Hey guys.. i have a 81 F250 that im building into a custom mudtruck. it has the 351m in it right now. the engine runs fine. i have never driven the truck before thou cause of the cab being off it, but it seems to idle and run very good. Ive been doing some reading and i hear the you can make a 351m into a 400 pretty simple. the only thing on my 351m that is junk are the heads. this week i found a 400. the only things good on it is the crank and heads. Im not sure what truck or car it came out of and im not sure of the year. does this matter? im wondering what is the first step to building my 351m into a 400?? keep in mind that this is a mudtruck engine im building. I also can get a edelbrock 4bbl intake and longtube headers.. I dont wanna put to much money into the engine but would like the best bang for my buck and put my money into the real weaknesses of this engine. lets me know where to start and other parts i should be looking for. thanks!!

 

To convert stock for stock, it's the crank and pistons. Now, imo, and many others here, it would be a waste of energy to take the motor down to that point and put it back together with the weak puppy stock specs.

So,

--pistons are available to get you 9.3 CR with stock heads.
TMeyer, Inc. Precision Automotive Machining

--almost ANY cam is better than stock, and you need springs etc as the stockers will float at 3600 rpm. (has your 351m ever been north of 3500?)

--then your manifold, 650 vacuum secondary carb, long tubes.

Background info and discussion--details are important:
https://www.ford-trucks.com/forums/1...-up-400-a.html

Note that is in the 335 engine series forum, where you will find many 351m to 400 threads and discussions and engine examples.

It's FREE to read, but remember, cheaping out on the engine will guarantee you short-lived pooch. Think about it.....

 

okay thanks. when you say 9.6:1 compression with TM pistons with stock heads. Do you mean 351m or 400 heads? pretty sure the they are the same anyways.. but ill be using the 400 heads.

 

351m and 400 heads are the same. The older 351 cleveland heads will fit also, and there are different varieties of those, but the good ones are rare and hard to find. You can also run Australian heads, which have smaller chambers that will raise compression.