with a 2 barrel and pollution stuff on it, my 79 F250 4X4 with the 351M and C6 got 9-11 mpg.

with all pollution stuff pulled off, and an edelbrock performer intake and a 600 CFM single feed single pump Holley carb, it now gets 13-16.
but if you use the secondaries for any length of time, the mileage goes back down to around 11.

 

Thanks alot guys for all the replys. I really appreciate it. I think I will go ahead and if a find a pickem' up truck with a 351m/400 in it I'll get it and see what I can do to it to make it better.

 

Does unhooking the smog pump have any negative effects on the engine?

 

If it's a CA truck with a cat and you hook it up later, you have a very good chance of plugging the cat with the carbon boogers that build up in the thermactor ports in the heads. They blow through and get stuck in the cat.
If you take this route and smell something that smells like a house heater that has not been run since last winter, look under the truck at the cat, and if it's glowing a nice cherry red (no kidding) it's plugged.
Can't see any negative effect on the motor itself but not entirely positive. Been there on the plugged cat though on a MkIV w/ a 460. Bummer.....

 

Not worried about the cat, already blew mine up. Took a big piece of pipe and punched right through that plugged POS. It's just there for looks if I get pulled over now. Thanks for the info, slstreit.
Don't know if I'll still be in Cali by next smog test (Found a really well paying job here, so not jumping to TX just yet). So I'm getting all the info I can on ripping out my smog garbage, for when I do move again. Hopefully picking up an Edelbrock manifold off of ebay soon. It won't have an EGR, so will that mess with my motor at all? Don't want to hijack this thread, but don't want to start a new one, as long as folks are on the subject... Cheers.

 

look for an edelbrock 3771 manifold so you you could use egr if you had to, or at least look like you have functioning EGR while your still in CA...

 

I'm not putting the edelbrock stuff (or headers) on until I know how long I'm still in Cali. Don't want to put it all together, and them have to slap the old stuff back on for smog. Too much trouble... Stupid Cali laws...

 

I have two `77 F150 4x4`s. One is bone stock 351m, NP435, 3.50`s and 31 inch tires. It gets 12 mpg every time I check it. Other one is a 400 with Edelbrock Intake and 600 cfm carb, c-6, 4.56,s and 35 inch tires. It can get anywhere from 9 to 11.

 

NO ONE SEEMED to mention reversing the cam timing retarded around 1973 and since then....
I'm in the same situation looking for any easy mods to increase engine performance and mileage for my 1996 351w in an EB e350 club wagon, 115k.
So far, when replacing the water pump, I fixed the timing chain cover coolant leak with new gaskets and found my timing chain had slack. I didn't want it to fail say at around 200k miles when I could replace it for
So I replaced with a chain and gears found on rockauto, CLOYES 91138Street TRUE for under $40 which allows an 8 deg advance option key slot on the crank gear. This translates to a 4 deg advance for the cam.
This increased the torque noticeably. Now I can chirp the tires on a right turn, the engine is more torquey now for sure even though I live at 6,000 feet altitude. Also the compression went up about 20 psi I tested before and after. Simply because the exhaust valves close sooner on the upstroke.
So far happy with the change. Supposedly in about 1973, most cam's were retarded for emissions and still are. I can't say about the fuel economy because I haven't owned this van for long and the gas tank hoses had a leak I finally repaired.
Of course you have to reset the ignition timing after a chain replacement.
I'm looking at possibly installing some short headers next and then probably calling it done for the lengths I'm willing to go may be the sweet spot for me.

It does seem for your situation, you should look for a newer truck from the late 1990's or earliest 2000 at the prices and see if you can upgrade for not much money.
I mean your truck should be a bit retired to low annual miles use because, not suitable for high annual miles use. Because of the age and gas mileage and reliability factors.
There are many people who put few miles on their vehicles because they are inactive, retired, used around a farm etc.
Additionally older vehicles are like time machines for people. So someone who is in their 50-60 years of age might love to have an older truck that reminds them of their family truck or late father's truck that they grew up riding in. You may find your truck can be sold for almost as much as the truck you could upgrade to.
Take care

 

#1: 14 year old thread.
#2: the 351M and 351W are two completely different engines.
the only thing the same between the two engines is the 351 engine size

 

So Tom you are saying that the 351m didn't have the cam timing retarded like the 351w did?
Well I would like to disagree at this point because I have read differently from more than one source.

 

i read to 351W and stopped because this is about a completely different engine.
as far as timing goes, i do not use a light on the 351M or 400. i set timing by ear.
advance till ping under hard acceleration, then back off a bit until it runs good and starts instantly hot.
this method has produced good running and also good fuel mileage, which stinks on the 335 series engines

 

I don't think that the retarded timing set was ever used on a 351W engine. That was used on the '73 up 460 and in the 400 but not anything else that I know of but with that said unless you checked the cam timing with a degree wheel before and after who knows where the centerlines are on your stock cam. I'd bet that they are probably pretty far back anyway on the stock Windsor cam so you might have it close to straight up with your 8 degree advance set on the engine.