i have a 79 f-150 and i have had trouble finding out what the motor is. The guy i bought it off of said it was a 400m but my brother in law said he thought it was a 351m. everybody i've talked to said that they dont think it is a 351 because it looks way to big to be a small block. I looked at numbers for the firing order and they matched up to the 351/400m and the motor only getting 9.5 mpg and people say that it should get better than that. I also would like to know how to bump the mpg up a bit.
351m or 400
- Thread starter 79ford4x4
- Start date
The 351 modified is a big block,I found out the hard way myself.Its the same block as the 400.
bigwhitebus
New Member
fuel injection only way I know of short of changen the gears.
79Crewcab
New Member
The 351M and 400 Blocks, Heads, Intakes, ect. look Identical. They, along with the 351C, are in the same Family aka the "335 Series" The only difference is the 351C (and a small,small percentage of the '73 400 with the FMX Trans)uses the small block bellhousing pattern. A lot of people think of the 351M as being a small block but it's the 351W that shares its genes with the 221( Yes the 221 was A V8 with a 3.5" bore and 2.87" stroke produced from '62-'63),260,289, and 302. The M has often been labled to mean "Modified or Michigan" but neither is really the case, it was simply M to differentiate it from the other two 351's.The Windsor block is smaller and the Thermostate housing bolts to the intake, where as the 351C,351M and 400 bolt to the top of the block. The Windsor has a cast aluminum timing cover like the other small blocks as well. The 351C, 351M and 400 block extends past the timing set and has a 1/8" metal cover. Bottom line you can't tell the diffrence between the 351M and the 400 (the 400 is just a 400 not 400M) without a tag or the casting numbers in the block and even then the rotating assembly is interchangable so you can "stroke" a 351m to 400 by simply changing the crank and pistons (they use the same Rods). Sorry this is so long winded but a lot of people often get this confused. I'm only trying to help clear things up. As to your MPG, the 400 I yanked from my '79 was only getting around 7.5 to 8 mpg after a thorough tune up and carb rebuild so I ran a Compression check and found the engine was in need of a rebuild so I put a rebuilt 460 in and get about 15-16 if I keep my foot out of the Carb otherwise I get 12 loaded or unloaded. Well I've rattled on long enough you might check out a book called "How to rebuild your Ford V8" by Tom Monroe. I hope this helps
>-79BroncoF250
New Member
Look at the Harmonic Balancer, The one on the 351 is smaller then the ones on the 400. The one on the 400 is about 1.5 inches wide and solid all the way around. The one on the 351 is about inches wide and is not solid all the way around. I hop this helps 
johnfacundus
New Member
johnfacundus
New Member
johnfacundus
New Member
look at the right head at the top right corner of the head casting to see if there is a "M" cast on the head. If it has the "M" it is a 351 cid. If there is no marking it is a 400 cid. The marking will be the same on the left head but in the rear at the top corner (since the heads are interchangeable right or left). This is assuming the heads were not changed by a previous owner. the only differ in a 351M and a 400 cid is the pistons and the crank (stroked from 3.5 inches to 4 inches). By the way the "M" stands for Michigan which is the manufacturing plant location. This is similer to "C" for Clevland, Ohio and "W" for Windsor, Canada. Hope this helps let me know.
fordtruckman
New Member
that is untrue. the 351m doesnt always have the m on the corner of the head. there are several markings in the front corners of those heads. since they aslo fit clevlands too. there is the blank head which is on the 351m i have. there is a the m head the 2 head meaning 2v and the 4 head meaning 4v.
The difference between the 351M and the 400 was the stroke. The 351M had the 3.5" stroke and the 400 had a 4" stroke. So the motors were virtually identical except for the crankshaft. Here's agood link for you if you want to investigate further...... http://www.fordcobraengines.com/Modified%20History.htm
BlackFord77
BlackFordTruck
If the valve cover says 351/400M it means it uses the 351Cleveland type block which is modified into a 400 from the factory. To gain power/economy drop the mechanical fuel pump for a good electric and try a new cam, lifters and pushrods. Really though if you want good fuel economy with a big engine use a supercharger.
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