I have a 351M in a 1977 F250 4x4 HiBoy. Very nice truck. I'm new to Fords and could use a little help on a couple of things, I've mostly been a mopar guy.

What is the stock carb for this motor? I am assuming it is a 2bbl autolite on a dual plane manifold, is that right?

How can I identify for sure it's a 351M and not a 400M?

If I keep the dual plane manifold what is a better carb to use that's smog legal. This truck has good low end torque but even the midrange power is not good. I think I should be able to get more midrange power than this.

A spread bore 4bbl 600 cfm is my thinking, how does that sound?

If anyone could suggest a carb to me I'd definately appreciate it.

 

OK, I learned how to tell the 351M from the 400 by reading through more of the old posts - you have to pull a spark plug and measure the stroke by marking off on a piece of coat hanger or stiff wire while slowly turning the motor. There is no way to tell visually on the outside of the motor, learned this from Dan something.

Still have the carb questions though. Can you even put a 4bbl carb on the stock manifold?

 

A lot of people use the edelbrock 600 cfm on an edelbrock performer manifold. The intake is available in an egr version if you need to stay legal. A decent cam makes a huge difference as well as a straight-up timing set. Also, just somantics, but it's called a 400, drop the M. Good luck.

 

The stock carburetor on all '71-'72 400s was the Autolite 2100 2V. Stock on all '73-up 400s and all '75-up 351Ms was the Motorcraft 2150 2V (the 2100's descendant).

Here's why you cannot reliably measure the stroke of a 351M/400 through the spark plug hole:

http://home.earthlink.net/~bubbaf250.../parts110.html

 

Thanks for that info Dreampiper. I've used the Performer before on my cuda and my charger (not sure if it's ok to mentiont that here...) and it's a good manifold that leaves you with good low end torque.

Is the stock 2v manifold on the 351M and the 400 a dual plane too? (And I'm assuming the 2v manifold has the wrong opening and bolt pattern to mount a 4bbl carb.)

 

Thanks BubbaF350.

Hope I can ID it by the carb then assuming it's stock.

You'd be able to use the rod though spark plug hole if you could get a measurement from known 351M and 400 motors for a given rod thickness keeping the rod as straight up as possible.

Can you decode the original motor size from the VIN code or from the Fender tag (if there is one).

 

The OEM 2V intake manifold was a dual-plane design, and without extensive modification, it is not compatible with any 4V carb.

The fourth character in the VIN is the engine code.

For '77-'79 trucks, the 351M code was H, and the 400 code was S. For '80-'82 trucks, the 351M code was G, and the 400 code was Z.

Note that in '81 and '82, both the 351M and 351W had the same VIN code, G.

Alas, if the piston tops were completely flat, you could just do a quick cosine calculation and determine the actual stroke. But as I noted in the article, keeping the angle of the measuring rod consistent at both TDC and BDC moves the measuring rod across the piston top as the piston rises or drops in the cylinder, and the piston's dish (and maybe a valve relief) will alter the measurement.

Unfortunately, Ford used several different pistons in both the 351M and the 400, with various dish sizes. So if you could find a known 351M or 400, with pistons known to match the ones in your engine, then you might be able compare measurements between the engines and determine what you've got.

 

Thanks for all the info. Between VIN code and carb identification, if they match then that'll be good enough for me for now.