WUR Vacuum Question

[Porsche_monkey]

I am still troubleshooting my rebuilt 75 2.7, but I am unsure about the WUR and the vacuum connection. I have the pressure test gauges coming tomorrow, but in the meantime...

I know there are several versions of WUR's. Mine has the atmospheric reference pressure on the side, vacuum line on top.

My question is: How should the vacuum supply to the WUR work? I have read so many conflicting things that I am confused now.

Lets start with two simple questions...

1. Should the thermovalve shut off the vacuum to the WUR for cold starts, or does the WUR always see full engine vacuum?

2. Is it logical that my WUR needs a major recalibration due to the rebuild (this negine is farly tight and I create a lot of vacuum)?

All suggestions appreciated.

[Porsche_monkey]

While we're here, how do you tell the difference between the two on the left?

[Paulporsche]

The parts are identified by numbers stamped into the case. Each is specific for a particular car/year, although there appears to be some crossover, and some original numbers have been superceded.

If you do a search, you will find charts with Porsche/Bosche numbers and charts showing control pressures. Also, my owner's manual for CIS (originally supplied w/ car) has similar info.

Good luck.

Paul

[alan911]

Well... I'll just add to your confusion.

I've got an extra WUR that looks like the one on the right. No side port. I think it came off of an 83 SC.

Now.....

The one that is currently on my '77 is the one on the Left. With the side atmosphere port although it goes to the throttle body.

Oddly, they both have the same part number. Does that mean they are interchageable ??

Someday, somebody will actually figure out how these things are suppose to work and why the connections are different when they are suppose to perform the same function.

What we need are pictures of the INSIDE of each device to see why they are connected differently.

[jdowty3]

It looks like the one on the far left has the atmosphere port in the bottom casting piece and the next one has the manifold port in the top casting piece.

[Porsche_monkey]

Well I've made mine adjustable, why can't Mr Walker chime in here on how the vacuum should be connected?

[Paulporsche]

I have read that the port on the bottom is for a chamber within and is to add more fuel for either high altitudes or for full throttle. If you have a lambda system, apparently the bottom port was eliminated. I currently have an 80 engine enlarged to 3.2 without lambda or DME, and am using the superceded 80 WUR without the bottom port and it works fine.

Paul

[Porsche_monkey]

SINCE NO ONE WILL HELP ME I've read a bit about the WUR and can contribute the following: Lower WUR pressures richen the mixture. The fuel pressure needs to be reduced for cold starting and this is accomplished with the bi-metal spring arm. It should reduce the pressure by conteracting the coil spring(s) when the car is cold. As it warms, either through electrical current or engine heat the arm should no longer contact the coil spring. The fuel pressure when cold should be between 0.5 and 1.5 bar depending on temperature. Colder temps require lower pressures.

The coil spring(s) without the bi-metal plate in contact should hold the pressure at 3.7 bar. If it does not you need to shim the outer spring slightly. This is your warm fuel pressure.

The inner spring (if it is present) is used to reduce the pressure (richen the mixture) under high load (ie low vacuum). Some WUR's use different connections and configurations, but this appears to be the basic operation principle. If your pressure is too low you can adjust the bimetal arm, but if the pressure is too high the arm might be scrap and you need a new WUR$$$$.

Without the main vacuum connection hooked up to your WUR you will not be getting full power on acceleration. The second port is the atmosphere reference it can probably be left open, but is ideally connected to a clean source of air above the throttle plate.

Some WUR's have only a single coil spring, I am guessing these cars do not use the WUR to richen the mixture under full load, these are probably cars with O2 sensors.

To set/adjust you should set the warm pressure first to 3.7 bar then adjust the pin on the bimetal plate to get your cold pressure for your current ambient temp.

Given the number of WUR posts, as these devices age more and more problems are cropping up. I hope people find this helpful.

PS - Wayne - find a source for rebuild kits please. The prices for WUR's are ridiculous

[William Miller]

Newer SC with lambda control have acceleration enrichment Via a switch on the throttle body and the computer. That is why there is no lower vac connection. K-basic (erlier SC's) Have the lower connection.
Mine is an 83 Euro SC and has no lambda control and does have the lower connection like a US 79 SC. This lower connection goes first to the thermovalve and then the throttle body. The thermo valve is closed when cold. It prevents the additional acceleration enrichment for about 30-40 seconds. This is at the same time the control pressure is and A/F ratio is lowered during a cold start (Via the bimetallic spring.)

The model that I have has a allen screw adjustment in the lower half that most do not know about. This has an effect on warm control pressure after the bimetallic spring is full heated and has no effect.
While turning this allen screw be careful to hold the outter piece and not strech or tear the diaphram.

I assume this is calebrated at the factory on a new unit.
The other adjustment is system pressure which also has a small effect on warm control pressure and A/F ratio.

You can do a search for CIS 201 where I put the link to the CIS PRIMER.
Add some info if you can to the CIS 201 thread. I have been trying to consolidate some info there for those of us with these cars.
Thanks!

[Porsche_monkey]

Maybe someone can confirm the 3.7 bar warm fuel pressure number for me.

[William Miller]

Here's what I found in the shop manual:
74/75 with vacuum control
with no vacuum 2.7 - 3.1 bar
with vacuum (520-546mbar) 3.4-3.8 bar
system pressures Test 4.5-5.2
Adjust 4.7-4.9


They have charts for cold control pressure but they are dependant on the model number.

My text above on the theremovalve operation was backwards.
With no vacuum the control pressure and A/F ratios are lower so the thermovalve is closed at initial start up for additional enrichment.

FYI I saw that the Cis primer now has a section on how to hook up the fuel pressure gauges.
Good luck I hope this helps. I don't thimk you need to convert the numbers most gauges have both PSI and Bar scales on their dials.