I would like to know if anyone has some info that they could share on the 391 engine. A gentleman has a early 70's 450 (or bigger) with this engine. It was rebuilt and haas approx. 600 miles on it. As I am comtemplating on using my 71' crew with a utility bed on it for work. I was wondering if this would be a better fit for my truck for the obvious reasons of putting a lot of miles in, and needed power to pull the truck. Comparing it to the 360 I have in it now.
A 391 is an FT engine, related to the 360FE you've got now. They're good engines, but they emphasize low end torque. It'll run out of wind by about 3000rpm. They have smaller ports and valves than the 360 you have now. If that's what you're looking for, then go ahead and use it.
If it were me, I'd take the heads, intake, cam and distributor off the 360 and put them on the 391. That'll wake it up a little. With a Crane 343901 cam it'll really wake up. Since the 391 was just rebuilt, keep the bottom end stock off of it. They have a forged crank, and the bottom end will be pretty much bulletproof.
I don't think the mileage would be too bad Greg. The small intake posts should promote very high velocity airflow at low RPMs. It should be as efficient as a 391 cubic inch engine can be. If I were to build and FE strictly for mileage (wow would that be dumb) it would have FT heads on it.
Guys: I went through this exercise lsst fall with a friend of mine, trying to replace a 391 with a 390 - we couldn't make it work without major modifications. I also just posted on this very subject in the '61 - 66 forum; to save you time, here is what I said:
The 330 is the smallest (in CID) of the FT engine family, which includes the 330, 361 and 391.
The FE motors include the 332, 352, 360, 390, 406, 410, 427, and 428.
The FT and FE motors share many similar dimensions, but they are NOT INTERCHANGEABLE without significant modifications.
The FT cranks are forged instead of cast, but the snout of the crankshaft has a larger diameter. This means the front timing cover has a larger hole and seal in it. Unfortunately, this means you cannot replace an FT motor with an FE motor not only because the front timing cover crankshaft hole on an FT is larger (the FE crank won't seal), but also because the front timing cover on an FT motor is the front engine support. (I've heard that a popular FE racing mod is to get the forged 391 FT crank, turn the snout down to FE dimensions, and use it in a 390 FE for greater reliability.)
FT rear engine mounts use the bellhousing, which has the rear engine support bosses cast into it - unlike the FE motor, which uses the side of the block for the mounts. The FT side engine mount holes are not in the same position as an FE.
Now, to go the other way, replacing a 390 with a 391 block might be a lttle easier. I see the following issues or possible issues:
1. The side engine mounts will most likely need to be fabricated.
2. Will the clutch/flexplate bolt to the 391 flywheel?
3. The 391 distributor will have to be utilized, if I remember right, the 391 block must be bushed to use the 390's distributor.
4. The front crank seal will be an issue.
Now this is where I haven't been able to get firm hold on this issue: Would it be possible to use the FE timing cover on a 391, and get a proper fitting seal on the 391 crank?
One possible - and easy - solution may be the answer to this question: IS the 391 crank snout OD the same as the FE crank snout diameter WITH THE FE oil seal sleeve in place???
Anybody know? Any other ideas?
The 391 does have a bigger snout. YOu can turn this down or just use the FT pulleys and damper.
The block does need bushed (part can be bought at ford) to use the FE dizzy.
I'm not sure about the clutch deal. I think you would have to use the FT flywheel for two reasons. 1) The 391 is externally balanced and 2) The flange is thicker by a tad. I'm not sure about the rest. I'm sure somebody has the answers.
Don't forget the exhaust manifolds are center down exit and will cause all kinds of problems with motor mounts if you are trying to use them in a pickup. The heads have 4 exhaust ports and a huge heat cross over that will leak bigtime if you try to use FE exhaust manifolds. The flywheel is larger and heavier and requires the FT bellhousing, which does have mounting ears. I agree with banjopicker in that the holes may not line up on the side of the block with your motor mounts, but that is doable. The clutch is 13 inches instead of 11 in your FE and splines may be an issue. IMHO the 391 is not going to be that big an improvement over your 360. It will have lots of torque but will feel slugish in a pickup. It would be alot easier to install a 391 in a pre 65 truck that had a 292 originally as it has a similar mounting system.I'd stay with the FE if it was me.
I'm glad I found this topic because when I bought my '59 F100 I was told that it has a 391. Which to my understanding is the commercial truck motor pretty much bomb proof in the lower end. However, I was told that everything internally will all work in accordance to 390 parts. Is this true? Just wondering because I recently dropped the Mallory Unilite distributor in it and it appears to sit high and now I also have a slight stumble at initial acceleration. From what I've read the 391 doesn't appear to be very desirable, especially mine which is a '74, robbed of power by emissions.. It does wind out very quick, and although used more for transportation I do want some power. I have only recently purchased it. What's your guys opinions on keeping this motor as opposed to finding something different before I drop coin into this thing. Also how can I tell for sure if it's a 391? Thanks.