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Discussion Starter #1
I asked this question on a heel/toe thread, but I don't want to hijack that thread, so I'm posting a new one.

Prior to rev matching, drivers had to use heel/toe to blip the throttle to get the rpms into the appropriate rpm range for the lower gear. So now the rev matching feature performs this operation automatically by using a computer to instantaneously calculate the required rpms, given the gear selected and the current speed. So before the clutch is fully released the engine is already at the appropriate rpm to support a given speed for the selected gear. This ensures a smooth shift each time.

So that's what happens in a typical downshift, but what happens when the driver inadvertently shifts to a gear that will exceed the engine's red line by let's say a 1000 rpm. Is the rev matching program written in such way that it doesn't consider red line and attempts to reach the calculated rpm regardless of how high it may be or is red line monitoring part of the program's algorithm?

Probably only the programmer knows the answer, but it seems to me that rev matching could offer another level of protection to minimize the damage for the inadvertent shift. If the program recognizes that the rpms required to execute the downshift exceeds red line and instantaneously cuts the engine's power, the car's speed will be reduced before the clutch is released. So in my hypothetical over-rev scenario, maybe the engine would exceed red line by only a couple hundred rpm instead of a thousand, thereby making it a harmless range 1 over-rev.

I've used the term "instantaneous" a few times since I don't know precisely how long the rev matching function requires. Typically computers require nanoseconds for calculations and milliseconds to execute a command. I can wrap my mind around tenths of a second, but when you begin dividing a second into a thousand parts or smaller, it becomes instantaneous in my mind. :)
 

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You are on target. I do not know exactly how the computer is programmed, however when Sport+ is active (rev matching enabled) if you mistakenly miss shift to a lower gear than you intended this is what actually happens in my experience:

The short answer is that the miss shift never actually happens when you use rev matching. You can thank both the Porsche engineers and your brain.

The longer answer explains what happens in that split second and why:
1. You approach a bend/turn.
2. You come off the gas, clutch (verb here) and down shift into a lower gear than you intended
3. Before you lift your left foot of the clutch pedal and engage the engine to the wrong/lower than intended transmission gear you will hear the engine rev out of it's normal rev matching sequence. Meaning the higher pitched engine sound tells you something is wrong and you do not let the clutch out.
4. Instead, you shift into the proper gear and then let the clutch out after hearing the "normal rev match sound". This happens so fast that it is almost involuntary. After it happens to you for the first time you will understand.

While the Cayman's manual gear box is really great IMO and mis shifts are pretty hard to do, when it does happen rev matching has paid for the Sport Chorno function when it gives you this early warning that you would not have received in the same circumstance noted above without this option.

Thank you Porsche!

(I am not an expert by any means but I drive with Sport+ on greater than 90% of the time so I hope I have done a good job of explaining what actually takes place. I am sure/hope others will chime in.)




I asked this question on a heel/toe thread, but I don't want to hijack that thread, so I'm posting a new one.

Prior to rev matching, drivers had to use heel/toe to blip the throttle to get the rpms into the appropriate rpm range for the lower gear. So now the rev matching feature performs this operation automatically by using a computer to instantaneously calculate the required rpms, given the gear selected and the current speed. So before the clutch is fully released the engine is already at the appropriate rpm to support a given speed for the selected gear. This ensures a smooth shift each time.

So that's what happens in a typical downshift, but what happens when the driver inadvertently shifts to a gear that will exceed the engine's red line by let's say a 1000 rpm. Is the rev matching program written in such way that it doesn't consider red line and attempts to reach the calculated rpm regardless of how high it may be or is red line monitoring part of the program's algorithm?

Probably only the programmer knows the answer, but it seems to me that rev matching could offer another level of protection to minimize the damage for the inadvertent shift. If the program recognizes that the rpms required to execute the downshift exceeds red line and instantaneously cuts the engine's power, the car's speed will be reduced before the clutch is released. So in my hypothetical over-rev scenario, maybe the engine would exceed red line by only a couple hundred rpm instead of a thousand, thereby making it a harmless range 1 over-rev.

I've used the term "instantaneous" a few times since I don't know precisely how long the rev matching function requires. Typically computers require nanoseconds for calculations and milliseconds to execute a command. I can wrap my mind around tenths of a second, but when you begin dividing a second into a thousand parts or smaller, it becomes instantaneous in my mind. :)
 

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I had the same experience, shifted into 2nd rather than 4th. Heard and saw the tach go beyond 7,000 on my 4 day old car! Thought I had an over-rev for sure. Recently got a Durametric and checked the over revs. Happy to find out I had only 6 events at range 1.
 

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As I mentioned in the other thread having the fuel cut might limit the magnitude of the over rev a little as the engine is providing compression braking to the car as the gearing is spinning the engine up. And as mentioned here the sound might clue you in if you're quick enough. The bottom line is the physics of the situation is the engine will over rev if the clutch is let out on too low a gear for the speed the car is going. Even if the ignition was off the gearing would mechanically drive the engine to over speed. Sorry, it might help you a bit but won't save you.
 

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Given how modern cars are so computerized, I don't get why car manufacturers don't program a warning beep if you shift down to a gear that could cause overrev. Just need a beep to minimize the overrev.
 

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Some gearboxes have "guides" that can block off gears, so it might be possible to block off gears that would over-rev.
That would stop you physically putting it into that gear.

As to the OP bit about software protecting, it sounds like it is trying to rev match, but also respecting redline.
Which means its helping it over rev, by already having the engine at redline.
It does seem sensible that it could rev it down, rather than up - to help avoid the over rev (or at least reduce it). But from what others say, thats not the case.

As for "instantaneous" - you can consider the computer decisions to be effectively instantaneous, but the engine itself does take some time to respond.
It can rev up or down very quickly, but it still has rotating mass in the engine (and a fly wheel) and would take hundredths of seconds (maybe a tenth or two) to do some changes in revs.
 

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Discussion Starter #7
Thanks to all who responded. So based on real life experiences rev matching will push the engine past red line but the sound of the engine at these rpm levels alerted the driver that something was amiss and the clutch was never fully released. I'm just surprised that Porsche programmers didn't write a script that would activate fuel shutoff any time a gear was selected that would require the rpms to exceed red line. That along with an audible alarm would be ideal, but I'm sure that Porsche considered this option but perhaps it may have some unintended consequences that I'm not considering.
 

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Discussion Starter #8
As for "instantaneous" - you can consider the computer decisions to be effectively instantaneous, but the engine itself does take some time to respond.
It can rev up or down very quickly, but it still has rotating mass in the engine (and a fly wheel) and would take hundredths of seconds (maybe a tenth or two) to do some changes in revs.


Here's the sequence of events as I see them in an inadvertent downshift, and how I would have written a script to minimize the possibility of engine damage.
1. The driver downshifts to a gear that will cause the engine to over-rev.
2. The car's computer determines the downshifted rpm based on the selected gear and the current car's speed and determines that the downshifted rpms will exceed the engine's red line. I expect that this calculation would take no more than a couple nanoseconds.
3. Once this condition is identified the computer would initiate two actions simultaneously: send a signal to cut off fuel supply and initiate an audible alarm. I'm expecting that this action would be executed in milliseconds.
4. I would expect that with the sudden loss of power and the audible alarm, the driver would immediately recognize his error and not release the clutch, but even if he didn't the fuel cutoff would occur at least a coupe tenths of a second before it does under the current process. The current process is to initiate the fuel cutoff when the engine actually reaches red line, and not before as would be the process that I recommend.
 

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Hi all,

In respose to above:
"4. I would expect that with the sudden loss of power and the audible alarm, the driver would immediately recognize his error and not release the clutch, but even if he didn't the fuel cutoff would occur at least a coupe tenths of a second before it does under the current process. The current process is to initiate the fuel cutoff when the engine actually reaches red line, and not before as would be the process that I recommend."

I decided to respond to this thread for my first post. I'm presently waiting for an alloaction for a Boxster Spyder. Perhaps I may be able to shed some light on the rev matching situation. Although, I have not driven a car with rev matching, I have been in many cars with rev limiters such as hard fuel cutoffs. That can even be demonstrated while sitting in the driveway and putting the throttle to the floor. a hard fuel cutoff will feel and sound like a misfire and the tach will not climb above redline. Disclaimer... PLEASE DO NOT DO THAT UNLESS YOU ARE ABSOLUTELY SURE YOUR CAR HAS A REV LIMITER!!! Indeed, one of my past cars shifted best by "bouncing" off the rev limiter.

If I am mistaken on how the rev matching works on the 981, someone correct me, but it WILL NOT AND CANNOT save you in the event of a "money shift" (the condition bolded above). I have quite a bit of track experience and (knocks on wood) have never had a money shift. But, I have been close. At those times, I was fortunate enough to put the clutch back to the floor after only partially releasing the clutch. It is quickly very evident when one misses the shift and is in too low a gear for the traveled speed (ie releasing the clutch in 2nd gear - when you thought you were getting 4th - at 100mph!). At the instant the drive wheels bite, the prop shaft and gearbox (in the wrong gear) catch up to the traveled speed and physically turns the crank shaft too fast for the valve train, rods... Game over.

Porsche can't create a rev limiter that can overcome the physics of a money shift. Unless they would be willing to protect the engine at all costs by locking up the drive wheels with the braking system. And, that would create a whole new set of problems when the rear wheels lock up at a high speed! Alternatively, perhaps a gate that locks you out of going into too low of a gear. But, that just overrides the reason you have a stick and a clutch to begin with. At that point, Porsche should only offer PDK and be done with shifting (meaning clutching and selecting your own gear) yourself.
 

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...

The longer answer explains what happens in that split second and why:
1. You approach a bend/turn.
2. You come off the gas, clutch (verb here) and down shift into a lower gear than you intended
3. Before you lift your left foot of the clutch pedal and engage the engine to the wrong/lower than intended transmission gear you will hear the engine rev out of it's normal rev matching sequence. Meaning the higher pitched engine sound tells you something is wrong and you do not let the clutch out.
4. Instead, you shift into the proper gear and then let the clutch out after hearing the "normal rev match sound". This happens so fast that it is almost involuntary. After it happens to you for the first time you will understand.

While the Cayman's manual gear box is really great IMO and mis shifts are pretty hard to do, when it does happen rev matching has paid for the Sport Chorno function when it gives you this early warning that you would not have received in the same circumstance noted above without this option.

...
Well said , Kindrunnermike.

I experienced this a couple of times last month at Circuit Mont Tremblant. Long back straight ("the hump") before right hand turn at end of down hill. Initially I had been down-shifting from 4th to 3rd, but after a few sessions and I got more comfortable and I starting shifting up to 5th on the straight , hitting 120mph before breaking and downshifting. A couple of times instead of going to 4th I started to shift to 3rd (since before I had been using that gear for that turn) but heard the auto rev matching "telling" me it was the wrong gear, before dropping the clutch all the way and engaging the gear.

Indeed, thank you Porsche!
 
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…because they rather install automatic transmission to solve the issue, not have to deal with manual guys/girls and make more profit on the automatic transmissions.

Given how modern cars are so computerized, I don't get why car manufacturers don't program a warning beep if you shift down to a gear that could cause overrev. Just need a beep to minimize the overrev.
 

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4. I would expect that with the sudden loss of power and the audible alarm, the driver would immediately recognize his error and not release the clutch, but even if he didn't the fuel cutoff would occur at least a coupe tenths of a second before it does under the current process. The current process is to initiate the fuel cutoff when the engine actually reaches red line, and not before as would be the process that I recommend.
One of the perils of driving a MT is there is no protection for accidental Money shifts. Cutting off the fuel will not help because when you select a gear, that gear is going to spin regardless of how much fuel you are feeding the engine. But as it was mentioned earlier, with the Rev Matching, you can hear the engine straining more than it should and that is enough warning to let you know you are about to Money Shift. No other audible alarm required.

To those of you interested in what the Rev limiter sounds like in the 981, skip to 1:18 in this vid below.

[video]http://www.planet-9.com/981-cayman-boxster-pictures-video/110137-new-video-running-my-local-backroad.html[/video]
 
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