What Really is an Over-Rev?
An over rev is exactly what is says it is. It is one or many engine revolutions over or above the maximum number of engine revolutions per minute the engine manufacturer has deemed safe. They typically occur in cars that have manual transmissions on down shifts when the road speed and gear tooth ratio are not in sync. When the road speed is faster than the gear ratio selected will allow the car to be driven at. It can also happen on the up shifts when a gear selected is not engaged and the throttle depressed. Racing is the most common arena where this happens.
It needs to be known that usually, there is some safety or head room built into the limits. It’s best not to prove the actual mechanical limits without a full understanding of the damage that can result. This paper serves to educate those that do not know the mechanical issues that can result due to “engine over-rev.”
In some software applications, over-revs are classed as numbers or letters. This is to determine the limit or revolution the engine runs at and the number of times the engine ran at that speed. This tells the person reviewing the software just how bad the engine condition could be. It also, in many instances, will limit the manufacturer’s warranty.
So what happens when you run at or above the engine speed limiters? In most cases, there are several engine speed limiters. They are categorized as soft and hard limiters. Different software will have different limiter features and within these features different limiting strategies can be set up. Soft limiters typically create ignition cuts to different cylinders, either bank to bank or odd and even cylinder numbers, on different engine revolutions. Cutting the ignition has the effect of a controlled high speed engine misfire. Hard limiters usually cut the fuel. The hardest of them all cuts all of the fuel completely until the engine has lowered its RPM well below the maximum limit. Strategies are in place to “cut” parameters at whatever RPM the software is programmed at. For those that have experienced a hard limit, without a restraining belt, your face probably met the windscreen in a violent fashion.
So what are the main areas of concern and the engine components that can be damaged? They are, but not limited to, valve train components, connecting rod components, rod and main bearings, drive belt systems and transmission internals.
Valve train parts are camshaft lash caps if the valve train is of a solid type, valve springs and, to some degree, the valves themselves and the seat inserts they come to rest upon. Lash caps are bounced off the end of the valve due to the violent nature of the misfire and the valves going into major surge. When this happens the valves and whatever mechanism is used to depress the valves loose contact with the camshaft. This loss of contact creates distance between the camshaft and the follower which losses contact with the lash cap and the lash cap can move upwards off the end of the valve. This is why continuous running in and out of the limiters is so dangerous. The valve spring going into surge is also very harmful. When this happens the spring goes in and out of coil bind. This is when the coils of the spring(s) (if it’s a dual spring) crash into one another causing surface damage that results in the spring breaking. If the spring gets close to its natural frequency, this too will break the spring. In dual springs there is always interference between the outer face of the inner spring and the inner face of the outer spring. This is done on purpose to help control the springs at high RPM. Another possibility with the valve train going south is the issue of the valves “floating” uncontrollably away from the camshaft and valve seats. This takes the valves and their partnered piston out of sequence and often they crash into one another.
In most cases, connecting rods are a two-piece component. The upper part is the beam section and the lower part is the cap section. These are held together by bolts or bolts and nuts. The most critical part of the rod strength is the fastener holding these two parts together. Bolts are assembled with a determined amount of tension by stretching the bolt close to its yield point. This puts the bolt in tension so it can act as a spring and react to the forces placed upon it with engine rotations. The bolt is subject to the most stress on the blow down stroke when the piston is on its upwards travel. This is when the piston wants to jump out through the cylinder head into freedom. Engine speed really puts a huge stress upon the rod bolt. As the speed increases the rod bolt wants to stretch as the stresses increase. This is the main cause of bolt failure. This is why when assembling rods, it is critical to measure the bolt stretch rather than rely upon a straight torque setting. Measuring the stretch removes the friction aspect that can affect the torque value significantly. If you only torqued the bolts, you could effectively have different amounts of tension applied to each bolt. Bouncing off the limiter puts a huge strain on the connecting rod. Big end diameters can go out of round and the clearances between the piston pin and bushing in the rod’s small end. As the engine misfires, the result causes an imbalance to the crankshaft, which causes a twisting effect similar to a harmonic imbalance. This may sound extreme, but is necessary in order to give a good account of the dangerous effects of running up against and into the engine limiters.
The main and rod bearings also take abuse when the limiters are in play. The pounding received by the upper bearing shells during engine misfires, on any piston down stroke, squeezes out the oil film between the bearing shell and crankshaft journal. On the upwards stroke, the same happens to the lower bearing shells. Inspection of the shells often show the telltale signs of an over rev. Wear patterns on certain areas of the bearing along with length dimension changes will be noticeable. Often on the back side of the bearing shells, wear patterns will show. This is evident where the rod’s big end goes out of round and the clamp (the shell was under) was not all the way around the circumference of the shell.
Drive systems, belt or chain, also take huge stress when the limiter is enacted. Both stretch and in the case of a drive chain, crash against the guides. The guides wear prematurely or even break. Basically, the effect on these parts is the same when the engine hits it’s harmonic resonance RPM. If the Camshaft is driven by a rubber belt, the belt can jump off the driving gear or the driven gears. This will result in the engine falling out of its supposed Camshaft timing, ultimately leading to Piston and Valve contact.
If you ever get a chance to watch slow motion video of the engine at speed, when the limiter is turned on, it will scare you to death. I believe, if the driver was aware of this phenomenon, they would never drive anywhere near the limiter. As an engine person, we want the best performance out of our engine. But, we also want the engine utilized and driven within its mechanical safety limits. This is why we are so upset when the engine is driven in an out of the limiting RPM’s. We know the damage that can ensue.
In the case of Porsche engines, its common to read about pre-buying inspection reports posting the limiter number and the number of times the engine has run into this rpm band. Should you worry? Yes, but also understand that in most cases, no damage is caused. Porsche has always given a lot of head room above the limiter RPM before any type of serious engine component damage would be caused. However, in a Porsche race engine, there is definitely more cause to be concerned. A street engine, is less likely to cause any damage. I know warranties are voided in some cases, but remember too, the limiter is not just a mechanical limiter, it’s also a responsibility limiter. I am not advocating dismissal of any over revs on an inspection report, just that often a lot of unwarranted emphasis can be placed on them and a perfectly good car could slip through your hands! I would suggest any inspection include internal inspection with a borescope along with the normal tests of Cylinder leakage and compression tests. Oil analysis can also be very helpful especially when the ECU reports over revs. Buying any car, race or street has some degree of risk attached. Due diligence is a must. If unsure seek out knowledgeable people to consult.
It is hoped this has shed some light on the matter of over revs and given you some idea of the damage that can result in the most extreme cases.
Experienced engine professionals know what to look for, whereas, the basic assembler will have no clue unless something is broken. Oftentimes, understanding of the issue never occurs and the same part is reinstalled. Off we go again! This is where my old saying comes from:
“There are those that see it wrong and fix it and those that don’t see it at all.”