924Board.org

[Benino]

I have a 1980 N/A which according to my Porsche factory manual has an “RPM relay”. I have heard something about this. I guess it enriches the mixture by setting the frequency valve at a certain RPM to a set duty cycle instead of adjusting based on 0XS readings. I also know that there is a full throttle switch which does a similar thing but based on a full throttle switch instead of RPM.
Looking at the diagram I see the full throttle switch wiring runs through the RPM relay before going to the OXS control unit (pin 7). I want to know how this system works. Any information would be good. Here are some specific questions I have as well. At what RPM does the enrichment occur? I want to know if the full throttle switch is independent of RPM? (Does it only enrich the air fuel mixture if both full throttle and certain RPM conditions exist? Will it enrich at the enrichment RPM if the throttle is not fully open?) Does anybody know what the duty cycle of the frequency valve is for full throttle and for RPM enrichments? (I’ve heard that on VW’s with an identical system but without the RPM relay, the duty cycle for full throttle is 60%). Here’s the wiring diagram. Does anyone know where the top two wires go (re/bk and gn/wt)?


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1980 Porsche 924 N/A USA
1980 Porsche 924 turbo USA
1987 Porsche 944S USA

[gohim]

Friendly advice is to leave the DAMN THING ALONE !!

Do you really think that you can get any more power from your engine then the guys that engineered the system could get after spending years on research, millions of D-Mark on development, and thousands of hours, and when they know what their doing you don't?

All you're going to accomplish is messing up your car's injection system.

[morghen]

well actualy the guys designing the 924 did not want a powerfull engine....see the head problem...it dont think you could get any more power out of litle things like that...anyway...i guess its worth the try if you have a car to experiment on...
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Supercharger and EFI kits
https://www.the924.com

[gohim]

Porsche did not design the 2.0L engine, it was an engine already in production in a truck, that was being manufactured by Audi.

The whole idea behind the 924 design was to take inexpensive parts, and throw them together with a ferw new parts to come up with a sportier car for VW for sell.

All the engine and suspension parts came from other cars already in production. If Porsche was paid to develop an engine for the car upfront, the cost of the car would have been more them $10K and the engine more like the 2.5L 944 engine.

[Benino]

I did not say anywhere in my post that I was trying to get more power out of this. I don’t believe I could increase HP any substantial amount at all by messing with the mixture. I believe that my full throttle switch is not working (switch works but I think wiring to the relay may be bad), and I'm not sure if the RPM relay is working, but not knowing how they work together or if they depend on each other I can't really test it. I don’t know when the enrichment is supposed to occur or if it depends also on RPM etc. . . well I already asked what I wanted to know.

the main question. if my car is idling and I manually click the full throttle switch (without opening the throttle), should the mixture enrich and stop adjusting based on OXS (I know this happens on VW cars, but as I mentioned before they don't have the RPM relay [scirocco, GTI]) Has anyone tested this on a 924 with the RPM relay? (1980 or later I believe). Thanks.
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1980 Porsche 924 N/A USA
1980 Porsche 924 turbo USA
1987 Porsche 944S USA

[-nick]

"Friendly advice is to leave the DAMN THING ALONE !! "

Wow, sometimes decaf can save lives you know.

Benino,
My limited understanding is that there is no rpm-based enrichment from the electronics. The enrichment is only in effect at WOT when the throttle linkage clicks the switch. I don't know quantitatively the changes the switch makes.

Conversely, the idle switch on the throttle should lean out the duty cycle.

Hope this at least helps a bit. If you know someone who understands VW's version, it should be practically identical to ours.

nick

[gohim]

There are three switches on the throttle body. They are "off-idle", "7 degrees" (accelleration), and "WOT".

"Off-idle" tells the injection brain exactly that: the throttle has been moved off-idle.

"7 degrees" (throttle open 7 degrees from the idle position) when this switch is actuated within a certain time window after the "off-idle" switch is activated, the fuel injection computer ASSUMES that the car is accellerating, and it disables the O2 sensor sampling for a period of about 10 seconds, which allows the engine to run richer for the short period of time since the O2 fuel injection brain will be ignoring the rich exhaust readings temporarily.

"WOT" (wide open throttle) disables the O2 sensor sampling, allowing the engine to run richer while throttle is wide open.

All of this information is available in the Annual Model Information/Servicing Booklet, and the Factory Workshop Service Manual. This is why it is important to own these documents is you really want to understand how your car works, and how to diagnose and fix it when it doesn't.

There in no active mixture enrichment circuitry (to allow more fuel than the fuel injection system is designed to supply at a given set of parameters), only circuitry that allows the fuel system to supply the maximum amount fuel programmed without interference from of the O2 sensor system.

The reason that I said leave the damn thing alone is because you obviously do not have the necessary documentation that would explain how the system works, and have no idea of how altering the current setup will change the way the system works, or how to trouble-shoot the system if you thought there was a problem.

If you want to mess with the system, buy the manuals, read them, and understand them, before you touch your car.

[timstar92404]

[Khal]

[D Hook]

Gohim: What model year does that info apply to?
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'80 924 n/a SOLD

[gohim]

This is the way that the 1981 and later model year U.S. cars are set up. The 1984-1985 Euro cars that I have seen that were privately imported were also set up this way.

I have seen some 1980 cars, and from what I can remember, this is the way that they were set up as well. There may have been some slight differences as the this 1980 cars did not have the electronic idle stabilization that the 1981 cars came with. Since 1980 was a transitional model year for many other features (door latch hardware, transmission, catalytic convertor, firewall/rain tray layout/drains (ever notice the difference between the 1980 and 1981 cars in the rain tray under the hood?), switch to dual fuel pumps, etc...), I have no doubt that some 1980 cars have 1979 features, and some the same features as the 1981 cars.

1979 cars definitely would not have worked this way.

[John Brown]

With lambda control and a 'frequency' valve the system does enrichen the mixture under some conditions. Will look for my 'other' orange manual. From memory:

The first micro-switch (1degree?) turns off the idle stabilizer if installed, changes the response time to lamdba changes (slower when throttle closed to prevent surges - not too effectively unfortuneately).

The second micro-switch (7 or 8 degree) does disable the lambda (O2 sensor) and enrichens the mixture via increasing the duty cycle of the frequency valve briefly.

Micro-switch WOT (54 degree 924 & 75 degree 931??) enrichens the mixture taking the frequency valve to 65% or so.

There is also a logic table in one of the manuals that lays all the above out and include engine temp. I seem to recall that for the most part once the engine is warm only the acceleration and WOT functions. At least I think that was why I stripped everything else off the motor

From a testing instruction about mid way:

The 'default' duty cycle is 50%

Warm up engine and run to about 4000rpm and hold. Bridge micro-sw 2 (full throttle). Duty cycle should go to 65%

If no: Pull plug off oxygen sensor relay and connect plug contact 7 with ground. Duty cycle should go to 65%

If no: check wire from contact 7 to control unit term 7. If good then control unit defective.

There's lots more banches but would be easier to just mail you a copy.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82