924 Turbo emissions

[tref]

Well some progress... I took the lines off my spare WUR, and blasted the airline through them, and connected it all up on top of the engine. Significant was that when I disconnected the old WUR, I undid the line on the front of the metering head first, and fuel dribbled out, but sprayed out when I undid the top connection. It suggests to me that something is blocked, or not opening. Now as soon as I started it with teh spare WUR connected, the gauge was reading 3 bar, rising to 3.5bar after a while. Close the valve, and the pressure increases to 5.5 bar, so I think this is acting more as it should. Only thing is... My little Gunson CO meter is now reading 6... which is probably right, because I would expect lowering the pressure through the WUR would increase pressure at the injectors... wouldn't it?

Anyway, I think one more thing working as the book says it should is progress! As always, any comment or advice welcomed!

Tref.

 

[924nutter]

As I suspected, the reading of 5.1 bar is too high even for a warm engine. The way the mixture enrichment works is counter intuitive; the WUR or control pressure regulator reduces fuel pressure when cold, thus allowing greater deflection of the internal diaphragm giving higher fuel flow for the same deflection of the metering plate. It should be 1.8 to 2.2 bar at 10deg c up to 3.6 to 4 bar at 40deg c. The plot on the graph is linear between these two values.

 

[924nutter]

With regard to the vacuum, I cant answer until I know for sure if you have one or two throttle butterflies, and whether your distributor has a vacuum chamber attached. I believe that you will have a plain disributor with a D.I.C.T ingition control unit underneath the heater matrix, as we have spoken about flywheel pulse sensors in the pat. That being the case I will have a look at the connections on my own turbo.

 

[tref]

ORIGINAL: 924nutter

As I suspected, the reading of 5.1 bar is too high even for a warm engine. The way the mixture enrichment works is counter intuitive; the WUR or control pressure regulator reduces fuel pressure when cold, thus allowing greater deflection of the internal diaphragm giving higher fuel flow for the same deflection of the metering plate. It should be 1.8 to 2.2 bar at 10deg c up to 3.6 to 4 bar at 40deg c. The plot on the graph is linear between these two values.

So does the WUR reduce pressure proportionally, irrespective of what the base pressure is, or absolute - i.e., irrespective of teh base pressure, it will allow 3 Bar out or what-ever it happens to be?

 

[tref]

ORIGINAL: 924nutter

With regard to the vacuum, I cant answer until I know for sure if you have one or two throttle butterflies, and whether your distributor has a vacuum chamber attached. I believe that you will have a plain disributor with a D.I.C.T ingition control unit underneath the heater matrix, as we have spoken about flywheel pulse sensors in the pat. That being the case I will have a look at the connections on my own turbo.

Mine has a single (rather large circular) butterfly, the distributor doesn't have a vacuum connection (as far as I am aware!) and indeed, there is an electronics box of tricks under the heater matrix.

Thanks,

Tref.

 

[924nutter]

As I surmised from one of the photos posted earlier. That is indeed the 177 hp engine, the same as my own. As far as the control presssure regulator goes (cpr as designated in the factory manual) inside is just a bi-metallic arm and a heating coil. The end of the arm occludes the the inlet and outlet ports when it is warm. It would appear that the vacuum connection can partially open the "valve" to effect mixture enrichment under hard acceleration. Looking at the graph, which I believe is reproduced in the Haynes manual, I would conclude that the enrichment is completely proportional within the minima and maxima plots, between 10 C ambient and 40 C ambient. To busy during the week to have a shufty, I'll check out the connections Saturday. If this fixes the problem then the expectation must be to see the wee beastie at the Royal Agg.

 

[tref]

Thanks,

Afraid it is unlikely to be at the RAC though - it is still not fitted out, and no roof! A lot of work yet still, and I have fuel lines on my S2 to worry about!

 

[Indi9xx]

If its been stood a few years it may have some rust in fueling system, including the fuel metering head.

Strip down the WUR, inspect it, ultrasonic clean the filter screens.. With the fuel metering head, some of the fittings have filter screens inside, take them out, ultrasonic clean them and flush out the head.

As the years have marched by, I have seen the WUR's going out of spec, many people then swap in another used one which will eventually go out of spec. Our solution has been to rebuild them as an adjustable unit so you can set the absolute exact correct pressure with the turn of an adjustment screw, we originally performed this modification in the 1980's to allow for tuning the fuel pressure to get the best possible AFR curve for modified 924 Carrera GT's and and 924 Turbo's, especially when running with hybrid turbo's, intercooler upgrades and increased boost, but now its more in demand for standard cars to get the WUR working within spec as they age.

Not seen you in ages Tref, pop your WUR down to me and I will sort it out for you, if you continue to have problems with the fueling or setting up the WUR, let me know and I will help you sort it out.

With the fueling on 924 Turbo's we tend to find the best results for air fuel ratio and performance throughout the rev range if the idle CO is set to 2.5%, this is as long as everything is working as it should of course!

 

[tref]

Thanks Jon, good to hear from you!

I've seen details of the "adjustable WUR mod" and am likely to have a go at that, as I now have a spare WUR on which teh heater is good. I have an ultrasonic cleaner too... Tempting to dunk the entire metering head in it in fresh petrol! Thanks for the figures for set-up - invaluable.

Biggest problem is time - working in London during the week, and a lot of private consultancy work at the moment mean little time to play - and an S2 with 200K+ miles on it takes some maintaining!

I'll look forward to catching up with you at some of the events this year though - Poole Quay is not far away now, if we don't see you at one of the regional meetings!

Tref.

 

[tref]

Gentlefolk,

I was expecting to find a "tunnel" through which I could insert a 3mm allen key between the metering head and the "air volume sensitive flappy thing". I didn't, I found a screw slot in a little spiggot... my question: Is this the mixture adjustment, or a blanking plug before I get to the mixture adjustment?

TIA,

Tref.

 

[CarreraRSR]

Blanking plug. Mixture allen screw is below

 

[tref]

You're a star!

I can now go out and happily check I can unwind it, rather than having to wait until I had a gas analyser connected for fear I had started to adjust the mixture! In my mind I had reckoned it must be blanked, or it would allow air to bypass the afore mentioned flappy thing.

Thanks,

Tref.

 

[tref]

Gentlefolk of the forum, I thank you all profusely...

The emissions are now at a much more sensible level, and today, the infamous turbo cab project passed it's MOT! It is believed to be over ten years since it was last road legal, and would probably have been a lot longer without your assistance.

Not finished by any stretch of the imagination, the plan is to hold a 924 class at the Dorset regional concour this year (to which, of course, you are all cordially invited - 19th June, Kingston Lacey, Dorset) and have it ready for that.

You heard it here first.

Best wishes,

Tref.

 

[tref]

Knowing you folks like pictures.[]

 

[Mike_Pollock]

Looks fantastic Tref - thought it might look like a 944/968 but the curves are very different.