Frank996

My apologies for, what some may find as a simple and strange question, but I have not much of a mechanical engineering background so thought I'd ask you tech savage people .

How come cars "allow" for over revving? I mean, the red area limit in the rev display should not be crossed right?

So, if over revving causes damage to the engine how come there is no electronic or mechanical limiter on it so over revving doesn't occur and saves the engine for damage, or am I missing something?

cuttinsod

2 main reasons...

1- Missed downshift. mechanical speed of wheels/transmission drives the engine well above engine rev limit (since they are all connected)

2- On newer cars, there is a rev limiter that cuts off fuel at/near the red line which prevents the engine from being run over redline, but you can still exceed the redline a little bit due to the momentum of the engine when the fuel cuts-out, but this should be minimal.

Frank996

Thanks for the reply cuttinsod.

One comment on the 1st point: Would it not be possible then to kind of put a limiter on the gear shift so a missed downshift could be averted? I mean, you are cruising away in 5th gear and trying to put the gear into a gear too high (so the rev would reach well into the "forbidden" zone") it wouldn't be possible. Like trying to put the gear in reverse when driving forward at highish speed... I know that you should still be able to slow the car down by using the engine as a brake by gearing down but should never come close to the red field...

I guess engine manufacturers would have had this in place if it was possible...

Thanks again for explaining cuttinsod

Imo000

This is not a bad ideat at all but the manufacturer has no ontrest in implementing this. Also, this would not work if the car is driven on the track. Heel-toeing would be hard to do.

Dennis C

Doesn't the new 991 have some electronic controls on the manual transmission just like Frank described? It's pretty advanced. I know it's 7-speeds, it rev-matches for you on downshifts and it does "block" 6th and 7th gear unless you are coming from 5th gear. I wouldn't be surprised if it blocked downshifts that would result in over-revs...

Imo000

The 3rd gen F-bodies (Camaro/Firebird) had an electronic lockout for the 2nd gear. Unless the rpm was high, it blocked the driver from using second but only during upshifts. GM did this to increase fuel mileage but it was anoying and pretty much the first this everyone did was to disable the lockout solenoid. So the technology has out there to limit gear shifts sine the early 90's.

cuttinsod

the transmission lockouts typically have been on high torque american cars (camaro, corvette), and was implemented to meet the EPA mpg reuirements forcing a shift from 2nd to 4th under light load (high load/fast revs/fast shifting bypassed this feature).

I am sure the technology can be implemented for preventing overrev downshifts....maybe some centrifugal lock-out or similar tech, but it would add a lot more complexity, and more failure modes to the transmission...and like said earlier, would make track driving (where you are more likely to encounter the "money shift") more difficult when trying to maximize power/performance.

Macster

"Would it not be possible then to kind of put a limiter on the gear shift so a missed downshift could be averted?"

See PDK.

Sincerely,

Macster.

wwest

Clutch "latch" lock out..??

Macster

My 01 Camaro had this and so did my 06 GTO. The mechanism didn't block/locktout second gear when shifting from 1st to 2nd under some conditions it shoved/forced the lever over at the "H" cross over to direct the lever with the driver's action into 4th.

For the Camaro I adapted a slightly more aggressive style of driving that resulted in this mechanism not being triggered.

For the GTO I bought a Predator OBD2 tool and used it to disable this mechanism.

Sincerely,

Macster.

DBJoe996

I have a rev limiter...my foot! I think in this case what is expected is driver/car experience. A few times of popping over the limit isn't going to damage the engine. I think sustained over-revving would be bad, or really aggressive driving that constantly pushes over the limit would also shorten the life of the engine. Frankly, I would rather have control over this rather than having it mechanically/electronically limited. If you want this...then why not limit it to 5000 RPM so you can never stray into the danger zone...just saying I would rather exercise my own abilities rather than having some corporate suit make that decision for me.

Macster

Well, I hope this doesn't come across as mean but I suspect your abilities to always take the engine right up to its redline and not exceed it are not perfect.

And they'd have to be perfect cause all it takes is one time over the enigne's max redline for the engine to go boom.

I like to pride myself on my ability to control my car at all times. That is the responsibility of the driver naturally.

This includes of course throttle control. I have always said the best traction control is the driver's right foot. However, as aware of this I am and as hard as I try to follow it once in a while my right foot can't work as fast as the engine revs. (This is especially true in my Turbo the engine can gain rpms in 1st gear like you would not believe.) As a result the rev limiter kicks in.

Thank God.

The real control, the real expertise in using one of these cars is in driving it in such a way to derive max performance from the car and the engine. There's nothing to be gained by taking a stock engine over its redline.

The quickest accel run the fastest lap the best passing is done shifting the engine at something under redline.

The rev limiter is there for those that don't believe that or can't always 100% of the time adhere to it.

Sincerely,

Macster.

DBJoe996

Not mean and realistic! I agree Macster and actually, I have never ever pushed close to the red line. Since I daily drive mine and depend on it for work/income, I would never drive like that. I have a whole lot more respect for the engine/car than that and since I do all my own DIY, I'm not crazy enough to blow it out like that. I was simply saying that a mistaken bump over the red line doesn't automatically mean it's going to blow. We know that daily driving around the city one would have to be crazy to push that red-line on every shift...there is just no way that is realistic. I agree with what you say...the real control and expertise is driving to achieve best performance within the abilities of both the car and the driver, and these days, gas mileage. LOL!

Frank996

PDK?

Dstutler

I've run past the power cut off point one time on my 996. The excitement of the revs and thrust of the car are intoxicating but certainly spoiled by the feeling of the gas being cut off, simulating a catastrophic mistake and engine failure. My head and right foot now work together much better and I protect the machine......