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Tuning With An Air/Fuel Gauge


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Hi guys,

I just purchased an air/fuel mixture gauge (https://www.amazon.com/gp/product/B00N3VGPYS/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1) and was wondering where to find the number the car should read when driving/idle.  Has anyone done this before?

The FSM shows fuel percentages but that isn't readable on these gauges it seems.

 

-chase

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Posted (edited)

There's no such thing as an air/fuel sensor. How does that gause operate?

 

Edited by TomoHawk
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6 minutes ago, TomoHawk said:

There's no such thing as an air/fuel sensor. How does that gause operate?

 

It has an oxygen sensor ("o2 sensor") that is welded into your exhaust.  All modern cars have this.

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Mine sits at about ~10.3 with the choke on to warm up, 12 on idle after warming up, 14.5-14.8 on load ~3k RPM, 14.7-14.9 @ 80-100MPH, cant get it over 100 it pops out of gear 😞

Running SM needles on a 4screw SU.

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42 minutes ago, chaseincats said:

It has an oxygen sensor ("o2 sensor") that is welded into your exhaust.  All modern cars have this.

Oxygen sensors measure oxygen.  How does the oxygen measure the fuel mixture in the intake manifold?

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1 minute ago, TomoHawk said:

Oxygen sensors measure oxygen.  How does the oxygen measure the fuel mixture in the intake manifold?

The residual oxygen in the exhaust is a reliable indicator of the ratio of fuel and air at the inlet.  This is true given a working and reasonable ignition and cam timing. Engine Management systems do work on this basis. I mean, as well as that being an assumption they make, they actually do work well in optimising fuel mixtures. 

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40 minutes ago, TomoHawk said:

Oxygen sensors measure oxygen.  How does the oxygen measure the fuel mixture in the intake manifold?

It measures oxygen in your headers/exhaust manifold's back piping (not your intake manifold as that's just air not air/fuel).  Jonbill also made some good points.

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I stand by my original statement that there is npo such thing as a sensor that measures the air-to-fuel ratio, in the intake manifold.

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3 minutes ago, TomoHawk said:

I stand by my original statement that there is npo such thing as a sensor that measures the air-to-fuel ratio, in the intake manifold.

Again, this is not measured in the intake manifold, there is an o2 sensor welded into the exhaust headers.

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Posted (edited)

The only place you can accurately measure the air-to-fuel ratio of what goes into the cylinders is to put a sensor into the intake manifold.  Even more precisely, is to have a sensor near each intake port.  Having an o-2 sensor (whatever "0-2" means) only measures o-2 in that particular spot.

Edited by TomoHawk
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5 minutes ago, TomoHawk said:

The only place you can accurately measure the air-to-fuel ratio of what gnes into the cylinders is to put a sensor into the intake manifold.  Even more precisely, is to have a sensor near each intake port.  Having an o-2 sensor (whatever "0-2" means) only measures o-2 in that partuicular spot.

https://en.wikipedia.org/wiki/Air–fuel_ratio_meter#Wide-band_sensors

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20 minutes ago, TomoHawk said:

The only place you can accurately measure the air-to-fuel ratio of what gnes into the cylinders is to put a sensor into the intake manifold.  Even more precisely, is to have a sensor near each intake port.  Having an o-2 sensor (whatever "0-2" means) only measures o-2 in that partuicular spot

What sensor can you put in the inlet tract to measure the air/fuel ratio? 

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Here is a very useful graph:

Measurement:

  • Same Car
  • Red O2 measurement at manifold
  • Blue O2 measurement at exhaust pipe.

Findings:

  • Both read the same value for steady state.
  • Dilution reduces resolution of transients at exhaust pipe.

 

AF monitor points.gif

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Mine sits at about ~10.3 with the choke on to warm up, 12 on idle after warming up, 14.5-14.8 on load ~3k RPM, 14.7-14.9 @ 80-100MPH, cant get it over 100 it pops out of gear
Running SM needles on a 4screw SU.


Those AFRs are perfect for part throttle / cruising.

You may wish to reconsider the mix if your definition of load is WOT.

My idle is also pretty rich at about 12.5-12.8 - which makes for a cooler idling engine but boy does it smell ;)

07ecf2ef766c69bff23724c6518ff4ac.jpg

For comparison, my AFR on WOT dips to a crazy low 10.5 at 3.1krpm where there is a reversion torque hole and then sits doggedly between 12.5-12.8 from 3.5k to the red line.



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Posted (edited)

@AK260 actually this is something that I would like to do, it needs more “ooofm” when you hammer the throttle. Perhaps dropping the jet down 1/6 of a turn at a time? Or do I have to start messing with needle profiles?
 

@240260280 I want to match that graph, what kind of setup are you running? 😬

Edited by heyitsrama
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[mention=32052]AK260[/mention] actually this is something that I would like to do, it needs more “ooofm” when you hammer the throttle. Perhaps dropping the jet down 1/6 of a turn at a time? Or do I have to start messing with needle profiles?
 
[mention=31602]240260280[/mention] I want to match that graph, what kind of setup are you running?


Well chap, the best tuning tool I have EVER used is the AFR gauge - going down 1/6 or 1/3 at a time on the side of the road and hammering up and down the same piece of road is a good way to go. The car will feel more responsive and lively when you have it right.

At 14ish AFR on WOT you’re definitely too lean for good power. I have found anything between 11.5-13.2 can work well but 12.2-13 is ideal.You will find that once you get it in this zone, your water temps drop too. Also you may aim for the higher end of that zone for better economy or a better compromise on idle AFR etc.

I too was running SM needles that felt a little lean on my modified engine. The jets were very far down. Once they were modified by a tuner on the rolling road, the car absolutely came to life!!!

I didn’t like their work so I set about experimenting and found MC needles with 2oz (stock) springs have superb throttle response and good all round AFRs.

The tinkering was hampered by C19 and will pick up again this summer - but here is my thread on it so far in case you are bored ....

https://zclub.net/community/index.php?threads/su-carbs-will-work-perfectly-on-my-modified-l28-and-give-dcoes-a-run-for-their-money.25839/

What is your engine spec?
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Posted (edited)

So I just finished wiring the gauge and will be taking it in tomorrow morning to have the o2 sensor bung welded in then I'll drive back and give you guys some numbers.  Stock l28 with EFI.

I pretty much followed this guys video (even got the same gauge): https://www.youtube.com/watch?v=XS_f2ciKVqo&t=9s

Edited by chaseincats
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Posted (edited)

Ok gang, here come the numbers on my stock EFI system as-is:

  • Idle: 13.4
  • 2500 rpm shift points: ~13.2
  • WOT: 12.4
  • Highway cruise: TBD (will update tonight)

NOTE: This is without the high altitude switch on (when enabled it seems to lean the mixture out 1 full point)

Changes made: leaned idle to low 15s, will adjust big drive mix gear at a later time.  @AK260 Is that chart (except for the ones labeled WOT) for idle, or highway cruise (4k-4500rpm)?

Edited by chaseincats
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1 hour ago, chaseincats said:

Ok gang, here come the numbers on my stock EFI system as-is:

  • Idle: 13.4
  • 2500 rpm shift points: ~13.2
  • WOT: 12.4
  • Highway cruise: TBD (will update tonight)

NOTE: This is without the high altitude switch on (when enabled it seems to lean the mixture out 1 full point)

Changes made: leaned idle to low 15s, will adjust big drive mix gear at a later time.  @AK260 Is that chart (except for the ones labeled WOT) for idle, or highway cruise (4k-4500rpm)?

TBH those values look pretty good. I wouldn't go leaner than low 14s at idle. Mid 14s at cruise, no need to go leaner unless you're doing big mileage and want to save a few pennies.

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2 minutes ago, jonbill said:

TBH those values look pretty good. I wouldn't go leaner than low 14s at idle. Mid 14s at cruise, no need to go leaner unless you're doing big mileage and want to save a few pennies.

So you're thinking I should lean out the drive gear and re-richen the idle a bit?  I thought the idle was supposed to be leaner than cruise, no?  I did notice after leaning the idle out that the car off the line is a lot zippier.  I almost smacked my garage door when turning away from my house the first time I drove it post-tune lol.

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I would yes, unless you've got to get through a strict emission test, in which case aim for 14.7 at idle.  Modern OEMs run leanest at cruise - lots of ignition advance and a little leaner than 14.7.

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1 minute ago, jonbill said:

I would yes, unless you've got to get through a strict emission test, in which case aim for 14.7 at idle.  Modern OEMs run leanest at cruise - lots of ignition advance and a little leaner than 14.7.

What rpm would you say is 'cruise' so I can have the pedal held when tuning it?

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well, depends on your gearing of course, but typically 2500 to 3500 rpm. But you do need to measure it on the road - it's not valid if the engine has no load. (just checking!)

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