Air/Fuel Gauge Experience ?

[ProfessorB]

.
I'm planning to do carb swap (Edelbrock to Holley) and decided a stand alone Air/Fuel gauge would help in tuning the new carb. I decided to install a "stand alone" gauge first....might as well see how the old cab was running before I yanked it.

The gauge is one of the cheaper models, an analog unit with a Bosch 4.9 O2 sensor. The brand is NOS (Holley, actually) if that matters. I went with the analog style as I think it's easier to view quickly at a glance.

The thing I notice is that the needle DOES jump around sometimes and in ways I wouldn't expect....other times it's relatively stable. And it sometimes does it (jump around a little) even at a steady throttle position, load, speed and RPM. Is this normal and just the way they behave? I did know enough to let everything warm up before I started making my observations. Thanks for any input.

[weendoggy]

Yes it's normal. You won't be able to consitantly keep a steady even flow 100% of the time. Even my Holley HP will flucuate. It becomes an issue when you have huge jumps in A/F ratio. Then you need to do something to fine tune it. I just hit a few keys on my laptop to correct. You'll be doing some jet swapping.

[ProfessorB]

Yes it's normal. You won't be able to consitantly keep a steady even flow 100% of the time. Even my Holley HP will flucuate. It becomes an issue when you have huge jumps in A/F ratio. Then you need to do something to fine tune it. I just hit a few keys on my laptop to correct. You'll be doing some jet swapping.

Hey, my friend Weendoggy.... thanks for the response. Yeah, I figured it might be normal. Actually, I'm thinking it might be just another "tool" in the arsenal. It is giving me a sort of idea of the "average" mixture I'm getting from the carb. According to the average of the readings of the gauge, the Edelbrock is a little rich....which matches what the conditions of what the plugs indicate.
.
Hitting a few keys on my laptop sounds much too easy. I'm looking forward to the pain and the mess of swapping jets! Merry Christmas :)

[Mike.Bray]

223241

[F500guy]

It does fluctuate a bit, here is a picture of my graph, I made a rolling average trace in the data to give a clearer steady state condition. Are you able to pull any data with your gauge? I recommend reading the lambda value as it will compensate for ethanol in the fuel, remember if you do have 10% blend the AFR is less than pure gas.


223242

[ProfessorB]

It does fluctuate a bit, here is a picture of my graph, I made a rolling average trace in the data to give a clearer steady state condition. Are you able to pull any data with your gauge? I recommend reading the lambda value as it will compensate for ethanol in the fuel, remember if you do have 10% blend the AFR is less than pure gas.


223242

Thanks, this is good info. I do not have a data logger involved (although there IS a port for one. And there is no option for Lambda values, which is unfortunate. Umm, when you say that ethanol will make it read less I am not sure I am understanding it correctly so I will pose the question this way. At a given setting, will ethanol mix gas read "richer" on the gauge than straight gas or "leaner" on the gauge than straight gas?

[F500guy]

Check this out, lots of good tuning info on the internet.

https://www.hemmings.com/stories/tech-101-blended-fuel-tuning-using-lambda/

14.7 AFR with straight gas is 1 lambda, the stoichiometric ratio target. 14.1 AFR for 10% ethanol added to fuel will be 1 lambda. Best power and best efficiency are not the same, less than 1 lambda is rich, more is lean and generally better power at a richer mix.

So answer is 10% ethanol will make it look richer.

Lance

[ProfessorB]

Check this out, lots of good tuning info on the internet.

https://www.hemmings.com/stories/tech-101-blended-fuel-tuning-using-lambda/

14.7 AFR with straight gas is 1 lambda, the stoichiometric ratio target. 14.1 AFR for 10% ethanol added to fuel will be 1 lambda. Best power and best efficiency are not the same, less than 1 lambda is rich, more is lean and generally better power at a richer mix.

So answer is 10% ethanol will make it look richer.

Lance

I.m struggling with this. So (bear with me) if 14.7/1 straight gas yields 1 Lambda but it takes 14.1/1 ethanol-mix gas to yield that same Lambda value than if all "settings' are the SAME (same carb)....but first you run a tank of straight gas....the carb setting and jets mix this straight gas at 14.7 and the Lambda value is "1".

Next, you keep everything the same (same carb, same settings, same jets) but you drain the tank and fill up with ethanol-mix gas. The carb will still be "mixing" it at 14.7 BUT....since it requires MORE (14.1) to achieve a Lambda of "1" and you are still using the same carb and jets WOULDN'T THE READING BE LESS with the ethanol mix hence looking LEANER on ethanol-mix and not RICHER?

[F500guy]

If you were tuning with E10 and looking for a 14.7 AFR ratio (not taking into account the difference), you are at risk of tuning it to lean. Maybe I am not really explaining well, sorry about that, but E10 will require larger jets/more fuel to run the same Versus straight gas.

I don't know exactly how the gauge would read, but ideally if it was 14.7 on straight gas, it would probably/potentially be lower value on E10 for argument say it read 14.5 (lower gauge reading). But if you did not understand the difference, you might think that below 14.7 is rich (for straight gas) , but if it was actually reading 14.5 on E10, that would be considered a lean condition.

https://help.summitracing.com/knowledgebase/article/SR-05230/en-us

[bobl]

To clarify a bit on this subject. The A/F gauge will read the correct ratio whether it be E10 or non ethanol gasoline. For example, If it's tuned to 14:1, changing fuel type will not change the reading. It will however change the A/F the engine wants. I've found after 100's of dyno pulls that (NA)maximum power for E10 to be 12.2 - 12.5 AFR, non ethanol likes 12.6-13. For cruise, tune it to where it runs best. There is no (ideal) ratio on a high performance engine, same with idle. Idle A/F gets diluted by cam overlap and may not even read correctly. I generally will set up cruise in the mid 13's and then lean it out a small amount at a time. until It's not happy, then back it up.

Bob

[ProfessorB]

To clarify a bit on this subject. The A/F gauge will read the correct ratio whether it be E10 or non ethanol gasoline. For example, If it's tuned to 14:1, changing fuel type will not change the reading. It will however change the A/F the engine wants. I've found after 100's of dyno pulls that (NA)maximum power for E10 to be 12.2 - 12.5 AFR, non ethanol likes 12.6-13. For cruise, tune it to where it runs best. There is no (ideal) ratio on a high performance engine, same with idle. Idle A/F gets diluted by cam overlap and may not even read correctly. I generally will set up cruise in the mid 13's and then lean it out a small amount at a time. until It's not happy, then back it up.

Bob

Thanks Bobi . You made the point I was trying to make (or, really, the question I posed). And F500guy sent me a pm which also helped me understand.Since I'm switching from an Edelbrock (which runs just fine) to a Holley double pumper (the carb of my teenage days) but am NOT going to be running different fuel, I'm just going to be comparing apples to apples. And since I DO want an A/F gauge, I might as well install it BEFORE the carb swap and learn a little about how the Edelbrock runs.
The plugs had indicated the mixture was a little rich on the Edelbrock....the A/F gauge confirms it. The needle DOES jump around sometime....often spasticlally for a second or two (under steady load, speed, and throttle position) then settles down again. Other times it behaves fine. I AM able to get an idea of how rich or lean it's running. Let's a good "general idea"....which I deem to be about 12.1. And that (12.1) would result in the condition of the plugs.
Too rich is better than too lean. Too lean can damage an engine. Too rich wastes fuel, fouls plugs, etc. SLIGHTLY rich is what I aim for. Somewhere around 13....but of course I'll start slowly (on the rich side) and go from there. Call me crazy, but I just love playing with carbs.

[bobl]

A/F ratio is constantly changing some. The gauge samples at a certain rate. Each sample will be slightly different, hence the movement of the needle. The higher the rpm the more steady it will read. Also, just a few seconds idle time can make a plug look rich, so be aware of what you are really seeing.