintince + registation and insurance by running two trucks and only driving one at a time.
you're already pushing 25 mpgs with your f250.
if you had the f250 setup with a 4 speed and 4.10's as a serious heavy hauler/tow rig i could see some savings.
but your still working with a truck with poor aerodynamics.
so even if you git 30 mgs.it will cost you more than just running the one f250 that gets anywhere from 20-25 mpgs.

 

Have to disagree about 6.9s not being capable of running smooth or quiet. Yes they are loud standing next to one idling but with my tall gearing, you can't hear the engine when cruising at high speed. Gear down for a hill or find that one spot on the range where the muffler drones then yes, it does make some sound but over all its a very quiet truck. Wind noise is my biggest complaint but the F150 seems to be quieter for some reason in that respect too. Weather seals are a little better on that one though.

I'm not sure if trim levels make any difference but mine is a lariat with all sorts of insulation through out the truck. The carpet is over 1" thick at the front of the truck and there is a bonded rubber(like) layer on both front foot wells. The firewall has insulation on both sides, the wheel wells have insulation and the hood insulation is also still in tact in mine.

I've never driven a 4 speed stick so I can't comment on that.

It probably would never pay off to build another truck almost from the ground up, but Oh well. As it is the F150 will probably just stay in storage as a backup vehicle.

4BT-F150; awesome build there! I saw a chevy version of that swap a while ago and he claimed similar MPGs which is why I think its possible.