Thanks.

My car and many others without the O2 sensor run just as well (if not better) than those cars that do use the O2 sensor.

Therefore I suppose the real issue comes down to engine longevity.

At the risk of stirring up a hornet's nest, I have to ask how you come to this conclusion? You've actually compared them?

People tend to lump all CIS cars into one category. If you dig a little deeper, you'll find that Porsche changed something just about every year. Even the O2 sensor-equipped cars had differences. An '80 SC and an "83 SC aren't the same thing. Toss in the O2 sensor-equipped Carreras and it's hard to discuss the relative merits of having an O2 sensor hooked up, or not. Too many variables to make a blanket statement, in my opinion.

I'd wager that 95% of the miles that are driven in a 911 are relatively low rpm, constant, part-throttle cruising. Why would not having a stoichiometric mixture be a bad thing, here? Do you think that Porsche forgot that a little richer mixture might be a good thing for wide-open-throttle running? Did they not allow for that?

I really think the average guy might be better off leaving the system intact and concentrating their attention on making sure everything is functioning correctly.

Just my .02 and change...

JR

Ya. It is a hornet's nest indeed.

My statement was solely based on my experience with my car. And I don't know that Porsche always had "best performance" in mind when it came to the US version of SCs. Just saying - they probably tuned these things for daily driving and not track/performance use.

However, I absolutely believe that it is best to use it on a regular basis. Like you said, most of the time we are not running WOT. I plan on hooking mine up as soon as I can get my hand on an AFR meter.

on an aftermarket exhaust system running two separate exhaust paths, one out of each header, and two mufflers, does having the o2 sensor reading from just one of the exhausts mess up the fuel ratio? Thanks
Scott

Don,
can you help me with wiring this into the car? I have the control unit and relay mounted under the passenger seat.
There are two long wires...one red and one red/white.....these go to the front fusebox according to Bentley.
And in the rear, there is a large-gauge green wire with a half-broken connector....this must connect to the O2 sensor.

this sound correct?

I disconnected my O2 during the summer and went for a drive, I personally thought my car felt easier to drive at slower speeds and on the highway I could notice a nice difference. That day I hit some fast back roads averaging about 120 + mph and my car felt great. On the flip side to having it disconnected, when I would come to a stop, my engine will continue to idle hi. My adjustment screw was as far as it would go-I suspect I have been running rich and didn't attempt to fix the mixture. I therefore plugged the O2 back in. I actually liked how it ran without it but my car drives great with it plugged in as well…

the solid red goes to 12v constant, the red w/white trace goes to switched 12v. (both to the front fuse box) in the rear, you should have a gang connector for the engine harness, a three wire plug with a red to swithed 12 (if you want, that rear red one is to power up the factory diog. tool) and the shielded green one is your 02 wire. If you think your car runs better without 02, something was been adjusted at one time or another to compensate. Most people forget that back in the 80's the 02 recomended replacement was ever 30k, and most have never been done. you can get a bosch one wire at oreillys for 20 bucks. the older and more covered with soot they get, the slower they work esp. if your mix has been a little rich. this is easly seen on a dwell meter.

Hi,
I was always trying to have my 81 911sc to run correctly and was never able to get good idle, good acceleration and smooth driving all at the same time until I fixed the timing and mixture properly.
I replaced the distributor because the other one was not advancing properly. If yours is ok, then you save 1500$.
I set the timing at 5 degrees BTDC at idle with both advance and retard hoses disconnected and plugged (golf T). I then plugged everything back in except for the ox sensor.
Make sure your relay works well under the passenger seat and that your fuel pump relay has good connections (ask me how I know)
Make sure the car is at operating temperature and put a voltmeter on the ox sensor lead and ground and check for the voltage output to oscillate around 4.5volts at a 900 rpm idle (that will give you proper mixture at idle without needing a gas analyser).
Once this is done, plug it back in and set the idle speed again if it moved. That gave me perfectly smooth acceleration and modern car like driveability around town. No more flat spots, no more hesitations under light throttle at lower rpm, a smooth running engine.
If your ox sensor is old replace it. If it is fairly new but the car has been running rich for a while, you might have to reset your mixture after a bit of driving as running it leaner now will clean it a bit.
Good luck and it is worth to plug that ox sensor back. Older cars were supposed to run without it and had a few more CIS components to adjust for partial throttle, the 81-83 CIS should run with the ox sensor plugged. Better driveability and better gas mileage.

I've been reading up a bit more on CIS. And here is a great quote:

"Fuel delivery to an engine is always a compromise. The factors which must be weighted against one another include power, starting, driveability, fuel economy, initial cost, maintenance and exhaust emissions."

My guess is that the USA tuned (think "de-tuned") SCs were meant to meet all of the above factors (daily driving reliable sports cars). So if someone is looking for better performance they may sacrifice other factors such as fuel economy and exhaust emissions.

There are two other changes to the SCs from 1978 - 1983 that are important to note.

1) Ignition Timing: Specs for 78-79 total advance is 26 degrees while the specs for the 80-83 is 21 degrees at 6000 rpms. Why do later SCs have less total advance? Can't be due to compression because 1980 has the same as 78 and 79. Could it be due to the emissions added? Oxygen sensors, vacuum retard, etc?

2) Warm Up Regulator: Earlier SC WURs had vacuum controlled full load enrichment while later SC WURs didn't (just had atmosphere hose). So do earlier SCs show richer mixtures at WOT due to the vacuum controlled full load enrichment? (don't know the answer).

I do know that the richer my mixture is at idle, the richer it is at WOT. To get my ideal mixture at WOT I have to set my idle mixture at 3% CO. Connecting the O2 sensor makes for a tough proposition in this case - a sensor can correct mixture by +- 20%. At 3% CO that puts me right at that -20% mark - causing a constant mixture correction and rough feel at constant acceleration. I'm making more power at the expense of driveability.

I'm just trying to sift through the information like everyone else. I keep finding that I have more questions than answers. I know that this is a bit off topic and may be suitable as a new thread. Sorry for the long-winded post.

the best way to "sift" is to think about when the system uses info from the O2 sensor and what it does with it

I don't know exactly for your car - or any 911 CIS motor, but that is the way to approach it.

I'm just trying to sift through the information like everyone else. I keep finding that I have more questions than answers. I know that this is a bit off topic and may be suitable as a new thread. Sorry for the long-winded post.


My advice would be to read the Bosch manual on mixture preparation and CIS lambda, you are missing the main points. The O2 sensor is irrelevent to performance. The CIS lambda 911 has a throttle position switch that ignores the sensor past 35% throttle. If you think you feel more performance without the sensor, it is only your overactive imagination. The sensor only makes minor trim adjustments at idle and part throttle cruise for emissions and converter life, and no engine management system makes 20% mixture adjustments to WOT under load. If you measure the AFR of a stock 911SC under WOT, you would know already that it is no where near 14.7. The O2 sensor allows you to tune for WOT mixture, and still have the benefit of stoich at cruise and idle and converter life. Running a cat with a disconnected O2 sensor is foolish. There is a substantial difference between an adjustment at idle and at WOT, think about the percentage difference in air mass.

The 80-83 cars have the same timing spec and the reason the timing is severely retarded is the higher compression. You need to understand the history of the engine changes and the factory goals during the period to understand the reasons. The 911 is a knock limited design, the unleaded 87 CLC fuel mandate required a drop in compression and/or timing to protect the engine. Porsche first chose to drop compression, but the drop in mpg was too much during the fuel embargo period, they changed direction and raised compression and retarded timing so they could advertise improved mpg with every change during the period.

CIS lambda does not use a vacuum WUR, enrichment changes can be made through the FV with much more accuracy.

Paul - please carefully read my comments again.

NEVER did I say the sensor had an impact on performance. It has an impact on driveability in my opinion. I want my mixture richer than stock settings. The O2 sensor tries to correct the mixture at idle, constant acceleration and part throttle which can be felt while driving. This isn't a deal breaker - just an annoyance in my opinion.

I also understand that at WOT (or beyond 35 degrees) the O2 sensor has no impact. Maybe I should have been more clear - but I thought everyone knew that already.


Don't have a cat. And I agree with that statement.

NOT TRUE! The 1980 engine has the same compression as the 78-79 according to page 100-1 of the Bentley Manual. So why the change in timing specs?


EDIT: THE 1980 ENGINE DOES HAVE SAME COMPRESSION RATIO AS 1981-1983 - The Bentley is wrong (thanks javadog!).

Well, for starters, you can throw the Bentley manual in the trash. It's riddled with mistakes.

Porsche upped the compression ratio on the 1980 US cars to 9.3:1. They also advanced the cam timing by 6 degrees. The warm up regulators changed, which altered the control pressure... the cylinder heads were different... in short, Porsche made a whole raft of changes.

If you want to learn the nuances of these cars, throw away your Bentley, quit reading the generic CIS books and get a set of the factory manuals. Then, track down the service training booklets that Porsche published every year, identifying all of the changes made from one year to the next. Those books are a gold mine of information.

JR

First, I'm not sure about the SC cars.

But I do know the 84-89 motronic system very well as I have dis-assembled all the program code in this system. The motronic only uses the O2 sensor to fine tune the mixture to a target of 14.7AFR during idle and Part throttle. The O2 is basically ignored at WOT and during acceleration enrichment.

The Euro motronic uses the same exact program code but the motronic box simply lacks the O2 sensor circuitry. This means the Euro cars run just like a USA car with the O2 unplugged. I have a USA DME and run with the O2 sensor unplugged.

Most O2 sensor circuits are simply designed to help adjust a mixture that already in the ballpark of 14.0AFR to 15.5AFR so that the target is 14.7AFR. In general if you unlpug your O2 sensor and you car runs very poorly it's a red-flag that something is wrong.

Try to answer this: do the Euro SC cars have an O2 sensors? my guess would be they do not - and if not how does the Euro CIS system vary from the US setup? This may help answer your question.

No, they do not.

Basically, all of the major parts are different. Fuel distributor, warm-up regulator, etc. Then, the distibutors are different, often the cam timing was different...

Keep in mind there is not one single version of the US and ROW engines. In fact, there are 4 or 5 of each. Documenting the changes from one to another would be a huge amount of work.

There's no question that you could change a US engine to be more like a ROW engine. It would require changing a lot of components, not just simply unplugging the O2 sensor. That's the fallacy here. People focus on the O2 sensor, as if it is the root of all evil. It's not. The changes that we are talking about here make more of a difference in driveability than outright power. To get the power of a ROW, you make a lot of changes, not all of which have to do with the CIS.

JR

hey craig,

you might check to see if the ignition timing changes followed the cam timing changes. they moved them around a good bit, then settled on someting in the middle.

i too would be curious of the mixture at WOT for a lambda car. mine, with a vac WUR, is in the upper 12's.

craig, go get the LM2 from tim if you want to check WOT. i need it soon though, my BMW's mixture is way off. just not sure when i can get down there to get it.

nicely put paul.

JR - thanks for that info. Freaking Bentley. That has puzzled me for a couple of years now. I've posed that question (about timing specs from 78-79 vs 80-83) many times here and nobody could clarify. I REALLY appreciate it.

Let me know if you have sources for better books.

I texted Tim last night to see if I could use the LM2 this weekend.

NEVER did I say the sensor had an impact on performance. It has an impact on driveability in my opinion. I want my mixture richer than stock settings. The O2 sensor tries to correct the mixture at idle, constant acceleration and part throttle which can be felt while driving. This isn't a deal breaker - just an annoyance in my opinion.

mca,

Constant acceleration ? Think about that for a minute........

My point is that it is better to read an accurate reference than to continue shopping for answers that agree with your misconceptions and erroneous facts. I think you have asked the same question 20 times on this list and continue to ignore the answers, but that is just my opinion.

The compression change was made in 1980 and coincides with the timing change. The CIS lambda system was changed more than once and all the changes were driven by emissions testing and the subsequent driveability issues. My advice is stick to the Bosch and factory manuals, the Bentley has many errors, especially about CIS lambda.

All beginners start learning about engines doing tune ups, adjusting idle settings and playing with an unloaded engine. Unfortunately, years of this behavior creates many misconceptions and false conclusions that are hard to overcome. The fact is that idle settings are largely irrelevent and have little to do with what actually happens under load. Most OEM's make full load WOT mixture settings richer than ideal for many reasons and the "richer makes more power" statement is largely false in practice. Most of the information in internet posts and in magazines is misleading and useless. If you actually want to learn about this subject, start with the Bosch manuals, the evolution of engine management systems teaches a lot about the problems and issues involves. More involved IC texts are available if you want to progress beyond the basics.

Look on this site for the books that are still available new, then look here (in the classifieds) or on eBay for the ones that are no longer available.

I'd suggest buying the factory manuals, the Technical Specifications booklets and the Service Training guides, for whatever car you have.

JR