Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > 911 / 930 Turbo & Super Charging Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
mark houghton's Avatar
 
Join Date: Jul 2008
Location: Central Washington State
Posts: 3,874
Decel valve delete and idle

Just a quick question, since I will probably be deleting my decel valve as part of my current BOV update project.

When seeing vacuum - such as in an overrun situation (off the gas coasting or shifting) the decel valve opens and directs additional air to the intake manifold (drawing that air from the stock secondary/boost recirc manifold)...thus bypassing the throttle valve and causing the rpms to hang up briefly. That's how it works, for some nefarious emissions reasons.

Now, here's the question: When at idle, that valve should be open as well due to the vacuum present, and a small amount of air should also be bypassing the throttle body valve and going straight to the intake manifold. Won't this additional air have an impact on idle rpm setting and maybe AFR's, such that once the valve is removed (deleted) the idle speed will have to be adjusted via the air correction screw?

Those of you that have done this delete, what are your experiences in this regard?

__________________
Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 12-31-2009, 05:34 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Mark,

Not totally sure but I expect the spring tension must be overcome by a combination of higher boost and vacuum.

It should not pass air at idle nor have any effect at idle.

I think you know my position on that valve. It may help boost recovery between shifts from the air bypassed around the TP and by acting like a smaller BOV and thus offers some insurance in the event the BOV fails.

Still, if I was building a track car I would probably work the BOV side of things and remove it for the sake of simplicity. Other than that I like what it dose and might still keep it.

JFairman I believe just vents it to atmosphere so it just works as a mini BOV. Not sure if that was a test or something he has stayed with but I find the idea intriguing and logical.

The best.
Old 12-31-2009, 06:55 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
R.I.P.
 
drmatera's Avatar
 
Join Date: Aug 2009
Location: Nicholson, Ga
Posts: 2,160
my drivability improved in direct relation to removal of that item along with all the emissions stuff. Then ofcoarse I went EFI
Old 12-31-2009, 07:02 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered User
 
mark houghton's Avatar
 
Join Date: Jul 2008
Location: Central Washington State
Posts: 3,874
Quote:
Originally Posted by 911st View Post
Mark,

Not totally sure but I expect the spring tension must be overcome by a combination of higher boost and vacuum. It should not pass air at idle nor have any effect at idle.

It may help boost recovery between shifts from the air bypassed around the TP and by acting like a smaller BOV

JFairman I believe just vents it to atmosphere so it just works as a mini BOV.


The best.

Could be that it won't open at lower idle vacuum...that's why I'm asking. And I may just keep it anyway for the small benefit of acting as a mini BOV.

As to Jim, I think he was just getting a woody with the blowoff noise it makes. There's a little bit of ricer in each of us. I may do the same thing, and hook a whistle to it.
__________________
Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 12-31-2009, 07:25 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
totle's Avatar
 
Join Date: Oct 2008
Location: Norway / Miami
Posts: 1,323
Garage
Tried removing mine as part of removing air pump etc.
Had too much popping when decel, so re-installed it.
Had idling problem, but after removing AAV it idled fine.
Old 12-31-2009, 07:44 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
totle's Avatar
 
Join Date: Oct 2008
Location: Norway / Miami
Posts: 1,323
Garage
Tried removing mine as part of removing air pump etc.
Had too much popping when decel, so re-installed it.
Had idling problem, but after removing AAV it idled fine.
Old 12-31-2009, 07:46 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Registered User
 
Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
Hi Mark,
If you put a mighty vac tool on the decel valve's small vacuum line connection and put a hose on the larger of the two hose connections then pump a vacuum into the decel valve while blowing into the large hose you'll notice that the decel valve starts to open around 19" of vacuum and is fully open and passing air around 21" vacuum.

The average 930 motor with it's very inefficiant low 7:1 compression only makes about 15" of intake manifold vacuum at idle so the decel valve stays closed at idle. Even real slow mild decelerations don't open it.

The average normally aspirated car with 9:1 to 10:1 compression is much more efficiant and idles around 20-23" of manifold vacuum when in good tune because each power stroke is making more power with the higher compression than a motor that has only 7:1 static compression.

I have mine vented to atmosphere and I like it that way.
Like Keith said it makes life for the turbo's thrust bearing easier and probably helps keep it spinning fast between shifts by acting like a second small blow off valve.
I like the little fooosh it makes as it blows out air pressure when you let off the throttle between high speed full boost shifts too.

I put a small valve I had sitting around on the actuating vacuum line so I can disable it or slow down it's action if I want but I've left that valve open because I like it.

Venting the decel valve to atmosphere also lowered the amount of exhaust popping on deceleration on my car alot. It still burbles and makes some soft popping but no more obnoxious explosive attention getting popping and I was sick of that anyway.

My cars overall fuel curve is different than a stock cars though because of the modified fuel head, and BL adjustable WUR that is set with high control pressure to lean it out on cruise and idle. This combination makes the car run a little richer on deceleration which lowers the exhaust popping.
My AFR gauge says I'm at 10:1 on decel with the decel valve working and vented to atmosphere and it said around 15:1 on decel when the decel valve is removed or disabled. A stock fuel head would be leaner on decel.

You have to try it on your car to see how you like the small change in the engines deceleration rate between upshifts and if there is a change in the deceleration popping.
A local friend tried venting the decel valve on his 930 with stock fuel head and WUR to atmosphere and it seemed to make his pop louder on deceleration, the opposite of mine...
So you have to try it on your car, and it's real easy to change it back if you don't like the result.
Old 12-31-2009, 07:54 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Quote:
Originally Posted by mark houghton View Post
... I may do the same thing, and hook a whistle to it.
Ya made me laugh with that one!

Old 12-31-2009, 08:11 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Quote:
Originally Posted by JFairman View Post
...
My AFR gauge says I'm at 10:1 on decel with the decel valve working and vented to atmosphere and it said around 15:1 on decel when the decel valve is removed or disabled. A stock fuel head would be leaner on decel.
...
It sounds like that valve must be flowing a surprising amount of air if it effects the AFR that much.

Very interesting about the difference intake vac with changes in CR. So higher compression motor suck more.

The higher compression also makes it easier to fire a lean mix and it should burn faster. Good for pre boost response.

Then we can add twin plugs for more pre boost response.
Old 12-31-2009, 08:19 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Registered User
 
Join Date: Apr 2007
Posts: 394
I trashed mine when I installed the digital wur. I can adjust the idle afr with it. If I set the idle afr at any thing higher than about 14.7 on idel I get a lot of popping on deacceleration. If I drop it down to about 14.5 I get very little popping. My idle is very stable with the digital wur even with temp changes from summer to winter. be fore with the adj. wur I would have to fiddle with it due to seasonal changes , but not now. I also run a larger turbonetics turbo and cat and muffler delete
Old 12-31-2009, 08:21 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
Quote:
Originally Posted by 911st View Post
Ya made me laugh with that one!

If you do that I'll have to hook up a saxaphone to mine...
or maybe just a set of harbor freight air horns.

I always liked the chirping noise the 962 blow off valves made on IMSA temporary street course races on TV in the mid and late eightees as the drivers let off on the throttle and downshifted before corners.
And I like the fooosh you hear on you tube videos of EFI turbo charged cars with blow off valves venting to atmosphere between shifts. It's a cool sound and part of the turbo car experience for me.

Alot of the BOV's made and marketed to the ricer crowd are desined to whistle when they open and I don't care for that. Some even have a horn on them to make them louder.
I just like a subtle fooosh when letting off the throttle under major boost and I got it for free with my unused decel valve reinstalled vented to air that had been sitting in a box of used parts for around a year or so.

If you decide to try it make sure you vent the smaller 7/16" ID center hose connection on the decel valve to atmosphere.
If you do it the other way around and vent the larger off center hose connection to atmosphere, turbo boost pressure could push the spring loaded internal valve off it's seat and it could leak boost pressure to atmosphere when accelerating... just what you don't want.

Here's a pic of what i mean...
Old 12-31-2009, 08:40 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
mark houghton's Avatar
 
Join Date: Jul 2008
Location: Central Washington State
Posts: 3,874
Thanks everyone. I'm clear on this now. I will probably make provisions to keep the valve and will probably first vent to atmosphere.

In my stock configuration, I get zip popping/burbling (very little, anyway), running about 14.2 AFR at no-load cruise.

Will try it out and just go from there. If it goes too rich on decel, I'll try hooking it back up to the manifold.

Harbor Freight air horns....now wouldn't that be seriously obnoxious!
__________________
Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.

Last edited by mark houghton; 12-31-2009 at 08:56 AM..
Old 12-31-2009, 08:53 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
 
Rocket Scientist
 
Join Date: Jun 2008
Location: Detroit
Posts: 948
DO NOT VENT THE VACUUM LIMITER TO ATMOSPHERE!!!!

The vacuum limiter (called decel valve here) is wide open during the decels, and you are sucking unfiltered air directly into the engine.

Here is some data with no decel valve. Notice that the manifold pressure is around 8 kPa:



Here is some data with decel valve in place. The manifold vacuum is now around 21 kPa:



With CIS injection, the limiter prevents the mixture from burning in the exhaust manifold/exhaust ports by leaning in out so that it can't combust and make the annoying popping sound.

By keeping the manifold pressure up during decels, the manifold takes less time to refill when throttle is re-applied, so response is slightly better.

There is also less oil sucked down the intake valve guides.

There is no mysterious emissions function
Old 12-31-2009, 10:56 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Registered User
 
Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
guess what speedy squirrel..
you're wrong and i don't care to argue with ya so thats it.
Old 12-31-2009, 11:07 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Banned
 
Join Date: Nov 2007
Location: Rockwall, Texas
Posts: 8,760
Based upon the discussions in this thread: Reseal for sure, Top end machine work? K27Turbo rebuild?
oil being drawn down the valve guides might be a good thing for our engines (other than the fact that it ends up in the combustion chamber) . . .

It's not clear to me how removing the decel/vacuum limiting valve causes this, though . . .

I removed my decel valve as it was defective (caused a continual air leak that resulted in an extremely high idle) and have been running without one for several months . . . thought I would try it out this way as a new valve costs about $500.00 and I had read all about this mod on this forum. . . throttle responce is much better (much more like a "normal" engine) but the off-throttle popping is quite excessive - I've grown accustomed to it (the noise), but could somtimes do without it . . .

I believe Brian (RarelyL8) has said that he has been running without a decel valve for over 10 years with no know adverse effects.
Old 12-31-2009, 11:22 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
You do not have to keep it.

I think of it as a performance part, a safety item, not an emissions part.

Only thing one gains by removing it is the RPM's drop faster with throttle lift and maybe more or less exhaust ignition depending on ones AFR's and ignition function.

Slowing rpm drop might be an issue for some with a poorly functioning D-valve but on a Turbo it also helps keep the boost up between shifts.

In my opinion.

---------

Speedy Squirrel,

Cool stats and pics.

As to sucking in air if vented, it is left attached to the area before the throttle body which is always pressurized by the turbo. If it was left attached to the area after the TB and vented to atmosphere it would not make much sense.
Old 12-31-2009, 12:07 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #16 (permalink)
Rocket Scientist
 
Join Date: Jun 2008
Location: Detroit
Posts: 948
Quote:
Originally Posted by 911st View Post
You do not have to keep it.

I think of it as a performance part, a safety item, not an emissions part.

Only thing one gains by removing it is the RPM's drop faster with throttle lift and maybe more or less exhaust ignition depending on ones AFR's and ignition function.

Slowing rpm drop might be an issue for some with a poorly functioning D-valve but on a Turbo it also helps keep the boost up between shifts.

In my opinion.

---------

Speedy Squirrel,

Cool stats and pics.

As to sucking in air if vented, it is left attached to the area before the throttle body which is always pressurized by the turbo. If it was left attached to the area after the TB and vented to atmosphere it would not make much sense.
I understand. I'm referring to just disconnecting the upper manifold end and leaving it open. That is a big problem. I though that might have been what Mark Houghton was going to do.

Taking it off, reversing it, and using it as a BOV presents no hazard.
Old 12-31-2009, 01:06 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #17 (permalink)
Registered User
 
mark houghton's Avatar
 
Join Date: Jul 2008
Location: Central Washington State
Posts: 3,874
Quote:
Originally Posted by Speedy Squirrel View Post
I understand. I'm referring to just disconnecting the upper manifold end and leaving it open. That is a big problem. I though that might have been what Mark Houghton was going to do.

Taking it off, reversing it, and using it as a BOV presents no hazard.
Speedy, thanks for watching. My intentions are to leave it connected to the upper manifold as normal (what's left of it, with my mods, though in a slightly different location), and rather than having it redirect the boosted air to the intake manifold, just let it vent to atmosphere. This will be taking boosted air from before the throttle body and intake manifold, and just venting it off when vacuum is sufficient to open the valve (when shifting). Same function as a BOV but it won't be recirculating back to the infeed of the turbo.

Sure won't be sucking no dirty unfilterd air in...that would not be cool!
__________________
Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 12-31-2009, 01:21 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #18 (permalink)
Registered User
 
Join Date: Oct 2006
Location: Chas, SC
Posts: 1,057
Quote:
Originally Posted by JFairman View Post

I have mine vented to atmosphere and I like it that way.
Like Keith said it makes life for the turbo's thrust bearing easier and probably helps keep it spinning fast between shifts by acting like a second small blow off valve.
I like the little fooosh it makes as it blows out air pressure when you let off the throttle between high speed full boost shifts too.

.
Just want to make sure I understand. You stated that you are now using the decel valve as more of a secondary BOV, this is because you vent the center port of the decel valve to atmosphere, and if you were to use it as a decel valve, you would connect the center port back up to the port on the intake manifold, so when the throttle is closed, vacuum opens the decel, sending a bit of air into the intake, which causes the idle to hang a bit, correct?
__________________
Tim
1986 930
Gone:71,2,4 914's, 70T, 71T(RS),77S
Old 08-19-2011, 09:21 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #19 (permalink)
Metal Guru
 
911nut's Avatar
 
Join Date: May 2001
Location: Michigan
Posts: 2,514
Garage
Mine's connected and I have mild popping in the muffler. When it was disconnected and venting to atmosphere I had loud popping.
My valve had become sluggish, letting the revs hang forever and then dropping slowly. A mechanic friend told me that this could be remedied by collapsing the vacuum side of the "can" 1-2 mm. It worked and I now have mine hooked up to the intake once again.

__________________
Paul B.
'91 964 Turbo
Port matched, SC cams, K27/K29 turbo, Roush Performance custom headers w/Tial MV-S dual wastegates, Rarlyl8 muffler, LWFW, GT2 clutch & PP, BL wur, factory RS shifter, RS mounts, FVD timing mod, Big Reds, - 210 lb
Old 08-20-2011, 04:04 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #20 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 12:25 AM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2020 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.