AFR Tuning- Old skool vs. New school

[SquareRoot]

All my life I was taught to read the AFR by the color of the plugs. I put new plugs in when I installed my PF4. After roughly 1000 miles the plugs look exactly the same as when I took them out of the box.

Sbc, mild cam, headers. Idle afr set to 14.7, cruise 13.9, Wot 12.

Truck runs great in all areas.

So, with fuel injection, if the AFR is set to a "safe" setting and the O2 sensor is telling you its running at what I set it at, why does plug color matter?

Why would I richen it up and lose milage and increase emissions if it runs good but the plugs are white?

 

[Frankenchevy]

I don’t know? Why would you? Trust your O2 sensor and leave it be. You can tell when your O2 sensor seems to be failing or fouled because it’ll close loop compensate more than 10% in either direction. Once your base fuel map, CTS compensation, MAT compensation, timing map, etc. are dialed, it shouldn’t vary much from that program. If it does, a sensor is going out.

Out of curiosity, what’s your AFR range? I’m around 12:1 at many throttle positions above 75kPa and in certain areas I have over 15:1 AFR at low throttle % under 45 kPa.

 

[SirRobyn0]

Trust your O2 sensor. It's also my experiance that todays gas leaves a lot less deposits on spark plugs even on carbed rigs, so in general plugs look cleaner / whiter than they did 20 - 30 years ago. Seriously a lot has changed in fuel formulation, and it's easy to run a rig to rich by reading plug color like we did in the old days.

 

[C10MixMaster]

I agree modern fuels dont give good plug readings.. go with the O2 AFR reading. It also gives you the ability to tune off idle and power circuits that plug reading could never show, for us guys with carbs.

 

[Vbb199]

This is seriously something I've never really considered much (the fuel changing)

I say that because even on my fuel injected vehicles that use O2 sensors, I still look at the plugs to determine how clean each cylinder is firing.

I know there's not alot you can do from the engine bay on a fuel injected motor like a carb, but I never once considered fuel may have changed.

I just have learned what I've learned from my dad in regards to watching afr.

 

[SirRobyn0]

This is seriously something I've never really considered much (the fuel changing)

I say that because even on my fuel injected vehicles that use O2 sensors, I still look at the plugs to determine how clean each cylinder is firing.

I know there's not alot you can do from the engine bay on a fuel injected motor like a carb, but I never once considered fuel may have changed.

I just have learned what I've learned from my dad in regards to watching afr.

I think fuel formulation can vary a bit from area to area. But take the ethanol that is in gas today. You hear folks complain all the time about how ethonal is harder on rubber parts, and some carb parts, but the other thing is it effectively lowers the boiling point of gasoline, there by increasing the risk of vapor locking and peculation. I think the ethanol because it burns cleaner the gasoline maybe at least part of the reason plugs look cleaner. And of course there is no lead, and MMT additives are long gone too. If you remember ever pulling plugs that had a dark reddish ash on them that was from fuel with to much MMT. MMT was used in some areas as a lead replacement. That's about the extent of my knowledge a fuel changes.

 

[Matt69olds]

When I had the Q-Jet on my Olds, I spent a lot of time getting it dialed in. I set the total timing to 34 degrees, and had a 18 degree adjustable vacuum advance canister. When it was all tuned, it would occasionally ping just a touch, easing into the throttle a little more would cause engine vacuum to drop, resulting in a decrease in vacuum advance, and no more pinging.


Once I had the timing dialed in, I worked with the carb. I kept leaning out the APT (adjustable part throttle) screw, when I ran out of adjustment I dropped 2 sizes on the jet. I then richened the mixture again, and on a steady highway speed, kept leaning it out until I got a lean misfire. I then slightly richened up the mixture to eliminate the lean miss. It wasn’t uncommon for the car to get 18-19mpg on the highway.

A couple years later, I got a Innovate LM-1 AFR gauge from a friend. After I put it on the car I was surprised to find that the cruise AFR was 15.8-15.9, far leaner than I ever would have expected. The engine ran great, the plugs always looked good, and was easy to drive.

My point with all this? Give the engine what it wants, not what you think it needs. If I had tuned it with the AFR, I’m sure I would have chickened out long before I found the “happy ending place”. I have no doubt I would have lost a couple mpg. BTW, the full throttle AFR was 12:9, I didn’t have real good selection of metering rods to really play. I do know that I tried a couple big changes richer and leaner, with worse MPH at the track. I knew I had to be in the ballpark of good WOT tune.

 

[SquareRoot]

That's some great info Matt. Thanks for sharing.

 

[1lejohn]

I just installed my wide band LMC2. I found my truck running rich at idel when hot about 12.9. , when its cold its 13.9 to 14. I need to work on that. Like the above post at part throttle or curise it's running 15-16. At full throttle it s 11.5-12. The truck runs better then it ever has. The set up is a Vortech headed 350. The cam is a GM Hyd roller from an HT 383 or Ram Jet 350. It has an Edelbrock 650. The exhaust has long tubes with dual exhaust out behind the rear wheels. I would like to have an X pipe , but the cost is not worth it. I was surprised how this engine likes the cruise setting lean. My vacuum advance is disconnected per the GM instructions for the HT 383. I have 14 degree advance and the rest is from the mech. built into the dizzy. I do have an MSD box for the ignition. Great info guys.

 

[Craig Nedrow]

What kind of sensors? Bosch UEGO LSU 4.9 excellant.

 

[C10MixMaster]

I just installed my wide band LMC2. I found my truck running rich at idel when hot about 12.9. , when its cold its 13.9 to 14. I need to work on that. Like the above post at part throttle or curise it's running 15-16. At full throttle it s 11.5-12. The truck runs better then it ever has. The set up is a Vortech headed 350. The cam is a GM Hyd roller from an HT 383 or Ram Jet 350. It has an Edelbrock 650. The exhaust has long tubes with dual exhaust out behind the rear wheels. I would like to have an X pipe , but the cost is not worth it. I was surprised how this engine likes the cruise setting lean. My vacuum advance is disconnected per the GM instructions for the HT 383. I have 14 degree advance and the rest is from the mech. built into the dizzy. I do have an MSD box for the ignition. Great info guys.

your numbers look alot like mine. one thing i noticed is as the weather gets hotter the richer it runs, looks like maybe as much as a full point. I was originally going to pull the wideband after it was tuned but think i will keep it now, that way I can tweek the idle and APT settings at the weather changes.

 

[Matt69olds]

Why would they suggest leaving the vacuum advance unhooked? It has ZERO affect on WOT performance. Properly adjusted, it improves part throttle drivability.

 

[C10MixMaster]

Why would they suggest leaving the vacuum advance unhooked? It has ZERO affect on WOT performance. Properly adjusted, it improves part throttle drivability.

GM has done that on several of their crate motors. The installation sheet on my zz4 is the same no vacuum . from what i can see they used the same off the shelf distributor on the 383 ,zz4 , big block etc. I believe GM just got lazy and instead of putting the proper curve and proper vacuum can they just decided to leave it disconnected to protect against detonation. You should run a can on all street driven vehicles. there are many different vacuum cans still available to get it right.

 

[wanderinthru]

Why would they suggest leaving the vacuum advance unhooked? It has ZERO affect on WOT performance. Properly adjusted, it improves part throttle drivability.

Great information gentlemen.
If I may, what is WOT?

 

[K201979]

Great information gentlemen.
If I may, what is WOT?

Wot=“wide open throttle”