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GMC 401 cid V6 Engines

All rating are for first year engine was used, many got bumped up in Horsepower & Torque during there run.

Engine Model

Gross Horsepower

Net Horsepower

Gross Torque lbs. ft.

Net Torque lbs. ft.

Bore - Stroke




401 V6

205 @ 3200 rpm

178 @ 3200 rpm

377 @ 1400 rpm

351 @ 1400 rpm

4.87 in. - 3.58

400.9 cu. in.

7.50 to 1


401M V6

237 @ 4000 rpm

210 @ 3700 rpm

372 @ 1600 rpm

348 @ 1600 rpm

4.87 in. - 3.58

400.9 cu. in.

7.50 to 1


Engine Application (Notes)
8. Used in 5500, 6000, & H-5000 and optional in W-and SP-5000.
12. M = Magnum series engines introduced in 1966.

401 Magnum V6 Engines

"Magnum" refers to 1966 & up 351M, 401M & 478M V6's with the large port heads. The
351E V6 which was available from 1966 to 1968 in 1/2 to 1 ton trucks is essentially a
Magnum engine, as it has the big port heads & bigger manifolds. If you ever run into
some old magazine adds from 1966 it mentions the 351 Magnum engine available as
a high performance option. The 351's were available both as Magnums & small port
head 351C engines up to 1972. In 1973 the 351 was stroked to a 379, but never got the

Magnum heads & retained the anemic little WW2 carburetor. Though GMC never
designated the 432 with an "M" it was a Magnum engine. The intake manifold design was
different on the magnum engines to give more power & better torque by adding an internal
baffle that acts to develop better fuel distribution & also pulses that help to charge the
cylinders better. The exhaust manifolds have bigger & smoother passages & are not
restricted at the collector. They are found in 4000 & up series trucks. Magnum Head
V6's have much higher flowing exhaust manifolds due to the larger ports & passages
& the collector is considerably larger like about 3 inches in diameter. The 351E is in

all reality a pickup truck version of the 351M engine & has large ports & passages
in the manifold, but it has the bottleneck at the collector just like all the 305A, D
& E V6 manifolds. These manifolds are identified by their I-beam shape. 351M, 401M,
432 & 478M manifolds have even bigger passages but won't match up to smaller port heads
on the 305 V6 but will on the 351E which has the Magnum Heads. The 351C & 379 heads are
basically the same small port heads as the 305-A, B, D & E engines. 1960-65 401 & 478
also have small port heads but larger valves. 1966 to 1974 351E, 351M, 401M, 432 & 478M
share the larger port "Magnum" heads, that's what the "M" designation stands for. The
bigger "M" heads will bolt to a 305 if you're into increasing the performance of your engine.

I have stared to tear down the GMC 401 Magnum V6 Engine, & have some good news already,
its not stuck! It wanted to turn when i was removing the Clutch & Flywheel bolts. I got
the back end apart & will be mounting it to the engine stand shortly. I really nees a
Shop tp work in, as it too hot in the afternoon to work in the sun & too many wasp.

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