1% CO Setting... Blazing Fast!
 

Hello fellow Porsche owners... There had been some confusion as to why my 81 SC goes from fast to slow and back again every time some other shop gets their hands on it.

Well, after a lot of input from this BBS, I had heard about a lot of 3.5% CO mixtures being set on the SC. Well, after going back to the one Porsche Technician at the Porsche dealership (a friend is the manager... I get "friend" rates! :-))who did the best and fastest engine setup... come to find out that my CO was not at 2.5% like I thought, but rather closer to 3.7% Anyhow, as I had thought, leaner is better for power, I was confused about whether or not to tell them to keep it rich at 3.5%... They came right out and said 1% is where it should be! The end result, the car is a totally different car! It is blazing fast compared to what it was set at before.

It's funny, I had it set to 2.5% and checked at 2 different shops (one said 2.5, the other 2.3)! How could they both be off by so much and Porsche comes in at 3.7%??? Someone is way off on calibration... My guess is that Porsche checks more often than the shop who uncovered their machines just for me!

Think about this:

Leaner has got to be better for performance. For example, low temp thermostats seen on a lot of performance cars from my understanding keeps the car cooler, and leaner for more power. Why wouldn't the same theory apply to the SC???

Thanks.

Cy
81 SC

The ideal mixture is 14:1 air/fuel (mass). Being on the leaner side results in higher efficiency, on the rich side results in better performance. Running leaner also results in running hotter. Can't explain why it seems you're experiencing the opposite, unless you were running so rich that your plugs were fouling.

Look at figure 2. from this link from a Bosch manual. Max power is on the rich side of 14:1.

http://persweb.direct.ca/aschwenk/page3.gif

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Bill Krause
'79 911SC Euro

[This message has been edited by wckrause (edited 06-04-2001).]

[This message has been edited by wckrause (edited 06-04-2001).]

Wasn't there a thread recently that detailed how, if you do not remove the Oxy sensor or something, might cause a reading like 1% where in fact it is higher, or at least different?

This setting seems low to me. I can't quite understand how all the conventional wisdom seems lost where a 1% setting would cause the Pcar to rocket. If this were true, then...

Jw

I wonder if the other shops were testing it at the tailpipe, and the dealer used the test port before the cat.

Tom

My CAT lost its guts this weekend (for track use only, of course) and I like the sound better. It also seems to have a little more power, though that's as subjective, and suspect a conclusion as you will ever see.

Lean mixures make engines run hotter. In fact, lean mixtures can burn exhaust valves. Usually, if your mixture is too lean, the engine will POP during deceleration.

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'83 SC

So is it just a crap shoot to find the setting that works best for our individual cars?? I converted to the pre 73 header setup and I think there are more ponies to be harnessed.
Pete

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'77 Carrera3.0 www.geocities.com/pcafaro2000/carrera30.html

By the way... I am running without a Cat, and without an O2 sensor. I'm worried now that I may be too lean! And may cause damage to my exhaust valves!!! What the heck does Porsche call for on these engines??? The dealer tech. says 1%

So what is the truth to all of this? Lean is better or is rich better for performance?
I hear both sides of that!!!

Mystery to me.

As far as the blazing fast thing goes... it may be a bit exaggerated. I'm saying for those who run too rich, the difference is noticeable for sure if they lean it out.

Cy

I would steer clear of "lean".

Cy,

Finally there is someone else out there that has seen the light!

The 3.2 Carrera also responds very well to lean; made mine much quicker and with more torque, feeling a lot like a 3.6 now. I know because I just tested a 90 C4 on Sunday and my 3.2 feels just as fast.

I think there was a discussion in and earlier post of someone else leaning out his air flow meter about ten turns and increasing his performance too.

Don't have to worry about burning your valves either because it just runs cooler just like you experienced. In fact, I'm even running hotter plugs because it runs so cool.

In case you're wondering, rich mixtures have a nasty habit of wearing valve guides prematurely, toasting your cat., prevents your rings from seating and overheating you engine$$$$$$ not to mention what it does for gas mileage...

Cheers,

Joe Garcia
86 Carrera

[This message has been edited by stlrj (edited 06-04-2001).]

[This message has been edited by stlrj (edited 06-04-2001).]

Please look at this link, and read it.

http://persweb.direct.ca/aschwenk/page3.gif

There is no mystery, that max. performance is achieved with a mixture that is richer than 14:1

It is possible that you were running extremely rich (far left of figure 2.) and that by making the mixture less rich, it increased performance.

Bill,

What you may not realize that no matter how lean you try to set you injection, it will always slam full rich at wide open throttle and ignore the 02 sensor signal keeping your mixture rich; well below 14:1 no matter what you do...OK?

I do believe that by leaning it out we are moving our mixture slightly right of Pemax in fig 2. and if eveyone else is slightly left of that point it could explain the lack of performance at that level; on throttle and WOT enrichment would move you left of Pemax and result in poor performance.

If, on the other hand, your mixture is set slightly right of Pemax then on throttle and WOT enrichment would instantly move you to to Pemax which would result in increased performance.

Joe

[This message has been edited by stlrj (edited 06-04-2001).]

[This message has been edited by stlrj (edited 06-04-2001).]

Joe- I know how that is done in a CIS system, by the vacuum connection to the WUR.

What you would want to happen, is to have the mixture at full throttle be at that peak point of Figure 2. on my previously mentioned link. The net result is during idle, and light throttle the mixture is slightly lean (best economy), but at full throttle the mixture is slightly rich (best performance).

My point is, lean does not mean best performance.

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Bill Krause
'79 911SC Euro

Joe - is that true? That no matter your setting, it is full rich wide open? So, basically, the setting only affects idle???

Cy

[This message has been edited by stlrj (edited 06-04-2001).][/B][/QUOTE]

I don't understand the term "full rich". A CIS system get's richer at full throttle. It's a function of the Warm Up regulator.

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Bill Krause
'79 911SC Euro

Joe - Ok, sorry, no such thing as "full rich"... I simply meant gets "rich" at full throttle :-)

So, it will really only affect idle. Also, I find the engine sounds a bit different (in the valve train perhaps) when leaned out... and at idle. I notice a very slight "chirping" when leaned and at idle and as soon as I begin to accelerate.

Not just at idle. As you open the throttle, the WUR gets more vacuum, which causes it to enrichen the mix. It's not an ON/OFF function. I think on some CIS systems with Lambda there is a full throttle switch that will enrichen the mixture further at full throttle, but I'm not sure about that operation.

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Bill Krause
'79 911SC Euro

AAAAAAAAHHHHHHHHHHH!!!!!!!!
http://www.pelicanparts.com/ultimate/confused.gif

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'77 Carrera3.0 www.geocities.com/pcafaro2000/carrera30.html

Joe - it makes sense. Thanks. By the way, no cat, no smog, no O2 sensor, or any other emissions stuff on my car. I have B&B headers and exhaust too. I do know from a different post that the lambda system under the seat does something (even without the O2 sensor in place) as temporarily disconnecting the relay there does make the car idle different.

I take it that your euro is setup like mine except for the larger intake runners and euro fuel distibutor... and no restrictive emissions crap.

Cy

Here's another finding I got a lot of criticism for... throttle linkage and leverage.

OK, my car was taken in for having only 1/2 throttle when the pedal was pushed to the floor. The porsche tech pulled the pedal far back, actually leaning slightly towards the driver's seat. I know it is said that will cause the linkage to bend. But here's what I noticed; that when the pedal was pushed it was achieving full throttle midway... at the rear of the car the linkage would stop at the throttle stop but the the lower section kept going and the spring just opened up until my pedal hit the floor.

The end result: was better leverage and hence, the quicker snap I felt with the car taking off and right up until 5th gear. The better leverage translated into quicker "flooring" response and quciker response overall. I swear it will amaze you!!! Pure physics here. I would try it if you never have (take the risk of bending linkage in your own hands). Unfortunately, my linkage did end up taking up slack again, and now full throttle is on the floor. I want to adjust it again making sure the nuts are tighter on the rods.

Just my finding and thoughts.

Cy

AAarrrrrggghhhhh.

Okay, here's the deal. the first generation of CIS (K-basic) is a purely 'mechanical' system. Incoming air moves the sensor plate and this mechanically sends gas to the injectors to match the air volume. It does not 'richen' the mixture, it just sends more gas because there is more air. On these systems several things contribute to the mixture, only one of which is adjustable. Unless you have an adjustable WUR. As most WURs are not adjustable, this and the fuel distributor are not adjustable...they are only working or not working properly.

The mixture control screw is between the fuel distributor and the air boot, as most of you know. This should adjust the mixture throughout the entire range of operation. If you get this adjusted too far to one side or another, there will be a flat spot, or poor running spot at some RPM.

the lambda systems simply add an O2 sensor, and what this does is to reset the mixture to correct spec even if it is not adjusted properly. Turning the adjusting screw without unplugging the O2 sensor will nto work because the frequancy valve will adjust re-acjust eh mixture, at the O2 sensor's command.

Both these systems, particularly the lambda systems, cause a car to run leaner than it really should. This means hotter and is a threat to your exhaust valves. If you richen the mixture, the O2 system will still keep your mixture lean. If you do this and unplug your O2 sensor I believe you will burn your CAT because the CAT will have too mcu gas to burn. If you don't have a CAT, unplug your O2 sensor and richen the mixture a little compared to spec.

Ont he K-Lambda systems, the O2 sensor is shut off during full throttle, to protect the engine from the pinging that will kill it during open throttle maneuvers.

Oh, and they key to power and performance is not 'richness' or 'leanness.' It is just an ideal mixture.

White spark plug electrodes mean the mixture is very lean. This is how they'll run when adjusted to spec. This setting does not kill CATs. The best setting for yoru motor is when the spark plugs are a tan color, but this is too rich for your O2 sensor and CAT.

I guess I had a little technical diarrhea here. Sorry.

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'83 SC

Bill,

I think the word lean, the way I use it, is a relative term relating to where I am in relation to Pemax and not necessarily in the 14:1 and beyond range.

I realize this discussion is based on CIS and I hope I am not confusing everyone when I talk about the DME system on my 3.2 Carrera which is completely stock including 02 sensor, Cat. etc.

Joe

Joe -- OK, when I say lean, I mean greater than 14:1, when I say leaner, I mean not as rich. I think we are in agreement as to which side of the 14:1 ratio to find best performance.

Superman - A wide open throttle WILL enrichen the mixture. There is a vacuum connection to the WUR that causes it to decrease control pressure to the fuel distributor. This results in the sensor plate rising higher for the same volume of air and thus a richer mixture. This is called Full Throttle Enrichment and is describe in all Bosch books on K-jetronic.

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Bill Krause
'79 911SC Euro

And don't forget how much enrichment you get due to contour of the air flow meter where the sensor plate sits. The steeper the angle the higher the plate is displaced upwards for a given amount of air adding more fuel and enrichment due to overswing.

So with all this enrichment going on, it would not surprise me that many cars end up with mixtures to the left of Pemax on chart #2 and actually loosing power.

It would seem that the best mixture to have would be somewhere to the right of Pemax so when enrichment does occur, maximum power would be generated instead of being reduced by richer mixtures that start at Pemax or slighly left that would result in a downward curve of the power band.

Joe

[This message has been edited by stlrj (edited 06-04-2001).]

[This message has been edited by stlrj (edited 06-05-2001).]

Joe,

I agree. The question then is, what is the correct CO level for a warm engine at idle. I would think that the factory spec would be slightly on the leaner side, because they were concerned about passing emissions, but I don't know.

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Bill Krause
'79 911SC Euro

Bill,

I guess the factory engineers must have known what they were doing when they purposely set the idle lean for emissions.

What scares the heck out of everyone is the assumption that lean at idle means lean all the time without any consideration of all the enrichment strategies that are built into our injection systems.

Ideally, the only way to dial in the setting for best performance is if you could manually adjust the mixture while you drive so you can stay within the window where Pemax is and slightly to the right of the power curve; so that the instant enrichment occurs it moves your mixture to Pemax.

I think this is what Cy and I have discovered and it all seems to make sense now thanks to your air fuel mixture chart.

Joe

[This message has been edited by stlrj (edited 06-05-2001).]

[This message has been edited by stlrj (edited 06-05-2001).]

OK, here is my 2 cents worth! When setting the mixture on a CIS system, what you are actually doing is changing the position relationship between the arm the airplate is mounted on and the lever that actually moves the piston in the fuel distributor. By turning the screw counter clockwise, the piston lever is moved closer to the airplate arm. In doing so, it not only reduces the mixture richness at idol, but also lowers the maximum richness that the fuel distributor can attain at full throttle because the fuel distributor piston can't be lifted as much. CO percent settings while a good measure of relative richness or leanness, (that is, greater than or less than the ideal 14.1 to 1 ration), in reality each engine probably has a "sweet spot" where it will obtain optimum performance. The engine in my 1980 SC is currently set at 2% CO at idle, and when revved to 3,000 rpm
the C)% increases to about 3.5%. The 2% CO at idle is approximatly equal to an air - fuel ratio of 13.8 - 1, so my engine is running slightly "rich". At the 3.5% reading, the a/f ratio changes to about %13.5 to 1, so I don't worry about going lean and burning valves. By the way, the acceptable CO% for my car in Georgia is 2.19% (at idle) so even with the O2 sensor disconnected, it is still "legal".

fred -- I'm curious as to how you figure out the mixture ratio from the CO%. I've been looking for that info.

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Bill Krause
'79 911SC Euro

Bill,

The %CO vs A/F ratio is not a big mystery.
I have seen the charts giving the relationship in several carburetor "how to" books. The chart looks something like this:
%CO A/F
.1 14.71
.5 14.27
1.0 14.10
1.5 13.93
2.0 13.76
2.5 13.55
3.0 13.37
3.5 13.19
4.0 12.99
5.0 12.63
6.0 12.24
You can take these equivalents and estimate
numbers in between them. The perfect A/F ratio (stochiometric) is about 14.8 to one. At this A/F ratio the CO would be 0%. However, due to losses and less than perfect engineering, this is usually too lean in the real world. On the other end of the chart, an A/F ratio much past about 12.5/1 is too rich. Anyway, apparently somewhere between 14.1/1 and about 13.0/1 is a good working range for most Otto (4)cycle engines. At idle the 14.1/1 will give better fuel economy but under a heavy load, a richer mixture provides more power so a ratio closer to 13.0/1 is probably desireable. The extra fuel not only makes more horsepower, but will help keep the combustion chamber cool. Hope this helps. I only recently (jabout 3 months ago) sold my 924S and bought a 1980 911SC so I am still trying to learn as much as possible about the CIS system. Personally, I think it is a very clever mechanical fuel injection system. By the way, my car gets about 17-18 mpg in town with the CO set at 2% (at idle) and the top end power is good for a 3.0 liter engine. Of course, it IS a Porsche 3.0!

I'm confused. How can you generate HP if the combustion chamber is cooler. I always thought that the expansion of the combustion gases inside the combustion chamber was due to the heat generated in the combustion process.

The expansion of the gases due to heat is what pushes the pistons down...right?

So cooler combustion temperatures is not what you want if your looking for power is it? Less heat, means less expansion equals less piston pushing power in my book.

Joe

Joe,

In reference to "cooler" combustion temps, I don't mean turning the combustion chamber into a freezer, just keeping it below the meltdown point. Yes, you are correct in that the 4 cycle internal combustion engine makes power by burning the fuel/air mixture, and heat is part of the equation, but if the A/F
mixture is too rich (too much fuel), you will actually make less horsepower. That is one reason that the most modern engine control systems are able to get gobs of horsepower out of small displacements. The computers are able to maintain a nearly perfect air - fuel ratio which not only makes
horsepower, but improves fuel mileage. The result of combustion is carbon monoxide (co),carbon dioxide (co2), nitrogen oxides (nox) and hydrocarbons (hc). In a "perfect" engine the air to fuel mixture could be set so that the carbon monoxide (co) would be 0%. However, as is said before, that doesn't work in the real world, so compromises must be made. It has been found through a great deal of experimentation that a combination of good power and good running characteristics can be obtained with air - fuel ratios of about 14.1 for low power requirements and about 13.0-13.6 or so for high power situations with the "adjustments" being handled by the carburetor(s) or the fuel injection system. Hope this helps.

All the above!

I just found some interesting data in the small Porsche 911SC Technical Specification
book. The factory setting for CO in a 1978-79 911SC is 1.5% - 3.5% (air pump disconnected), and 0.4% to 0.8% (measured in front of cat. conv. with O2 sensor disconnected) for 1980 and 81 models. This and a lot more injection tuning data can be found on pages 54 - 59 in this book. Don't know who might still have these, but mine came from *********** (ask for George) at
(770)427-2844. I think these books are discontinued so if you wan't one, don't wait too long.

I've been wondering about this lately. I have a '78 SC, no cat or air pump...oxygen sensor? I did a major tune up, a month back, and the plugs I removed (Bosch Platinum) were uniformly carbon caked. On start up, I get blue smoke that smells of fuel...could get better mileage. I'm running too rich, no?

I know where to put the allen wrench to adjust, but how do I go about getting the adjustment at least close to optimal, except by taking it to a Porsche dealer and having them do it? (Closest is ninety minutes away and expensive!!!) Is there an affordable tool for this? And I wonder if maybe I should be running hotter plugs than the NKGs I put in.

Hey Fred,

I just picked that book up, it is out of print now....

Randy

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Friends don't let friends drive RICE!

1978 911 SC

7.3 What is the effect of changing the air-fuel ratio?

Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel
ratio, but was very engine specific. Modern engines, with engine management
systems, now have their maximum octane requirement near to 14.5:1. For a
given engine using gasoline, the relationship between thermal efficiency,
air-fuel ratio, and power is complex. Stoichiometric combustion ( air-fuel
ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum
power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal
efficiency - which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio
is controlled at part throttle by a closed loop system using the oxygen sensor
in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio
is used during full throttle operation to reduce knocking while providing
better driveability [38]. An average increase of 2 (R+M)/2 ON is required
for each 1.0 increase (leaning) of the air-fuel ratio [111]. If the mixture
is weakened, the flame speed is reduced, consequently less heat is converted
to mechanical energy, leaving heat in the cylinder walls and head,
potentially inducing knock. It is possible to weaken the mixture sufficiently
that the flame is still present when the inlet valve opens again, resulting
in backfiring.

http://www.faqs.org/faqs/autos/gasoline-faq/part3/section-1.html

Joe