[Prev][Next][Index][Thread]

Definitive word on Overrev

I have read the overrev thread with great interest these past few months
since I am a proud owner of a 95 M3.  I found the following article posted
on the Houston BMW CCA web page.  It was featured in their January 1996
newsletter "The Torque of the Town".  IMHO it is an excellent discussion on
overrev.  The author James Williams with Advantage BMW in the Houston area
relates his knowledge on this subject and his specific experience with E36
M3s.  Since the article was not copyrighted, I'm posting it for those
individuals who are interested in this subject.

                        Beyond Overrev by James Williams

Each and every rotating mechanical device on the face of the earth has a
maximum rpm at which it can operate safely. From lawnmowers to Lear jets,
engines
have a safe limit, with a margin to boot, commonly known as redline - it's
clearly marked, right there on your tach, for all to see. Older cars with
distributor-and-points type ignition would make spark at any engine speed,
no matter if that engine speed was way over that engine's efficient redline.
Our more modern engine management systems have the capabilities to omit
spark and fuel at predetermined rpm. This is commonly known as a rev
limiter. Rev limiters keep you from blowing the engine, right?

Let's clear up a misconseption: Rev limiters keep you from blowing the
engine up with the throttle. The throttle could be held wide open with the
gearbox in
neutral and, in theory, the engine would not suffer. Don't try this at home.

So, what exactly is over-revving and how does it occur? Let's start with
some definition of terminology.

Overrev: to greatly exceed the given mechanical limits of rotating equipment.

Reduction Gearbox: the standard transmission is a reduction gearbox,
converting engine speed down to a lower speed. Automatics do the same thing
but are
not in discussion here. Transmission output drives the driveshaft, sending
power to the differential, which is also a reduction gearbox. Thus, the
engine has great leverage over the rear wheels.

Downshift: to shift the gearbox from fifth gear, to fourth, to third, etc.
Motorcycles have a sequential gearbox, meaning the shifts only precede from
one gear to the next. Automobiles have shifters that can select any gear,
any time. Can you see a window for mistakes here? Can you say "miss a shift"?

Three times this year alone, I have had M3s fall into my hands . One came in
running on five cylinders, the other two had been overreved so bad, engine
replacement was required.

All three had bent exhaust valves, with the fatal two having bent intake
valves as well. The piston crowns had round imprints and cresent shaped
hammer marks
from the valve heads. In both cases, the final blow was identical: each
engine had valves bent over so far, a spark plug was broken and, a piece of
the porcelin center cone had lodged itself beside the piston and scarred the
cylinder wall too deep to rebore. From one moment to the next, from a
healthy engine to expensive junk.

How can this happen so often? What caused this to begin with? There is only
one way possible and that is to miss a downshift and accidently stick the
gearbox
into too low of a gear at too high high a roadspeed. In most cases, I
believe this to mean missing a fifth-to-fourth downshift and making a
fifth-to-second
downshift. Ouch!!

When this happens, the events that follow are swift and sure. For the sake
of demonstration, let's say we are motoring along at a nice, safe 100mph.
All is well and we are in top gear, with a corner in sight. We slow down and
attempt to downshift to fourth gear. The shift is fumbled, second gear is
selected by mistake, the clutch is re-engaged. Suddenly, the leverage the
engine had over the rear wheels is history. Those big, wide rear tires have
quite a bite on the road and do not slide. The transmission becomes an
induction gearbox as the car and rear wheels are now driving the engine. The
hapless engine is now being spun far faster than it's mechanical limit. What
does "mechanical limit" mean in real life terms?

Let's step inside the engine and see what happens. An internal combustion
engine of such high specific output is a well choreographed procession of parts
narrowly missing each other and vulnerable pieces. Let's keep our eye on one
cylinder to simplify things.. When the valve is closed, there is a small
amount of clearance between the cam lobe and the lifter. As the camshaft
turns at half engine speed, the cam lobe touches the lifter, raising the
valve off its seat to full lift. The cam continues to turn and the valve
springs keep the lifter in contact with the backside of the cam lobe. The
valve is well. In our situation, however, the engine is suddenly spun so
fast, the cam actually throws the valve open and the valve springs cannot
overcome the inertia to keep the lifter in contact with the backside of the
camlobe. Known as "valve float", the valve is open longer than the cam's
intended duration. This also means that the exhaust valve is still extended
into the cumbustion chamber as the piston reaches Top Dead Center (TDC). The
piston slams into the valve head, bending it over like a nail. At these high
rpms, the piston and valves crash into each other hundreds of times per
second. By now, the driver has realized his mistake, kicked in the clutch
and, as they say, it's all over but the shouting. The loudest sound to be
heard now us the sound of his wallet screaming.

Can this horrible outcome be avoided? Yes, with careful and deliberate
placement of the shifter. Does this happen only to M3s? No. Any car with a
standard
gearbox is a potential victim. Know the top speed of each gear and learn it
well. Must shifts be in sequence? No. Professional race drivers have
different driving styles including downshifting. Two prime examples come to
mind: Check out old Indy in-car camera shots of Nigel Mansell. Nigel
downshifts so fast in sequence, the engine speed is never out of the
powerband or far from the redline. Michel Andretti, on the other hand, slows
for each corner in whatever gear he is in, then shifts to the proper gear at
the apex of the corner and accelerates out at full-tilt-boogie. Either
method is correct when applied correctly. Novice drivers new to sport
driving, or and old pro, anyone can miss a shift in the heat of the moment.
Presence of mind is the key here. Learn your limits and the limits of the
car and mark it in your mind as clearly as that redline on your tach.


Hope my $.02 helps other drivers to avoid the consequences of overrev.  I
have also almost missed shifts on two occasions.  Mind you I've seen my life
flash before my eyes.  No damage to the engine.  Shifting gears get all of
my attention all of the time.


Tony Dainora
1995 M3