The Ford 351M from my understanding is basically a torque-monster. It was designed to generate large amounts of torque for heavey cars and PU trucks. I also understand that from a HI Performance Racing (Automotive) perspective, the engine is limited to about 5500 RPM. This really doesn't get it up there in the 6500 to 8000 RPM range necessary for drag racing.

What is the limitations within the engine that shuts it down at 5500 RPM? I have some thoughts on this, but would like to confirm with others.

Is this problem related to the weight of the rotating assembly, thereby limiting the speed at which the engine can be turned.

If this is the case would it be possible to chuck the crank up in a lathe, turn the counter balances down by an eighth or a quarter of an inch or so, and then rebalance the crank for a set of light weight pistons and rods. By doing this would decreasing the weight of the rotating mass allow increased RPM.

It seems to me that this engine is built very heavey duty on the bottom end. and anything that has been adapted from a cleveland motor should be able to generate a lot more horsepower. The fact that it has an inch+ more deck heighth should make a canidate for a serious stroker motor.

Just some thoughts.

What do you nall think?


Kent Mathew
Dallas, Texas

 

Welcome to FTE Kent! You've asked some very interesting questions several of which have been debated at length in the 351C/M, 400 Forum which is devoted exclusively to the 335 engine series. There are some very knowledgeable and helpful folks over there who have explored the upper limits of what can be done to make one of the most misunderstood and therefore maligned engines that FoMoCo ever produced put out some serious power. How does a naturally aspirated, 800+ HP stroker using a stock (351M/400) block sound to ya? Well there's a guy over on the forum who's built one and runs it regularly in a pulling truck.

FTE 335 series forum

 

Crank journal size is one of many considerations when calculating a max rpm. A bigger journal means a stronger crank, but also a higher journal "speed" which lowers the max safe rpm. Note that the 351C has smaller journals...

 

who is the one who has a 800 hp 351m/400? i want to talk to him, i have a 351m and i am looking to get alot of power out of it

 

The 351W and 460 have the same size journals as the 351M/400 and you don't hear people saying their journals are too big for high RPMs.

 

Kent, where did you get the notion that the 351M has problems above 5500 rpms ? The 351M has larger than average main bearings, but this can be addressed with more clearance on the mains and plenty of oil. The 351M and 400 are not the most popular engines to build, but there is plenty of potential there. DF

 

I have the 5500 rpm info from several post that I've read as well as info that I have read both on the inet and other literature. I think that since the motor was designed for heavey hauling ie big cars and pickups, torque was more important then performance.

I think the best explaination I've heard for it is the fact that the motors were put togeather at the beginning of the epa's smog restrictions. The timing on the original motor was retarded considerably (I think 6 degrees) and this doesn't help the engines performance. In addition the intake has a bit of a restrictive flow to it, but I understand the heads flow close to some of the aftermarket heads, just depending on at what amount of lift the valve is at at the time of reading. Changing the valves out to the larger valves from the 4v heads and a little port and polishing and I think they would kick some major butt. Yes, they are cast iron and heavier then aluminum, but if there is the potential to outflow the aluminium 351w heads.
I guess it really depends on how much work you want to do to them and how much money you want to spend.

 

That pretty much sums it up.
The 5500 RPM limit that is refered to doen't have anything to do with the bottom end , except for maybe the strength of the crank and rods which can be upgraded.

The main RPM restriction is the cam and intake. Yes they were designed for torque at low RPM's. So adding a good intake, cam and headers, these motors will make plenty of power with an extended RPM range.

Of coarse if you want more the sky (money) is the limit.

BTW - If this were going in a very light car the 4v heads are the way to go for upper RPM use. Most combo's however are better with the 2v heads.