Hi, Really pondering rebuilding the engine in my 79 crew. Now I have a 400 with a edelbrock 4 barrel, and a weiand 8010 intake. The old girl is getting to the end of her rope and I'm gonna rebuild within the next couple years. What I'm having trouble deciding on is whether to find another 400 and build it (the truck is my dd so spending as little time in the shop is a good thing) or find a 4.9 and build it. What I'd really like is 300hp/400lbtq. I realize that isn't really doable with the average 300 build so I could settle for 200hp/300lbtq. That's what my 400 is rated at now but I don't think it's making that much. Its wore out, kind of a dog. I can't get it to even rev to 4 grand. Have to push it to around 3 grand just to climb a couple hills around my house. Doesn't feel like it's running very well.

My question is, could I make the 300hp/400lbtq out of the 400 relatively easily, reliably, get around 11-13 mpg and at a low enough rpm to be easy to tow with?
I'm ok with having heads cleaned, better vavles installed, new cam, higher comp ratio Pistons, and such. How reliable will it be? How long until I have to rebuild it? (Assuming I built it with quality components). What kind of mpg could I expect? And finally, will rpms be low enough to be able to get into the torque curve easily without having to ring it out?

I'm ok with 200hp/300lbtq number I could get out of the 300, the mpg would be awesome too, and at a low rpm that I could haul with and still hear my own thoughts? That would be great with me. But Id need a bellhousing adapter, new radiator, engine mounts, etc. but if I could get more power, easier, for ok mileage with the 6.6 that would work to. What do you all think?

 

I would stick with the same size engine the truck came with, simply for cost effectiveness, plus as they say there is no replacement for displacement. Pick up a jy 400 and take your time with the rebuild on that. Have a machine shop work the block and heads. Get a rebuilt kit with new pistons, a Comp Cam 268H cam kit that comes with gear set and valve springs, a set of headers, and use the intake and carb you have. You will be around the 300hp/400tq you are looking for. To help with the mpg, look into a 3g alt and an electric fan. I did my alt and efan for less than $100 with very noticeable power gains and better mpg.

 

I agree on finding a decent junkyard 400 and having it built while you drive your truck.. Then you have the leisure of dropping it in when its best for you..

And yes, the 400's make very good power with a few upgrades

 

I was definitely gonna find a jy engine and build it (although id rather have the original block in the truck but oh well). But what kind of mileage could I expect with a light foot? What about longevity? I don't want another 9 mpg engine, especially if I'm gonna have to rebuild it in 100,000 miles.

 

Well the 9mpg issue comes in with your right foot as well

At least with you rebuilding the junkyard motor you can our your 400 up and get it back under the hood later

 

Numbers matching in a 1969 Mustang Boss 429 is important. Numbers matching in a dent side, not really important at all. I wouldn't worry about that. I dd my '78 with a cam, intake, and carb and get 11 to 12mpg with a 4" lift and 33s. I just installed the efan, so i expect that to go up a little. And I am still running exhaust manifolds with a single 2 1/2" pipe, for now.

 

Compression ratio is critical to get the Hp up. Tmeyer pistons if you can still get them or aussie Cleveland heads. I got a pair of aussie heads from a buddy so I went that route. Comp xe262 cam on paper is better than the 268 but I had the 268 in a previous motor and liked it. A 600 cfm vacuum secondary holley is a good fit for a dd and headers and a good exhaust is a must to round it out.

I'm expecting around 350 hp and 450 tq from my build. Link is in my signature if you want to read up...