Hey guys.. I know this has been brought up before, but I can't find the threads.. Just need a quick response, nothing indepth..

My `79 F-150 Supercab 4x4, 4:10/4.09 Lockers, C6 Custom Tranny, 351M stock 250k miles, needs a new motor, My 3rd cylinder has 0 compression on it, while the rest of the motor varies between 130 & 145 (is this good!?).

I can get either a 351M or 400
here from our shop for about $90 difference. My questions are this...

1) What is the stock HP difference between the two!? This truck is an everyday driver for a work truck, so I don't want to go too radical, buyt maybe a gear drive, that's why I'm asking for stock ratings if anyone has a book and knows...

2) What are the torque differences!?

3) What is the big gas mileage difference going to be!?

4) Are there any mods that will have to be done to the truck!?

Both motors are long blocks. I just need to swap the intake, headers, pumps, etc, and it's a go!

Thanx in advance!

 

This is posted in the wrong forum.
it should be in thr 73-79 forum.


*Please note*

There is no 400M is is strictly a 400.

Notice on the top left of the emissions sticker I attached.

 

I meant the differences between 351M & 400. Sorry!

 

The 351M only makes 140HP, the 400 makes closer to 350. The 400 is a way more powerful engine and you will probably get the same mileage as well because your engine won't be struggling so much. In my opinion you might as well go with a 400. If you go with a 351M you can stick a blower on it because it will rev higher, then it should make some decent power.

 

350hp in a stock 400? There are alot of variances in HP between the years in the 70's but the most a 400 ever made was around 210hp if I remember correctly in 71 i think. That is because it used smaller dished pistons (think it was rated at 9:1 compression ratio) didnt have retarded cam timing, and were NOT measuring power using all the emisions equipment. I beleive in 72 it was mandated that all motors be rated in SAE NET horsepower wich required the engine to be tested with the same restrictions as it did when in the vehicle (air filter assembly, exhaust, emissions equipment, etc.). Horsepower dropped to 174HP if memory serves. Between the years ford used slightly different piston dishes and retarding cam timing after '75 i think, plus different depending on if it was a car or truck motor. 351M was rated at 145 hp on average. I dont have the torque figures commited to memory.

I would definately go for a 400. They are practically the same motor (thats probably why there is less than $100 difference) and if you ever wanted to get aftermarket pistons they are very hard to come buy with the 351M but there a several options for the 400. the 400 has a much lower torque peak too, if its going in a heavy vehicle that is good. A stock 400 devolops peak torque at a lower rpm than even the 460 (stock, equivelent years), mostly due to the long stroke the 400 had and the longer rods. Motor mounts, brackets, exhuast are all identical between 351m's and 400.

Just my $0.02

 

It is in fact, going into a heavy truck! It's a `79 F-150 Supercab 4x4, 35x12.50's, stock high boy, and it my everyday work truck! When I registered it here in california, with no equipment was a little under 7k. with everything now, at the same scale last week, just for kicks, it was weighed at 8176. So I would say it is definately a heavy everyday truck! A lot of traffic driving, over 600 lbs. in equipment and tools alone.

I appreciate the info. I was close to thinking that the 351m developed around 125, and the 400 was around 200. that's enough power for me in that thing anyways. With a detroit locker in the rear end, left turns and rainy weather here in Sunny So Cal are fun!

The only other thing... The sticker that I have in my truck for the Cal. Fed. Emissions says Family is 5.8L/6.6L, so that means that I can in fact drop a 400 in there and still pass smog, since it was in fact an option from the factory for that specific vehicle, correct!?

 

Well first of all there's no such thing as a 79 F-150 "stock high boy". The "high boy" suspension was never installed on F-150s, only F-250s and up and it was discontinued in mid-MY '77 production.

Second, there is visually no difference between a 351M and 400 from the exterior so even if the inspector can tell from the V.I.N. that the truck came with a 351M, he won't be able to say that you've swapped engines with a larger model. I haven't kept up on the smog laws in CA since I left in 1990 so I can't say for sure what's a "legal" swap and what's not.

 

So then the "blocks" on my truck is there for what reason!? I was told by someone, that it was a "high boy", he said because on a "stock" F-150, you wouldn't be able to fit 38" tires without doing any kind of a lift to it at all.

Plus, remember the only difference between a 250/350 & a 150 Supercab is 9"/D44 & the engine size!

 

Well I don't know anything about the history of your particular truck but I do know that Ford quit using the "highboy" suspension in 1977 and it was only on F-250s & 350s. All F series trucks after 1977 1/2 were "lowboy" type suspensions. Maybe someone took an older set of springs & blocks and put it on your truck.

Maybe you could find more information in the 73-79 Series forum.

 

Just wanted to make a quick correction... I stated above that in '71 the 400 motor was 210 hp... I just found a printout I had showing the hp & tq and info on these motors and in '71, before the stingent emissions, SAE net hp measurements, low compression, and retarded cam timing etc. etc. the 400 was rated at 260HP. Just wanted to correct myself since I said it was 210hp. But this is a bad comparison because although the 400 and 351M motors are virtually identical except for stroke, that there was never a 'stock' 351M similar to the '71 configuration mentioned above.

And to further what Bill_Beyer said I think the only way you can tell a 351M from a 400 is to check the actual displacement of the cylinders. I dont know how you actually do that, but in racing it is done to verify that motors are allowable in certain classes i beleive. To my knowledge I think all the external casting numbers are the same too.

 

the only real way to tell the difference is to check the part num on the chrankshaft. the harmonic balencer is thicker on a 400. a 1k on the crank is a 351m. the 400 was made from 1971-1982, the 351m was made from 1975-1982. The 400 had a S code for an engine, the 351 had an H i believe.

 

Well on the Emissions tag under the hood, it says "Engine: 5.8L/6.6L". So isn't a 6.6L a 460? If so, is it really worth the extra whatever it is $700 to replace the stock 351m with a stock 460? The truck is basically just going to be a daily driver, and in a couple years, my nephews will start driving, so I am going to let them use the truck as well... Just curious!

 

A 6.6L is a 400. The 460 never was installed from the factory in 1979 4X4s.

 

Okay, kewl.. Then I guess it brings me back to the 351m or 400? I guess I'll just go with a 400, leave it virtually stock, except put a 4bbl 650 on it, and call it a day...

Anyone else think that's an okay setup!? The 4bbl I don't wanna go too radical, and with a motor like that, I don't think you will ever get all 750cfm pouring through there, so it would just be a waste of carb.. I think a 650 or even a 700 max would be a good setup...

 

Having never been subject to California emissions testing, the only thing I would add is this: I don't think it will pass visual inspection as these trucks didn't come with a factory 4-barrel carb option on the 351M/400.