Wideband AFR tuning help.

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idrift

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So I am tuning my 340 with a wide band afr and have
a couple of questions for you guys that use them.

I have two different carbs I have been going back and forth on. A 3310 750 and a 670 Street Avenger.
The 3310 has 72's in the primaries and the 670
came with 65's. The 65's are way too small as at
cruise I am running about 17. The 750 at cruise
runs about 13.5

So given that here's the couple of questions I have:

Will advancing the timing change the afr to become
more lean? I am putting 68's or 70's in the 670
to richen it up, but will also be advancing the
timing about 8 more degrees before once I get my
recurved disty back. I am not sure the 3 sizes would be enough, and am thinking the slightly rich
3310 might be about right on if the advance leans
it out.

Also, under partial and full throttle, I have always understood you make the most HP around 12 on the afr is that correct?

Thanks.
 
Under load yes you want around 12.5:1. But at cruise I think 16:1 is what it should be; this will give you the best gas mileage especially if you have your vacuum advance hooked up.
 
I think I am rich with the 3310 with the secondaries also rich running 10-11
under throttle, and the 670 is lean running over 17 at cruise and also around
11 at throttle. I think the 670 is easier to tune with the jets having secondaries
also.

I think I am still in a bit of a crap shoot until I get my timing set up right.

The AFR sure has helped so far though showing my stumble off idle as way
lean. Changed the acc pump cam and it sure woke things up.

Isn't 16 a little lean for cruising at?
 
I always thought that when cruising you want as close to 14.7 afr as possible. where did you mount the wideband gauge into your exhaust?
 
Thats what I thought too, around 14.7 at cruise. My gauge lights up
red when I am above like 15.5

I mounted the sensor in the header collector, was the suggested location
for headers but seems a ways back.
 
On my turbo 4 cylinder,Multiport fuel injection car...I'd always tune for 14.7 A/F at cruise, light throttle, and 11.7-12.2 A/F @ WOT...with a wideband.

there's my .02
 
OK...I give..:read2:...what is wideband AFR...LOL....:dontknow:

Is it something to do with the air/fuel mixture :scratch:
 
Prine said:
On my turbo 4 cylinder,Multiport fuel injection car...I'd always tune for 14.7 A/F at cruise, light throttle, and 11.7-12.2 A/F @ WOT...with a wideband.

there's my .02
Click to expand...

I think that seems about right. It's easier to tune yours than a
carbureted car though! I think 16+ is too high because my plugs
are bright white, and from the old days we would jet that heavier.
 
6pk2goDemon said:
OK...I give..:read2:...what is wideband AFR...LOL....:dontknow:

Is it something to do with the air/fuel mixture :scratch:
Click to expand...

A wide band air fuel ratio gauge senses the exhaust gasses and tells
you if your car is running rich or lean basically. Quite the tuning tool.
 
Do you know for sure your getting out of the idle circuit when cruising? At moderate speeds you may still be running on the idle circuit. I see you say it's running fat up top. Remember the power valve channel restrictors (PVCR's) also come into play at WOT so you have to figure them in. It's possible to have the a/f ratio perfect in the idle circuit and main jet circuit but either fat or lean at WOT. To make sure it's the secondaries that's running rich you can wire the secondaries closed and go for a top end blast and see what the a/f ratio shows. You might be surprised to see it's really the primaries that are causing it to run rich at WOT due to PVCR's being too big. I've ran into that before even on non modified carbs. Just last week I took apart a 3310 and found it had some huge PVCR's.

BTW: 17:1 is real lean but that's what alot of the little econo boxes are running at for good fuel mileage. I personally don't like to go over 16:1 just for safety sake. As long as your not overheating or surging it's fine. 14.7:1 is stoichometric (sp) which is the supposed ideal perfect ratio for best combustion, not necessarily best for fuel mileage or power.

Definetly get the timing ironed out before you waste any time trying to tune the carb. because timing does impact a/f ratio to a degree. 12.5:1 at WOT is pretty close for best power usually but every engine is different. Just have to dyno it or run it at the track to see what it really likes.
 
i am having a hell of a time figuring it out. i can adjust my carb to read anything i want as of afr's(by drilling air bleeds idle circuit and high speed) but still cant get my stroker to pass smog. hydrocarbons read a little differant on a smog machine.

plugs do look a little white if you run over 16. i pretty sure its suppose to be 13.5 to 12.5 under load.

id like to help but thats all i got.
 
That 16:1 A/F ratio at cruise is how Street Avengers give you better gas mileage than the other Holley carbs. Same with the Edelbrock/Carter carbs with metering rod circuitry. That lean of a mixture won't hurt anything (especially if the timing advance is there) because at that point the engine should be under very little load.
 
I was going to bump the jets from 65's to 68's on the primaries. I was getting
some detonation running 18 before with the street avenger that I did not get
with the 3310 holley. I thought it might bring me down into the 15's. My temp
was also running about 10 degrees hotter. Does that sound about right?
 
So I swapped out the jets to 68's from 65's and that got me down into about
14-15 cruising above 50 mph. Below, I am pretty sure I'm on the idle circuit
still because she will run 15-16. I was also able to tune in the secondaries
opening with a lighter spring to eliminate a lean spot. I still think I am running
a little rich under WOT after the secondaries come in because I am running
about 11. I think I can change out secondary jets a size or two smaller. Boy
what a difference I am from where I started!
 
My 2 cents...

Disconnect the secondaries and wire them shut. Tune the primary side. Once you get that sorted out, then go to the secondaries.

Check to make sure the transfer slot in the secondary is properly positioned at idle.
 
crackedback said:
My 2 cents...

Disconnect the secondaries and wire them shut. Tune the primary side. Once you get that sorted out, then go to the secondaries.

Check to make sure the transfer slot in the secondary is properly positioned at idle.
Click to expand...

The primaries are pretty dialed in, and the transfer slots are good otherwise
the idle screws would do nothing for me.

I'll have some final tweaking to do next week when I get my performance
disty and ignition installed.

Widebands are very cool.
 
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