With so many expectant drivers now having their babies delivered, I
think its time to review what we know about breaking in (or, as we
Brits say, running in).

The owner's manual from my (United Kingdom) Prius has this to say:

"Break-in period

Drive gently and avoid high speeds.

Your vehicle does not need an elaborate break-in. But following a
few simple tips for the first 2000km (1200 miles) can add to the
future economy and long life of your vehicle:

o Do not drive over 120 km/h (75 mph).
o Avoid full throttle starts.
o Try to avoid hard stops during the first 300 km (200 miles)."

Now, I've read that the Stateside owner's manuals have the same
wording, except that the distance is given as 1000km (600 miles) and
the maximum speed is given as 100 km/h (65 mph). See post 20292 -
Thanks Michelle!

So, which is it to be? I've seen it said that the break-in
instructions are the same as those given for other Toyota models.
But my opinion is that there are just enough differences between
ordinary cars and the Prius that we ought to change the rules a
little.

BRAKES

Why do we have to break in the brakes? That's an easy one - it's
because a new set of brake pads does not make perfect contact with
the disks. There are normally some high spots on the pads (and on
the disks) which make contact first. It takes a few hundred miles of
driving to wear the high spots off so that the pads make contact over
the whole of their surface. Until then the heat generated by braking
is concentrated on the high spots, and it can burn the pads if you
brake hard and long.

With a Prius, some of the braking is done by electrical
regeneration. This means that the brake pads are not used as much
during mild braking, but it is not easy to tell when the regenerative
braking has reached maximum and the hydraulic brakes come into play.
So, how should we modify the usual break-in procedure?

What I did was to use the brakes extremely gently at the beginning,
then gradually increase the breaking pressure until the point where
(at about 300 miles) I was braking normally and I no longer paid them
any special attention. I felt I ought to wait longer than the
standard break-in distance because, with regenerative braking, the
brake pads are not used as much as they are in a normal car.

BATTERY

You may have heard that nickel metal-hydride batteries need to be
conditioned before they will deliver their full capacity. This is
true, but it only takes about five charge-discharge cycles to do it.
When I collected my Prius, I noticed a pre-delivery check sheet for
the battery that gave three options: Fit a known fully charged
battery; charge the battery in place from an external source; or run
the engine for half an hour and let the computer sort the battery
out. So your battery has probably already been conditioned when you
collect your car and you won't have to do anything about it.

But just in case, I drove home with a feather light touch on the
pedal. I was probabably just being paranoid. Even if your battery
does need conditioning, it won't take longer than a few miles driving
to do it.

ENGINE

This is where I am not so sure of my ground, but I'll give it a shot.

Over the years I have tried to find out just what happens to an
engine during break-in, but I have never been given a definitive
answer. By piecing together little bits of evidence, I have come up
with some suggestions which I invite others in this group to comment
on.

It appears that there are two reasons why an engine must be broken
in. The first is that newly made bearings have a somewhat rough
surface which needs to be worn smooth before the proper amount of oil
will flow in the gap between the two metal surfaces. (See post 19919
for more on this). Until then you should avoid applying full force
to the bearing. This was certainly true in the past, but nowadays
bearings are manufactured to much tighter tolerances and the need to
break them in is greatly reduced.

The second reason concerns the cylinder walls, which are normally
made of cast iron. During manufacture, the cylinder walls are honed
to a special surface texture which allows just the right amount of
lubricating oil to escape past the piston rings. To begin with they
are soft and easily damaged, but over the first thousand miles or so
they glaze over, becoming hard and durable. So it is important not
to do anything that will damage the cylinder walls before they harden.

The usual advice is: Do not allow the engine to run at very high
revs during break-in; do not use the maximum torque during break-in;
do not allow the engine to labour at wide throttle opening and very
low revs during break-in.

I believe that this advice is still valid for todays engines, but how
do we apply it to the Prius?

I reasoned that in a Prius the engine revs are determined more by the
power I was demanding from the engine than by how fast I might be
moving at the time. So to avoid high revs I did not use high
acceleration. (Maybe I could avoid the need for maximum torque the
same way, but I'm not sure about this). By listening to the pitch of
the engine I could tell how much I was demanding of it, and could
ease up on the accelerator pedal if it seemed to be working too hard.

My stategy was to use the engine very gently to begin with, then
gradually make increasing demands upon it until the end of the break-
in period, at which point I was willing to take the engine to maximum.

For the first few journeys I absolutely feathered the pedal, pushing
the engine nowhere near its maximum. (I chose some quiet back
streets to do this). Then I went for a 50 mile run at about 50 mph,
accelerating (and braking) gently. After this I started to push it
harder. At about the 600 mile mark I was travelling routinely at the
UK motorway speed limit (70 mph), but was still going easy on
acceleration. I continued pushing it harder until I reached 1200
miles, when I celebrated by flooring it for the first time. Wee Hee!

SUMMARY

So, here's my proposal for a break-in procedure specially crafted for
the Prius:

o Drive gently to begin with. Avoid strong acceleration.
Listen to the engine, and ease up if it sounds as though
it is working hard. For the first 600 miles, try to stay
below 65 mph, but don't worry if you stray above 65 once
in a while. It's more important to accelerate gently than
it is to stay below a given speed.

o Be gentle with the brakes at the beginning. Use them
gradually more firmly until, at about 300 miles, you are
using them normally. (If an emergency happens before
then, forget this rule and use the brakes as hard as you
need to).

o After 600 miles, gradually increase the demands you make
upon the engine. At 1200 miles, feel free to use everything
it has to offer.

Ok guys, there are my thoughts. Have at it!

Regards,
Julian.