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Might be the de-accel valve that passes air around the throttle plate between shifts and with throttle lift.

I would probably move my boost gauge read to the other side of the throttle plate.
Old 12-11-2009, 02:23 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #41 (permalink)
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Early 911 turbo's had a second saucer shaped throttle body bypass valve that looks alot like a deceleration valve without the 3rd small accuating intake manifold vacuum line going to it.

It was mounted back behind the auxiliary air slide cold start valve and you can't really see it with the factory aircleaner in place.

I've never seen one other than occasional pictures and I don't know what opens it so i can only guess air pressure in the intercooler when you let off the throttle quickly opens it letting air bypass the throttle body during upshifts and initial deceleration.
Probably an emissions and anti turbo compressor surge device.

They got rid of it by 1986 when the turbo came back to US dealers with more emission controls.
Old 12-11-2009, 02:32 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #42 (permalink)
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Originally Posted by 911st View Post
Might be the de-accel valve that passes air around the throttle plate between shifts and with throttle lift.

I would probably move my boost gauge read to the other side of the throttle plate.
This VDO gauge I have doesn't read vacuum - in-fact there is a check valve to keep it from getting a vacuum. Which hopefully it does not see considering its above the throttle plate. The check valve looks ancient but I will most likely verify it works properly and move it behind the throttle plate.

I actually removed the decel valve (and plugged up the ports) on my '87 all that stuff was attached to the "pop off valve" or blow off valve as I'm used to calling it.
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Old 12-11-2009, 02:34 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #43 (permalink)
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Quote:
Originally Posted by 911st View Post
Might be the de-accel valve that passes air around the throttle plate between shifts and with throttle lift.

I would probably move my boost gauge read to the other side of the throttle plate.
On all 930's the deceleration valve was not plumbed into the intercooler, instead it was bolted to and plumbed between the big intermediate manifold or origonal blow off valve housing and the intake manifold.

..and definately install an aftermarket boost gauge line to the intake manifold or brake booster line so you can see what the motor is getting.
That way the origonal boost gauge in the tach reads pressure from above the throttle butterfly, while the other one reads below the throttle butterfly and you can sort of see the pressure differential.
Old 12-11-2009, 02:42 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #44 (permalink)
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Originally Posted by JFairman View Post
Early 911 turbo's had a second saucer shaped throttle body bypass valve that looks alot like a deceleration valve without the 3rd small accuating intake manifold vacuum line going to it.

It was mounted back behind the auxiliary air slide cold start valve and you can't really see it with the factory aircleaner in place.

I've never seen one other than occasional pictures and I don't know what opens it so i can only guess air pressure in the intercooler when you let off the throttle quickly opens it letting air bypass the throttle body during upshifts and initial deceleration.
Probably an emissions and anti turbo compressor surge device.

They got rid of it by 1986 when the turbo came back to US dealers with more emission controls.
Now that I think about it I believe I have seen this in a picture posted on here but thought that was only on the early cars mines an 87.

So if that thing exist my '87 actually has three devices to alleviate compressor surge when the throttle quickly closes.

1: the pop off valve (Blow off valve)
2: the decel valve thats bolted to the back of the "pop off valve"
3: the one were talking about

I'm going to take her all apart during the holiday so I should be able to figure it out once and for all. I wonder if the car ended up with an earlier intercooler than it originally came with and whats why I have the two ports on the stock unit. I have been running the B&B intercooler for a few years it certainly does not have and 2nd outlet in that spot just the one for the idle valve.

charlie
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Old 12-11-2009, 02:45 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #45 (permalink)
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Originally Posted by JFairman View Post
On all 930's the deceleration valve was not plumbed into the intercooler, instead it was bolted to and plumbed between the big intermediate manifold or origonal blow off valve housing and the intake manifold.

..and definately install an aftermarket boost gauge line to the intake manifold or brake booster line so you can see what the motor is getting.
That way the origonal boost gauge in the tach reads pressure from above the throttle butterfly, while the other one reads below the throttle butterfly and you can sort of see the pressure differential.
Jfairman - you have the right idea (you have been very helpful to me on these forums! many thanks!) During the holiday season I am going to be fixing a few little issues with my car and installing an AFR meter and sensors for my Megasquirt, one of which is an map sensor I'm going to put behind the throttle plate. Once I have that all in I'll spend months with it in the car before it actually controls fuel or ignition so basically i'm just using it to log data to a palm etc while I save for the PWR fuel kit and decide if I want to go EDIS or try and run the stock ignition from megasquirt. I have considered trying to control the stock Frequency valve but I doubt i'll mess with it and instead just go straight to full control with the PWR fuel kit.

sorry to derail this thread...
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Old 12-11-2009, 02:54 PM
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Your '87 didn't have the extra saucer TB bypass valve.

It was alot easier and cheaper for the factory to simply put a plug on that port instead of making a new end tank casting without it for the intercoolers.
Old 12-11-2009, 02:55 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #47 (permalink)
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