big wheels and O2 sensors - 3.2 Carrera

[philly1263]

will 9 inch wheels fit on the back of a 87-89 non-wide body Carrera and if I bolt on a Euro pre-muffler with no O2 sensor bung and leave my O2 sensor unconnected what are the implications?

[Por_sha911]

There have been many posts on both of those issues in the past. For instance: Carrera Wheel Fitment
The search button is your friend.

[rayjak]

I have 17/9" twists on the back of my 88. Fits fine, looks amazing

[jtratza]

The issue is going to be the O2 sensor or lack off.

[ischmitz]

Why not have a muffler shop put in a port into the exhaust or pre-muffler where you can put the O2 sensor. Having it will make the car run better and more ecconomic even w/o a cat. I would not run it without O2 sensor unless you get yourself the proper chip for a DME without O2-sensor.

Your current DME will move the mixture around at random and it will be off.

Ingo

[jtratza]

Why not get a pre-muffler with an O2 bung? There are lots out there. I've used one for about 10 years with 8 inch wheels. Lots of room and no problems passing smog until they checked for NOx in Houston. Moved to the Texas Hill country and no more problem.

[rusnak]

I have two O2 bungs, one capped off until I want to connect an air fuel tester to check the dme and co% adjustment.
As noted, running with no O2 sensor is a bad idea.

[911st]

I ran my car with a cat bypass, SW chip, and the O2 unpluged to disable the Lambda function.

May or may not effect MPG a small amount. Also may help throttle response a small amount.

Bassicaly it makes it operate like the Euro Carerra.

No issue I could find except when it is smog time.

Also ran 9's w 245's on stock fendered 85 with about -2.5 deg of neg camber on euro look height and stock non rolled fenders with tons of room.

[rusnak]

Originally Posted by 911st

I ran my car with a cat bypass, SW chip, and the O2 unpluged to disable the Lambda function.

May or may not effect MPG a small amount. Also may help throttle response a small amount.

Bassicaly it makes it operate like the Euro Carerra.

No issue I could find except when it is smog time.

Also ran 9's w 245's on stock fendered 85 with about -2.5 deg of neg camber on euro look height and stock non rolled fenders with tons of room.

You probably lost all performance advantage of having the SW chip. The dme would not run in closed loop, which is where all of the advantage lies in having a dme controlled system.

There is NO good reason to not have the O2 sensor connected, and if you have a performance chip, then you are throwing money away by not having it connected and running healthy. It is doubful that SW would find any reason to tamper with the pre-loaded map, except maybe WOT.

[911st]

rusnak,

We will probably have to agree to disagree on this one.

My understanding is unplugging the O2 only takes the Lambda correction away. Unplugging it just moves the O2 input to a range where there is no correction to the part throttle maps settings.

I believe SW makes a lot of improvements with his chips. The basic is more ideal AFR and ignition ratios to both the part throttle and WOT maps. Further the later part throttle map entry's more approach the WOT map's. Unplugging the O2 should not effect these more ideal settings.

However, the cool improvement to his chip is I believe he changes the algorithm that triggers the WOT map acceleration even when acceleration is not enough to trip the TPS WOT switch. That is, the WOT maps can be triggered by the TPS WOT switch OR by the rate of change in AFM values sensed.

None of these improvements should be effected by unplugging the O2.

I was also told by SW that when I dynoed my car to un plug the O2 if I used the stock O2 location for the dyno's Wide Band. To me this tells me it dose not effect his improved WOT programing settings.

Further it was a master Porsche mechanic that unplugged my O2 and told me it had the potential to improve performance a little further. It could be this was just on a stock chip. He also said it would probably effect gas milage. I think his belive is it trigers the more agressive WOT settings.

I think I talked to SW about this and he has no issue with this but am not tottal sure. I know he was offering a chip with the O2 function zeroed out. If someone wants to confirm this with him it would be grate as there is always a chance I could be mistaken.

[ischmitz]

Gents,

I have to respectfully disagree with several statemtents 911st made:

1. Chip tuning & software changes
If you wanted to change the way the DME responds to the WOT switch input it would require extensive re-compiling of source code for the 8051 microcontroller. Unfortunately, none of us mortals has access to that code. So we are left with changing RPM values were something happens (i.e. rev limiter).

Some have disasembled the entire assembler code for the Motronic and gained deep understanding how things are calculated internally but even that only allows relatively crude ways of changing the behaviour of the software. The assembler code is not a simple thing to work on when it comes to software flow. The main program loop basically calculates fuel and timing values. Interupt routines are used to measure the engine speed and the TDC.

The maps that define timing and mixture are nothing but look-up tables. And that is where the chip tuning takes place for the most part. Arguably SW's maps are amongst the best and help improve throttle response and performance. He has accumulated tons of experience and his chips are top notch. But still, the basic code is never changed.

2. O2 sensor
As you might know some 3.2 DME versions came without the O2 sensor function. And the code on the EPROM of those DME's reflects that. So to disable the O2 sensor function you simply start out with another code base and modify its maps. Again the code itself isn't changed because that would be way to complicated.

If you run a DME with O2 sensor code and have the O2 sensor disconnected the mixture can vary up to 35%. I tested that on my DME test station. The code can influence the mixture based on the O2 sensor signal by up to 35%. See here for an example of injector timing at 3000RPM with the O2 sensor signaling full rich or full lean. And once the sensor is disconnected there will be random noise on the O2 sensor input sending the mixture all over the place. Best case it stays in the center but this is not a given and largely depends on what type of EMI noise you have in your engine bay.

The O2 sensing works pretty simple. A comperator checks the voltage of the O2 sensor and compares it to a reference voltage of about 0.5V. It then integrates the times the signal spend above or below with a time constant of about 5 seconds to decide if the engine runs rich or lean. In other words if the O2 sensor value bounces between 0V and 1V nothing happens to the mixture. If the value is constantly at 0V (lean) the mixture starts to get richer. If the sensor output is constantly at 1V (rich) the mixture starts to become leaner. The full swing from one to the other side takes about 5 seconds. See here an example where the similator was used to suggest fully lean and fully rich while all other parameters were kept constant.



And yes, this also happens at idle. The O2 sensor is influencing the idle mixture as well. Later DME's starting with the 964 have the ability to detect a damaged or missing O2 sensor and will automatically assume a standard value. But the 3.2 is pretty dumb in that respect. It doesn't check for plausability of any sensor signals. The code and uC used weren't powerful enough for that at the time.

[911st]

ischmitz

Some cool info here. I love what you are doing. Thanks for the new data points.

It is impressive that the AFR's can be effected 35% in the idle and cruse maps.

I assumed the Lambda control system when to a fixed point when unpluged. I think you are saying this might not be the case.

Again, SW told me to unplug the O2 on my dyno runs when we fit the wide band. I am not sure if this blocks the Lambda function or trigers the WOT map that is not effected by Lambda.

I do not think SW changes the architecture of the chip but that he has identified the cells that effect it's response other than the map cells.

I believe SW has identified the cells or locations used by the O2's and can effectively disable its function if he chooses. If that number is changed to say 0 (maybe it is the other way and a large value), any modifications the O2 might make are mitigated and changes in the O2 voltages no longer effect AFR's.

I recall that when I bought my chip I talked to SW and he was offering a "euro" chip that disabled the O2 function on a non euro Lambda system. He felt it was his best raw performance chip but it had a potential cost on the MPG side.

From talking to SW it is my understanding that not just the TPS switch determine which of the three maps are selected. There is apparently some type of methodology in the system that triggers the WOT map without the WOT switch being engaged. This can most likly be adjusted by changing one of these values. There is also seems to be a location to change for how long it stays in the WOT map once triggered.

Thus, he not only dials in more ideal ignition and fuel settings, he also takes control of the lambda, map selection and other functions with changes to the values used by the system as input to compute control of such functions.

[aston@ultrasw.c]

Originally Posted by philly1263

will 9 inch wheels fit on the back of a 87-89 non-wide body Carrera and if I bolt on a Euro pre-muffler with no O2 sensor bung and leave my O2 sensor unconnected what are the implications?

9" wheels will fit, but only if the offset/backspacing is correct.

[Jeff Alton]

Yes, but a wide range of offsets work on a 9.0 depending on the alignment....

Cheers