Deleting the decel valve

[JFairman]

"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.

[mark houghton]

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....

[full quack]

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

[JFairman]

Originally Posted by full quack

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.

[16Volt]

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.

[Ronnie's.930]

Originally Posted by full quack

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

Originally Posted by JFairman

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.

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

[16Volt]

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).

[16Volt]

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.

[oilonly]

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.

[silverlock]

Originally Posted by 16Volt

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

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?

[sundevil64]

Originally Posted by Ronnie's.930

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).

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?

[Speedy Squirrel]

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.