Air Fuel Ratio gauge and now have question?

The other day I installed a AFR gauge but did not take it for a test drive till this morning and it has brought up some questions.

When searching the WWW it gives 14.7 AFR as the magic number to shoot for.
Is this a good number or should you go richer (lower number) or leaner (higher number)?

The gauge has a outer ring that changes colors, green / yellow, as the numbers change and from what I can see the green/yellow is at 13.9/14.0 area so if I was to lean it out to get 14.7 it would be in the yellow not the green.

I still need to check my timing and get that dialed in before messing with the carb but just the run to work, 37+ miles on the high way (65-70 MPH) the gauge was between 13.2 and 13.7 so an average of 13.5 in my book.

Now holding the throttle steady on flat ground it was still bouncing around the above numbers is this some what normal?

If I should leave it where its at, average 13.5, what else could be done to get the MPG up more than the mid 14's I am getting now? Its not like I am burning rubber at each light, 300 six, 2.75 rear gear and T18 is not a speedster, I also have over drive so the RPM is not killer high at the 65-70 MPH maybe 1800 RPM?

I was really wishing for the AFR to be like in the 11's or 12'sand could say that is why the poor MPG.
Dave ----

To say Big Blue's AFR bounces around would be a huge understatement.  But I have it set for mid-14's at a steady cruise at ~65 MPH with no wind on level ground.  Note that I didn't say "14.7" as that's way too precise.  And the conditions make a huge difference.

But 14.7ish is a good target as that supposedly is the ratio to allow all of the fuel to react with all of the oxygen - assuming you are running pure gasoline.  This page says that 10% ethanol lowers that to 14.04.

Then when I ease into it for a slight hill or to compensate for a bit of wind it'll creep up to into the 15's or maybe to 16.  And about then the vacuum will go low enough to allow the metering rods to pop up and the AFR will drop, and if I'm pushing it a bit it'll drop to around 13.

But when the pedal gets down very far and the engine starts to really wake up the AFR will be closer to 11 - 12.

As for your timing, it will change the AFR 'cause it will change the amount of power the engine is generating.  In Big Blue's case a bit more timing leaned things out a bit.

Last, I saw 17:1 at times on David's 1986 300, and it was turning in excellent MPG and drove well.  So we left it alone.  However, that was on my old AFR meter and each meter reads differently, so don't think that you can't lean it out a bit more.

Thanks for that information just wanted to make sure what I was seeing and thinking that I was on the right path.

Because I am getting pinging I have to deal with that first and now that I have a timing light I can.
Once I have that under control I will see where the AFR is and adjust from there.
Dave ----

Son made me put the timing light to the truck and came up with:
10* BTDC @ 700 RPM vac. disconnected
20* BTDC @ 2500 RPM vac. disconnected
46* BTDC @ 2500 RPM vac. hooked up

I tried to adjust the vac. can and was thinking it would limit the total vac. but my son was looking up what size wrench to use and found it is not total but at what HG it starts to pull in at.
Being I have been up since 3am I did not want to start pulling out tees and vac gauge to see what I was getting at what HG.
I did give it about 1-1/2 turn up to that point and will see if I still get pinging and maybe back it off a little.

Then I can see what the AFR is. I also want to go back over the "MPG post" to see if I can pick anything up to try on my truck.
Dave ----

Dave you raced...
12.5:1 for best power, where you are at 13.5:1 is "safe".
If you go too lean (16:1 or more) you are never going to get rid of the death rattle, and that is why we use EGR.
It allows for what would be a much too lean mixture under light throttle openings.

Yes, the vacuum can screw is only adjusting the spring preload that the vacuum has to work against.
If you want to tune your distributor get a Crane cams adjustable vacuum advance and a set of Mr Gasket springs.
Follow the instructions and review the tutorial Scotty has on his old Reincarnation High Performance web pages.

Remember, the meter is only measuring oxygen in the exhaust, not hydrocarbons.

An EGR works by adding inert exhaust into the intake charge.
This not only slows flame spread, but limits NOX emissions, because it keeps the combustion chamber cooler.

FuzzFace2 wrote

Now holding the throttle steady on flat ground it was still bouncing around the above numbers is this some what normal?

If I should leave it where its at, average 13.5, what else could be done to get the MPG up more than the mid 14's I am getting now? Its not like I am burning rubber at each light, 300 six, 2.75 rear gear and T18 is not a speedster, I also have over drive so the RPM is not killer high at the 65-70 MPH maybe 1800 RPM?

Dave,

I am running an Air/Fuel Ratio gauge also, and it oscillates a bit like yours. If I'm running along on the hwy at very light load, it's in the 14.2-14.7 range, but it probably bounces overall between 13.5-15.0, but I can get it to hold relatively tightly around 14.5 +/- 0.3 or so. It runs around 12.5 at idle.

This is with the Holley 4160 4bbl 600CFM Vac secondaries. When we did the break-in on the dyno, we did bump the jet sizes up from the stock #66 to #68. If I recall correctly we were hitting upwards of 13.0 on the dyno pulls and he wanted to get it back down around 12.5, and the #68 jets worked.

He said at that time that I could probably put the #66 jets back in the truck for street use...assuming that one, I wasn't going to be doing heavy load redline pulls with it, and two, it wouldn't breath as freely on the truck as it did on the dyno with their longtube headers and basically no exhaust. Howver, the #68 jets seems to be OK. I was going to put the #66 back in to lean it out a little more to see if there was another MPG available, but the darn thing just works so nicely where it sits now, I figured it is best if I simply don't touch it at all.

And, after all that....with regards to my typical light load easy driving, I'm probably running on the idle jets anyway, and not the main jets, right?

PS: I should note that the truck did get better MPG with the large open element air cleaner installed. I didn't have the AFR gauge installed at the time, but the truck got a solid 2MPG more. I will have to try it this summer for a while and see what the AFR is with the open element air cleaner.

Cory, any time you are off the throttle stop you are running in the transfer slots, and from there into the boosters.
The mains are the upper limit of fuel in the two primary bores.

If you step on the gas you get a shot from the accelerator pump.

If you stay in the gas and vacuum drops, the power valve opens.

And (of course) if the throttle is open far enough to release the secondary butterfly, then the rear barrels open in response to vacuum.

This is how all 4160's work

ArdWrknTrk wrote

Dave you raced...
12.5:1 for best power, where you are at 13.5:1 is "safe".
If you go too lean (16:1 or more) you are never going to get rid of the death rattle, and that is why we use EGR.
It allows for what would be a much too lean mixture under light throttle openings.

Yes, the vacuum can screw is only adjusting the spring preload that the vacuum has to work against.
If you want to tune your distributor get a Crane cams adjustable vacuum advance and a set of Mr Gasket springs.
Follow the instructions and review the tutorial Scotty has on his old Reincarnation High Performance web pages.

I know I am in the "safe zone" at 13.5 but with only getting mid 14's MPG I want to try and get a little more out of it if I can.

On the Crane vacuum can how is it's adjustment different from the factory one?
The factory will adjust at what vacuum HG it will start to pull in.
So I can have start at say 5"HG or 10"HGbut if I want to limit the total, I have 46* now, to say 36* I would need to add a limit stop inside the dist.

When I first got the truck on the road the pinging was pretty bad and I adjusted the vacuum can and it helped a lot but has come back, could be the gas blend?
I also did not know at the time what the adjustment did back then so got a little better handle on it now.

The springs seam to be OK but now you got me thinking if I go heavy on the springs and back the vacuum can off so it pulls in advance ASAP that may also cure the pinging?
It will take some playing / adjusting to get it right.

You are right I am not running a EGR or any emission junk so adjustments are needed.
Thanks
Dave ----

Rembrant wrote

FuzzFace2 wrote

Now holding the throttle steady on flat ground it was still bouncing around the above numbers is this some what normal?

If I should leave it where its at, average 13.5, what else could be done to get the MPG up more than the mid 14's I am getting now? Its not like I am burning rubber at each light, 300 six, 2.75 rear gear and T18 is not a speedster, I also have over drive so the RPM is not killer high at the 65-70 MPH maybe 1800 RPM?

Dave,

I am running an Air/Fuel Ratio gauge also, and it oscillates a bit like yours. If I'm running along on the hwy at very light load, it's in the 14.2-14.7 range, but it probably bounces overall between 13.5-15.0, but I can get it to hold relatively tightly around 14.5 +/- 0.3 or so. It runs around 12.5 at idle.

This is with the Holley 4160 4bbl 600CFM Vac secondaries. When we did the break-in on the dyno, we did bump the jet sizes up from the stock #66 to #68. If I recall correctly we were hitting upwards of 13.0 on the dyno pulls and he wanted to get it back down around 12.5, and the #68 jets worked.

He said at that time that I could probably put the #66 jets back in the truck for street use...assuming that one, I wasn't going to be doing heavy load redline pulls with it, and two, it wouldn't breath as freely on the truck as it did on the dyno with their longtube headers and basically no exhaust. Howver, the #68 jets seems to be OK. I was going to put the #66 back in to lean it out a little more to see if there was another MPG available, but the darn thing just works so nicely where it sits now, I figured it is best if I simply don't touch it at all.

And, after all that....with regards to my typical light load easy driving, I'm probably running on the idle jets anyway, and not the main jets, right?

PS: I should note that the truck did get better MPG with the large open element air cleaner installed. I didn't have the AFR gauge installed at the time, but the truck got a solid 2MPG more. I will have to try it this summer for a while and see what the AFR is with the open element air cleaner.

When I heard the pinging the AFR was around the 14.0 - 14.2 mark and after what Jim posted if I try to adjust AFR to mid 14's pinging may get worst.
But I may be able to get to the 14.5 and adjust the timing curve to stop the pinging but like you the motor runs so good I hate to start messing with it other than to stop the pinging.

It just hit me I think Gary posted what the AFR would be for 10% blend over real gas.
I have been looking at the AFR as real gas and not a blend so maybe I am ok AFR wise now?
If that is the case my MPG sucks at mid 14's!

I will do more adjusting to see what I can get out of her
Thanks
Dave ----

Yes, the "best" AFR for 10% ethanol blend is 14.04:1, not 14.7.  So you may not be that far off.

But, I suspect that your pinging problem is due to having a vacuum advance can that was designed for EGR, which was meant to come in at high vacuum.  But the exhaust gas slowed the flame front such that they had to give it a whole lot more advance, which they did with the vacuum can.  So if you are running w/o EGR but with an EGR vacuum unit you have too much advance at high vacuum.  And while you can determine when it comes in you can't change how much you get.

I don't know what the options are from Crane, but I'd be looking for a vacuum unit that gives less advance.

Limit stops affect mechanical advance.
Bending the spring tabs affect spring preload.
With no EGR you probably don't want the mechanical advance 'all in' until 3,600-4K

Go to Jegs.*    Arrrgh!  
Find the Crane advance for '68-80 Ford's and look in the listing for the instruction sheet link.
Print it and read it.
Go to Scotty's old pages and find the two pages of distributor recurve instructions.

Both will help you understand what you are doing.

Gary Lewis wrote

Yes, the "best" AFR for 10% ethanol blend is 14.04:1, not 14.7.  So you may not be that far off.

But, I suspect that your pinging problem is due to having a vacuum advance can that was designed for EGR, which was meant to come in at high vacuum.  But the exhaust gas slowed the flame front such that they had to give it a whole lot more advance, which they did with the vacuum can.  So if you are running w/o EGR but with an EGR vacuum unit you have too much advance at high vacuum.  And while you can determine when it comes in you can't change how much you get.

I don't know what the options are from Crane, but I'd be looking for a vacuum unit that gives less advance.

Well if 14.04 for blend then I am not off all that much as you say.
I did 1 full turn of the rod / seat on the carb to get were I am at now and that was before I had the gauge. It did seam to help with a little more power and the motor ran smoother just nothing on the MPG front

As for the "while you can determine when it comes in you can't change how much you get." from what Jim is saying below I need to slow down the mechanical. I was all in by 2500 RPM so if I slow this down and / or limit it I would be changing the total.
My thinking is keep the vacuum to come in as it was, low vacuum, and install heavy springs so the mechanical will come in later. It is rare it get the RPM up to 3000 RPM, I shift between 2000 & 2300 RPM and maybe 2500 RPM but the truck does not shift well (NP435) at the higher RPMs.

ArdWrknTrk wrote

Limit stops affect mechanical advance.
Bending the spring tabs affect spring preload.
With no EGR you probably don't want the mechanical advance 'all in' until 3,600-4K

Go to Kegs.
Find the Crane advance for '68-80 Ford's and look in the listing for the instruction sheet link.
Print it and read it.
Go to Scotty's old pages and find the two pages of distributor recurve instructions.

Both will help you understand what you are doing.

I will look up the information to get a better grip on this timing thing

So right now my mechanical is all in by 2500 RPM and as I posted above if I was to install heavy springs and return the vacuum can back to where it was that would limit the total timing, remember I have 46* BTDC @ 2500 RPM.
The vacuum timing would kick in as soon as the carb was off idle but the mechanical would not start to come in till say 2000 rpm.

Now this is just numbers I am throwing out and would be adjusted for best running and no pinging.
Remember I shift around 2000 RPM and cruse RPM is around the 1800 in OD in the high way and under 1400 RPM on back roads.
Or am I way off in my thinking before I read up on this
Dave ----

I am NOT a guru on this, so take what I'm saying with a pound of salt.  But I didn't think they changed the static nor mechanical/centrifugal timing all that much for EGR vs non-EGR.  That the real change was done in the vacuum advance because it was only at high vacuum that the exhaust gas came in.

In other words, when you are accelerating hard the EGR will be off and the vacuum low so you are mostly getting your timing from the static + mechanical.  So they left that alone and just dialed in lots of advance via the vacuum can.

Again, that is my very limited understanding and may well be wrong.

Dave, if your "Redline" is 3k, then you're fine with the springs you have (and you may not ever have to limit mechanical)

BUT, if you want more vacuum advance then you might have to trade some spring for vacuum.


Gary is correct!
Also note that Ford set all these DSII trucks up with manifold vacuum to the dizzy.
So it was all the way there when the throttle was completely closed.

Autoincorrect screwed my post up.
Go to Jegs and get the instructions.
You need to complete 'A', before you can address 'B', and so on...
You cannot fix a spring issue with vacuum, or vice versa.

But you don't say what exact conditions cause your pinging*, so I can't help until you better define the problem.
If you're only concerned with leaning out the carb, re-install the EGR and you'll be fine.

Took truck to work this morning and have an update but on phone so hard to type it out.
Will do when home on pc.
Dave  ----

I was trying to use talk to text to reply to Staci yesterday.

It just wasn't having Jon Kaase.
I gave up and used his initials.  

Dave, you can ignore my post in your project thread... I'm following along here with interest!

Ok, Think Jim said something about the EGR may help with the pinging.
Well I cant hook it back up. A freeze plug was installed in the intake under the EGR plate, a plug where the tube would connect but nothing for the tube to connect on the exh. pipe side as I am running EFI exh. manifolds.

I must have gone the wrong way with the vacuum can adjustment it had the death rattle really bad all the time at speed. I had to pull off the first exit 10 miles from when I got on, road work so no place to pull over. I pulled the vacuum hose off the dist. and not being able to find anything on the ground at 1amI left it open.

No more rattle and it seam to run pretty good.
AFR was about 14.0 but I don't know if that is the open hose or change in timing?
I could not find anything before I left work to plug the hose so it was left open for the ride home.
I did find it odd that the AFR was about 14.0 going to work OAT was 61*f
On the way home the AFR was high 14's to low 15's with OAT of 48*f.

Normal driving keeping the RPM shift points to 2000 the truck felt good but I had to run her out a bit getting on the high way and it was a dog with no vacuum advance or was it just running that lean?

Someone wanted to know what the AFR was at WOT. From 65 MPH It went so lean the gauge went ----
only when it got up to about 75 - 80 MPH did I start to see any numbers, 16's & 17's.
It did have a little "miss" every now & then getting up to that speed.

Going to read up on re-curving the dist. as I feel this has to be under control first then the AFR.
Dave ----

Sac79 wrote

Dave, you can ignore my post in your project thread... I'm following along here with interest!

I should do a link in the project thread pointing to here
thanks'
Dave ----