corvette-kyle

I noticed today while driving when coming to a stop and not downshifting, just putting it into neutral and coasting to a stop, while coasting the engine will bump up between idle and 1100rpm. The only time I have had this happen before is if I downshifted and rode out a gear too close to idle so that when I pushed in the clutch the engine bumped it back up. I assumed this was normal and was the engine's way of preventing itself from stalling. However I have never had it happen more than once, or while coasting. It did not happen while completely stopped, only when moving. I attached a poor video so you can see what was happening if it would help. Pardon the music at the beginning, I didn't think about turning it down at the time.

Also, this is my first manual car so I know you can't tell much from the clusters but if you notice anything I'm doing wrong or could improve on let me know. (If I'm cruising at too low rpms, or downshifting incorrectly, ect.) My 1-2 shift is pretty rough sometimes even on a normal acceleration so I don't know if it's me or the car. I have never driven a manual car before the corvette. Thanks!

http://www.youtube.com/watch?v=m1Z1cMsM4Nw

I started the video getting off of the highway so give it a minute or two before you see what I'm talking about. The squeak noise is the actual clutch pedal.

Car has Kooks long tube headers, X-pipe and high flow cats + NPP with fuse pulled and has been tuned by Chuck COW if it matters.

thesubfloor

No, that's normal operation.

You need what's called the "Airflow Final Minimum" table adjusted and/or the Overspeed/Underspeed spark tables, any of which can cause the idle to fluctuate like that. My guess however is that it'll likely be a combination of all three that need adjusting.

Basically what's happening is this:

The Airflow Final Minimum table helps moderate the target idle of the car. If the values in this table are more than they should be it'll idle too high and if they're too low the RPMs will drop and the car may stall depending on how out of whack they are.

Where the Overspeed/Underspeed spark tables come into play is that when they detect the idle is too low they'll add a bunch of spark timing to the engine to keep it from stalling and if it's idling too high they'll pull timing to bring the RPM back down to where it should be.

Ultimately these tables need to work together because otherwise you get a bit of a ping pong effect - the airflow table causes the RPMs to shoot up, the overspeed table tries to pull timing to bring the idle down and then the underspeed table senses the RPMs being too low and adds timing, sometimes too much and the cycle can easily repeat itself.

Hope that helps.

Christopher

0Chuck CoW

Corvettes, Especially sticks, have airflow added into the minimum idle airflow tables (called different things in different tuner packages)
to prevent stalling and to also prevent customers from "jabbing" the clutch and trans.

By holding the airflow up just a bit, (similar to blipping the throttle on a shift) it cushions the apply into gear for the drivetrain and the synchros)
and helps prevent drivetrain/trans warranties in the big picture for GM.

Not many, but a few times, I've had customers ask me about this.. Most guys never notice it, but the autocross guys are usually hit to this
sorta thing.... They always ask about the "floating" throttle....

If it's a problem for anyone, it's really easy for me to just remove a gram or so of air in the 1000-4000 rpm area to help the car return
to idle quicker.

If you FREE REV a C6 to 3K or so and let off, you'll notice how slow it falls back to the base idle.

It's easy to fix, but if you've got an 08 and never Vette or Camaro (with the 6060 trans) you don't want to kill the synchros cause they
were not very good in the first place.

It's an easy fix.
Chuck CoW

corvette-kyle

Thanks for the answers, they are very helpful. I don't mind it at all I just wanted to make sure it wasn't an initial sign to a problem down the road.

Just another question - when shifting any gears, up or down, when the clutch is completely in is the shift **** supposed to feel completely smooth when choosing another gear? Because specifically 1-2 but also 2-3 when shifting at low rpms (under 2500) feels like it has some resistance all the way into the next gear (Not just at first but almost as if I'm pushing the **** through a metal jungle sometimes).

I also get some grinding and/or chattering sometimes while engaging first. Obviously it doesn't happen if I give it more gas while taking off but I don't want it to seem like I'm launching and burning the clutch everywhere I go. Is this chatter and shudder normal or is it just something I have to get used to?

Thanks again.

Vet

I noticed, in general, the higher I rev, the slower the throttle returns. Shifting below 3,000 rpm is usually fine, but when I shift (upshift) above 3,000 rpm, the revs tend to "hang" for a moment after letting off the accelerator, then drop... and it's not necessarily consistent.

And, for those with stock exhausts (where you often can't hear the engine well, especially when the top if off), if not paying attention, this results in sloppy shifts since you can't really predict exactly when the revs will drop. It's hard to impossible to execute near perfectly rev-matched shifts if you can't hear or predict where the revs are at.

I've actually gotten used to watching my tach in these cases so I can shift better. I often note the tach needle "hanging" during 3500+ rpm shifts, and thus I wait a little bit longer to re-engage the clutch for a more closely "rev-matched" shift. But, quite honestly, it's a little bit of a pain to have to quickly glance down at the tach on many typical street shifts.

I much preferred my old pre-computer cars where the rate of rev drop was almost perfectly consistent all the time, hot or cold, low revs or high revs, once you learned the throttle behavior, you could predict every time what would be gong on and make perfect shifts almost every time... without looking at the tach.

I like the throttle behavior on my C6 in the lower rpms, I would not want the throttle to return to idle any quicker... but it would be nice to eliminate the "delay" that occurs in the throttle return at higher rpms and just get the throttle return to be more consistent across the rpm range.

I was told that the slow throttle return at higher rpms had something to do with keeping the stock cats from overheating or getting damaged... I would not wish to alter anything with respects to throttle behavior if it would have any adverse effects on the stock components.