911SC air to fuel ratio question

[myink]

Hello,

I have a 1979 911SC with SSI heat exchangers and a performance muffler. The exhaust modifications should allow more air to flow in to my engine. This means my engine would need more fuel to burn at the proper air to fuel ratio.

Is my car (with no O2 sensor) smart enough to know that there is more air coming in and inject the additional fuel needed to maintain the ideal air to fuel ratio for combustion?

I am thinking not and would like to know the best way to richen up the fuel mixture. Is it as simple as just turning the air/fuel ratio screw or are other adjustments needed?

Any input would be appreciated.

myink

[Targalid]

That's a good question. The fuel metering system on the CIS that you have is dependent upon airflow. The more air flowing past the air metering plate, the more fuel is released by the fuel distributor. You should probably read up on CIS to get a better feel for how the system works but basically I think you should just drive the car and enjoy the sound of your new exhaust.
Bosch - fuel injection systems
BOSCH CONTINUOUS INJECTION SYSTEMS

[jeffs9146]

On my 83sc I added a headder and two in two out exhaust, it runs great and sounds even better!!

[don gilbert]

check into an adjustable wur for afr mods.

[Peter Zimmermann]

myink; I think that you're making assumptions that don't fit the situation. You want to find someone with an exhaust gas analyzer and set your idle CO% at 2.5-3%. You will then have to plan to prepare the car to pass its bi-annual smog test, which will require a catalytic converter and probably a smog pump. I think that you should talk with Marc at Red Line Service, 310-280-0700. He's on Livonia just off the 10 at Robertson.

[Ramathorn]

being ssi's and a sport muffler (all exhaust parts) is allowing your engine to exhale (meaning air out of your engine). Doing intake related parts will allow your car inhale (Air into your engine).

[myink]

Thanks for all of the input.

Sounds like adjustments need to made, preferably with something that can measure my exhaust gases. Which confirms that CIS is not smart enough to make fuel adjustments for the new heat exchangers and mufler.

myink

[stlrj]

Originally Posted by myink

Sounds like adjustments need to made ...

myink

CIS, like any modern fuel injection system is designed to compensate for any additional air as easily as stepping on the throttle when introducing more air into the engine, the correct proportion of fuel is also added.

[T77911S]

improving exhaust flow can lean out the upper RPM's. i would connect something like an LM2 that can record or put it on a dyno that can check the AFR's across the rpm range.

with the lambda system, the O2 is not used past 3k roughly. from there on up, it is strictly based on WUR control pressure. BUT, you cant just lower the CP to make it richer. there is a balance for open loop and closed loop that the WUR CP has to be set at. IE, richen the upper RPM's and the O2 will never get the mixture riight at lower RPM's.
the same thing for just turning the mixture screw, richen it too much and the O2 cant lean it out enough at idle. this is a very basic explanation.

[schumicat]

you should learn more about CIS before making that statement. there are explanations of it on youtube and online. it is a pretty freaking amazing system (when it is in good shape and working properly).

Originally Posted by myink

Thanks for all of the input.

Sounds like adjustments need to made, preferably with something that can measure my exhaust gases. Which confirms that CIS is not smart enough to make fuel adjustments for the new heat exchangers and mufler.

myink

[Bob Kontak]

Originally Posted by myink

Which confirms that CIS is not smart enough to make fuel adjustments for the new heat exchangers and mufler.

See shumicat's post above.

[moneymanager]

The CIS will adapt. You need do nothing extraordinary.