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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.
Decel valve delete and idle
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Old 02-18-2014, 07:51 PM
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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
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Old 02-18-2014, 08:30 PM
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Quote:
Originally Posted by 16Volt View Post

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.
Decel valve delete and idle
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.

Last edited by Ronnie's.930; 02-18-2014 at 09:21 PM..
Old 02-18-2014, 09:18 PM
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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.
Old 02-19-2014, 06:03 AM
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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
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Old 02-19-2014, 06:12 AM
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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.


IMG_5742 by thecrashingdoor, on Flickr


IMG_5770 by thecrashingdoor, on Flickr


decel2 by thecrashingdoor, on Flickr

Fear my epic MS paint skills.
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Last edited by 16Volt; 03-05-2014 at 01:27 PM.. Reason: updated for correct instructions/pictures
Old 02-19-2014, 07:14 AM
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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 -

Changing 930 dump-gaskets, pictures galore

Last edited by Ronnie's.930; 02-19-2014 at 08:51 AM..
Old 02-19-2014, 08:28 AM
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Quote:
Originally Posted by JFairman View Post
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.
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.
Ditto all that. I just got tired of the rpm's hanging up between shifts.
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Old 02-19-2014, 09:24 AM
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Quote:
Originally Posted by Ronnie's.930 View Post
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.
Yep, Ronnie, you're totally right. I was a little confused. So here is what I did.


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

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?
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Last edited by 16Volt; 03-05-2014 at 01:27 PM..
Old 02-20-2014, 10:18 PM
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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).

Last edited by Ronnie's.930; 02-20-2014 at 10:56 PM..
Old 02-20-2014, 10:23 PM
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Argh.

Yes, your totally right again. Boost plumbing is slowly coming back to me. Welp, back to the garage. Grrr.
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Old 02-20-2014, 10:27 PM
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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!

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.
Old 02-20-2014, 10:59 PM
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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.
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Old 02-20-2014, 11:10 PM
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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.
Old 02-20-2014, 11:20 PM
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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.
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Old 02-20-2014, 11:42 PM
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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$$. It is like anti-speed shifting. Feels like it hits a wall between shifts. An for the 10,000th time Fairman, it is not a emissions device.
Old 02-21-2014, 04:24 AM
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Congratulations! Your Porsche will now sound like it ate a Taco Loco for a week. The only way to "tune it out" is, well there is no way. Normal throttle response my a$$. 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"?
Old 02-21-2014, 09:21 AM
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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.
Old 02-21-2014, 10:26 AM
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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.
Old 02-21-2014, 10:46 AM
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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.
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Old 02-21-2014, 10:56 AM
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