1981 SC enrichment

[mikemdd]

I've searched the archives, but am still not clear on the how the acceleration enrichment process works on my 81 SC. My understanding is that the earlier WUR's have a vacuum enrichment system. However, my 81 does not seem to use the WUR for acceleration enrichment (after it is warm, of course). My specific questions:

1) How is the enrichement controlled for this 81 SC model?
2) Is enrichment only at WOT, or does it enrich throughout the throttle range?
3) Is the enrichment level adjustable?
4) If I adjust the mixture using the normal allen screw adjustment on the fuel head, does this affect the mixture only at idle, or also throughout the throttle range?

My reason for asking is that I am using an Innovate LM-1 for testing AFR/Lambda. I'm trying to understand what adjustments are possible other than at idle. I can measure the AFR, but don't understand how to change it. Sorry in advance for all the questions.

Mike

1981 911 SC
1987 930

[Porsche Head]

The mixture screw changes the relation of the air flow sensor to the piston which allows/varies the fuel volume to the injectors. Pressure is constant +/-.

A change there means a change at ALL position sof the air flow sensor plate (any RPM).

I got a bigger gain from bumping the timing on my 3.0 than actual mixture but I did go another 1/4 turn to account for the dual out exhaust and SSI.

Cliff

[Richard C2S]

Cliff,

How much did you advance your timing?

I've got an '81 SC with SSIs and a Bursch 2-in-1-out and am wondering if advancing the timing a little and adjusting the mixture a little will make any noticeable diff.

Thanks in advance.

[Superman]

Yes it will, Richard. Many folks time these engines at 35 degrees BTDC at full advance, which means about 5000 rpm! With everything hooked up. Then drive it. If it pings, then back it off. Mixture should be set using a gas analyzer.

Mike, if you're asking about the 'full throttle enrichment,' then I think what happens is the computer goes into a closed loop. That is, the oxygen sensor system leans the mixture out terribly (for emissions purposes, not performance) at idle and possibly at any steady throttle position....but this system stops working when you goose the accelerator pedal. I'm not even close to an expert in this however, so please get a second opinion.

[mikemdd]

Thanks everyone for your comments. Still trying to understand how part throttle enrichment occurs (or does it) and how to adjust it (if it is possible). On the later Carreras, the ECU does all this, I suppose, but these later year SC's are a mystery to me. Clearly, I can adjust AFR at idle through the mixture adjustment. However, can I adjust it other than at WOT, which as noted above, is by triggering an open loop signal through the Oxygen sensor system?

[aigel]

Superman, what is the typical idle timing that leads to a full timing of 35deg. at 5000rpm?

Would it be beneficial to mess with the advanvce curve (i.e. install different springs / weights in the distributer's centrifugal advance?)

Any concerns about pinging? If I adjust my advance now and it works, how will it change during a 105 degree track day? Will I be able to hear that and fill up with 105 octane before my rings are cracked?

Thanks,

George

[emcon5]

Originally posted by aigel
Superman, what is the typical idle timing that leads to a full timing of 35deg. at 5000rpm?

What Super-Jim is saying is set the timing so full advance is 35º and let the idle advance take care of itself.

You will have to reset the timing at smog check time of course.

Tom

[aigel]

Mike:

You need to take the o2 sensor out of the loop to have an effect on fuel mixture. Otherwise, except at idle at wot the o2 cirquit wants you to be at lambda=1 and regulate it to such.

As far as I know, the idle adjustment influences the whole rpm band as soon as you got the 02 sensor out of there. WOT is switched and seems to be at about 12.5 a/f ratio in my car.

I have had good luck tunign the car for the track by disconnecting the O2 sensor (physically removing it), adjusting the mixture to around 13 a/f at idle. I try to go as rich as I can, without the rpms dipping too much under 1k rpm when you are coming off throttle.

With the above procedure, fuel economy will suffer a bit, but throttle response is greatly increased, even if you floor it. Your system doesn't have to go from 14.7 to 12.5, rather than already being at 13.x.

George

[mikemdd]

George,

That is the type of information I was looking for..thanks. Your procedure is clear. Please expand a little on the comment about the rpms dipping too much under 1k rpm when coming off throttle. Your experience would be helpful.

Thanks again.
Mike

[john walker's workshop]

the idle adjustment is such an infinitely small part of the whole range of the fuel distributor piston that it makes pretty much no difference beyond idle and when you just start to accelerate.

[Superman]

I always listen to John, and you should too. I've heard John say this, and at one time I even understood it. I think he's saying essentially that while a fine adjustment of your mixture at idle is possible, the A/F mixture along the rest of the throttle positions and driving conditions is essentially not adjustable.

George, I don't want to give you advice that destroys your engine, but John simply times these engines at 35 degrees full advance and drives them. If they ping, he backs it off. I've even seen him forced to return the ignition timing to factory spec because of pinging. 35 degrees full advance happens to be, at idle, about 7 degrees more advanced than factory spec (which is 5 degrees BTDC). A little temporary pinging is not likely to hurt anything, so it might be worthwhile to advance and see. Make it ping, so you are comfortable that you can hear it. You can hear it. then back off 1-2 degrees away from "ping point." Two degrees would be safe. I think John may have discovered, over the decades he has been working exclusively on 911's, that a slightly rich mixture combined with advanced timing....makes these cars funner to drive.

[Porsche Head]

Okay, what I forgot to say was "My 3.0 is a 79", so no O2 sensor and no brain

Of course the ECU overrides on later models based on the O2, TPS, MAP etc., but isn't the overall function the same as on the 79? So what is happening is if you overset the mixture is that you are now running to upper/lower end of the operating range of the ECU's overide to obtain 14.7:1 A/F.

Timing, let's just say that it runs so much stronger now with more advance, especially if you make mods to improve the pumping of air you need to compensate with fuel and timing. And I am running 7:31 gears with posi so it rev quicker. After I added the Permatune Coiless/dual dansk 84mm/timing/mixture I no longer wanted to a) get rid of the damn thing b) add a supercharger, well the charger is still in the plans but no rush.

Remember it's a 72, no sunroof, fiberglass rear&front-end with a 3.0 and 7:31 posi gears, so it takes to mods real well.

I agree with using a meter for the mixture, I have used the mix-till-fluctuates-@-idle approach then back off a bit, not so precise but seems others are doing it too.

The one complaint I have about the 3.0 is the top end compared to a 4 valve motor (like my M5), it's a high lift short duration and overlap cam I guess. But because of the gearing and light weight I can lug her around at 2k RPM even in 5th! Never a single complaint or noise, feels real responsive.

[mikemdd]

John,

I was not aware that the idle mixture adjustment is so insignificant in the scheme of things beyond idle. I have heard that very small adjustments (ie 1/16 to 1/8 of a turn) have a very large impact on the car runs (idle only?). However, it sounds like you can really mess up the idle mixture and have very little effect throughout the throttle range. On my turbo, it ran much better at all rpm ranges with small changes to the idle mixture. However, maybe something else was going on. Anyway, I will do some testing with the LM-1 at different throttle ranges, changing the idle mixture to see if I can see any difference off idle on the AFR. Interesting points.
Mike

[Porsche Head]

Well Mike good luck, let us know what the meter shows across the RPM range after an adjustment. I bought the BOSCH Fuel Injection book which covers all the types, there is a simple diagram which shows how the mixture screw changes the relationship of the sensor plate to the piston. It's very mechanical, but when you add an O2 and a brain on top of that these will try to reset everything back to an effective 14.7:1.

So it seems to work on your Turbo ah, my brother wanted to do some changes to his 930 as well.

Have you worked the timing at all?

Cliff

[Paulporsche]

Mike,

One of the common symptoms of a too rich mixture is a dip in the engine speed almost to the point of stalling before swinging back to the correct idle speed when you abruptly let off the gas. Sometimes the engine will actually stall. Sometimes you will get an oscillation in revs while idling if the mix is too rich.

[aigel]

I am surprised to read that the idle mix adjustment procedure has so little effect on the entire rpm band. I swear that this woke my car up significantly, together with disconnecting the o2 sensor. I can see that it makes no difference if the o2 sensor is still in there.

I do not know how the CIS works, so if someone could please explain, that would be great. Is it like on a carb, where you set the idle mix with a adjustable bypass, where you just feed more or less air? And then, once you hit the throttle, other cirquits become dominant?

What Paul said on the dipping of the RPM when engine runs too rich.

[mikemdd]

Good points by everyone. Getting back to the enrichment question, I found an interesting point in the Bosch Fuel Injection and Mangement book. I quote: "In all systems, rapid opening of the throttle increases air flow and causes the air-flow sensor plate to over react... Overswing of the sensor plate allows the plunger to momentarily rise higher and enriches the mixture enough to prevent lean-mixture flat spots and hesitation." They go on to show a graph that indicates this response. This is the enrichment process I was wondering about. However, it is obviously not adjustable. Non-lambda systems use vacuum to control enrichment, but is clearly not environment friendly. With lambda systems they use the O2 sensor system to keep things at environmentally friendly levels. Anyway, maybe this is interesting to someone other than just myself.
Mike

[john walker's workshop]

[QUOTE]Originally posted by aigel
[B]I am surprised to read that the idle mix adjustment procedure has so little effect on the entire rpm band. I swear that this woke my car up significantly, together with disconnecting the o2 sensor.

a richer idle adjustment does pep up the idle and low rpm acceleration, but after 2K or so, it's too small of an adjustment to affect the rest of the RPM range very much if at all. the disconnected O2 sensor contributes to a smoother cruise too. gets rid of that light surging.

[Porsche Head]

Hope this helps....


Fuel distributor- Control center, contains a regulator to keep system pressure constant. The main moving part is a central pin which is controlled by the air sensor plate. When the throttle is opened the increase of air into the manifold causes the air sensor plate rises and pushes the control pin into the FD which allows fuel out to the injectors. Mixture adjustment is via the 3 mm allen screw, turning the screw clockwise makes the mixture richer, remember lean "out".

[Porsche Head]

Oh and the Warm Up Regulator WUR..

Warm-up regulator- An electrically heated bi-metallic strip operates a valve that controls fuel pressure to the fuel distributor. Many WURs have a vacuum diaphragm that provides enrichment during acceleration.