Keer Kolloft

Hi there all,

I've received a rev range report for a Carrera GT I'm looking to buy and wanted your opinions. I know a number of you here are very knowledgeable in this area!

The report reads as follows:

Rev range one: 3939
Rev range two: 227

The latest ranges have occurred at 104.7h and the car has totalled 118.2 h, with a mileage of 5496km which gives us an average of 46.47 mph.

I know rev range two activity is BAD, but how bad is this relatively small activity? The rev range one is stated as MAX LIMITER <8400 and rev range two is above that (mechanical over rev). Does that equate to around 2 seconds at rev range two?

I guess the question here is, is this one I should walk away from?

Thanks in advance

911SLOW

range 1 is rev limiter plus 600rpm
range 2 is 9000+

If it has a serious over speed event the Carrera GT's DME (7.1.1) should throw the code P0219:

[I]
P0219
Engine Overspeed Condition
Diagnostic conditions
• Engine running
Possible cause of fault
♦ Engine is racing
Affected pins
-
This fault is recorded if the engine speed is well above the maximum
rpm value. This can be useful in determining the cause(s) of engine
problems.
Diagnosis/troubleshooting
Note!
Work instruction Display OK If not OK
1 Are there any other faults or customer complaints? No

Step 3
Engine damage/low
performance/other
fault entries

Step 2
2 ♦ Document the fault memory entry (vehicle analysis or OBD log)
♦ Carry out a damage analysis and talk to the importer about what to do next
3 ♦ Document the fault memory entry (vehicle analysis log)
♦ Do not erase the fault memory if this is the only entry. Since
this fault does not activate the CE light, the fault memory
entry can be retained for documentation purposes.
→ End



Porsche when analyzing the VAL (vehicle analysis log) in a damage or to extend a warranty pays attention to the operating hours:

The information below is intended to help you to evaluate engine damage, which may occur as a result of one or more overspeed events. You can also use the values entered in the VAL to improve the assessment process for granting pre-owned car warranties, for example, and to examine entitlements for carrying out repairs under warranty.
Engines are designed to operate at a maximum permitted speed. This engine speed is not exceeded when the vehicle is driven normally. However, driving and operating problems (e.g. “changing to the wrong gear” on vehicles with manual transmission) or manipulation (e.g. tuning) can cause the maximum permitted engine speed to be exceeded when the vehicle is driven.
Overspeed events are stored in the DME control unit. The following values are also recorded:
- Number of ignition attempts in each overspeed range 1), and,
- Status of the hourmeter during the last overspeed event 2).
1) Overspeed range = Defined rev ranges with classification of expected engine damage, e.g. engine damage possible, ... probable, ... very probable, engine damage has generally occurred; see section ⇒ 'Rev ranges'.
2) For technical reasons, overspeed events are entered in a preset time interval. As a result, there may be entries in a higher rev range (e.g. range 2), while no entries are stored in the lower range.

Typical faults and symptoms as a result of overspeed events may include the following:

* Damaged valves and/or pistons (impact marks on the valve, valve impressions on the piston, valve bent).
* Cylinder liner damage (even “piston seizure”).
* Damaged or displaced connecting rod bearings.
* Loosened or loose fastening screw(s) on the crankshaft pulley.
* Loosened or loose fastening screw(s) on the camshaft gear.
* Loosened or loose connecting rod bolts.
* Cracked timing chain(s).
* Changed timing.
* Irreparably damaged clutch elements (pressure plate and/or drive plate).
* Vibrations and/or engine imbalance due to irreparably damaged clutch elements.
* Vibrations around the transmission.
* Defective synchronisation.

Information
If such faults or symptoms are present together with engine damage, the VAL that was created must be checked for overspeed entries.
If overspeed entries are found and if the difference between the current hourmeter and the status when the last overspeed event was recorded is less than 50 operating hours, there is a high probability that the damage was caused by overspeed event(s).
The value “50 operating hours” should be seen as a guide. The possibility of engine damage occurring much later due to previous overspeed event(s) cannot be ruled out.

Keer Kolloft

Thank you for the detailed reply. I'm checking on the presence of the PO219 event in the DME.

Generally though, is that a high number of ignitions in the rev range 2? Is this a car to be steered clear of?

Plastique999

Can anyone help interpret some DME #'s?
Don't have full report, or see rev ranges...some numbers are:
# ignitions at rpm > max rpm, lower range = 5123
# ignitions at rpm > max rpm, upper range = 554
repeated data = 5137
and = 567
Thanks!

Leadfoot01

Those are the number of ignitions that occurred in that rev range. 227 and 567 ignitions in Range 2 would only amount to a second or two, from what I've been told....

superyota

I highly doubt there is any engine damage, these engines are built proof and can easily handle 9k rpm. But the dealer would be able to tell you.

smokeygt

I've heard these engines were tested at 20000 rpm for 24 hours straight! If that is true, then a couple seconds or even hours at 10000 rpm would be a piece of cake. Has anyone heard of the 24 hour test, or is this just folklore?

nuvolari612

Yet to hear anyone have engine problems - the engine is top notch and wouldn't doubt the 20000 rpm.

Porsche took this build F1 serious.

E-Man

I've read of this 20000rpm test in Panorama magazine's article of the development of the cgt.

I wouldn't worry of a few stage 2 overevs at all.

Petevb

^^ Big misconception here guys.

The engine for the CGT came out of a 1994 engine prototype built to F1 rules. This was a small 3.5 liter displacement V10 with pneumatic valves. The bottom end of this motor was tested to 14,800 rpm, and the cylinder heads (with those pneumatic valves) were tested to 20,000 rpm for 20 hours.

When it evolved into the CGT motor this motor changed dramatically: displacement and stroke nearly doubled, the pneumatics were replaced with conventional valve springs, the cams changed, etc.

This means that the entire motor was re-engineered for a lower rev limit. Doubling the stroke means the piston moves twice as fast at a given RPM, while conventional valve springs also can't come close to the rev capability of pneumatics. Thus it's absolutely certain that anything close to 20k rpm would instantly destroy the motor catastrophically- think not only bent valves, but pistons punching through the block...

I don't know exactly how much margin is built into the CGT motor, but I do recall at least one journalists destroying a motor on a test drive by selecting the wrong gear. I thus I would personally not take type 2 over-revs lightly.

E-Man

I've personally logged thousands of miles in a cgt with a couple hundred stage 2 revs, and I've never experienced a stronger running engine.

Petevb

I'm sure that's true. But that doesn't mean you haven't shortened the life of your engine at least somewhat in the process, and this is something I'd want to take into account.

Every engine will eventually need to be rebuilt if it's run long enough, even if it's cared for perfectly. The time between rebuilds depends mainly on wear and fatigue life.

Wear is straightforward- things like piston rings rub against the cylinder bore and slowly wear out until they leak and need to be replaced.

Fatigue life is more complex: unlike steel, metals like aluminum (pistons) and titanium (rods) lose a little bit of strength every time they are stressed. If the stress is low the fatigue life is so long that it's virtually infinite. In things with medium stress, say an aircraft's airframe, fatigue life might be 30 years. In high stress applications, like a Porsche RSR's engine, fatigue life is 30 race hours, after which the pistons and cylinders must be replaced. A top fuel dragster's rods are under extreme stress, and need to be replaced after less than 10 passes- that's less than 5,000 total revolutions under stress. If these components are not replaced after their fatigue life is exceeded, they will fail, probably catastrophically.

From an engineer's point of view the nice thing about fatigue life is that it's very predictable. Engineering materials have a known curve, so if you're designing a race car with 200 race hours between rebuilds at 8,400 rpm (10^8 fatigue cycles) you can look at a chart, like below, and see what stress level you need to stay under:


So knowing this, what happens if you over-rev? If it's a very bad over-rev, you'll exceed the allowable stress of some part, it will fail, and the engine will blow almost instantly. A less bad over-rev will see the valve springs unable to close the valves before the piston comes back up in the cylinder. The valves will "float", the piston will hit them, the valves will bend and you'll get a get cylinder leakage and power loss at a minimum. Either of these will give you instant feedback.

Below this is a range where nothing instant happened, but you did effect the fatigue life of the parts. Consider an engine revving to 10,080 rpm with a design redline of 8,400. That's a 20% over-rev, perhaps not enough to provoke one of the instant issues above. From a fatigue life point of view, however, it's quite noticeable. Stress increases at the square of RPM, so 20% more RPM means 44% more stress. And what does this do to fatigue life? On the chart for aluminum above, if we were at 16.5 ksi stress before, roughly 10^8 cycles for our 200 hours, increasing stress 44% takes us to 24 ksi, which is down just over 10^6 cycles. Meaning we're going through fatigue life 100 times faster- our 200 hour engine is suddenly a 2 hour engine! If it's an 11,000 rpm over-rev it's suddenly a 30 minute engine! So things get pretty serious pretty quickly.

All of which may or may not be so bad. Porsche does build a lot of margin into these engines, and general use likely doesn't threaten the the fatigue life of the engine unless you're tracking the car or putting on lots of miles. Treat it like a Cup car and expect it to last like a Cup car, treat it like a street car and it's going to last a lot longer. But this is why over-revs are tracked, why I wouldn't take them lightly, and why your strong running engine does not mean nothing resulted. Something has, the question is simply to what extent. Unless the over-rev was bad it could be minor, but I'm mentally calculating: every type 2 over-rev is equivalent to how many more miles on the car? And were they the 9,000 rpm type, or the 11,000 rpm type, which are obviously very different?

It may be impossible to tell with the CGT's ECU, so if people are not in general having engine problems with type 2 over-revs that's obviously a good sign that there is good margin there for the average usage.

On some other cars there's sophistication behind this equation: my Cup car engine, for example, has a separate data logger, and my rebuild schedule is determined by exactly how "nice" I am to the engine. Or not, as the case may be...

nuvolari612

[QUOTE=Petevb;11729902]I'm sure that's true. But that doesn't mean you haven't shortened the life of your engine at least somewhat in the process, and this is something I'd want to take into account.

I think there is a big differences between a RSR and a CGT. Why didn't Posche simply install a rev limiter?

The V10 isn't nearly as stressed as the 6 cylinder RSR 60 hp 4.0-litre six-cylinder.

I agree that Porsche re-dedesined the engine and components for a lot of reasons but the titanium internals are still very impressive.

Extremely low centre of gravity reason carbon composite clutch with a dry sump and oil tank is in the transmission housing.

Forming part of the chassis means the engine will be subject to forces beyond those of just providing power and accordingly it has to absorb energy from the chassis without the bores distorting

CGT engine with the lack of flywheel - true racing engine. Making it very hard to find one with no over rev's. CGT patent hollow transmission input shaft with inner solid rod acting as a torsional spring to damp any power fluctuations from the engine. Thanks to the compact clutch and the innovative " no -flywheel" the engine sits 3.9 in. above the car's underfloor.

Borrowed some info IMO the only car in the CGT league is the F50 and it's an extremely harsh ride vs Porsche with the dampers are brilliant. All someone has to do is look at the overall CGT under the skin I do not know of a more impressive design in a road car.

Even if these engines wear - the amount of hp isn't going to punch a hole in the block so what's the big deal with using and enjoying the BEST road car for 100,000 miles and rebuilding the engine.

Sorry for the edit - computer issues.

Petevb

I didn't mean to imply that it was- I suspect the CGT engine would be good for something like 5 times more high RPM track use based purely on an educated guess. But it's not as different as you might think.

One of the easiest ways to estimate the stress on an engine is to calculate "mean piston speed". This is usually expressed in meters per second, and the formula is 2 x stroke x rpm / 60. The mean piston speeds for high performance engines are remarkably consistent even across very different engine designs. Some examples:

Carrera GT: 76 mm stroke, 8,400 rpm- 21.3 m/s
2009 GT3 RS: 76.4 mm stroke, 8,400 rpm- 21.4 m/s
2010 GT3 RSR: 80.4 mm stroke, 9,000 rpm- 24.1 m/s
2006 F1 V8: 39.8 mm stroke, 20,000 rpm- 26.5 m/s

So real race cars do have higher piston speeds and stresses, but not by as much as you'd think. And the biggest single difference between the GT3 Cup Car engines, with ~100 hours between rebuilds, the WC cars at ~50 hour rebuilds and the RSR engines at ~30 hour rebuilds is the ~12% difference in peak RPM.

Bottom line, these speeds are governed by the strengths of metals, and there is no room for a large margin of safety. The CGT motor is clearly less stressed than an RSR motor, but not by very much. And spin a CGT engine to 20k rpm, as was mentioned earlier, and you'd be doing a piston speed of 57.5 m/s, over twice what the best F1 engines did. Pop...
There is one installed, but type 2 over-revs are generally caused by a missed shift or an early downshift. A rev-limiter won't help there.

stout

Yes, big difference between a small-displacement F1 engine tested to high rpm and the large-displacement Carrera GT street engine.

At the original Carrera GT press launch at a former MIG fighter base, a French writer missed a shift. The last reading on the ECU was 14,500...

pete