351C OILING PROBLEMS
Copyright Douglas B. Baker 6/97
Although the 351C engine was only in production from late 1969 through 1974, due
to the performance associated with the Mach I Mustang and the popularity of the 4V
head with various successful drag racers, the engine was and still is a very popular
engine to build for various street performance vehicles. Since this engine was
installed in a huge number of Ford vehicles during the approximately 4 years of
production, there are a surprising number of these engine still floating around in
boneyards, and the resurgence of popularity in restoring "muscle" cars from the
'60's and '70's continues to generate demand for these engines. Unfortunately,
these engines also have a certain black cloud hanging over them regarding their
reliability, some of which is justified and some of which is just a lack of knowledge
on the part of the average enthusiast.
For the era, the 4V heads available on the 351C had flow potential well beyond the
other production small block heads of the day, and would make big HP when
prepared for racing. As we know, the large port volume in this type of head requires
either a big engine or a lot of rpm for the port to work correctly, and the high rpm
required is where most of these engines had problems. The valve spring technology
of the day simply could not control the weight of the large valves (2.19"-2.24"
compared to the 2.02"-2.08" diameters in the competition) leading to more spring
and valve failures, and the thin cylinder walls in the production block castings tended
to crack under high cylinder pressure conditions. Another area that was harder to
evaluate for the average racer was the oiling system in the block, which differs
significantly from the 289/302 and 351W based engines. Understanding where the
oil is going, and where it is supposed to go is half the battle in getting these engine
to live.
In the diagram, I have drawn out the flow of oil through the engine block as it enters
from the filter boss. Oil crosses the front of the block from driver side (Left) to
passenger side (Right), intersects a drilled passage that angles up to intersect the
lifter galley on the right side of the engine. Here is the first problem area: This galley
feeds all the main galleys and intersects all the 8 lifters on the right side. As the oil
passes down the block, there are many other places for the oil to go than to the
mains, leading to a cumulative loss of oil pressure from the front to the back of the
block. The second problem area is the cam bearings, which are fed from the
intersection of the main feel galleys at the top of the main bearings, and also offer
an opportunity for oil pressure and volume loss relative to the main bearing. The
third area is the #5 main, which has an extra galley drilled from the backside of the
bearing back up to the left side lifter galley, and all 8 lifters on the left side of the
engine are fed though this galley.
If you monitor the oil pressure on one of these engines (unmodified) at the back of
the block where the stock oil pressure fitting is located, that you might see the oil
pressure drop significantly as the engine exceeds 6500 rpm. I have seen as little as
25 psi at that point, while the oil pressure at the front of the block is reading 70 psi at
7500 rpm.
Some of the problems associated with this oil system layout: Since the oil pressure
is very unbalanced relative to the left and right lifter banks, hydraulic lifters tend to
make more noise in these engines, and are even harder to control at higher engine
speeds without lifter pump up or bleed down, sometimes occurring simultaneously
and causing the infamous "S" shaped pushrods that anyone who has owned one of
these engines probably is familiar with. Obviously, if the oil pressure is falling, there
is not enough oil going to the mains and rods, and that usually leads to bearing
failure. The higher pressure and volume in the right lifter bank tends to pump
excessive oil volume into the valve cover, leading to loss of pan volume, and lack of
oil volume to the left front lifters tends to induce rocker failure due to lack of volume.
With a reasonable investment these engine can be built as reliably as any other if
you take the time to detail the oil system, and maintain good volume and pressure
without resorting to a 100+ psi oil pump as was popular to recommend years ago.
The very high loads on the pump drive and distributor gear generated by the
ridiculous oil pressure usually caused the drive to snap or the gear to wear out
quickly, leading to more failures, especially if the drive failed at high engine speed.
There are several other ways to fix the system so you can run the engine reliably,
regardless of engine speed. The simplest and easiest to install for the average
builder is to increase the oil pan capacity, and install an external line (1/4" to 3/8"
dia.) from the threaded boss on the front of the timing cover to the back of the block
where the oil pressure fitting is. You can construct this line using steel brake line and
fittings, or with braided steel line and AN fittings, and the extra volume from this
auxiliary line will help provide additional volume to the mains and to the left lifter
bank. As long as the pan has oil in it, this set up works pretty well up to 7000 rpm.
As a enhancement, restrictors can be installed in the passages feeding the cam
bearings to direct more oil volume to the mains (Point A in the diagram). For
applications where the engine is using a solid lifter cam and roller rockers, a
restrictor can be installed in the oil galley feed to the left lifter bank to reduce the oil
volume to the lifters on that side of the engine, and help feed the #4 and #5 mains
(Restrictor kits are available from Canton Products and Moroso for around $10.00).
With the restrictors and a solid lifter cam the engine can be run to 7500+ and
maintain good oil pressure. For the real hard core racer, the right side lifter holes
can be bored and sleeved with a bronze lifter bushing with a small .060" diameter
orifice to restrict the oil to the right side lifters and rockers. This usually eliminates
the need for the external line, but is a very costly modification and not commonly
done anymore except in pure racing applications. With a dry sump oil system and
the bushed lifter bores, these engines are run over 10,000 rpm.
351C_OIL.WPS 1153 words