CIS AFR at idle vs at WOT

[Walt Fricke]

The wide band sensor I bought last year apparently didn't work, because my Innovate LM2 would count up to 99 and stop. Bought another one, and it works. But I am puzzled.

I was shooting (based on some advice) for a WOT AFR of 13.2. I thought that if I set the idle AFR at that value, it would be that at WOT, since the frequency valve in this 82SC motor (with the O2 sensor not connected) should run at the same default setting at idle as at WOT.

But there is some flaw in this thinking. The engine was running at about 10.9 at WOT, and well below 14.7 at idle. I kept leaning the idle, eventually to about 15, but the WOT values on the track stayed pretty much the same, maybe more like 11.9/12. This is a race motor, so power is all I care about.

But why won't the mixture screw lean out the WOT values? Obviously I can't adjust it at WOT, so have to rely on the Innovate to see what is there.

Plugs looked, if anything, to be quite lean - bone white ceramic with a tiny hint of brown at the tip, though the read didn't follow the prescribed protocol and was taken in the garage.

So maybe I wanted to shoot for an idle AFR of 13.2, and let the WOT value fall where it may?

[dhanl82]

Hello,
I have a 1982 SC and have noticed similar behaviors - have an AEM air/fuel gauge installed. I disconnect my O2 sensor ( as per directions) to set idle mixture so that the Lamda system will not be correcting as it is designed to do. I have my idle mixture (open loop) set at about 13.2 - which WITH the O2 sensor reconnected and lambda system correcting fluctuates around 14.7 as it is designed to do. At wide open throttle (open loop) my A/F ratio is 11.2 – 11.6 depending upon ambient temperature. I also have noticed that there does not seem to be a significant correlation between idle mixture and WOT mixture, as I have attempted to lean out the WOT mix without much success. I realize that my WOT is a bit rich but the car polls extremely strong and since at idle and cruising the lambda system leans it to around 14.7 I am not concerned with having excess fuel washing down my cylinder walls, and being in Florida with the heat, I definitely am OK with it being rich at WOT. I am not sure, but I think I read somewhere that the air sensor intake funnel angle was designed in such a way that it prevents adjusting the mixture screw so lean at idle that there would be a corresponding overly lean condition at WOT. Although I fully understand that adjusting the mixture screw does impact the mixture throughout all ranges, it just does not seem to have a very significant impact on WOT mixture. I also seem to remember that before I totally rebuilt my CIS to remedy significant air leaks my WOT mixture was about 13.2 due to unmetered air getting in.
Anyway, just thought I would share my thoughts.

Dave

[Walt Fricke]

Shared thoughts are what I am hoping for. Thanks.

Did higher ambient temperatures translate, at least qualitatively, into leaner WOTs?

[tirwin]

Walt,

I need to go look up the values but I believe the duty cycle is different for idle open loop and WOT.

Idle is 50% and WOT is 60%. I believe this varied depending on model year. I think 82-83 this was the behavior and 80 & 81 were different still.

There is a picture that is floating around here from the workshop manuals that shows the differences.

[dhanl82]

It is my understanding that duty cycle at idle is 50% and once the 1/3 throttle position switch is triggered,"open loop" duty cycle goes to 65%. I also believe that colder air is denser providing more oxygen therefore a leaner mixture, my a/f gauge shows leaner in colder weather.

[Walt Fricke]

Well, I was only part right about the throttle switch in the FV system.

Here is what Jim Williams measured. The throttle switch on these CIS systems is basically a double pole single throw (if I have that right - center to pole 1, or center to pole 2) switch. The center pole is to ground, so at idle one of the other poles is open, the other to ground. Mid-range I think both are open. WOT the other pole is grounded. One connects to the mini-brain directly, one to the acceleration enrichment unit.

Here is Jim's diagram for hot running:



So 50% duty cycle at idle and part throttle (no O2 sensor), and a richer 65% at WOT.

Maybe I set the hot idle AFR at 13.2? I can do that. At idle 14.7 or thereabouts the plugs look rather lean.

[tirwin]

Yes, I had to reset my idle AFR recently after an exhaust system change. 13.1-13.4 at idle was what I had in my notes and what I reset it to. I landed on 13.2.

[dhanl82]

As I stated in my post, with my O2 sensor disconnected "open loop" idle mixture set at around 13.2 , my WOT mixture is 11.2 - 11.6. I would be interested in learning what another car's WOT fuel ratio is when that cars's idle is also set around 13.2.

[Walt Fricke]

Wikipedia has this to say: "In naturally aspirated engines powered by octane, maximum power is frequently reached at AFRs ranging from 12.5 to 13.3:1 or λ of 0.850 to 0.901.[citation needed]

Air-fuel ratio of 12:1 is considered as maximum output ratio, where as the air-fuel ratio of 16:1 is considered as maximum fuel economy ratio."

Which suggests my WOT values of high 10s and low 11s are too rich?

But also that this may vary from engine (model?) to engine? So dyno tuning may be the only way to get it just right for your engine's max power?

[AndrewCologne]

Originally Posted by Walt Fricke

I was shooting (based on some advice) for a WOT AFR of 13.2. I thought that if I set the idle AFR at that value, it would be that at WOT, since the frequency valve in this 82SC motor (with the O2 sensor not connected) should run at the same default setting at idle as at WOT.

No. With the sensor not connected and at warm engine state, when driving with throttle opened below 30° angle (cruising and idling) the duty cycle will be kept at static 50%. If you accelerate and throttle opens beyond 30° angle then the duty cycle switches to static 65% means enrichment of the mixture. So at WOT your mixture should be richer. But as you're very in the 911 engine concept, I guess you already know that.

But there is some flaw in this thinking. The engine was running at about 10.9 at WOT, and well below 14.7 at idle. I kept leaning the idle, eventually to about 15, but the WOT values on the track stayed pretty much the same, maybe more like 11.9/12. This is a race motor, so power is all I care about.

You report a behavior which I also figured out already a year ago when testing my CPU based ECU.

At idle, engine was set to default 0.8% CO with lambda regulation switched off, means with static 50% like when unplugging the sensor if using the orig. ECU for doing the initial CO setup.
Then, when switching to lambda regulation (plugging the sensor) at idle a duty cycle of about 60% results which is normal as here the lambda regulation enriches just a little cause 0.8% initial CO is below lambda 1 where Lambda 1 actually (on this engine type) equals to approx. 1.2 - 2.0% CO.

Then "at idling" the 60% duty cycle results in a perfect, smooth stoichiometric AFR based engine running.

Now, .... with warm engine when driving the duty cycle immediately drops down to about 30% and if rising the rev to about 2000 RpM on a freeway or highway the duty cycle even goes down to about 10-15%! Means in AFR words: the sensor noticed in this case at approx. 2000 RpM that the AFR was noticeable too low = mixture was too rich. So this was hereby compensated by the low resulting duty cycle of 10-15% which makes the mixture stoichiometric again, means 14.7:1 (14.4:1 with E5 gas).

This means that the construction of the K-jet let's the mixture enrichen very much already at middle revs and for sure at higher revs.
So IF in that state of driving at medium/higher revs a duty cycle of 15% means an optimal combustion of lambda 1, means AFR of 14.7:1 (or 14.4:1 with 5% ethanol added fuel) then here your engine when're the lambda operation is switched off (sensor disconnected) with static 50% duty cycle will already result in a much richer mixture even before WOT.
Now in your case when hitting the gas pedal even more with a wide open throttle beyond 30° angle a duty cycle of 65% will push the enrichment even more. And thats why you get an approx. 10.9:1 AFR at WOT in your case.

Now we come to the next thought:

But why won't the mixture screw lean out the WOT values? Obviously I can't adjust it at WOT, so have to rely on the Innovate to see what is there.

Yes, the idle initial CO adjust via screw does affect the WOT mixture as well, but proportionally in a very small way cause the air flow sensor funnel comes with diff. angles. A modification of the sensor plate initial adjustment here doesn't help neither. As here when adjusting the initial state of the plate in a way so far off that the WOT will result noticable leaner, then the middle load/rev states will lean out extremely and will be totally messed up.

Means, without lambda operation working on that engine type you can't modify the WOT AFR in a way without messing up the AFR of the middle load/revs state. Thats the very downside of the static 65% enrichment of the original state of the orig ECU.
I used in my ECU an option that beside the static 65% enrichment you can choose a "enrichen by XX%" in case of WOR. Cause I found out when driving at 2000 RpM and here a perfect 14.7:1 mixture would result in a 30% duty cycle then a higher duty cycle by "+25" will result as the best "switch" to obtain a perfect 12.5:1 WOT mixture when accelerating. Means in that case/example a duty cycle of 55% and not 65!.

[Walt Fricke]

Andrew - thanks, I was hoping you would have ideas about this.

Makes sense that the mixture screw will have less effect at part throttle, and even less at WOT. Sort of like the idle screw has a lot to say about the idle mixture, but just about nothing at WOT. Luckily, I haven't considered messing with sensor plate height, which seems like a good thing.

Since it seems the O2 sensor doesn't affect the WOT 65% duty cycle, am I on track by thinking that - for race track purposes - I won't get better performance by hooking up the O2 sensor? I could do that, since each exhaust bank has a bung, but I'd rather not, especially if it won't give me more power at WOT.

Assume again no O2 sensor (except for my Inovate wideband to see what is happening). What if I disable the WOT part of the switch? I think that would be green/red from pin 3 of the switch, either at T10/12, or pin 4 at the acceleration enrichment unit? No ground on that line (or would it be pin 3/T9/12 without disconnecting the temperature switch?). That way 50% duty cycle all the time? A bit off of the 55% at WOT you found good? Maybe better than 65, though?

I could compensate there a bit with the warm control pressure? Adjust it to a lower value within the range given? Or even a hair lower? Seems I could use the mixture screw as needed to get idle in a decent AF range if it got too high?

Usually I run US pump premium, which is 91 or 93 RON/MON octane, and normally has alcohol in it (E5?). I'd want my WOT AFR to be 0.3 lower with E5? I have not tried any dyno tuning to see exactly what WOT mixture gives the most power (and, given that you can't adjust that with the emissions screw, it would be a bit tough to do other than by making the control pressure adjustable - which isn't all that hard but I haven't tried that on a dyno). Looks like with your electronic bits you could make changes like that on the dyno, though - very nice.

But you view 12.5 as perfect for WOT, and earlier in this discussion there are suggestions going up to 13.3 or 4. Guessing (based on indifferent scales) from Bosch manual graphs, 13.2 looks like where they thought it was, though that would not have included alcohol in the fuel, which didn't happen when they were designing and testing the CIS.

Further thoughts earnestly solicited.

With my current lean idle settings, I'm going to have to be careful next time I am at the track to do a proper spark plug cut: WOT approaching the exit, then turn engine off, coast to a spot in the hot pit, and pull a plug. When I get around to it at the track garage, the engine has been idling or nearly so for quite a while. Last year (Innovate not working) even then plugs showed rich. This year, quite lean, but maybe due to bad procedure.

Walt

Reviewing the various Bosch manuals, I see that the air measuring system gives a momentary enrichment when you "hit the throttle." The initial extra rush of air causes the plate to rise up farther than it would at cruise at that RPM, which adds more fuel until the plate settles down and continues its more linear climb.

So perhaps rich spikes on my Inovate data indicate the start of accelleraions out of corners?

Walt

[icarp]

Don't forget that you can change the effective AFR with control pressure and system pressure.
Ian

[Walt Fricke]

Right - I've thought of that. I'm not keen on reducing system pressure - affects the injectors some? And would call for finding a thinner washer, or leaving one out, which might be too much.

Reducing control pressure I can deal with. I modified a previous WUR with two threaded holes, and a sort of raised bottle cap I could use to have the nuts on the two studs or bolts raise or lower the main pressure slug. To lower CP I usually, after loosening the appropriate nuts, used a punch in the longish Allens I had screwed in to tap down for higher WCP

[AndrewCologne]

Originally Posted by Walt Fricke

Right - I've thought of that. I'm not keen on reducing system pressure - affects the injectors some? And would call for finding a thinner washer, or leaving one out, which might be too much.

Reducing control pressure I can deal with. I modified a previous WUR with two threaded holes, and a sort of raised bottle cap I could use to have the nuts on the two studs or bolts raise or lower the main pressure slug. To lower CP I usually, after loosening the appropriate nuts, used a punch in the longish Allens I had screwed in to tap down for higher WCP

Affecting the pressures here – no matter if control pressure or System pressure – is not the way to solve your problem with correct idle AFR but too low AFR @ WOT.

Because if you rise the Pressure to obtain a higher AFR at WOT ... this automatically will lean out significantly your idle. Because in case of a 930.16 engine the idle and WOT pressures both do stand in direct relation ... as no manifold pressure connected to the WUR affects the control pressure at acceleration. or WOT.

[AndrewCologne]

Since it seems the O2 sensor doesn't affect the WOT 65% duty cycle, am I on track by thinking that - for race track purposes - I won't get better performance by hooking up the O2 sensor? I could do that, since each exhaust bank has a bung, but I'd rather not, especially if it won't give me more power at WOT.

First of all, ... its the biggest myth that the lambda regulation "itself" on the 930.16 engine is responsible for any power loss.
As you said, hitting the gas for WOT makes the throttle switch turning the lambda control off.
So, if your are very into this lambda system, its even a wonderful part of the engine for tweaks & Co.

So what you can do is: Get your lambda control fully working and for the following CO adjustment do NOT unplug the sensor as you should do normally.
Now connect a DMM with duty cycle option or a dell meter to your test port. With connected O2 sensor, engine warm and idling turn the CO screw CCW till the duty cycle on your DMM is at 60% or 54° on a dwell meter.
By doing this you will get a leaner initial mixture which directly affects the AFR at WOT (65% duty cycle).
And .. as the lambda regulation now is doing its job, you won't suffer from a leaned out idle cause the lambda control brings the idle back to stoich! :-) So idle will be ok at 14.7:1 and WOT will be leaner ... here you can play a bit with CO screw for setting up the initial idle duty cycle, maybe istead of 60% actually 55% or even 70% is needed for obtaining a 12.5:1 AFR (or whatever you want) at WOT.
Let me know if this helps

Or ... if you got a wideband sensor controller like the innovate one, you can do the throttle switch relay trick.
LM-1 WOT closed loop trick with freq. valve CIS

Or use the CPU controlled Digi SC ECU I showed you, where you can set up everything for obtaining an on target AFR@WOR with a narrowband sensor, ... Wideband sensors from diff. brands are supported as well.

[tirwin]

Walt,

How long are you observing AFRs in the 10-11 range? I find that even tuning idle AFR in the garage is challenging because of the fluctuation. Blipping the throttle causes the AFR to bounce and then settle. Are you looking at log data or just eyes on the gauge?

If you’re seeing momentary fluctuations in the 10-11 range I think that is different from sustained periods of time in that range.

If log data, what is the sampling rate?

[dhanl82]

I thought that perhaps my experiences might be helpful.
When adjusting my a/f ratio at idle with the O2 sensor unplugged, my AEM fuel gauge hardly fluctuates at all due to the constant open loop 50% duty cycle, with the sensor connected in closed loop,at idle, the gauge fluctuates about .5 percent around 14.7 as it is designed to do as the O2 control unit is constantly reacting to mixture reads from the O2 sensor in its attempt to maintain 14.7
When my throttle exceeds 1/3 and activates open loop via the throttle position switch the mixture switches to around 11.8 and stays there as long as I am past the 1/3 throttle and accelerating or WOT.

[Walt Fricke]

I have recordings from the Innovate LM2. They are very squiggly on the track, even with a fair amount, say 2 seconds, of smoothing. And lots of spikes, which I think happen a) when I hit the brakes, b) when I go from cornering to WOT (lean spikes?), and maybe also c) when I blip the throttle downshifting. Haven't got it quite figured out.

On my Racepak data logging I can figure out where I am on the track by knowing the track - this must be the straight, see these two small dips in the increasing speed trace, etc. I haven't figured out the Innovate's log.

I'll use my "take a photo of what you highlight on the screen" function to make JPGs of a couple of representative ones. Idle (like waiting on the grid) is easy to spot, and is relatively smooth. Katie bar the door when I get on the track.

I can't go to digital etc control - rules don't allow it.

Andrew - my thinking was that if I set CP higher (starting carefully, just moving it to the leaner edge of the band on the graph), I can compensate at idle with the CO/mixture adjustment screw, plus the idle air screw. Because what I am hearing is that at high RPMs (on track typical range is from 4,000 RPM or maybe 5 depending on the track, to 6,700, with upshifts on straights at 6,250 (best fit to the torque curve and gearing). All WOT stuff, well above 30% throttle opening. I just want a stable idle for convenience before and after being on the track.

And I perceive I can get some power gain by keeping the WOT mixture in the 12.5-13.2 range. It for sure isn't there as things are - high 10s, 11s, occasional low 12s (where the trace isn't spiking like crazy).

Since at WOT (and all else above 30%), the O2 sensor contribution to what the FV does is nothing (right?), and the WOT programming built into the system gives a 65% FV modulation.

My second (right side header) bung is buggered - I installed a sensor without anti-seize, and damaged the threads. I'll have to see what I can do about that so I can hook up a narrow band sensor (I have one or two of those hanging around). How important is it to have the factory wiring and plug to hook up to the still factory wiring from the engine to the control units?

[AndrewCologne]

Originally Posted by Walt

Andrew - my thinking was that if I set CP higher (starting carefully, just moving it to the leaner edge of the band on the graph), I can compensate at idle with the CO/mixture adjustment screw, plus the idle air screw.

ok, but the compensation with the CO screw will also affect the WOT.

Since at WOT (and all else above 30%), the O2 sensor contribution to what the FV does is nothing (right?), and the WOT programming built into the system gives a 65% FV modulation.

Yes, the lambda control driven by sensor reading is switched off and a static 65% duty cycle modulation of the FV is given.

With all my respect, but I think with all that procedere above just for trying to achieve a better AFR at WOT, ... at the end you won't feel a significant rise of power.
What has already been done else on your engine? (exhaust system, timing, cams, P/C Replacement ... etc?)

[don gilbert]

For total AFR control, I use the Innovate 02 kit, dumped the wur for a regulator mounted between the seat. Dial in whatever afr I want. In the morn...dial down to 15-20 lbs bust off as soon as you engage the starter...a few blocks up the road...dial up to 55. Need power? Just dial down to 12.8 in my case. The WUR is prehistoric..never in spec and the problem with most driveability issues.