sly1

Just wondering if the rev matching option would make engine damaging over revs impossible or at least very unlikely. I have no idea how GM programmed rev matching, but it wouldn't be difficult to write a script to have the computerized rev matching feature immediately activate the over-rev option as soon as it detects that the selected gear would push the engine past red line.
Computers work in the nano second world, whereas mechanical systems operate in tenths of a second. My reasoning would be that the instant the computer recognized that the selected gear would push the revs past red line, it would for all intents and purposes instantaneously cut off the fuel supply, well before the engine reached red line which is the current method of operation.
Any thoughts?

NSC5

Even with the fuel shutoff the engine will be forced into over speed if you shift into too low a gear at high speed, the power will be supplied by the moving mass of the vehicle. Fuel shutoff can prevent accelerating to an engine overspeed condition and can prevent it from happening with an open throttle in neutral/open clutch condition but not when the engine is being driven by vehicle momentum.

sly1

I understand and agree with what you posted, but just wondering how rev matching is programmed.
For example let's say that I'm on the track in 6th gear and I want to downshift to 5th but inadvertently select 3rd. I would expect that in a couple of tenths of a second the engine would be at red line and fuel shut off is engaged, but as you point out engine momentum will likely push the engine revs into the danger zone. On the other hand if rev matching is programmed as I suggested, the fuel cut off would occur a couple of tenths of a second sooner, thereby making engine damage far less likely.

Skidplate

I assume you have a manual transmission but that is not stated. If you have an auto trans and try to downshift into too low a gear for a given speed, the downshift will be denied and Shift Denied will be displayed.

mksz51

If you have a manual rev-matching will do nothing to save you if you shift to too low of a gear and let the clutch out fully. Your RPM will skyrocket in a heartbeat and your wallet will open shortly thereafter. Common sense is still more important than rev-matching.

Bucknut2006

The only thing that would prevent a mechanical over rev would be something like the CAGS kicking in but for 3rd/4th gear.

meyerweb

I would think that fuel cutoff would prevent rev-match from over-revving the engine before you release the clutch, but nothing will prevent the engine from mechanical over-rev if you select the wrong gear and release the clutch. That's true whether rev match is on or off. Heck, it's true even if the ignition is switched off. The driveshaft is driving the engine, not fuel and spark.

pickleseimer

Being in 6th gear on the track doesn't make much sense to me. I'd think that 5th gear would be the last gear you would be using, and that would be on a long straight after fourth gave all it had to give. Car reaches top speed in 5th IIRC, 6th and 7th won't make it go any faster. Seeing that the shifter is spring loaded to the 3-4 gate, it would be hard to select the wrong gear from 5th on a track downshift. Just my opinion.

At Spring Mountain, most of the driving was done in 3rd and 4th, with the very occasional use of 2nd.

4GS7

Money shift is a money shift. You can't stop the rear tires from overspinning the motor with fuel cut.

fireinv38

Just learn how to drive stick. We were lucky to have synctomatic on the older Vettes. Guys must have stick. DUH

sly1

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 part of the program's algorithm?

Probably only the GM 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 the trouble 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.

motomanvette

It's easy to drive a stick. I've had 50+ cars with them but this 7-sp is the first stick where I miss gears.

sly1

I just had a conversation with a Porsche owner, and he told me that he experienced an inadvertent downshift with rev matching engaged. He said that the rpms did exceed red line before he released the clutch, but the sound of the engine was so loud that he immediately recognized his error and never fully released the clutch and immediately shifted to the correct gear. He went to the dealer a couple days later to have an over-rev report run, and it just showed a few ignitions in range 1 which are harmless to the engine.

I expect what happened is that fuel shut off was initiated as soon as the engine reached red line and since the clutch wasn't fully released, the engine rpms dropped quickly.

JerryU

Suggest your example is flawed! Assuming you're planning to go from 6th to close to red line in 5th, which is what you would do when tracking, 5th at red line is 209 mph! 3rd at that rpm would be more like 13,000 rpm!! (note, all speeds are approximate and for a Z51.)
Putting in some realistic values, suppose your in 4th (max speed is 150 mph but your at ~4500 rpm and your tiring to go to 3rd near red line as you come into a fast corner, which is 100 mph. Instead you go into 1st! That would be ~13,000 rpm as well! Sure the fuel would cut off at 6500 but if your racing you'll shift much faster than taking a few 10ths!
I think a more likely case is when passing on an open road! Your close to red line in 3rd and want to shift to 4th, i.e. ~100 mph in 3rd going hopefully to ~4300 rpm in 4th and you inadvertently go to 2nd. That would be about 8000 rpm. Again the fuel shuts off at 6500 or there abouts and you've just had pushrods bend, valves hit pistons and hear this terrible noise.

That would work but require more solenoids and blocking pins! Afraid for now just don't make the mistake! In those situations I'm shifting very fast, the clutch engages, push-rods bend, valves hit pistons and I would hear this terrible noise!!

NSC5

Easier to implement would be a single solenoid lockout that would prevent the clutch from being engaged when an incorrect gear is selected. However this potentially opens a new issue of the case where someone feels the need to shift into too low a gear selection to provide sufficient braking if a failure of the brake system occurs.

If a missed shift is a valid concern for a individual driver then serious consideration of an automatic transmission is a good idea since they are very good at protecting the powertrain. I am not sure how the automatic is programmed to behave if a lower gear is selected and the vehicle is allowed to coast to excessive speed on a steep grade, this might be the one case where it would allow excessive engine speed and I am certainly not going to test it-maybe a dealership salesperson will When I was getting ready to buy my first GMC pickup with the Duramax Diesel/Allison auto in 2001 the salesperson wanted to demonstrate the engine braking capability and pulled the shift lever into first at 70 MPH. It crisply downshifted through the gears as quickly as possible while keeping the engine from going into an over-rev condition and I credit the transmission with being smarter than the guy demonstrating it.

meyerweb

No, it can't. You're missing the point that if you downshift too many gears and let out the clutch, the rear wheels are now driving the engine, not the other way around. EVEN IF YOU SHUT THE IGNITION OFF COMPLETELY, if you're going (just as an example) 150 mph in 3rd gear the rear wheels are going to spin the engine at a higher speed than redline. No amount of electronics can prevent that, unless there were an automatic clutch disengager (which there isn't).

sly1

Maybe I should have highlighted before the clutch is released, but I clearly stated that in the post that you referenced in your reply. Once the clutch has been released than there's no electronic aid that will help, and we all recognize the fact that the drive wheels will dictate the engine's rpm.

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.

JerryU

Clutch release lockout is a good idea! I'm reminded of driving from Northern Scotland to London many years ago with our international manager. He rented the car but I was driving since he was not that familiar with a standard shift. He insisted on driving for a while and after going on the wrong side of the road a few times he tried to shift back to first from 3rd instead of going to 4th! I could hearing the gears screaming and yelled "Leave the clutch depressed!" I pulled the gear shift to neutral and drove the remainder of the two day trip!

NSC5

You are lucky to have survived that trip

At least he listened to you and didn't destroy the engine. There are some people that you could give them a hammer made from foam rubber and a cast iron anvil and somehow they would manage to damage the anvil-unfortunately some of these folks are the low level dealer service people that do oil changes.

meyerweb

Sorry, I did miss that phrase in your post.