I have a 79 bronco with a 400 it gets 9-10 mpg, which has made it somewhat of a yard ornament lately. I've been thinking of replacing the V8 with a 6, But I'm not sure if this will produce the gains I want that will be worth the exspence. The V8 runs good but the bottom end is a little weak so I'm looking at have to go thru it in the near future.I have a 300 in a 78 E-150 that runs strong but has lots of miles on it. I am looking at rebuilding it with some mild mods. {head work, intake, headers, mild RV cam and either a 2 or 4 bbl carb} The Bronco has a C6 trans which I will be replacing with an AOD. It has 411 gears and 33" tires. I would like to get mileage up around 15mpg or better. I mostly just hunt and fish out it with some towing a small fishing boat and occasionaly pull a 23- 26 ft travel trailer. I quess what I'm wondering is if the 300 is up to the task. The last thing I want is a dog that eats alot but wont hunt. Can ya'll offer some advice or point me in the right direction?
Thanks

Pat

 

I would just rebuild the 400. Improving the performance of it will usually net some better mpg's. The 351M/400 were dogs because they had weak compression (~8:1), 4° retarded cam, and lots of emission crap hanging off of them. This reduced power and used more fuel. I would rebuild the bottom end to stock specs. Increase CR to 9:1 - 9.5:1 to be compatable with cheap gas. Have head work done, and throw on an aftermarket intake with a 4V carb (525-600 cfm) For the cam, I wouldn't go too far above stock, and get a nice wide lobe seperation angle, 112°-114°. Then make sure that your gears are adequate for what you do with it.

The 300 is great, but built similarly, it will net about the same mpg's with less power, plus you will have all of that swap work to do. Your Bronco is a 5000lb. brick with the areodynamics of a garage door. It takes the same amount of power to push it through the air, whether that motivation is via a 300 I6 or a 400 V8. Both engines have great torque when properly built, but the 400 has a 100 cubic inch advantage. Yes, more cubes need more fuel, but smaller isn't always better. It the overall efficiency of the system as a whole.

Good luck. BTW, My 300 I6 4wd truck, gets ~12 mpg hiway/town mixed, and that's with 31" tires and 3.50 gears. If I get into it, it'll suck down fuel like a V8.

Also, I wouldn't want an auto behind a 300, it's powerband is too narrow, and an auto eats more HP, something the 300 lacks.

 

I won't disagree with 77300i6. A built 400 is a good way to go. since I emailed hooker bob about my 300 swap into my 79 bronco, I corrected the timing and changed the vacuum lines around, and now get closer to 15 mpg....31" tires, 3.55 gears.

 

I agree, keep the 400. I pulled a 400 out of an 80 Bronco because it was knocking and replaced it with a built 300. I just like the six, that's why I went with it but for practicality your probably better with the motor you have, you won't need to swap mounts, accs crap rad etc. The 300 is a stump puller but by the time you get done building it yor mpg isn't anything to write home about.

 

I've been weighing the pros and cons on this swap so far I'm leaning toward the 300.
Pros
One point for the six is my personal bias, I like this engine.
I also like the idea of less weigh forward.
Much easier to work on due to more room under the hood.
I already have the engine complete with acc.

Cons
Will have to find engine mount brackets for the 6.
I can build the 400 cheaper than the 6.
I already have performance intake and carb for the 400.

The trans becomes a non issue as the truck will be getting an O-drive trans reguardless which engine I use.
Although I can build an AOD for the 6 cheaper than any thing that will fit the 400.

Pat