351m/400 question
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Tuned
Joined: Feb 2007
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From: Kingman, Ks
351m/400 question
I've been researching this engine and have seen that it isn't supposed to make a lot of horsepower and doesn't do very good on mpg. So first of all is this right? So would it do anything significant to put on a 4 barrel carb, new intake, headers, open dual exhaust, and take off any smog components? How much would this help mpg? How much would this help mpg? I'm talking about a 400 in an f150 with a 3.5:1 gear ratio going about 65 mph. Can you get it anywhere close to 13+ mpg or am I just shooting for the stars? Would the 351m make any better gas mpg?
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400 in my Crew Cab, 10mpg uphill, 10mpg down, 10mpg against the wind, 11mpg with the wind, if I'm going downhill. Hook up my trailer, 5mpg. Varying the speed doesn't help either. My motor is cammed, Wieand, bored 030 over, headers with duals. Re torque and horsepower, I'm very pleased with the results, tows the travel trailer loaded at 7000lbs. I won't say that you don't feel it, because you know it's there, but it certainly doesn't strain alot doing it.
Last edited by Bouts21; Mar 2, 2007 at 09:25 PM. Reason: adding information
Posting Legend
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My stock but very worn 400 in an F250 with T-18 and 3.07 axle made 12MPG uphill downhill etc etc up until the time the engine really started going downhill due to oil pressure loss. It had a 1406 Edelbrock carb. Face it, these trucks are heavy iron bricks. The addition of an overdrive and other expensive modifications can help the mileage but unless you can make the modifications for a very reasonable cost they are not cost effective. Even a used overdrive unit will run around $500 and most run higher. New units are thou$and$... You can buy a LOT of gas for $1000 considering it is not the direct cost of gas but the cost of the mileage increase etc.
10,000 miles at 10 mpg at $2.5/gallon = $2500
10,000 miles at 15 mpg at $2.5/gallon = $1667
That $833 difference over several years can pay for a lot of improvements but that 5MPG gas mileage improvement is also HUGE which would cost a lot of $$$$. Run the math yourself and add up the costs of improvements.
10,000 miles at 10 mpg at $2.5/gallon = $2500
10,000 miles at 15 mpg at $2.5/gallon = $1667
That $833 difference over several years can pay for a lot of improvements but that 5MPG gas mileage improvement is also HUGE which would cost a lot of $$$$. Run the math yourself and add up the costs of improvements.
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Tuned
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From: Kingman, Ks
Would there be anything to gain if it was a 351w or 351c? Or are those even worse on mpg?
So if you took an old stock 400 that was making 9mpg going 70mph that had 110k miles on it and put on a edelbrock 1406 with performer intake, headers, dual exhaust and took the smog off of it would there be any difference at all?
So if you took an old stock 400 that was making 9mpg going 70mph that had 110k miles on it and put on a edelbrock 1406 with performer intake, headers, dual exhaust and took the smog off of it would there be any difference at all?
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My truck before the engine rebuild was getting 10mpg, after the rebuild it gets 10mpg, if fuel economy is the overriding consideration, a smaller truck or diesel car will be the answer. Cleveland, Windsor, M, 400, expect 10ishmpg. Also maybe slowing down a wee bit might net 11-12 mpg, maybe, big maybe. I see your ratio is 3.07, slowing down a wee bit with that ratio and 11-13 numbers might be doable. Guy around here has a 300 six in a 78(?) he runs mid teens for mpg. So there's another option.
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Sorry, read wrong response! A 3.07 gear would lower your engine RPM on the highway, but it wouldn't make a noticeable difference in fuel mileage. Being a 4x4, a 3.07 wouldn't be too good for offroading where a numerically higher gear would be prefered. I have 4.10s in the Crew Cab, run between 2500-2700 on the highway giving me somewhere around 60mph. Works for me. The 3.55s in your truck are a good compromise gear.
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From: chicago burbs
try a comp 240 or 252 cam with the stock intake and carb. make sure your choke is opening all the way. running 65-70 down the road is going to eat gas, leave a few minutes earlier and save a few $$$. a numerically lower gear ratio is going to turn the engine slower and save gas, but how long would it take to break even after the cost of re-gearing? if you don't need the low gear ratio for off road or pulling weight up a grade, the 400 should have the low end to pull even a 2.50 gear on the road, but it's a compromise. a Ford 4-by ain't no honda civic, just the facts. i wouldn't put a 4bbl, intake and headers on to try to save gas.
what trans?
what trans?
Last edited by grclark351; Mar 5, 2007 at 12:54 AM.
Laughing Gas
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From: Tri-Cities, TN
I get 13-14mpg unloaded from my '79 F150 4x4 T18 3.50s 400 riding on 33x9.50-15s. The only mods are the smog pump is unhooked, I installed a thermo fan clutch and run synthetic lube in tranny and tcase (and diffs, soon). I don't hammer on it, though. When hauling a lot of material, I have seen as low as 11 mpg. My F250 w/351M (4x2 auto) got 11 no matter what unless heavily loaded. The fan clutch and 33s made about 1 mpg difference to the good. Also runs better with a Robert Shaw 193 thermo.
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From: saskatoon,sask
my dad had a 79 f100 custom supercab 2wd 351m he swapped to a 76 460 gained hp,trq lost 1to2 mpg, best it got was 13mpg , 9mpg fully loaded pulling 21 foot camper
351m arn't worth the trouble inproving, for the same money if not less you can swap
351m arn't worth the trouble inproving, for the same money if not less you can swap