Thanks for all of the great info! I think I'm gonna go with rebuilding my 351M into a 400. I've got the Aussie heads lined up, as well as the 400 crank and pistons. I'm shipping my block out at the end of the month for the build up. I especially appreciate the cam info! I'll post the dyno results when I get the beast on the road. Thanks again.

Rob

 

Out of curiosity what kind of performace increase could you expect if you left a 351M at 351 and not 400? Is the increase to 400 really that big a difference? I ask because I have a 351M I'm getting ready to to a total rebuild on and wondered if spending the extra money on the crank and pistons was really required.

N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford

http://home.earthlink.net/~eqbauldor/index.html

 

>Out of curiosity what kind of performace increase could you
>expect if you left a 351M at 351 and not 400? Is the
>increase to 400 really that big a difference? I ask
>because I have a 351M I'm getting ready to to a total
>rebuild on and wondered if spending the extra money on the
>crank and pistons was really required.
>
>N3


Well 49 cubic inches and .5" of stroke is nothing to sneeze at. Besides if you're doing a complete rebuild you're going to need new pistons anyway so the only real additional cost associated with converting the 351M to a 400 is the new crank. I got mine off of eBay for $65.

 

Going out on a limb here and showing my true ignorance, but why do you have to change pistons if you were going to leave the 351 as a 351? I thought you could just mill the piston top and change out the rings? Man somedays feel like I drove my canoe right past the river and launched it in the Atlantic....

N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford

http://home.earthlink.net/~eqbauldor/index.html

 

Most high mileage engine cylinders develop taper in the cylinder walls. The action of combustion during the power stroke forces the rings out against the cylinder wall which helps seal the piston and force it downward. The pressure dissipates as the piston travels downward consequently so does the friction between the rings and the cylinder. In addition the action of the connecting rods converting the reciprocal motion of the pistons to the circular motion of the crankshaft places stress on the "sides" of the cylinders which are perpendicular to the axis of rotation. Got it so far? :-)

Bottom line is if you were to measure the I.D. of the cylinder at the top and then at the bottom both front to back and side to side you would find that not only is the cylinder now shaped like an ice cream cone but it's also slightly oval. The difference in the measurements tells you how much the cylinder has to be "overbored" in order to make it round and straight again. That's what it means when you see "400 .030 over", etc. When you overbore an engine you must get the proper pistons to fit the new diameter of the cylinder.

 

The 351M is probably the easiest, least expensive engine to stroke that was ever made. The 400 was made for 4 years before it was destroked to the 351M version.

Ford should have made longer con rods but made taller, heavier pistons instead. There is a lot less torque available and gas mileage was almost the same.

The question should be, "of those who started with a 351M, has anyone regretted rebuilding it as a 400?" I bet it's a short list.

 

The link that 58custom put up, the one about engine weights, is a real eye opener. When you look at the cubic inches for pound of engine weights, all the modern Ford V8's weigh about 1.55 lbs per cubic inch. The Lincoln 460 comes in slightly below that.

The 400 has had a reputation for being a heavy engine, but considering the displacement it provides, it's not bad at all. That table has me reconsidering engine possibilities.

Side note, the stroker 351/393 and the 460/514 look like they would be the displacement per pound winners.

 

Bill your right about the bore wear, but that is not the reason why you need to use the 400 pistons when changing from the 351-3.5" stroke to the 400-4.0" stroke crank. The only difference between the 400 and the 351 is the crank and the pistons. Because the connecting rods are the same length in both engines you need to use the 400 pistons that have the piston pin relocated .500" higher up on the piston. If you were to try and use 351 pistons on the 400 crank they would be slamming into the heads. If your 351 meets the standards for bore ID and taper there is no reason to overbore it. Just use the correct 400 pistons.


>Bottom line is if you were to measure the I.D. of the
>cylinder at the top and then at the bottom both front to
>back and side to side you would find that not only is the
>cylinder now shaped like an ice cream cone but it's also
>slightly oval. The difference in the measurements tells you
>how much the cylinder has to be "overbored" in order to make
>it round and straight again. That's what it means when you
>see "400 .030 over", etc. When you overbore an engine you
>must get the proper pistons to fit the new diameter of the
>cylinder.

 

Ok got it Bill thanks for your patience and knowledge. I hope when I obtain the knowledge that some of you folks have I can have the patience you folks do with us "new guys". Thanks again all, the amount of knowledge on this board never ceases to amaze me.


N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford

http://home.earthlink.net/~eqbauldor/index.html

 

>Bill your right about the bore wear, but that is not the
>reason why you need to use the 400 pistons when changing
>from the 351-3.5" stroke to the 400-4.0" stroke crank. The
>only difference between the 400 and the 351 is the crank and
>the pistons. Because the connecting rods are the same length
>in both engines you need to use the 400 pistons that have
>the piston pin relocated .500" higher up on the piston. If
>you were to try and use 351 pistons on the 400 crank they
>would be slamming into the heads. If your 351 meets the
>standards for bore ID and taper there is no reason to
>overbore it. Just use the correct 400 pistons.

Yes I'm aware of the fact that the 351M and 400 require different pistons but the question asked was "If you're going to leave the 351M as a 351 then why would you need new pistons?", so I was explaining the reasons behind putting new pistons in a rebuilt motor even if you don't plan to change the stroke. It's true that if the engine is close enough to not require an overbore then you would still require new pistons to make it into a 400.

 

.. Did you ever do these mods?

.. The pistons only need the pin moved up 1/4" (not " .500" ") for 400 pistons vs 351 pistons, because the pin moves 1/4" higher and 1/4" lower in a 400 than in a 351. (Piston moves 1/2" lower)

.. To get some decent power and MPG out of a 351M (& 400), the pistons need changing because the actual static compression ratio is horribly low, nominally 8.0:1, prolly actually down in the 7.xx:1 range... like a late 1940's engine... very inefficient/ineffective by today's standards... adding a bigger cam to that only bleeds the dynamic compression ratio down even lower... maybe adding a few highest RPM HP but weakening torque down lower... the opposite effect of what's desirable for a long stroke low RPM torque motor which a 400 must be with stock connecting rods... Also, the piston at top dead center is well down into the bore giving horrible quench effect in both 351M and 400 engines. There's no better pistons available for a 351M, but pistons from a 351C engine can be made to work in a 400, are readily available off the shelf in different compression ratios/designs, and offer higher compression heights (wrist pin lower) which moves the piston slightly higher in the cylinder bore and improves quench and compression ratio. Adding just Aussie heads can't do all that. Also, the stock heads have good sized valves, bigger than the vaunted Chevy 'fuelly valves'.

.. Also, of course the bigger 460" can always make more low RPM torque than the smaller 400", but the 400" can have nearly doubled HP and much stronger torque with just a compression ratio/quench boost from pistons and/or heads plus a budget cam like a 204/214 for trucks(heavy) or 214/224 & valve springs for cars (lighter). If you have the later 1970's & up stock heads with air reactor bosses/bulges blocking most of the exhaust port flow, a $10 air (or electric) die grinder with a $10 carbide bit can be used to remove those.