02 Sensor function on CIS car?

[sbmackie]

Does the 02 sensor on a CIS car with no cat work in all throttle positions? Is there really anything provable to be gained by unhooking it other then maybe a smoother idle when warm, or maybe reduced "flat spots" (whatever that is)?

I've "heard" you get more power/better throttle response. Anybody dyno'd it to find out?

Thanks
Scott

[VaSteve]

Originally Posted by sbmackie

Does the 02 sensor on a CIS car with no cat work in all throttle positions? Is there really anything provable to be gained by unhooking it other then maybe a smoother idle when warm, or maybe reduced "flat spots" (whatever that is)?

I've "heard" you get more power/better throttle response. Anybody dyno'd it to find out?

Thanks
Scott

Flat spots are typically described as when there just doesn't seem to be power with the application of the big pedal....a little falter.

The O2 sensor's purpose is (my understanding) more a mission of controlling emissions. It monitors the exhaust and tweaks the mix via the frequency valve on the backside of the CIS system. Mix on a CIS system is affected by the control pressure controlled mostly by the WUR. The frequency valve is plumbed into the return line after the fuel distributor. My understanding is that it helps to balance out the mix for emissions by adding or reducing to the control pressure already set by the WUR.

There's a million threads on here about disconnecting it. Mine has been disconnected, but I was advised by a Porsche mechanic to reconnect it.

[Scott R]

Which year is your SC? 80' and up US you want it connected for optimal AFR's

[sbmackie]

It's an 82. The reason I'm asking this question was addressed in another thread re: a hunting idle when warm. Turns out when I unplug the 02 sensor...it doesn't hunt. So, now I'm deciding if I want to leave it unplugged or not.

If I get real ambitious, I'll dyno it both ways with mixture adjusted properly and find out.= if there really is a power gain to offset increased fuel consumption and (alledged) cooler running.

sbm

[psalt]

sbm,

Your engine has a throttle position switch that disregards the O2 sensor after 35% throttle. Anyone that tells you that performance is increased by disconnecting the O2 sensor does not understand how the system works, and does not understand how engines produce power.

[psalt]

The frequency valve is plumbed into the return line after the fuel distributor. My understanding is that it helps to balance out the mix for emissions by adding or reducing to the control pressure already set by the WUR.


This is incorrect, the FV has nothing to do with the control pressure or the WUR. The CIS fuel distributor works on a pressure differential. The FV bleds off lower chamber pressure to effect the delivery of fuel to the injectors. Air flow sensor height is not effected (control pressure). The higher the duty cycle of the FV, the lower the pressure and the richer the mixture. This is why a closed FV, bad fuse, relay or no ECU, causes a lean mixture. However, when you graduate to actually tuning a CIS lambda engine, you have to reverse your thinking. A high closed loop duty cycle means the actual mixture is too lean, and the system is trying to compensate. A low duty cycle means the actual mixture is rich and the system is trying to lean it out. My experience is these cars produce the most output with the O2 sensor connected and the mixture adjusted to a 30-40 closed loop duty cycle.

[VaSteve]

Originally Posted by psalt

The frequency valve is plumbed into the return line after the fuel distributor. My understanding is that it helps to balance out the mix for emissions by adding or reducing to the control pressure already set by the WUR.


This is incorrect, the FV has nothing to do with the control pressure or the WUR. The CIS fuel distributor works on a pressure differential. The FV bleds off lower chamber pressure to effect the delivery of fuel to the injectors. Air flow sensor height is not effected (control pressure). The higher the duty cycle of the FV, the lower the pressure and the richer the mixture. This is why a closed FV, bad fuse, relay or no ECU, causes a lean mixture. However, when you graduate to actually tuning a CIS lambda engine, you have to reverse your thinking. A high closed loop duty cycle means the actual mixture is too lean, and the system is trying to compensate. A low duty cycle means the actual mixture is rich and the system is trying to lean it out. My experience is these cars produce the most output with the O2 sensor connected and the mixture adjusted to a 30-40 closed loop duty cycle.

Ah thanks! I guess that would make sense since it's downstream of the fuel distributor.

The fact that all of fuel delivery issues in this primitive fuel injection system is controlled by upping or lowering pressure, or bleeding off fuel is pretty amazing. Very little computer involvement at all....that computer under the seat only performing one piece of this complex equation, compared to something like motronic and the modern systems.

[sbmackie]

Paul,
I will show this thread to my tech. I will post the results of any dyno testing we do and parameters.
Peace.
sbm

[psalt]

The fact that all of fuel delivery issues in this primitive fuel injection system is controlled by upping or lowering pressure, or bleeding off fuel is pretty amazing. Very little computer involvement at all....that computer under the seat only performing one piece of this complex equation, compared to something like motronic and the modern systems.


Yes, Bosch call the K jet a mechancal injection system. After years of timing the injections, there were lots of skeptics at Bosch who thought a continuous system would never work. CIS lambda is kind of like the compound turbo engines in the last gasp of the piston aircraft engine, when the turbine made more power than the gas generator. The FV is essentially a pulsed injector, and once you have that, why bother with the hydraulics. CIS does have some advantages over EFI, the higher pressure and the ingenious rattle pin, gives better atomization. Some people confuse the injector chatter with pinging, but it is a good thing.