1% CO Setting... Blazing Fast!

[stlrj]

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

[wckrause]

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

[stlrj]

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).]

[wckrause]

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

[stlrj]

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).]

[fred cook]

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".

[wckrause]

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

[fred cook]

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!

[stlrj]

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

[fred cook]

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.

[fred cook]

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.

[Lee]

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.

[kamikazepilot]

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

[stlrj]

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

[stlrj]

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