I would have to agree with that.Go bigger if rebuilding as the cost is the same other than buying the 351 long block. A good RV type cam would not hurt either to give a little more low end grunt. Long tube headers and full exh system as part of the rebuild.

Now with any motor you need to look at the HP & torque bands and the RPM this happens in, then look at your trucks RPM at the speed you normally drive at are they close? My guess they are not and most likely below it so you may get a little better MPG just going witha lower gear like 3.55. Yes this will get the RPM higher but because you are inthe power band the motor does not have to work as hard so will not burn more fuel as your foot is not deeper in the pedal.

I do like the fuel injection (FI) for may be getting better MPG. Gary said Eddy 4150? Carb and getting better MPG. My guess it has smaller primaries than the Holley carb and why the better MPG. I thinkHolley makes a carb like it also but most you see are not like that. If stayingwith a carb I would look into one of the smaller primary type carbs.

Depending where and how much (time) I tow that much weight I might look into 3.73 gears, small primary v4 carb, long tube headers and a Gear Venders OD unit. https://www.gearvendors.com/index.htmlI would check the power band to see if the 3.73 gear & the OD unit would put the RPM where it needs to be.

BTW I bought an 81 F100 with a I6/ 4sp grannylow hopping it gets better MPG than my DD 02 Durango w/5.9, auto OD, 3.92 gears. I bought it to pull my car trailer. I will try pulling the trailer local first tosee ifit will work before pulling anydistance with the F100.

Dave ----

 

I need to check the local yards for 351s quick craigslist check is a bust(however tons of sixs for cheap) I know I'd need the engine(obviously) acc brackets, intake, and to either widen my current y pipe or build a new exhaust.

Fuzzface- I believe your referring to a spread bore design which doesn't seem to be common with Ford's, if I were to take that route I have several Q-jets laying around, which I know some people hate but as my first car was a Z/28 I learned that thing inside and out. I'd love to stay carb'd for the fact that I wouldn't have to change anything but can't deny the benefits of fuel injection

 

QJet is my favorite carb. Lots you can do with them. But the new Street Demon brings the best of the QJet and ThermoQuad. Bolts where a standard 4bbl goes.

 

Oh god that's an ugly carb.

The throats certainly mimic that of a qjet. Is there a specific model you'd suggest?

 

I'm going with the 750 CFM with the polymer bowl. I have it but haven't built the engine yet. It'll be a 400 with lots of deep breathing capability, so you might not need that much carb.

 

The desire to have my cake and eat it too as far as hauling capacity and mpg is what made me get the IDI diesel. Prior to that I had I-6's. Both engines have very similar characteristics as far as supreme low rpm torque in exchange for a limited power band, but the IDI feels like two I-6's under the hood while still beating an I-6 with respect to fuel consumption. However... the key to good mileage with the diesel is tall overdrive gearing and engine tune up. Guys with a worn out injection pump/injectors (and therefore timing issues) and have a C-6/4.10's complain of 10-12 mpg while someone with a maintained engine and ZF-5/3.55's hits low 20's all the time. IDI's transplanted into light F150's can approach 30 mpg with the right gearing and tail wind. As far as engine weight in a light truck, the IDI is only slightly heavier than a 460 which was offered in F150's. I have not heard of any issues with IDI transplants into F150's or Bronco's.

As far as my personal mileage experience, my 1986 F350 4x4 crew cab 6.9L NA 4.10's T19 4 speed + Gear Vendor overdrive (3.20 final drive ratio) got 17mpg empty. All the time. No matter how I drove it was almost always the same. Loaded and pulling a trailer I was ~12-14 mpg. Not bad for a high suspension 4x4 weighing almost 8,000 lbs empty. With the overdrive off (4.10 drive ratio) I would get 15 mpg.

Last year I helped a member here ship an IDI truck to Europe. It was a 1986 F250 Supercab 4x4 6.9L NA 3.55's and a C6. He drove it to Arizona from Oregon and was getting 14-16 mpg. We swapped in a ZF-5 (2.73 final drive ratio) and when I drove it to the Los Angeles shipping dock I had over 2,000 lbs of cargo in it (~9,000 lbs total vehicle weight) and got 21 mpg driving 60-65 mph. On the interstate I was driving 70-75 and was averaging 18 mpg. My truck is getting a ZF-5, and with double overdrive (2.43 final drive ratio) I should be able to hit 20+ mpg with my 4x4 crew cab.

I am not trying to talk you into or out of anything, just offering information not yet covered in this thread since I have diesel experience. They are not for everyone, but I love mine. If I had a gas engine in my crew cab I could not afford to drive it. If I wanted a gas truck again I would go back to the I-6 (still able to pull a load) and try to keep the truck as light as possible for mpg. My I-6 trucks with T-18 and 4.10's got 15 mpg empty, but I kept the speed down around 65-70 max because of the gearing.

Nice low revs and a slow fuel needle

 

I would think the 750 a little big if trying for MPG as the air/fuel is not mixed as well over a 650.
Now if top speed/HP numbers then yes a 750 and even then you may not see a difference.
Had a 360/auto drag car started with a 650 DP, swapped on a 750 I had and no change in times but had a BIG bog off the line I had to work out. If I was not so lazy I would have put the 650 back on.
So something to think about on carb size.


And yes it was the spread bore carb. I have not messed with them but hear if looking for MPG and good top end (speed & HP) numbers that is the way to go.
I have not compared carb to carb replacement FI on the same motor so cant say if any better or not other than what you read form people selling them.


As for diesel I did have other make both NA & turbo and loved that truck body just fell apart on it. with the turbo it was like driving a gas motor truck on take offs but had the MPG of the diesel. Now days I don't know how much that option in on a new truck but last I looked (2002) it was 5K and just could not swing it.......I am kicking my self not getting it.
But switching a gas truck to diesel is a little more work than I would like to do.


Dave ----

 

Swapping in a 6.9/7.3 into one of these trucks is really easy. Not any worse than one gas engine to another. It's all mechanical, so the only wiring you need is the shut off solenoid, a manual glow plug push button and the starter. You need the engine brackets and a return fuel line to the tank, and a different core support if you want to keep the huge diesel radiator (not a necessity in a light truck but a good idea if you're going to tow heavy). Used turbo kits can be had on eBay Craigslist and Facebook groups for ~$500 ish and get you in the 200 wheel horsepower ball park which is similar to the early stock Powerstroke trucks. Main thing about a diesel swap is legality if you live where there is emissions testing. I would absolutely love to have an IDI Bronco, but I don't want to build a truck that I can't register, or end up in MVD hell trying to get some janky state-assigned assembled vehicle VIN, class III inspections and other overpriced nonsense. In some states it's just flat out not allowed, but the swap itself is pretty easy. There are a lot of guys putting them in dentside, bumpside, and earlier F-series trucks.

 

Great photo, Jonathan! Makes me want to "hit the road"!

 

Yes, great photo and great post about IDI's. Wish there was a "like" button.

On the carb, I went with the 750 CFM Street Demon on Tim Meyer's advice. He said he's seen 650 CFM carbs on 400's like mine (aluminum heads, high flow intake, and roller cam) actually pull enough vacuum to bring in the vacuum advance - and cause detonation. Since I'm building this to be bullet/grandchild proof I opted for the bigger carb. And, by the way, Tim dyno'd an engine with my carb as well as a Holley 750 and the Street Demon came in a very close second.

 

Wow, that's really fantastic info on the diesel. Thanks! Is there one you would suggest over the other 6.9 or 7.3 when all my ducks are in a row I should be able to find one fairly easily, hell there was a 6.9/c6 for $600 but it went pretty quick. Would the crossmember perches be the same as the v8? Would I have to change the tank? This one is brand new although later I'd like to install a larger one. Last time I went to a dump my truck weighed about 4500lbs even with my liftgate, which will of course go up once I swap the cab and add a/c and hopefully a late model seat.

 

I will say if money/time weren't a concern my ultimate goal for the truck would include a 4bt, 47rh, sas and 35's and as much fuel tank as I could fit under the truck so that my wife and I could go anywhere and explore, from the back woods of west Virginia to the Colorado river.


However with all my other projects and her time consuming(yet successful) career, a solid dependable truck is all I need for now.

 

The 4bt idea isn't far fetched as many bread trucks were sold with them and a Ford tranny. So they have the right bolt pattern to accept at least the C6, E4OD, or ZF5.

 

Being a 4cyl Cummings I'd rather grab the engine plate and everything off a 6bt as the 47rh is based off the 727 with od and a lockup converter which doesn't require a controller, you just need to ground a pin for each function which makes it very simple

 

You obviously know more than I do about it. Would love to follow along on that conversion.