Mysterious adjustable boost/vacuum valve?

Ringmaster · 03-19-2009, 04:58 PM

My 1978 930 has a mysterious valve mounted under the dash! It is clearly not an original Porsche part so I am hoping that someone can help identify it’s purpose! Here is what I know:

-The valve receives a boost and vacuum signal via a rubber hose that is connected to a manifold pressure source.

-This pressure hose enters the valve via the port labeled “OUT”.

-The center of the valve has a threaded knob with ball end. Let’s call this the adjuster knob.

-Two hoses exit the valve. One goes to the in-car VDO boost gauge and one goes to the vacuum brake booster.

-Both of these exit hoses (post signal adjustment one would assume???!?!?!) come out of the “SUP” port on the valve which I would assume means SUPPLY. It seems to me that the hoses routing is backward and the pressure hose should be attached to the "SUP" port. Perhaps this is a one-way check valve and the person who installed it connected it backward for a reason?

My best guess is this valve has some effect on the total amount of vacuum boost assist the brakes get???

equality72521 · 03-19-2009, 05:31 PM

It looks like a manual boost controller for dialing up or down boost as needed. Otherwise known as dial-a-death.

Ringmaster · 03-19-2009, 05:50 PM

Quote:

Originally Posted by equality72521

It looks like a manual boost controller for dialing up or down boost as needed. Otherwise known as dial-a-death.

Looks like one but likely isn't. Maybe my description was a bit fuzzy but basically a hose runs from the engine (manifold pressure) to the valve and then from the valve to the brake vacuum booster. Another small hose goes to the boost gauge.

An in-car manual boost controller would have a hose from the engine (manifold pressure) to the adjustable valve and back to the wastegate.

equality72521 · 03-19-2009, 05:53 PM

Hmmm, odd.

A930Rocket · 03-19-2009, 06:58 PM

Never seen or heard of this. The only thing I can think of is limiting the vacuum to the brake booster as you say to limit pedal pressure or someone thought they were hooking up a manual boost controller

Ringmaster · 03-19-2009, 07:03 PM

Quote:

Never seen or heard of this.

That makes two of us

Quote:

The only thing I can think of is limiting the vacuum to the brake booster as you say to limit pedal pressure

That is my only guess, although I haven't wrapped my head around whether or not this valve would actually have any effect on the brakes

Quote:

...or someone thought they were hooking up a manual boost controller.

LOL, the joke is on them

. FWIW the installation job looks quite professional.

A930Rocket · 03-19-2009, 07:13 PM

If you have a part number, check it here: http://www.coastpneumatics.net/search.php

Also, check out this site: http://www.coastpneumatics.net/

I was looking at the site to try adn figure out what exactly the device does.

A930Rocket · 03-19-2009, 07:15 PM

It's also strange that the lines seem to be installed backwards on the device...

Whtnkls911 · 03-19-2009, 07:53 PM

Is it possible it adjusts brake bias?

kycarguy 935 · 03-19-2009, 07:56 PM

My DP 935 car has a adjustable brake bias lever made by Tilton. They are to help keep the wheels from locking up especially when raining and wet out. The rears with larger wheels rubber tend to lock up fast. With it you can control the brake bias from inside the car while driving.

I don't know if that is what you have but could be.

lucittm · lucittm's Garage

This is an adjustable air regulator made by the SMC corporation in Japan. http://www.smcetech.com
It is a good product based on what I saw on their web page. Keep in mind we are dealing with vacuum and the regulator works with pressure. That's why it appears to be hooked up backwards. The brown cloth covered hose is the gauge fitting on the regulator.
The person who installed this regulator knew exactly what they were doing. It is designed to limit (or regulate) the amount of vacuum that the brake booster can receive, it has nothing to do with boost. So, Ringmaster, you were correct in your guess.

Mark

Ringmaster · 05:58 AM

Quote:

Originally Posted by lucittm

This is an adjustable air regulator made by the SMC corporation in Japan. http://www.smcetech.com
It is a good product based on what I saw on their web page. Keep in mind we are dealing with vacuum and the regulator works with pressure. That's why it appears to be hooked up backwards. The brown cloth covered hose is the gauge fitting on the regulator.
The person who installed this regulator knew exactly what they were doing. It is designed to limit (or regulate) the amount of vacuum that the brake booster can receive, it has nothing to do with boost. So, Ringmaster, you were correct in your guess.

Mark

Thanks Mark

Since the boost gauge hose comes off the side that has been regulated I have to wonder if adjusting the knob will alter my boost readings. Since the valve appears to be a "bleed" type maybe it just slows the signal to the gauge but the same pressure still arrives in the end. Hold on!! Maybe you are are saying is this is a one-way valve that only regulates vacuum, not pressure due to the way it is hooked up? Boost pressure can flow freely to the brake booster and boost gauge but I can adjust how much vacuum is pulled back through the valve to change the brake feel??

I suppose a good old fashion test drive is in order to see if the knob makes a pronounced difference of the pedal pressure required to stop the car.

Has anyone ever seen this type of regulator install in a Porsche before? What is wrong with the factory vacuum assist system? Keep in mind this is not a brake bias valve.

As others have noted, this SMC valve appears to be very well made and it quite robust and heavy.

A930Rocket · 03-20-2009, 07:48 AM

I think it was installed to prevent locking up the brakes, especially at the track. I've not seen this, but have seen where a wooden plug with various size holes have been inserted in the brake boost line to limit vacuum.

Ringmaster · 03-20-2009, 08:24 PM

Quote:

Originally Posted by A930Rocket

... but have seen where a wooden plug with various size holes have been inserted in the brake boost line to limit vacuum.

Now that is strange! Where does the wooden plug go? In the booster return line?

I drove the car today and tried the brakes with the knob turned all the way in and all the way out. There might be a small difference but it sure isn't anything major. Maybe no difference at all and my mind is playing tricks on me

A930Rocket · 03-21-2009, 11:11 AM

Yes, it goes in the line to the brake booster. By limiting vacuum, it lessens the boost, thus making it less likely to over brake.

I wondere what driving your car with the valve turned both ways would do. I know when I had the lever on my brake bias valve on backwards, I had no braking power to the front brakes at the track. It sucked until I figured out what happened...

Quote:

Originally Posted by Ringmaster

Now that is strange! Where does the wooden plug go? In the booster return line?

kycarguy 935 · 03-23-2009, 08:34 PM

any updates on this part?

Ringmaster · 03-29-2009, 09:50 AM

Quote:

Originally Posted by kycarguy 935

any updates on this part?

No. I drove the car with the knob all the way open and all the way closed and I couldn't really detect any difference in the brake feel