Whats the difference between the the run of the mill 390's and the truck 391.

 

One extra cubic inch! . Based on some quick googling, the 391 is an FT engine (idk what FT stands for) and the 390 is an FE series engine, which is the Ford-Edsel series. So they're basically just different series and it has an extra cubic inch. Hopefully, NumberDummy or someone who knows better will swoop in and correct any misinformation I may have presented.

 

Quite a bit of differences actually. While both engines "look" similar, not that many parts interchange.
The heads are different, the exhaust crossover ports are in a different location, the intake manifolds bolt on but the crossover ports won't line up, the crank snout is a larger diameter.
That's off the top of my head.

 

There is a ton of difference between the FT and FE motors. Blocks: the dizzy shaft is slightly larger on the FT. The dizzys won't swap. As mentioned the intake system is different with the exhaust cross over. Means the manifolds are different, heads are too. The original valves were sodium filled. The FT is externally balanced, not internal like the FE. So, balancer and fly are different and don't interchange. The crank is forged, not cast. Ideal right? Well, not so fast. The snout has a larger OD, so the timing cover is different, water pump is too. Come to think about all the accessory brackets are different from the FE. Lastly, with the crank, the lip for the flywheel is slightly longer.
The FT motor was made with lower compression in mind heavier reciprocating mass. It's great for lots of hours of continuous lugging power, not great in a pickup. The pistons are deep dished and stock ones have full skirts and an extra set of rings.

So what is useful? Not much. The block can be FE re-purposed by bushing the dizzy hole with to accept a standard FE bushing. The crank can be re-used, but the cost of turning down the snout and re-broaching for a standard build is probably prohibitive (yet, not complete junk like many other parts off the motor). The rods are compatible for a 390. The pushrods might be too, depending on wear and length needed for the new build. Pistons are not of much use. One plus is the large oil pan and mid sump pickup. These are useful, but can be challenging to fit in certain chasis. If converting, one has to decide whether to keep the stick in pan or go to the oil stick in the block. The valve rockers, shafts, and pedestals are interchangeable. The springs and keepers could be, depending on the intended cam for use. FT CAM is of little use. Oil filter adapter may or may not be of use. Some have the large grooves, that are desirable. I can't think of the term, but the sheet metal oil diverters in the heads are good to re-use as well as the lifter galley pan. If you're trying to save money, which I don't know who can on a FE, the fuel pump, oil pump, oil pressure sending unit, and coolant sending unit are re-usable. The spacer plate between the motor and tranny as well as the starter are also keepers. Main bolts, head bolts, oil pan bolts, and valve covers and bolts are also re-usable. The cam retainer and fuel pump eccentric are re-usable. The rest (which is a lot) is pretty much scrap material. Another complication is the transmission. Typically, the big trucks had Clark MTs. These are not the best for a light truck application and the mounting will be different off the bellhousing. This is the same for the front center mount as compared to the two side perches used in FEs.
This isn't to write that one can't use a 391 motor in their pickup, but it really won't perform very well, given the amount of modification to work.

Don't ask me how I know these details.

 

Thanks for the replies.I'm afraid to ask decaruby how he knows all this. So... I'll leave it alone. I am however curious if the distributors are interchangeable. I realize I won't have the tach drive but I can put in an electric version.

 

There was a great story called "The Poor Man's 427" that was based upon the 330 FT which is in the same family a yours. I found lots of links to it but the ones I tried were all dead.
The YouTube version is still there at: but that doesn't tell the story at all.
Please post the address if anybody finds the text and images for this story. I should have archived it as a PDF or something when I first encountered it.

 

The dizzies won't swap between the two different motors. The FE dizzy's shaft pilot (part that fits into the hole in the block, right at the bottom at the oil pump drive pin) is smaller than the FT opening. So, to use a FE dizzy in a FT block, one has to install a bushing (there are people that sell them) into the block. In order to use a FT dizzy in a FE block, the hole would have to be enlarged. Neither of these options would be easy or recommended unless you're working on the bare block.

 

Thanks for the info. I was just thinking the other distributor would perform better. I changed out the carb to an Edelbrock in hopes of getting a little more performance out of it. I didn't like the governor set up on it. I hope to sell the Holley. The truck just seems to be gutless. The throttle response is just slow. Even when you hit the gas in neutral. Maybe I'm expecting too much out of it. After all it weighs about 25,000 lbs loaded with water. It does have a 2 spd diff and a 5 speed tranny, which in low it will climb real well but slow...

 

Does anyone know if the rear main is a typical seal or the rope type on the 1977 391FT. I have a pretty severe leak on the truck. It would be nice to be able to pull the trans rather than the pan.