Is the rate of RPM dropoff adjustable?

[autobonrun]

I got a chance to drive a BMW Z3 roadster for a while this week. Very enjoyable car but the clutch is not as stiff and the steering softer than my Porsche. One thing I noticed and liked was that the rpm's didn't drop off as fast when the clutch was engaged during upshifting or downshifting as they do in my 911. This eliminated a lot of the need for double clutching during downshifts and I found the rpm's closer matched where I should be as I was applying gas.

My question is whether there is an adjustment that can be made on a 911 that will increase or decrease the rate at which the rpm's drop off when the clutch is engaged ( that will not affect idle speed)?

Have any of you modified this rate to hold the rpm's slightly longer?

79SC

[82SC]

get a flywheel made of solid iron...that would slow down the spin down as well as the spin up!!!!

a fast reacting fly wheel means better acceleration...but I know what you mean...

upshifting into 2nd can be a bear...and when it does not go in, I have to rev the engine while upshifting while getting it to grab second!!! uggg

[type930]

Autobon- Yes this can be adjusted by turning the threaded post on the deceleration valve in (clockwise) causes the decel to come down faster: counterclockwise will cause the car to come to idle slower. Im sure one of our othe board CIS gurus will chime in should you have any questions

[jkcrewsn]

Or just do what I do..... don't take your foot off the gas when you shift.

[masraum]

I'm not a CIS guy, and I don't have a CIS car, but from reading some books I thought the decel valve was only functioning down at the low rpms? I thought the engine spun at it's own rate until it got down to 1500 or 2000 and then the decel valve kept it from stalling. Maybe that's what the (my) DME cars do.

If the rpms are coming down too far before you get to the next gear you aren't getting there fast enough. The quick rev up/down is a good thing a feature not a bug. Just make sure you don't ever go to a lightweight flywheel clutch pressure plate combo or you'll really be hating life.

[stormcrow]

Actually, the decel valve is a pollution control device, and it is adjustable for that purpose.

Steve

[VenezianBlau 87]

I was wondering the same thing as autobon...rather, what causes the revs to drop so fast. So, it's mainly the light flywheel, right?

This car forces me to concentrate. Nothing better than getting spirited sets of crisp, efficient shifts (up and down) on those back roads. Funny how I prefer listening to engine/gearbox than new Blaupunkt I installed. No stereo upgrade on next P-car.

Since my exhaust is stock, the best exhaust note I get is with driver window down. This negated my A/C upgrade in the summer and a little chilly, and odd looking, in the winter.

Please excuse my ramblings, it's just love.
Anyway, good day to all...
----------------------
Bob Sauerteig
'87 Carrera

[jsmarcus]

Do you also experience a bucking even in 1st while crawling at 12-1500 revs? Sometimes while in traffic, it's almost as if the throttle cuts in and off, but never a problem during acceleration, just cruising at low revs......I know, so don't do it, but I have no choice in my commute....

[Ted Stringer]

My solution is to blip the throttle just as the transmission goes into gear. This way the engine is on an upswing and it matches the revs better. The only problem is when you don't time it right, you look stupid.

[Jdub]

Storm has it right: it is an emissions device. I don't have my CIS booklet in front of me, but can say that, when RPMs come down quickly, unspent mixture is created and that causes bad emissions to spike through the cat and out the tailpipe, both bad things.

I had the problem of the "soft" letdown such that, especially after a long highway run, I would hit the first stop sign and the idle would not come down. Turning the post in (clockwise) caused the RPMs to come down to heel quicker. I'd assume the reverse would be true, but I really think that the "float" idle is a bit of a pain and may not solve the problem you are having.

Will say that the locknut holding the post on the decel valve (this is a '78sc here) was on very, very tight. Be prepared to hold that decel valve tightly.

Jw

[autobonrun]

Originally posted by jsmarcus
Do you also experience a bucking even in 1st while crawling at 12-1500 revs? Sometimes while in traffic, it's almost as if the throttle cuts in and off, but never a problem during acceleration, just cruising at low revs......I know, so don't do it, but I have no choice in my commute....

Mine does the same. I've assumed this is normal for this car. It may have something to do with the amount of HP. Some higher HP cars continue to pull without lurching even if you don't press the accelerator. There comes a point in my SC where the idle RPM is not capable of pulling at all and I have to go in on the clutch and pressing the accelerator to keep it from dying.

I think what I'm hearing from this post is that there is no easy adjustment that will manage the rate of decel on an engine other than for emissions purposes. It would be nice though. I'll just continue to shift quickly and double clutch. I found shifting the Z3 less "frantic" in order to keep the rpm's up and thought I could adjust my 911 to respond similarly.

I just ran a test in the Z3. It idles at about 800 RPM. On a level surface, I was able to go down as low as 700 rpm in first with no stall and a smooth slow progression with my foot off the accelerator. I didn't go lower searching for the stall point but this is about as slow as you'd need to go in traffic without stopping. Maybe someone with a newer Porsche can respond if the issue you bring up is limited to the older Porsches.

[Gary Myers]

Just a quick question, guys. I've got an 80SC that doesn't always bring the idle down. It will hang at between 1500 and 2000. Does the '80 decel valve have an adjustment screw??

Thanks

[RoninLB]

I sometimes have a problem with bringing my '77 eng to idle. I think it is deceleration valve. Will try disconnecting the worthless Smog $hit