Beyond Overrev
March Trax 1995
by James Williams
Houston Chapter BMW CCA Technical Advisor
Advantage BMW
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 misconception: 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 wheel.
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 sifters 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 crescent 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 porcelain 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 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 100 mph. 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 slice. 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 that 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 close, there is a small amount of clearance between the camlob and lifter. As the camshaft turns at half engine speed, the camlob 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 camlobe. 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 combustion 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 is 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 an 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 you tach.