box986

Just flashed the car with a new tune with Fuel Cut off set at 7500RPM and bumping in to the rev-limitor (not mis-downshift) gave me level 3 over-revs, is this normal?

I know engine inertia will sometimes carry the engine RPM beyond fuel cut off, on a stock tune Carrera with limiter at 7300 RPM is it normal to go 1 or 2 ignition ranges above the cut off?

Range 1-RPM Range 7,300-7,500
Range 2-RPM Range 7,500-7,700
Range 3-RPM Range 7,700-7,900


Durametric is showing my car as a GT3 but it is actually a 06 C2S.

utkinpol

I run softronic flash with rev limiter bumped to 7500 and yes, a lot of 1,2,3 ranges. It is racing setup, so, you do rev it high. If you do not want that - ask for a flash with stock rev limit.

box986

I am fine with 3s but not 4s, just wonder if the 200rpm over-shoot is normal. My car did have a Range 5 early in its life but runs normal now so I think I should be fine.

Sharkys

If an engine over shoots post fuel cutoff, how can there be an ignition? I assume the coil energizes and fires the plug in the absence of fuel and that is considered an ignition of the plug as in spark?

Skibum

Overshoot is a myth.
It is not possible.

gota911

If that ^ is the case, how and why did the OP get ingitions in Range 3 after he reset his rev limiter/fuel cutoff to 7,500 RPM, which is 200 RPM above the "new" rev limiter/fuel cutoff?

gpjli2

Fwiw, it should be clear that there are overrevs that damage the engine and those that do not cause damage. Regardless of where the overrev falls numerically if, after 40-50 running hours there is no cel or other problem there has been no damage. It is afterall engine damage we are concerned about, not the rev itself. Don't get crazy if your great running car shows a few bad #'s. During the shopping process however a motor which has a sloppy printout is less desireable than one that is cleaner just based on the general principle that the previous owner did not show a lot of mechanical sympathy or care. Whether that should disqualify an otherwise fine car requires a crystal ball imo. My own prejudice is that a car coming off a 2-3 year lease may have had little or no service. A car that has no service records and a lousy printout is probably a poor risk.

Edgy01

Late for a shift can run you past redline due to engine inertia.

Skibum

Can any of you that subscribe to this "inertia" thing point to the physics behind it?
Have you EVER seen any object go faster due to inertia? Example please.
Do you believe in cold fusion too?

Skibum

Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, iners, meaning idle, or lazy. Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states:[1]
The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.
In common usage the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context. The term "inertia" is more properly understood as shorthand for "the principle of inertia" as described by Newton in his First Law of Motion; that an object not subject to any net external force moves at a constant velocity. Thus an object will continue moving at its current velocity until some force causes its speed or direction to change.
On the surface of the Earth inertia is often masked by the effects of friction and gravity, both of which tend to decrease the speed of moving objects (commonly to the point of rest). This misled classical theorists such as Aristotle, who believed that objects would move only as long as force was applied to them.[2]

RadiusTim

Hi,
First post here, greetings from a 997 owner in the UK.

I had no part in the design or development the Bosch Motronic engine management system, so although this is informed by more than just speculation, I've no interest in trying to be a web forum pundit, the world's got enough of those already.

There's plenty of empirical evidence that these 'overshoots' do happen with 997.1 M96 and M97 engines, where DME overrevs are recorded after bouncing off the rev limiter while accelerating.

The cause of this has been tagged 'inertia,' a word that implies a mechanical effect of the crankshaft/flywheel's angular momentum, which somehow makes the engine's RPM increase with no fuelling. As folks have said already, this not physically possible.

The observed overshoots happen because the DME rev limiter is a reactive process and hence is subject to a lag between the engine speed going above the rev limit, and the fuel cut-off being completed.

The DME overrev recorder is a separate process that runs independently, to detect ignitions in the defined 'overrev' ranges, and record a cumulative count of these with a timestamp for the most recent.

Of course it's not beyond the wit of man to make a smart predictive system that completely prevents 'overshoots' when accelerating into the rev limiter, by dynamically monitoring the rate of change of engine speed and applying variable advanced-triggering of the fuel cut-off to overcome the delays. And it's also possible to design a system where the limiter and overrev recorder are perfectly in step with each other. But the Bosch 7.8.1 DME as used in the 997.1 doesn't do this.

On the 997.1, with sport chrono used in sport mode the DME sets a hard cutoff: it allows full power right up to the set rev limiter.

The rev limiter gets input signals on engine speed from a transducer (crank sensor), processes them using its internal clock to calculate engine speed, compares the calculated result to a set threshold value, and if/when that value is exceeded it sends signals to electromechanical actuators to cut the fuel (injectors and throttle butterfly, not ignition coils).

All this happens very fast indeed, but it still takes a finite time to complete the whole process (as long as ~300ms according to data-logging posted here), while at full throttle in 1st or 2nd gear the engine speed rises very fast (by >1000rpm per second). Hence 'overshoots' in the region of 300rpm are plausible.

Accelerate hard into the rev limiter in low gear and in 'sport' mode then, and within the fraction of a second it takes the limiter to detect an overrev and the actuators then to complete the fuel cutoff, the DME will already have detected and recorded overrev events.

This happens with a stock 997.1S, even though the 7300rpm redline is well above where maximum power is produced (~6500rpm). With a 997.1S X51, there's more power and torque right up to the limiter (7450rpm), but the DME overrev counter is left unchanged, so more overrev events get recorded whenever the rev limiter is hit hard. Same goes for cars with aftermarket tuning mods and maps with a raised limiter, as the OP has found.

With all that said, it's really not good driving practice to crash into the rev limiter, any more than it is to mess up a downshift. (apologies for stating the obvious there - I know only too well how it's easily done in the heat of the moment)

Even though it's 'just' exercising the engine's own defense mechanism against damage, it can't do the engine any good, and if nothing else, bouncing off the rev limiter badly affects progress and laptimes.

Cheers

Skibum

Well said Tim. Spot on.... and welcome!

MICHAELWWW

Pundit!

Skibum

Sorry... I should have more correctly said over-rev due to "engine inertia"