M-Block 351M/400 History

Many owners of Ford trucks that were originally equipped with M-block engines choose to upgrade their vehicles’ performance by replacing the M-block engine with a 385 series (429/460) big-block engine. In fact, this particular engine swap is so popular that an aftermarket industry has emerged to cater to it. (L&L Products, Inc. in Rowlett, Texas was originally founded specifically to manufacture and market components for Ford 385-series big-block engine swaps.)

On the other hand, an M-block engine in factory trim has vast, untapped power potential. It is smaller and lighter than a big block (which is why it was developed in the first place), and when properly built, the M-block 400’s power output is comparable to most big blocks.

Yes, yes, yes... when all other things are equal:

You can always build a bigger engine that will stomp a smaller engine, but so what? When you factor in the costs and time (and hassles) involved in swapping engines, and the “real world” handling performance penalties of excess weight on the front end of the vehicle, it’s easier to put the benefits of the M-block engine in perspective.

M-block power vs. big-block power

I have to admit it, one of the things that bugs me most is when a Ford big-block enthusiast drops numbers to compare the 375 hp (gross) from a '69-'70 429 4V with 11.4:1 compression and no emissions equipment, with the 158hp (SAE net) from a '75 400 2V with 9.1:1 compression, retarded cam timing, exhaust gas recirculation (EGR), and outrageously retarded ignition timing to keep it from pinging on 85-octane unleaded.

So let’s get real! Funny how you don’t hear big-block enthusiasts bragging about the 200hp from a '72 460 4V with 9.3:1 compression, or about the 198hp from a '73 460 4V, or about the 195hp from a '74 460 4V, all of which were vastly exceeded by horsepower numbers from the 351C 4V engines made those years.

The point is that it’s simply ridiculous to compare pre-smog, pre-SAE numbers to the numbers produced during Detroit’s darkest days in the mid-to-late-'70s, with retarded cam timing, retarded ignition timing, low compression ratios, and low-octane unleaded gasoline. So let’s look at some real numbers.

In 1971, before SAE “net” power measurements, the 400 2V was rated at 260hp @ 4400 rpm. By comparison, the 1971 460 4V was rated at 365 hp @ 4600 rpm. Keep in mind these numbers compare a 400 with a 2V carburetor and 9.2:1 compression to a 460 with a 4V carburetor and 11.0:1 compression.

If you consider that the non-Boss 1971 351C 4V was rated at 280 hp @ 5400 rpm with a 9.0:1 compression ratio, you can get an idea of the 400’s awesome power potential. I would estimate that with a 4V carburetor, a decent cam (like the 351C had), an 11:1 compression ratio (like the 460 had), and 1971-era emissions controls and leaded high-octane gasoline, the 400 could have easily put out 325+ hp straight from the factory.

The next year (1972), when SAE net power measurements were used, the difference between the power output of the 400 and 460 was even narrower: 172 hp @ 4000 rpm (400) vs. 200 hp @ 4400 rpm (460).

Comparing maximum torque figures for 1972 (SAE net) shows even more dramatically how close the two engines’ power output was: 298 ft lb @ 2200 rpm (400) vs. 342 ft lb @ 2800 rpm (460). At its torque peak, the 400 produces almost 90% of the peak torque of the 460, and it does it at less than 80% of the 460’s engine speed.

The 1972 comparison is a little more fair because the 460’s compression was dropped. Now we’re comparing a 400 with 9.2:1 compression to a 460 with 9.3:1 compression. Don’t forget, though, we’re still comparing a 400 with a 2V carburetor to a 460 with a 4V carburetor.

M-block power vs. big-block power