The 351m or 400 (not 400m) it is of the 335 family. It is neither a big block, small block, or Cleveland. However it does share parts from a few. It has an FE bell housing pattern and shares quite a few parts from the Cleveland series.

The modified series, especially the 400 is a torque oriented engine and with proper modifications can make impressive HP and torque numbers.

As stated before emissions put a big crunch on these engines. One main part being the compression ratio. The easiest way to bump compression is through a cam and shaving the heads. More compression means a more complete burn of the fuel- better fuel economy. However, when doing this you might end up with detonation or "pinging" on 89 octane so premium might be required.
Pistons would bump up the compression but they are very hard to find in anything other than the stock form. You would have to buy 351C or Chevy rods and pistons- then have the rods sized to match the crank. Its much easier just to deck the heads and live with that compression.

Again, like others have said, there are a number of bolt on parts to wake up the engine. Intake, cam, carb and headers will help quite a bit as far as power is concerned. Getting the proper sized carb and tuning it with a vacuum gauge will yield the best MPG. Straight up or advanced timing will help the engine run better as well.

The transmission department is what will make or break MPG. You have an automatic, so that hurts things a bit. The good news is that you can modify a few things and get a 460 ZF transmission to fit. Manuals get better fuel economy and having an overdrive will help on the highways.

With the 400-C6 you should expect 8-12mpg. 8 being stock form and 12 with a few modifications.

 

Not hard to find at all for a 400. Tmeyer makes them new that provide higher compression.

 

It has a 385 series (370/429/460) bell housing not FE. The FE bell housing is shared with the MEL (Mercury, Edsel, Lincoln) series engines (383 Mercury, 410 Edsel, 430 Lincoln, 462 Lincoln)

 

I get about 12-13 out of my 351M (bored up to 400, with a 650cfm Edelbrock).

 

Only having built one engine many years ago, (250 ci Chevy Inline six), I'm far from even a novice at these things but I'm still going to ask a dumb question; how can a cam choice raise the compression?

I'm leaving open the possibility that this was simply an oversight but I am curious.

Dennis

 

You should be able to get 14 to 15 mpg with your truck I got 14.5 on the highway long 800 mile trip with a 78 Bronco with a 351 4 spd 3.50 rear gear ratio.Got 15 to 18 mpg with a 2wd F150 with 351 4spd 3.25 rearend ratio on a 4000 mile trip so if you keep your foot out of it 12 to 15 mpg is dueable.

 

I haven't heard of his products before, I'll have to look into him. I had a hard time finding hardly anything when looking into rebuilding my engine. Luckily, that was not needed.

I always thought the 429/460 was part of the FE family. I guess we learn every day! My apologies for the misinformation.

 

Go take a look at what he offers. He knows how to make a 400 scream.

 

Easiest way that i have found to improve mpg is just lie to anyone that asks.
I bought the truck a year ago, and using this method, i am honestly up to 14mpg easily, running 35s!!

Also, installing new wheel bearing, greasing regularly, and flush and fill your front/rear ends will help with mpg, as well as proper tire pressure.

 

Well that's unfortunate, my brother in law has an 08 f150 with a 5.4L triton and he gets 16-17 on average and that's mixed city and country driving. I didn't know much about the motor itself. I know these old motors do poorly on mileage and do to the limitations put on them for emission reasons really hides their potential. I don't want a smart car. I want a classic truck that doesn't do too bad on gas. Thanks for the input though

 

Hahaha thanks!

 

To be honest, if i could live in a perfect situation, i woulda kept my v6 Honda and bought my truck.
I had the Honda and a V6 Ford van. I sold the Honda due to moving into an apartment that had 1 parking spot. 3 months later the van engine started knocking and so i off loaded it and bought my truck.
Its a daily driver but i notice my life has transformed around it. I drive lesser long distances and make sure i multitask my errands.

Am i glad i bought it? Yes, more times than not.

Kinda fun to see you at this stage, lol. Once you get it, i wonder how you will feel