How to bypass decel valve?

[Ronnie's.930]

Mark, like you, I was skeptical about getting rid of the valve (though was interested in doing so after I had read about how eliminating it would allow the off throttle revs to quickly drop like they do on a "normal" engine) . . . when my high idle problems arose, due to a failed decel valve, I asked many of the same questions you are and was assured that the car would be fine, and even improved, without it in place . . . since removing it, I no longer have idle problems, the engine decelerates like a "regular" one and I have experienced no problems thus far . . . going without the decel valve does increase off-throttle popping considerably, however - and I can see that to some, this could be considered a negative (quite a bit more noise) . . . one of my friends thinks it gives the car even more "character" (like it needed any more, right !!!!) . . . but I can see where all the noise could almost be considered "embararssing" to some folks . . .

Ron

[Ronnie's.930]

Originally Posted by Speedy Squirrel

If I have a properly functioning decel valve and you don't, under hard braking I will not be in the ridiculous position of standing on the brake AND giving it a little gas at the same time while I downshift.

When you are upshifting, the throttle slams closed and all that momentum for the airflow converts to pressure. You have a big boost spike, and little airflow. Now, go look at a compressor map at a low airflow and 2.0 pressure ratio. What do you see? Nothing. It is to the left of the surge line. The compressor can't run there so it stalls until it gets to a pressure it can run at, and you've just lost your turbo speed. Not the fast way out of the corner. Don't think your blowoff valve is going to save the situation either. It's way too slow. The decel valve is the "first responder" and keeps you out of trouble from the boost spike 'till the blowoff valve can bypass the compressor.

Also, without the decel valve you can get 35-40 inches Hg vacuum under decels, which sucks oil down the intake valve guides and mucks up your plugs.

WOW, "Speedy" - interesting points - definitely something to think about . . .

Ron

[JFairman]

"going without the decel valve does increase off-throttle popping considerably"
Thats true on a stock motor.
An odd side effect of having a Flowtech modified fuel head that gives more fuel and a BL adjustable control pressure regulator set for high steady speed and idle speed warm control pressure to lean out the rich midrange mixture of that fuel head to the mid 14:1's is I get alot less popping in the exhaust. Very little deceleration popping at times.

"Also, without the decel valve you can get 35-40 inches Hg vacuum under decels which sucks oil down the intake valve guides and mucks up your plugs."

With bizarre logic like that then all engines need a vacuum limiting deceleration valve and that is not so.
I've had vacuum gauges installed on several normally aspirated cars and during high speed deceleration I've never seen vacuum go higher than around 27 or 28" Vacuum gauges only go to 30" of mercury.
A 930 would have no reason to be higher than that and because they are low compression are going to be less.

[beepbeep]

Deccel valve has zero, zip, null influence on longevity of turbo or spoolup performance. It's just there because CIS didn't have fuel shutoff on engine brake which caused popping on overrun so they added the valve to decrease the vacuum and HC emissions. It also caused "lazy throttle" syndrome.

If it was such a good idea it would be fitted on other cars...unfortunately, there is lot's of lore and mystique around every shiny gadget found under 930 rear hatch. In reality, most of it is half-a$$ed band-aid cr*p installed only to let the engine pass the strict NA emission inspections. Next thing, someone will say that thermal reactors are really good beacuse they increase the temperature of gasses going to turbo or something


Get rid of the valve and never look back!

[RarlyL8]

most of it is half-a$$ed band-aid cr*p installed only to let the engine pass the strict NA emission inspections

You only have to look at the evolution of CIS equiped engines to see which parts have no performance value. Many can be replaced with a simple hand throttle as in the pre-'75 911.

[Speedy Squirrel]

JFairman, all cars do have a vacuum limiting device. It's called the IAC and it's open all the way on hard decels.

[Speedy Squirrel]

Enjoy your hand throttle, and standing on the brake and throttle at the same time.

[beepbeep]

Originally Posted by Speedy Squirrel

JFairman, all cars do have a vacuum limiting device. It's called the IAC and it's open all the way on hard decels.

Idle Air Control valve is usually driven by stepper motor and as such way too slow to be opened fully by ECU on deccel. And why should it, the whole problem doesn't really exist anymore as EFI replaced CIS. Only thing you have to do is to zero out all fuel cells under certain vacuum (for example, everything below 30kPa and above 1500 RPM) and ECU will shut down fuel on engine-brake, thus completly removing any popping on deccel which has plagued mechanical fuel injection systems before.

On some luxury cars, factory fitted a dashpot on the throttle to cushion the last few mm of throttle travel and prevent sudden RPM drop.

Which cars open idle valve on deccel?

[964 T #304]

They did the same thing in the 60's on some GM cars with a little selenoid valve on the throttle linkage that kept the throttle open a little on de- aceleration on the automatic cars to keep them from dying when you let off the gas .

[911st]

Lets all agree to disagree. Or, to each there own.

There is a lot of good points here for someone that wants to take the time to actually ready through it and make there own decision.

There are valid reasons for keeping or discard and it is not that hard to try one's car both ways.

I found a lot more opportunity to keep my compressor wheel spinning between shifts in the Compressor Bypass Valve but I believe keeping the Throttle Bypass Valve did contribute to my boost recovery between shifts. Further, my car had no issue with lazy rpm between shifts like my old 914 with a de-accel valve. Lastly when I had my set up I had no/nada drop in boost between sport shifts without having to do a speed shift (keeping foot on the gas with the shift).

To be or not to be. It is just a choice.

[RarlyL8]

You can't make an informed choice without good information.

This is a simple non-issue. It is common to ditch the TBV when it goes bad or you simplify your CIS. Mine's been that way for 10 years. Stand on the throttle and brakes at the same time? Why? The only time I do that is to exit a corner on boost when under the threshold, which has nothing to do with the TBV. That little gold disc changes very little in the driving experience. I have noted that burble on decel is increased in some circumstances. The reason I match revs to shift is because I like to, not because I have to. I don't do that just driving around town, only when boosting part throttle. Don't need to do it at all on full throttle as the gears shift fast enough.

CIS is a simple mechanical component system so you can test removal of all equipment yourself. Remove it and drive it. If you don't like it then put it back on. If it's busted get a used one. No short supply as these things are removed often.

[JFairman]

Originally Posted by 964 T #304

They did the same thing in the 60's on some GM cars with a little selenoid valve on the throttle linkage that kept the throttle open a little on de- aceleration on the automatic cars to keep them from dying when you let off the gas .

Yep, I remember those too. I've been around a while..

The 12 volt solenoid on the throttle linkage of those old USA cars was energized when you turned on the AC and it mechanically pushed the throttle linkage idle speed stop open a little to keep the rpms from dropping or the engine from stalling while decelerating to a stop when the AC compressor came on.

There was also a little saucer shaped gadget with a diaphram in it called a dashpot on some carburator linkages and it acted as a little pnuematic shock absorber in the throttle linkage that kept the throttle butterfly from snapping shut if you let off the gas quickly all at once.
It would make the linkage delay final closing of the throttle butterfly for about 1-2 seconds so the engine wouldn't stall after reving on heavily smogged cars and some muscle cars running a rich mixture and huge carburator that would almost stall after reving the engine.

[JFairman]

Originally Posted by Speedy Squirrel

JFairman, all cars do have a vacuum limiting device. It's called the IAC and it's open all the way on hard decels.

Nope...

The idle control valve used as the idle speed controller on motronic fuel injection since 1983 is an electronic idle speed air bypass valve that is opened and closed by a stepper motor that is controlled by the motronic ECU.
They don't last long either. I replaced those often on motronic BMW's when I worked as a street car mechanic.

It is there to maintain a steady even idle speed under all conditions. It keeps the idle speed the same when the engine is cold or when you turn on the AC or whatever.
As someone else said, Bosch L-jetronic and Motronic ECU's both shut off the opening of the fuel injectors while decelerating until the engine rpms drop to 1500rpm. At that speed they start opening and injecting fuel again.

My first car with a standard transmission was a '72 BMW 2002 way back in the late 70's that had totally shot 1 - 3 synchros in the transmission when I bought it so I learned from the beginning to always match rev double clutch on downshifts. It's a little extra effort that feels normal and satisfying to me because I learned to drive a stick that way over 30 years ago.
It is the only way you can downshift without grinding the selector teeth on the gearset and synchro hub slider if the synchros are worn out.
I'm saying this because later on I learned to heel and toe double clutch during downshifts while driving fast into corners.
When you do this you are standing on the brakes with your toe and blipping the throttle with your heel to match the revs in the gearbox while you are double clutching between downshifts.

ALL competitive roadrace drivers heel and toe downshift into corners because it is the fastest and smoothest way to brake and enter a corner while matching revs in the engine and gearbox so there is no jerk in the drivetrain when you let out the clutch and get on the gas again smoothly a little before the apex of the corner while at the limits of adhesion.
They aren't always double clutching while heel and toeing on downshifts if the car doesn't need it because that takes a little more time, but they do match revs of the engine on downshifts while braking by heel and toeing.
Motorcycle riders also do that with the hand throttle so there is no jerk when letting out the clutch on downshifts too.

So yes squirrel, sometimes I do stand on the brakes and blip the go peddle at the same time while decelerating and downshifting into a corner. I do it well and I enjoy it.

[ertech]

just removed mine 2 night ago the idle would creap when stopped at a light .the problem looks like it went away . the car does not stall .I tried pushing it in 1 st and second and release the gas and the idle just drops to 900 rpm instantly . there is a little more crackeling on cecel . I would not replace it drives great as is.
Thnaks

[totle]

Deleted min as well today, but I had alot of crackeling when decel.
Could this be a timing issue as well?
Some here say that they did not notice much more crackeling when decel valve removed.
I had much much more.

Could my timing be wrong?

[911st]

CIS dose not stop delivering fuel with throtle lift like EFI.

That vavle is to supply air so that fuel has a better chance of burning in the cylinder than in the turbo or muffler.

Some like it, some don't.

It is not a timming issue.

[s5uewf]

If you notice more or less crackling than others I am guessing that is jsut a factor of your WUR being a litle richer or leaner than the other guys'. Or, something in your fuel/ignition system is affecting your AFR's (plug condition or heat range, valve gap, air leaks, etc.).

Timing could also affect this, but I am assuming you have set your timing properly.

[mooney265]

I'll second those guys... I deleted mine a couple weeks ago [with help from JFairman] and very happy with the shifting!

Now, mine pops a whole lot more and a whole lot louder on decel, though... If i have a hard decel in the neighborhood [like I did earlier today whilst showing off to Brian, the mailman], the sound just bounces off the houses and prob scares the ***** out of my neighbors!! I'm talking glock 9mm emptying 6 rounds in 4-seconds!! That's not a good sound to hear in New Orleans!!

As I pulled in to the garage [and quickly closed the door] to let the turbo cool off, I promised myself I wouldn't do that again in the neighborhood... I know my neighbors will appreciate it...

[mark houghton]

Originally Posted by mooney265

I'll second those guys... I deleted mine a couple weeks ago [with help from JFairman] and very happy with the shifting!

Now, mine pops a whole lot more and a whole lot louder on decel, though... If i have a hard decel in the neighborhood [like I did earlier today whilst showing off to Brian, the mailman], the sound just bounces off the houses and prob scares the ***** out of my neighbors!! I'm talking glock 9mm emptying 6 rounds in 4-seconds!! That's not a good sound to hear in New Orleans!!

As I pulled in to the garage [and quickly closed the door] to let the turbo cool off, I promised myself I wouldn't do that again in the neighborhood... I know my neighbors will appreciate it...

Damn neighbors....always getting in the way of a good thing. I've trained my neighbors on my loud Harley; they've just learned to look the other way and say: $hit, there goes Mark again in one of his toys. When will he ever grow up?

[JFairman]

Originally Posted by mooney265

I'll second those guys... I deleted mine a couple weeks ago [with help from JFairman] and very happy with the shifting!

Now, mine pops a whole lot more and a whole lot louder on decel, though... If i have a hard decel in the neighborhood [like I did earlier today whilst showing off to Brian, the mailman], the sound just bounces off the houses and prob scares the ***** out of my neighbors!! I'm talking glock 9mm emptying 6 rounds in 4-seconds!! That's not a good sound to hear in New Orleans!!

As I pulled in to the garage [and quickly closed the door] to let the turbo cool off, I promised myself I wouldn't do that again in the neighborhood... I know my neighbors will appreciate it...

yeah.. now put one of these on with a hose where the golden saucer valve was and you can really impress the mailman and neighbors. heck, maybe you can even get inserts for it so it'll play tunes!
http://cgi.ebay.com/ebaymotors/SSQV-blow-Off-Valve-LOUD-BOV-Sound-Style-No-Res_W0QQitemZ220478503701QQcmdZViewItemQQptZMotors _Car_Truck_Parts_Accessories?hash=item33558afb15&_trksid=p4506.c0.m245

you will still use your regular recirculating blow off valve too so no worries, just more noise