I discovered some information on the Type 1 and Type 2 over-revs that I would like to share. I purchased the Durametric OBD-II software and cable set a week ago and have evaluated my error codes and Over-Rev Counts The count that is recorded, and cannot be erased to my knowledge, is a count of the ignition firings above the redline threshold. It is not the total of the times that the engine has breached the redline value, as I was led to believe. The count is broken down into the two categories and I'm not sure how it does this, but it just does.

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.

After reviewing the number, I concluded that my numbers were not that bad and although it was not lily white clean, my engine was not thrashed because what I saw as a high count.

Comments?

 

Keith - that was by FAR the most detailed explanation on type I and II over-revs that i have EVER seen.....and it's been mentioned alot in the last couple of years namely because Porsceh likes to limit or flat out deny warranty claims because of Type II over revs

Thanks for the detail!

 

Another general type of failure that can occur with type 2 over-revs... the valve springs cannot keep the valves tight to the cam profile and the valves can "float". In most engines (not sure about 996TT) the pistons can hit the floating valves and damage both. Very bad.

 

Keith,

thanks for the very informative review

 

Where I come from, we call that the pistons entertaining the valves. A little humor...

Thanks for the nice comments. I enjoy the research and writing.

Cheers,

 

I agree with your conclusion one Type 2 OR is probably not a problem. I think it also depends on the engine hour the last one occurred. Early in the engine life and the engine is still ok, seems better than in the last few hours before it was sold. But that is just IMO and has no basis in mechanical fact.

If a car has some Type 2's, what is a reasonable number? I am waiting until Monday for a PPI result from Champion on an 03TT with 8K miles. The tech needed an "interpretation" of the type 2 OR's from the shop manager. I didn't get the number, so I'm concerned about where to place the cut off for deciding the car was abused?

Opinions from the experts?

Thanks!
Carmen

 

so if the rods are primarily designed for compression, then the intake stroke must be killing them.

 

Exactly, at least the power stroke has some compression coming from ignition. But I think the increased force generated from the simple change in the rate of acceleration (TDC to mid stroke) in an over rev is detrimental.

 

Keith,

I just got the number back from Champion on my PPI and it was 96, close to your number. The Service Manager told me they found no documented explanation from Porsche to interpret the count but if you are correct the 96 is one missed shift of less than two seconds. He felt it would not be a warranty issue.

Carmen

 

Carmen,

Please be very careful with any verbal assurances from any Service Manager... get it in writing. They will hose you at their firrst opportunity.

 

At 7200 RPM there are 360 firings per second. 96 doesnt sound too bad.

 

"At 7200 RPM there are 360 firings per second. 96 doesnt sound too bad"
A minor point, but I believe there are 180 firings per second at 7200 RPM on a 4-stroke engine that fires every other strroke.

 

Keith, thanks for a great write up. When I recently saw my Type 1 count ~ 5000 (I have 29k miles on the car)...I nearly flipped. No Type 2's (knock-on-wood).

 

On major big ticket items the dealership's Service Advisor and/or Managers don't have the authority to make that call. It will either come down to the PCNA regional/district rep or the people in Atlanta/Germany that go over the data that the POSES system sends in when your car is plugged into the dealer's computer.

 

7200 rpm = 120 revs/second
A 6 cylinder fires 3 times pre revolution.
3x120 = 360 ignitions/sec