924Board.org

[Piper]

Just a quick question on exactly what the Vent and Vacuum ports on WURs do? Mine has neither on my 924 NA and I thought that it may improve fuel economy by having the vacuum adjustment of the WUR. Does it just enrichen the mixture(lower the control pressure) on manifold vacuum? Or can it lean out the mixture under certain circumstances also such as engine braking?

I see VW Golf Mk2's have vacuum control and seem to get much better fuel economy. I'm only getting 24-26 mpg depending on driving conditions.

And does the vent port compensate for altitude?

I thought I'd fit one with both ports then set up the hot control pressure higher than the manual states, so when cruising it would run leaner, and run fine under throttle.

Does this sound feasible?

John

[ideola]

The 931 has a boost-actuated WUR that provides enrichment at WOT.
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erstwhile owner of just about every 924 variant ever made

[Piper]

So I guess it must be different than the VW golf set up which is also naturally aspirated?

[ideola]

Yes, and the NAs never had WOT-enrichment features. I don't know if the 931 unit would work well on the NA, but there is a snippet in the 924.org tech section that discusses the possibility of using one of the Audi WURs. I think you would want one specifically setup for vacuum, as opposed to the positive pressure configuration on the 931.

However, I'm not sure it would have a positive effect on your mileage. If anything, you would consume more fuel on WOT than you currently do, and I doubt you would be able to compensate enough on the off-throttle side of the equation to offset the extra fuel consumption from WOT enrichment. I'm theorizing, of course, but it would be great to have somebody give this a go!

FWIW, 26 MPG out of a 26-year-old CIS equipped car is pretty remarkable.
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erstwhile owner of just about every 924 variant ever made

[Piper]

The MPG is in Imperial Gallons - 4.54 litres per gallon, so 26 isn't that great. About 20 us MPG!!

Cool I'll have a look at the Audi WUR's. I'm sure half the battle with the fuel economy on these cars is the fact the fuelling in contant no matter what load is on the engine!

[Piper]

Right it looks like it is the same idea/part for a 931 or an N/A car, as the load sensitive WUR will lean the mixture under Vacuum and enrichen it under boost. So it could well work to my advantage! I'll have to try and get a second hand one of these I think!

[ideola]

[Rasta Monsta]

Piper, I agree that your mileage sounds low, and your 931 WUR idea is interesting. However, there are a number of things that can cause poor economy in these cars you may wish to check first.

The first to come to mind:

1. Bad WUR (check fuel pressures cold, during warmup, and hot)
2. Leaking CSV
3. Leaking injectors
4. Cap, rotor, wires, plugs
5. Wrong idle mixture

And one minor correction. . .you are right that CIS does fuel constantly, but it actually does use a mechanical airflow meter to change fuel delivery rate as engine load changes. . .FWIW.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[Piper]

Rasta, I've not long set the control pressures up and given the whole CIS system a thorough test, apart from the CSV though right enough, I'll check that out!

Plugs, leads, cap and rotor are all new. The idle mixture could be off though, how am I best to set it? Find someone with a CO meter or is there an easier way to get it roughly right?

I think i'll try out the 931 WUR idea though, would be interesting to see what happened! It surely couldnt make it worse as the NA car will never give a positive Manifold Pressure!

Ideola, To clarify - in the picture of the 931 WUR you posted, is the side connection the Vac connection and the top one is open to atmosphere? Or do they both get hooked up somehow? My car has neither.

[ideola]

The side connection is the vacuum/boost port, and the top one is open to atmosphere.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

Piper,

Great job on being thorough with your car. We sometimes have a hard time convincing people that a CIS tester is a critical diagnostic tool.

FWIW, the 931 shop manual, at 25-13, describes testing the WUR under boost pressure, but does not address vacuum testing. This may be an indication that the WUR doesn't alter control pressure under vacuum. . .but it may not. I suspect you'll find out if you pursue your experiment.

Good luck!


rasta

P.S. On the 931, the hose fitting from the WUR goes to manifold vacuum, and NOT throttle plate vacuum. You may need to tap your own fitting for this on an NA.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ic932]

Rasta has lots of vaccum experience...