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,

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.

my drivability improved in direct relation to removal of that item along with all the emissions stuff. Then ofcoarse I went EFI

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.

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.

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.

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.

Ya made me laugh with that one!

:D

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.

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

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...
http://forums.pelicanparts.com/uploa...1262281219.jpg

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!

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:

http://forums.pelicanparts.com/uploa...1262288904.jpg

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

http://forums.pelicanparts.com/uploa...1262289038.jpg

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

guess what speedy squirrel..
you're wrong and i don't care to argue with ya so thats it.

Based upon the discussions in this thread: http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/518736-reseal-sure-top-end-machine-work-k27turbo-rebuild.html
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.

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.

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!

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?

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.

I think JF Fairman suggested that, too. About 1.5yrs ago on this forum...

I may put mine back on and do that, too. I removed mine because the rpm's were hanging for too long - - felt like I was going to run into the back of the car in front of me after I lifted...

Newb question, how does one collapse the vacum side?

I've never done that to a deceleration valve but I've done it to stock bosch EFI fuel pressure regulators on BMW's in the past. You're compressing the spring inside and raising it's preload against the internal vacuum diaphram.

With the bosch FPR you put it in a bench vise with a correct size 3/8 or 1/2 drive socket on each side and squeeze it a little at a time to compresss the vacuum line end side of the steel canister inwards concave a little to compress the spring on the inside to raise the preload tension on the vacuum diaphram a little at a time until you get it where you want it.

Same procedure might work on a deceleration valve with correct sized sockets or whatever.

So Mooney, are you saying that I don't get any brownie points for my response?
I hate it when that happens :D

he suggested it [i didn't say he 'did' it...] i'll find it... http://forums.pelicanparts.com/support/smileys/pc2.gif

And, yes you do deserve credit for reminding me of the fix...

Had it removed on mine, no real difference for me at least.

When I took mine out of the equation (well, it's still there but instead of recirculating the boosted air, it just vents it) what I have found is that the rpms drop instantly when shifting vs. hanging up there...which is why I did the mod if the first place. Yes, it will draw air into the mainfold in an overrun condition (off the throttle, vacuum opens the valve). No problem, I simply installed a small air filter on the end. Now, when fully off the throttle, I see my AFR's leaning out as would be expected.
Will this mod make your 930 a ricer and gain all sorts of performance? Not....Just vents a little more boost pressure between shifts and minimizes the 930 "cackle" when decellerating off throttle. That's been my experience anyway.

I have been having a hanging RPM and was looking for a solution. I spoke with the head guy at the Dealership i have been working with and he said that Porsche back in the day had the guys compress the valve 1 mm to fix the hanging RPM problem. its funny that i stumbled upon this thread as i had been searching for a while for the solution. this is a great knowledge center!

diaphragm regulators acts as a damper
 

Interesting read on the experiments and the zeitgeist of the BOV / diaphragm regulators. I've had the notion the diaphragm regulators acts as a damper so removing generally causes crisper on / off throttle response. But when removed for whatever reasoning, different, non original configurations are there to factor..

Some other factors worth mentioning are effects of the mass of the flywheel / clutch assembly, intermediate volumes and catalyst / muffler systems, etc. I view it as a drivability and safely component of the emissions control, as a system.

Having machined and sold thousands of billet repro boost control valves over the past 12 years, I've had some time to question designs and argue with the correct way to use these.. Now, I generally believe following the engineering examples set by the constructors. So what did Porsche / Bosch do on the 965-or motorsports? Bosch really designed an engineering marvel with the original 930 system..but with emissions constraints. Now later or Motorsport / Bosch commits to recirculation type valve design ("Over-run cut-off valves"). Some reasons being emissions which not many care about or noise levels or not wasting metered air. The most important reason is reducing boost surge, thus protecting the turbocharger! Now if you wanted to stick with venting to atmosphere this will be louder but also because they are metal where as Bosch re-circulating type valves are a polyamide and thus quieter.

Some time ago (around 1999), ZucZ Performance had a “Long Neck” conversion package which consisted of a billet 3D machined adapter and updated to the 965 cut off valve. It was a slick, expensive package with some ugly machine work on the adapter but they worked (with the diaphragm regulator needed) and were sold to Performance Products / Automotion at the time. Unfortunately, not much empirical testing or feedback was there with just that part only replaced.. so never knew if it was worth it.

Mark,
Now that you've had your decel valve vented to atmosphere for some time now, what do you think? I'm considering doing the same thing to my 930. Mine's a little different because I have an Andial IC. I still have my whole re-circulation valve.
Did you cap off the intake manifold?
Are you getting more or less exhaust popping?
Do you ever have a problem coming to a stop and the engine dying?(something I heard about from doing this)

I run a small air filter attached to the free end of the decel valve, just as insurance against drawing in any unfiltered air.

http://forums.pelicanparts.com/uploa...1334758535.jpg

I have to replace the gaskets in my DV and have been following these related threads. This may be a dumb question but, it sounds like it would be easier just to go to a long neck intercooler and do away with this problematic area all together. Am I wrong or am I heading down an expensive road?

There aren't any gaskets associated with the DV (deceleration valve). You're probably referring to the boost recirculation valve housing, where worn gaskets cause a clunky sound when getting on/off the throttle.

If you still are running with the stock IC and plan on going with something larger, then yes....a long neck IC will allow you to delete the boost recirculation assembly all together..with a few minor add-ons to complete the project.

Like mark said you might be talking about the whole BOV assembly that the decel valve is connected to. The decel valve is the Gold colored disk that I retained when swithching to a long neck to get rid of the whole BOV manifold.

Video here.
930 engine.3gp - YouTube

You guys were right, I was thinking of the BOV. Still a great thread and I'm enjoying reading it.


Oilonly... went to your video and where did you get the air cleaner?

I bought it from a poster on this site, JFairman. Don't know the name brand.

Mark,
Thanks for your input back. I can't wait to hear what it sounds like. The car's that I've had before this 930 were a few fixed up turbo charged Mitsubishi's including an Evo IX, that all had BOV's that vented to atmosphere. Not to mention, I just went to Rennsport Reunion IV, where I got to here the BOV's of the 962's. What an awesome sound. I'm sure it won't be that loud but...