A Type 1 is an over-rev condition that thru to engines own inertia or power stroke has exceeded redline and the DME has fired the ignition during above redline and the condition has recorded the individual firings. It also records the time (Total Hours) and is revised at each event.

A Type 2 is an over-rev condition when an external force has caused the engine to speed beyond the redline threshold such as down shifting to a lower gear and engine matches the rpm of the transmission input shaft.

The difference between the two may sound subtle, but is more involved.

In a Type 1 condition, such as when one misses and up-shift. The engine is in a free-wheeling state and the internal energy is acting on the rotating and reciprocating mass. The crankshaft is unloaded for a brief moment and there is still some force on the top of the piston during most of the 4 stroke cycle.

Type 2 conditions are not healthy for the rod and rod bolts, although there must be some design reserve before yield. When one down shifts to a gear which leads to an over-rev condition, there is little energy pushing down on the head of the piston to keep the rod in a compression mode. The crankshaft is pushing violently on the rod and then jerking the piston and rod assembly back towards the crank centerline. The rods are primarily designed for a compression mode, meaning forces enacted on the top of the piston and imparting energy into the crankshaft via the connecting rod. The connecting rods and bolts really don’t like the dynamics in a stretching mode and this condition leads to material yielding. This is the condition that causes the rod bolts to fail. They yield during the stretch. The connecting rod can also yield due to stretch, but that is seen more with Aluminum Connecting Rods.

When I saw the number of type 1 and type 2 over-revs, my heart started to race. I was thinking that the engine was going to explode at any time because of the previous driver was thrashing the engine. Well when I came to my senses, I realized that the counts were ignition firings, and that at 6,700 rpm’s there are 335 firings in a given second, I felt a little better. Consider the actual time the engine is in an over-rev’ed mode. The DME caught the condition, responded with the appropriate shut down to protect the engine. This would be the dwell time of what, may a second or two. So at 2 seconds there were 670 firings. So if your count was 1435 over-rev firings, then it would have been 5 times that the engine over shot the redline. The type 2 over-revs were 91 and means that maybe someone jammed it into a lower gear just once.

A type 1 over rev occurs when you linger on the throttle a bit too long when making an upshift. The rev limiter will prevent any damage at this point, but the event is recorded as a type 1 (actually, each "event" will show dozens or hundreds of counts).

A type 2 over rev occurs when you are shifting, say 4th to 5th, but accidently grab 3rd instead. At this point, the engine is forced by the wheels to rev higher than its design limits, and serious damage can occur... like pistons running into valves, or connecting rods being stretched. Even a single type 2 over rev is bad, and often a dealer will deny warranty coverage to an engine that has recorded a type 2 event.