Looking at a 79 F350 supercab longbox 4x4 highboy with a 400 and a c6, not sure of gearing (important piece of this question I know so for the moment assume a lower gear ratio in truck). I plan to spend a lot of drive time in this truck mostly highway and I'm not too worried about going much over 65mph. It will be hauling my old 80's 8' camper through mtns a few times/year which weighs pretty much what a lot of older campers did.... lots. I also hope to a few times a year haul my aluminum enclosed car trailer with car which loaded is ~7700lbs. They wont be hauled together though. If I threw headers with full exhaust, rv cam, and fuel injection at the motor what are your thoughts on couple things:
- unloaded highway cruise mpg. Has anyone done the above and seen good results? I feel like it may be a 2-4 mpg gain? But maybe someone has tried something similar?I don't really have a baseline 400 mpg with a stock truck like this but am guessing 13ish??
- Will that 400 (if in reasonable condition/compression etc) with those mods leave me wishing for more when hauling either the camper or the trailer when I hit a solid hill? I drive a 2000 7.3 at moment so realize it wont haul like a diesel but.... curious on if it'll be a painful experience (with a wife in the truck haha)..


Thanks for any thoughts, appreciated.

 

the 400 is a very capable motor, but unfortunately it got a bad rap because it came out at the start of the emissions crap and so came retarded and really tuned down. There are some great articles out there on waking the 400 up. Headers cam and say a Holley or Edelbrock throttle body EFI system would be a great start. But you need a little more. One of the biggest down falls of the 400 except the retarded cam, is it only has like a 7.4:1 compression. So that is another area to look at.

I am not a Highboy expert but one will chime in, I believe they quite making Highboys in 76 or 77.

You are gonna get crap gas mileage. I think 13 in a supercab 4x4 350 is a dream. It probably gets 10 or less, and 5 or 6 pulling. The upgrades will help some. Mine is mostly stock, 2wd SC F250 400 c6 and it for sure needs a 4 barrel. Can't mantain 55 uphill with camper on it.

Just my $.02

 

An Edelbrock intake with a 4bbl carb and dual exhaust will bring smiles and miles of happiness. If you go deeper, an RV cam and new timing chain from a 70-71 400 will eliminate the 8 degrees of retarded timing bringing more smiles...but none of this will help fuel mileage. 8-10mpg and maybe 11 on a downhill would be all you can squeeze out of a 400.

 

I run a stock 400 in a '78 F250 4x4 standard cab. 4.10's and 31" tires. Does well with a 6500 # trailer... wouldn't be afraid to do more but it would be on the slow side.

Timing set, cam, 4 bbl and headers would help a bunch. If re-building, more compression with better pistons would help more. Have a look at Tim Meyer's offerings. With mods described it would do fine... not like a Diesel but OK in your application. Fuel consumption would most likely be single digits.

 

hmmm, thanks for the input. If the mileage is going to be that kinda low anyway wonder if I might be better off finding one with a 460 in it and do similar adds (F.I., headers/exhaust, and maybe cam).

 

the 460 will be no better on gas, i dont care what any body says,little more pulling power but that is it.

 

460 will be considerably less mpg when you have to step on it.

 

I had a 79 f250 with mild cam, headers, 4bbl, optimized ignition timing and fuel curve...truck had no gearing...3.55's and 37" tires. Best empty mileage was 12mpg and avg was 10.5-11 mixed driving.

Truck made 240rwhp and 355rwtq on a mustang chassis dyno.

Most I pulled was 9500lbs and it did it well enough. 7k was a good bit better. Power actually wasn't as much of an issue as simply getting pushed around some at 9500.

Modern diesel truck is going to pull way bette though.