AirCOOLme

I have a 77 911 with 3.3L in it. The fuel distributor is rebuilt and the injectors have been tested.

When i make a run with the wide-band O2 sensor hooked up the AF ratios are;

before playing with the mixture; 12.5 idle 12.5-13 at 3000 under a light load..

on boost it started at 12.5 and soon as the PSI started climbing the AF dove into the 10.3- 11 range and stayed there till red line..


after playing with the mixture: idle to 13.5 13-13.8 on the freeway
when i got on it again the mix dove down to 10-11 and stayed there... leaning a bit up top but not going over 11.2
is this TOO rich? any way to cut back on the fuel up top? do i even need to?

AirCOOLme

oh... and my name is Nick... nice to meet you guys. thanks for any help you can give.

BTW it has SC cams,8-1 comp and stock boost

right now i only have the SC fuel pump in the front... will adding the second pump in the rear make the condition worse?

this have anything to do with the car starting hard when its cold and idling like crap? its like it runs on only a few cyl for a min or two.......and the idle is lower then its normal900rpm and if you touch the gas in the first 15 second after it starts it give you a muffled backfire...

at idle it has an intermittent "pift" sounding cough out the exhaust and it feels like its on 5cyl.. but as soon as your off idle its fine..

but then randomly sometI'mes you hit the key and it jumps to 1400rpm and idles like a champ for the first 30 seconds till it returns to its crappy idle..

the fuel system has been checked at DON JACKSON ENTERPRISES and the plugs have been changed.. they said they fixed a vacuum leak but this condition existed before and after my car went in.. and i adjusted the valves to try and fix the condition...

any ideas? the motor is less then a year old but I'm not near the builder so i cant really drop it off and let him deal with it.

Brent 930

Adjust your A/F at idle to 14.5.

DonE

Other than the idle issue, does the car run OK? If so, leave it. The only way to tell if it makes sense overall is to put it on a dyno.

For example, my car likes to idle at 12.8:1 AFR. Any leaner and its rough. At cruise, it likes 14:5. Any leaner and its rough. Under acceleration, it dives to 11:1 and I have it come up to 11.5:1 to red line.

My car has A LOT of cooling to it, so richer mixtures make sense (atomization, wetting cylinder walls, etc). And, it produces great, reliable HP.

AirCOOLme

alright... i will see if i can lean it out a bit.. the car is fine off idle and runs like a raped ape on boost..

thanks for the help!

-nick

SGOGT4

Isn't that a bit lean for a turbo?!

jeff91C2T

Good topic...

I just installed the innovative gauge on my C2 Turbo. Details are as follows;

I run w/ the stock O2 sensor plugged in so the computer is regulating mixture at idle and part throttle. At idle the AF bounces around between 14.6 and 14.8. With the stock sensor unplugged the system is in default mode and I set the mixture at 14.7, where it holds a steady value. Runs well with this setting.
Under boost the mixture drops. At low RPM's going full throttle to build boost I'll see low 10's and sometimes high 9's (like in 4th or 5th gear tests). Then it leans out to high 11's/low 12's at ~6K. Now in second or third gear pulls starting at a higher RPM it drops to high 10's then leans out slightly.

This seems to be a nature of the WUR and how it works. You get enrichment with boost independent of RPM. BTW...I did the WUR mod to allow more on boost enrichment to address a lean condition. Now I've got too much fuel! But a much better problem to have....

Brent 930

14.5 AF at idle is fine turbo or no turbo.

SGOGT4

I have been told otherwise.

Can anyone else confirm?

jeff91C2T

Well....the 91 Turbo is shooting (by design) for ~14.7. Motor is essentially the same as the earlier Turbo's. The 86-88's run in closed loop like the 91's. Who told you otherwise???

SGOGT4

I had called down to Imagine Auto on Thursday and was suggested that mid to high 12s was more appropriate for the 930. Could also be dependant on the level of boost that you are running. Stock vs. 1.0 bar

Maybe Stephen can jump in.

JoeMag

930 Ldr -- Your comment from IA on mid to high 12's, is that idle?

NSXTC

Brent's right...the key word is 14.5 at IDLE, which is where is should be. @ mid 12s, that should be under boost.

PorschePhD

Folks, Who cares what it does at idle? The concern is under WOT and boost. If the car is at 12, 14 or 19 at idle and you are at 13 at WOT you have issues. Please make sure you know you are safe under boost, at redline and WOT. This will be what is most important. Consider 12-12.3 your safe range. Personally I like to see nothing more than 12.1 on our motors. Typically my suggestion at idle on CO is a ballpark of a safe range under boost. With the amount of WBs we sell I honestly expect everyone to have one in their tool box as a necessary tool for owning a 930 that is not stock and worked on by the owner. Please don't read my post as being a smart *** but I want to make the point not to get stuck in what is the car's AFR at idle and cruise. These are important, but all are trumped by the boost and WOT reading. If this isn't right you will have a hole in the piston. Then you won't idle at all

Now, who ever in my office said high 12s is going to get kicked square in the **** tomorrow. Which one of them was it!!???