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turbo ride 01-25-2013 12:22 PM

Air pump - affect AFR readings?
Hey guys-
So my WUR (964T 3.3) is off to BL to get rebuilt and made adjustable. I'm going to increase my boost - no more than 1 bar fwiw. While the WUR is away I'm installing a boost gauge and AFR meter.

Here's my question: Does anybody know how much the air pump affects the AFR readings (since it pumps raw air into the exhaust)? It may not be much but I'm thinking the AFR meter must read leaner than what the combustion chamber is actually seeing?


stevie 77 930 01-25-2013 03:03 PM

On my 77, i set my idle mixture at 13.5 with the air pump disconnected per the factory manual. When I connect the air pump, my afr s will read 14.1-14.3

timspu 01-26-2013 12:06 PM

On my install , When wide open throttle it did not have any effect on AFR' s . At Part throttle and idle it will indicate lean, With my idle set at 13.4 with air pump disconnected this then reads anywhere from 17.5 to 19 AFR with pump connected. When on boost it reads 10.8. The best bet is to verify with punp connec0ted and disconnected you can then confirm for yourself.

I have a 930 all standard apart from BL WUR which is not yet set up.



Speedy Squirrel 01-26-2013 12:47 PM

You are both correct. The air port injection shuts off above a certain rpm, above a certain throttle position. The air to the catalyst is shut off above a certain oil temp.

turbo ride 04-07-2013 07:28 PM

Hey Guys-
Still a little confused about the air pump - (I do plan to keep the system in place FWIW - 964 Turbo) - I'm NOT not worried about how it will affect readings on the AFR meter - what I don't get is doesn't it affect what the normal O2 sensor sees?? - cause the lambda system to think it is running lean and try to correct?

timspu 04-08-2013 07:06 AM

My car is a little different as i don"t have a Lambda sensor.
My understanding is that if you set the AFR with air pump disconnected this will set your base line CO level. When the air pump is connected the Lambda sensor and ECU will change your fuelling to the correct air fuel ratio 14.7 or so. I believe the Lambda only controls the fueling up to 3K rpm then it runs off the metering plate for wide open throttle.

turbo ride 04-08-2013 08:31 AM

Yes - in my particular situation I’ve got my baseline CO set so my warm idle with O2 disconnected and air pump disconnected (pump is just blowing into engine compartment, line to ports plugged) so that AFR is 13.5. O2 sensor plugged back in so now it idles at 14.7.

My Big Question is -- So when I reconnect the air pump – won’t it throw everything off (at idle and low rpm) because its pumping raw air into the system pre-O2 sensor? Won’t it think the car is running lean and thus increase fuel – causing it to run rich in the cylinders but look normal at the O2 once the extra air is pumped in?????

I’m missing something somewhere as to how it all functions….

( Yes I know many just dump the whole system but I don't want to do that - for hassle factor and future emissions issues should I ever sell - so let's leave that solution out - thanks.)

timspu 04-08-2013 08:44 AM

I understand where you are coming from. The ECU will control Idle fuelling, Your main concern is when the Lambda is not in control which is at high RPM and WOT. Ensure these are correct and not leaner than 12 AFR.
I am afraid i can't answer your question, hopefully one of the Genius's on the forum can advise.

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