Ok here's the deal.... i am restoring/building a 1978 F-250 , i have a 390 and a 400 that are both good engines. I had the 400 in my F-250 (that some of you know that i rolled and am currently building it back up) and loved the it was strong as hell and put it through hell and it held up , and i have the 390 but haven't fired it but i know it's healthy and i wanna rebuild one or the other But i have come to my point where i can go either way now. I need decent fuel economy and power. Which one of these engines is most fuel efficient when built to similar specs???? I live in california and we have emissions testing but i don't have to worry because the truck is going to be regestered as a 1973 F-350 so there won't be any emissions on this motor . I'm probably going to go with Edlebrocks performance package or something like that. But i need some helpful suggestions and replies..... Thanks in advance!

 

There are some big design and building theory differences between the two motors. Let me start by saying the FE is easier to screw up if you dont know what you're doing with it, and where it's design advantages are. It also yields better results if done correctly though. With the FE, you can run higher compression due to the quench style head. This directly translates to more horsepower, torque, and fuel efficiency, if you take advantage of it. It's been a long time since anyone came to the FE forum wanting to build a motor for fuel efficiency. It can and has been done though. I'm running a correctly built up 390 in my '70 F100, that gets around 15mpg on the highway. It also produces around 400hp, and will slam your head against the back window. Now mine has a fairly big cam, and you could build one for better fuel efficiency than that.

I could get into other design differences, like stronger parts and so on, but that IMO is beyond the scope of this thread.

The downside to the FE is in this case you'd have to swap it in. But since you're asking, it doesn't sound like that will be a problem.

I'd put the 390 in. But I would definately visit the FE forum first to avoid the common mistakes!

 

What are some common mistakes, like cam and piston choices or building.....? i have the truck done to the frame and it's all painted and ready to be built back up, so yeah swapping the FE in is not very hard at this point.... you get what i'm sayin. I have 4.10's or 4.56's (can't remember which) and i'm running 36's so gearing is arount 3.85 or so so i'm not gonna get the best milage but i used to get around 6-8 with my 400, but i drive with my foot 3/4 into the pedal most of the time with a few full throtle punches a couple times a day. i run back and forth to town (4-6 miles each way) and to jobs (10 miles or so each way(own a construction company)) a few times a day. And this is going to be my main driver so it needs to be somewhat fuel efficient.

Keep the help comin, please....!

 

F250, I will agree with the FE390 recommendation, because you can do it with easily available parts. But it is VERY easy to do it wrong. About 999 out of 1000 390s are rebuilt with the wrong pistons in them. Ford took a cheap and easy way out in the mid 70s trying to lower compression in the 390 truck engines. They used leftover mercury 410 pistons. The look almost exactly like other 390 pistons, but they only have a 1.66 pin height instead of a 1.76 that 390s ought to have. Most rebuilders repeat fords low compression mistake. The 390 built with the wrong pistons will give about 7.6 to 1 compression and that doesn't do power or economy any good. If a 390 is built with speedpro H395P or H304P or KB150 or Silvolite 1116, 1130, or 1131 pistons, you can get 8.75 to 10 to 1, depending on you choice, and have good quench and good economy and performance. If you do run an FE, headers are an absolute MUST. The FE's factory exhaust manifolds are worst ever. Be sure to get a distributer with a vacuum advance and use it. You should be able to get a solid 10 or 12 mpg easily. If you try to build a 400, you have the problems of a very large, open chamber in the head. It is tough to get enough compression, and there is no quench. Now, if you can find a pair of austrailian small chamber quench heads, the 400 would then have some real possibilities. But I don't know if those heads are hard or easy to find, and don't know if they are exspensive or not. DF, @ his Dad's house

 

Check with the FE engine forum for FE building tips please. Do the same for 335 series tips. Your cooperation will be appreciated.

I got 4-8 MPG with my 390 but it was built with 12.5:1 compression and 500HP. Either engine can be built for economy. The 400 was designed after the 390 was already in production. In a heavy vehicle the 400 will probably come out on top built for low end torque and economy since that is what it was specifically designed for. -That is b4 it was gutted for the EPA.

 

I would say the 390 will make the best results. The 400 was a mickey mouse motor to be honest....it doesnt have the rod ratio that 390 has. Its a very poorly designed motor, from what I understand they made the stroke longer and used the same length rods and used taller pistons to make up the distance. This does not make for a good motor unless you invest cash in quality engineered aftermarket parts like proper rods and pistons.

 

Oops, sorry, to be honest it sounds like you don't know anything about the 400 and are very confused. You need to check out the 335 series engine forum for some good information. You can start here for a little history etc: High Performance Options for the 351M/400 Engine by Dave Resch

Happy reading!

 

Thats right it was the 351M that used the taller pistons......either way the motor is a total boat anchor until you put money into it to make it over come the design flaws. Regardless of anything I have read I use life experience to show me the way, and I have never seen a "M" series motor that was really worth its weight without some $$$$$ flowing into it. Im not bashiing it though I just think that its not worth it for the money. Then again it sounds like you already have some good parts to start with on your 400 so it might not be bad. I guess im just looking at the money side, its proboly not an issue for you.

 

Again, check out the 335 series engine forum and the above link. No boat anchors around there.

 

No doubt.......when cash is king the motor will sing.

 

Again. parts are easy to come by now and it is not any more expensive than any other Ford motor of the era.

 

Common FE mistakes:

1. Failure to take advantage of the quench heads. Silvolite pistons and Fel-Pro head gaskets are notorious for causing this. I hesitate to really call it a mistake, since all you end up with is the equivalent of an open chamber like the 335 series motors, but you have so much more potential if you take advantage of the head design.

2. Intake sealing. Since the intake extends under the valve cover, it is exposed to oil. Where another engine would have a simple vacuum leak, the FE is now sucking in oil. Very annoying if you dont know how to prevent it.

3. Failure to check lifter preload. Hydraulic valve lifters need to be compressed in a certain range in order to function correctly. Later FE's came with non-adjustable valvetrains. So when people put bigger cams in them, now the lifters are compressed too much or not enough, causing bent pushrods and other problems. The adjustable setup is easy to convert to, and makes it a snap to set lifter preload.

Those are the big ones. The FE's do have a number of advantages though.

1. Weight. 625lbs for a fully dressed all iron FE. Remember the intake alone weighs around 80lbs, put an aluminum intake on and that number goes waaaay down.

2. Strong bottom end. The rods, crank and block are EXTREMELY strong. Get aftermarket rod bolts and put your foot down. Standard reconditioning is all that's needed. No need for aftermarket cranks, rods, or blocks unless you're building some odd stroker or something.

3. Quench heads. All FE heads flow great, and respond well to porting. Some flow better than others, but there is no bad FE head!