Deleting the decel valve
 

I am considering leaving out this valve when I re-install the upper manifold. From my research it seems there are two ways to go about this. Can you folks please check my logic.

Option 1
Remove the valve completely and cap off all vacuum ports associated with it. Two on the upper manifold and one on the throttle body.

Option 2
Leave everything in place with the exception the center hose on the backside, ala JFairman I believe.

Option 3
There seems to be some chatter about reversing it and venting to atmosphere?

Pros to removal
1. simplicity - getting to the hose clamp on the backside was a pain

Con to removal
1. No boost hold between shifts

Personally the added noise theater isnt a concern of mine. Will any post change adjustments need to be made, idle setting etc?

Thanks for the help folks, sorry for all the noob posts.
http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/519065-decel-valve-delete-idle.html

You can keep it all connected and plug the small hose with a bearing or marble and reconnect it so it looks as it was made to the casual observer,

This will also ensure you don't get any vacuum leak from that area,
I don't now why people would vent it.
your throttle response will improve, but it engine may pop a bit more loudly and more noticeable on lift

hope this helps

The decel valve (Porsche calls it the "vacuum limiter") doesn't hold boost between shifts - it does cause RPM do drop more slowly, which some have theorized is some kind of benefit, but for me, having "normal" throttle response is much nicer.

I removed mine entirely, as you mention above, a long time ago as it was damaged by water and leaked air bigtime (talking a 2K plus idle). I'm sure you would like the throttle response, but good thing you say you won't mind the added noise, because the increase in closed throttle popping is very dramatic.

I removed mine and sold it for 40 bucks years ago. I did vent it to atmosphere for a little while before removing it because I like the little woosh sound of it when you let off the gas during boosted acceleration upshifts. Sounds alot like a 962 during upshifts in the in car you tube videos I've watched.
With it removed and the hose connections plugged the motor decelerates quicker between upshifts.
Then RPMs match perfectly with the heavy stock clutch/flywheel assembly and the slow acting Porsche steel synchros in the gearbox during upshifts.

It bypasses air around the throttle body when intake manifold is high during deceleration so it's the same as not taking your foot off the gas pedal all the way during upshifts or decelerationg. It's just another miserable CIS emission control to help lower nitrogen oxide emissions while decelerationg.

With a CIS Flowtech modified lambda fuel head the mixture is not as lean during deceleration so the popping in the exhaust doesn't increase and become obnoxious in my car during deceleration, it just burbles nice.

sorry can't for the life of me try and tell you how I removed mine as it was 1000yrs ago, but I know I've got an uberhighspeed / highly desirable yellow golf tee jammed into one of the lines in there somewhere

I got 'some' more popping on decel but honestly not that much really

however

properly tuning my AFR's once I installed a gauge & BLWUR rendered all those pops (and there were a lot, coupled w/ the removal of the OE exhaust) into what ^Jim notes^...

...nice burbles.

AFR's set right, it shouldn't pop obnoxiously but emit a mellow burble

IME, if it's popping - it's lean

Good to hear. It will go into the "included with sale" box then.

Just so I am crystal:

1. Do not change the BOV output on upper manifold route to the lower manifold.
2. Plug throttle butterfly bypass on TB
3. Plug the decel port on the underside of the upper manifold.
4. Plug the upper port of the Y pipe on the lower manifold.

https://v4s2.yimg.com/sk/3778/126700...18b205b8_b.jpg
IMG_5742 by thecrashingdoor, on Flickr

https://v4s1.yimg.com/sj/2834/127090...9fc9bbcf_b.jpg
IMG_5770 by thecrashingdoor, on Flickr

https://v4s.yimg.com/sj/2873/1263643...e00c6351_b.jpg
decel2 by thecrashingdoor, on Flickr

Fear my epic MS paint skills.

The second "plug" from the left of the top picture = no. That is the spout for the diverter valve, that I referred to earlier, and must be left functional as the diverter valve/blow off valve, keeps boosted air from packing up during closed throttle situations - without one, the boosted air would "hit a wall" when you close the throttle, and thus, stall the compressor fan in your turbo (potentially causing much damage).

The other thing which you need to plug, not pictured above, is the port on the primary intake manifold that the decel valve hose attaches to. Actually, I can see the silver colored spout for it poking up in the left corner of your second pic.

Here's what the diverter valve looks like inside -

http://forums.pelicanparts.com/porsche-911-technical-forum/71370-changing-930-dump-gaskets-pictures-galore.html

Ditto all that. I just got tired of the rpm's hanging up between shifts.

Yep, Ronnie, you're totally right. I was a little confused. So here is what I did.

https://s2.yimg.com/sk/3745/12669924...2432abfe_b.jpg
IMG_5745 by thecrashingdoor, on Flickr

1. Output from diverter/bov left open to atmosphere
2. Decel valve vacuum line from TB capped
3. Port on the upper intake manifold capped
https://v4s.yimg.com/sk/3778/1267002...18b205b8_b.jpg
IMG_5742 by thecrashingdoor, on Flickr


That correct?

I took the car out after re-assembly and experienced no added noise from the BOV valve and no additional popping upon lifting. The car is very sluggish and while the gauge says I am making some boost there is zero power. Off boost the car feels pretty normal.

Did I do something wrong or has the change just made it so I need the mixture re-adjusted?

As you have it set up right now, going by your pictures, you have completely disabled the diverter valve/blow off valve. You should have a hose that runs from the downward curved portion of pipe (you have a cap on it in the pic above) to the spout on the diverter valve end cap. That hose is sort of difficult to rout and attach, but must be in place or the diverter valve will not work at all.

* Let me amend that - it will "work" sort of , but you can not vent a BOV to atmosphere with CIS/K-Jetronic - creates a terribly rich condition - there's the cause of your described sluggishness.

The cap on the secondary manifold is correct (second pic) and so is the cap on the straight portion of the silver pipe. Also, you have to have hose clamps on those caps or they will blow off with boost pressure (you probably already had them on there because I just saw that clamp assortment in one of your pictures above).

Argh.

Yes, your totally right again. Boost plumbing is slowly coming back to me. Welp, back to the garage. Grrr.

Nice "grrrrr" - these cars still make me feel like that some time! :D

Did you remember to cap the tiny, now unused vacuum port on the throttle body? I ask, because if not, that will be a juicy vacuum and boost leak.

Ok, just so I am crystal clear.

1. Reconnect line from lower manifold to diverter
2. Leave all other lines plugged (small TB line, upper manifold and the upper of the split lower manifold).

That old line was pretty crusty, I assume any correct size modern silicon boost hose will work here. I ask since the old line actually steel re-enforced.

Thanks for all you help.

It reads like you have it all straight now.

Heater core hose works fine, too (only sees about 11-12 psi with stock wastegate spring) - you really want hose that is thick walled so it is less likely to kink/flatten out when it curves up toward the diverter valve spout. I was so concerned about this that I inserted an appropriate sized spring into the hose to that it would not flatten out where it bends/curves.

No problem about the help - glad to do it.

Cool, going to swing by the shop tomorrow and see what I can rummage up.

The I am going to update all the pics in this thread so no one follows my calamity train.

Congratulations! Your Porsche will now sound like it ate a Taco Loco for a week. The only way to "tune it out" is, well there is no way. Normal throttle response my a$$. It is like anti-speed shifting. Feels like it hits a wall between shifts. An for the 10,000th time Fairman, it is not a emissions device.

Speedy, have you driven one of these cars that has had its vacuum limiter removed? I ask, because frankly, your comments above don't make sense. "Normal throttle response" means that when you close the throttle, engine RPMs drop immediately (as 99.9% of all vehicles I have operated do - including motorcycles) - to have engine speed hang up, after closed throttle, would be considered by most to be "abnormal" and an annoyance. And I definitely don't understand what you mean by "hits a wall" . . . close throttle, gear change, open throttle, accelerate - where's the "wall"?

Rudeness aside, I think I know what he's trying to say.
With the TBV defeated the revs drop more quickly between shifts which can cause the turbo to get out of boost. That creates a moment of lag when you get back on the throttle. You hit a wall so to speak. This phenomenon is easily compensated for with your right foot modulating the throttle between shifts.

With the 10 ton stock steel sachs pressure plate the revs still drop slowly which isn't all that bad because you can't upshift quickly with the stock 4 speed anyway but getting rid of the deceleration valve/vacuum limiter/anti backfire/throttle body bypass valve helps.

I don't discredit/disbelieve the valves intended purpose. For me personally however, not being someone who tracks the car and not being Ayrton Senna I am not concerned with a couple seconds of re-spool.

After a easy drive around the block I prefer the throttle response.

"An for the 10,000th time Fairman, it is not a emissions device."
Then lets just agree to disagree :)
I've seen them on other older motors with carburetors and mechanical fuels injection with no engine management and they are named different things on different cars. They are always there to lower emissions and I can't imagine or think of any other use for a vacuum limiter or throttle body bypass valve that is opened by high intake manifold vacuum during closed throttle deceleration.

My car runs so much better with it removed. Fuel injection and igntion timing is not stock on my car either.

Wouldn't it make sense that when the throttle body is suddenly closed, that the lack of air would cause a very low AFR and associated gasses puking out the exhaust pipe...said gasses being emmissions concerns to some federal agencies? Let some air get in there past the throttle body to lean things out to a more acceptable emissions level?

Maybe Porsche put them on for an entirely different reason, so perhaps they serve two functions. Just saying....

Ummmm....don't forget to put some hose clamps on those capped off ports. Remember, that manifold has full boost pressure in it.

Just sayin.....they just might prevent you from having an embarrasing moment out on the streets :}

Mark

I was thinking the same thing when I saw that. They might not blow off first time you hit boost without hose clamps but when the motor gets hot, being rubber they will soften up and there's a very good chance they eventually will blow off at the worst possible time.

Oh dont worry, I would never not put a hose clamp on. Though from me fumbling through this I can understand why you would think I wouldnt. :)

So the line I need to reconnect I had to "persuade" to come off initially so the next step is finding a replacement. Swung by the shop and rummaged around and found some candidate and orders some other new bulk hose to fix it permanently.

You gents must be similar to the naturally aspirated 911 version - way too slow (see post #10)!!! :p :D

Ok, so i updated the pics/info since I had the incorrect info posted.

In case anyone is curious as to sound/rev fall after this mod here is my car (without the mixture tuned).

<iframe width="560" height="315" src="//www.youtube.com/embed/-_ZlAPv6Xe4" frameborder="0" allowfullscreen></iframe>

Thought I would provide a small update here for those looking to delete the decel/vacuum limiter. I have put about 2K miles on the car since removing it.

As other state the revs between gear shift fall much more quickly. Being that I am used to and enjoy driving older cars the rev-drop feels allot more normal to me. Perhaps the only drawback is I did notice that predictably it does feel like it takes a little longer to build boost when your going for it. Really though its almost imperceptible.

Sound wise I enjoy the burbling and popping when you lift and coast down in gear. There are of course situations (extended city traffic, stop and go) where it becomes obnoxious. Its just added theater to an already obnoxious car though.

One small thing to note though. I am running a B&B dual out muffler and I think that since I removed the valve that it has gotten louder overall. Its not a stretch to hypothesize that all that popping has loosened some of the mufflers packing. It very well could be just age as well.

There you go. Would I do it again? Absolutely.

Just my opinion but with the stock muffler i preffered to keep it disconnected since the popping was not so noticable. But when i went with an aftermarket muffler the popping noise was too much for me so i connected it back up. Yes the hang up in RPM's between shifts is annoying sometimes.

Bit of a thread necro but in an effort to keep related questions in the same place for future readers...

- I have a '78 SC with the decel valve marked in yellow (no. 14 in the PET)
- I want to delete it
- It seems the laziest way is to plug the hose (no. 13A) that goes into the "top" (circled in red) of the decel valve, and the hose itself, correct?
- Do I need to plug the nipple on the decel valve itself, or can I leave it open to atmosphere?
- A slightly better way would be to remove the length of hose (no. 13A) and plug the t-socket (no. 14; circled green) entirely.

Is that right?

https://r2.questionable.services/decel_valve.png
https://r2.questionable.services/vacuum-tee.png

decel plumbing
 

I am in the middle of doing this, and if I understand correctly, the hose below should still be attached to the diverter valve end cap hose? Everything else plugged in?
http://forums.pelicanparts.com/uploa...1745977748.jpg
http://forums.pelicanparts.com/uploa...1745977748.jpg

Yes, correct. All air going through the airflow flapper must go into the engine. When the throttle is closed the boost pressure is relieved to the compressor inlet. This helps keep the turbo spooled up.