A/F ratios???

[RickWI]

After doing a ton of EFI work over the years you can't dial in fueling without compensating timing. I have never had an engine like 30 degrees of advance at idle either. Maybe 20, with a big hairy cam and 10" of vacuum.

[Motor Daddy]

...I can go up a hill and watch the vacuum gauge drop slowly, and when the gauge reads below my power valve rating of 6.5, the A/F ratio changes about a full point from 13.8-12.8..]

BTW, MD, if fuel economy is of interest (and why wouldn't it be these days?) have you considered reversing the tune sequence on your car? Dial in lean best cruise AFR (depending on cam, etc., perhaps 16.5:1) with the primary main jets (likely # 55 -60), then drill and tap the primary P.V.C.R.s to 6-32 for Holley's 142xx emulsion jets (available from 00 to 0.078") and tune the primary W.O.T. AFR with them. With this approach, PV ratings need to be fairly high to prevent weak tip-ins, but it's usually good for several miles per gallon minimum..

MadBill, Believe me when I say, I pulled my hair out tuning with the wideband.

I have a Tremec TKO, so at cruise on the highway at ~75 MPH my engine performs the best at ~13.5-~13.8 at 2250 RPM. I get 15.5 MPG.

If I make the cruise ratio more lean, it has noticeable lean misfires and generally doesn't run very good. My engine simply likes to cruise at 13.5. I have a 240/248 solid cam, single plane manifold, headers, 14 initial, 35 total, and 14 degree vacuum advance hooked to a manifold source. The engine puts out about 430 HP. I'm kind of happy at 13.5:1 with 15.5 MPG. When I cruise at a leaner A/F ratio my mileage goes down.

Your suggestion is a good one, though. The best thing to do is adjust the main jets and air bleeds for cruise, and then adjust the PVCR size for optimum WOT.

I have the Innovate Motorsports LC-1, SSI-4, and the XD-16 system in the car. The engine runs the best ~12.8:1 WOT. I adjusted the primaries to idle at 13.5:1, cruise at 13.5:1, and with power valve open (no secondaries) the engine runs at 12.8:1. When the secondaries open the A/F ratio remains at 12.8:1.

I pulled my hair out for months trying to make the engine run at the numbers I wanted it to run at. I finally learned to give the engine what it wants. Success was achieved at that point and not a minute sooner.

[MadBill]

To briefly continue the hi-jack I started MD, thanks for that info! I've tuned a number of moderately high perf engines to cruise in the mid to high teens, but none with as much cam as yours or such low cruise RPM, and since my BBC will be ~20 degrees more, I now know what to expect!
You might be able to run a little leaner without misfire using a bit less vac. advance and a killer long duration spark (short CD spark was terrible for cruise misfire with vac. advance on a couple of engines I tuned), but likely not much better economy.

Now back to our regularly scheduled programming...

[Motor Daddy]

To briefly continue the hi-jack I started MD, thanks for that info! I've tuned a number of moderately high perf engines to cruise in the mid to high teens, but none with as much cam as yours or such low cruise RPM, and since my BBC will be ~20 degrees more, I now know what to expect!
You might be able to run a little leaner without misfire using a bit less vac. advance and a killer long duration spark (short CD spark was terrible for cruise misfire with vac. advance on a couple of engines I tuned), but likely not much better economy.

Now back to our regularly scheduled programming...

You may have a good point about the CD spark. I use a MSD Digital 6 plus box. I noticed when I first installed the box years ago that the idle was a little cleaner. It also fires as soon as I turn the key, regardless of the air temps. I don't know how it effects cruise, though.

I also have a in-dash vacuum gauge. I cruise at about 18" at 75 MPH at 2250 RPM. With the larger cam and larger overlap, I'm sure there is a fair bit of exhaust mixed in with the fresh mixture, especially with the high vacuum. That may be another reason why the engine has lean misfires at ~14.5-15.0.

Like I said earlier, the engine loves to cruise at 13.5-13.8, and I'm all about giving the engine what it wants, now.

[camaro_fever68]

I tune with a wide band (Innovate LM1)

They are pretty responsive. A 1/16 and less turn of the mixture screws makes a difference.


My 383cid 10.80's Luv engine gets 18mpg. No overdrive, 4.11 gears, 30" tire. I cruise in the 15-16 afr range. WOT is flat 12.3 afr.

I'm running 11:1 compression and a Comp 288 solid roller cam. 288/296 252/258 .630/.630 106/104 Timing 18 initial 36 total, no vac. Holley 750 4779.

I've run a few test also to prove the old wives tale of main jets and/or power valves making a difference in idle. I took the main jets out and it didn't change the idle.

[MadBill]

Hmmm... you must be turning ~3,450 RPM at 75 MPH, Vs. MD's 2,275. Maybe there's enough less exhaust dilution at that RPM to permit the leaner mixtures. Doe this mean MD needs to tune and cruise in 4th?
BTW, EGR requires more spark advance; some mid-eighty GM's used 30 degree vacuum cans..

[Motor Daddy]

Hmmm... you must be turning ~3,450 RPM at 75 MPH, Vs. MD's 2,275. Maybe there's enough less exhaust dilution at that RPM to permit the leaner mixtures. Doe this mean MD needs to tune and cruise in 4th?
BTW, EGR requires more spark advance; some mid-eighty GM's used 30 degree vacuum cans..

Or increase my speed to 90.

[David Redszus]

An oxygen sensor cannot provide air/fuel ratio. The sensor can only read the partial pressure of oxygen in the exhaust; it does not recognize the presence of fuel. We have often seen a sensor display a lean reading while raw fuel is dripping from the exhaust.

The idea that a O2 sensor can read mixture is based on the assumption that the fuel has a stoich value of 14.7-1. This is almost never true.

Sensors were formerly calibrated on a test engine using a specific test fuel under controlled test conditions. This became expensive and the test engine was replaced by an synthetic exhaust gas to simulate the exhaust stream. The technical documentation for Bosch lambda sensors states that milivolt output readings will vary depending on stoich value of fuel, temperature, composition of exhaust gas, lean shift, etc.

Since we cannot accurately measure either air or fuel, the air/fuel ratio is useful only for calculation purposes. In actual practice, the engine should be tuned for best power across the entire operating range. The sensor readout should be noted and used for future reference to see if either fuel or air flow has deviated. It can tell us relative readings, whether one setting is richer or leaner than another, but accuracy is non-repeatable and poor.

An ECU uses sensor rich or lean milivolt signals to toggle mixture back and forth about the stoich point which is why it can use an oxygen sensor as a rich/lean switch and control mixture ratio.

There are many more technical complications regarding the accuracy and usage of oxygen sensors but that is a long complex tale.

[Tuner]

An oxygen sensor cannot provide air/fuel ratio.

Yes, it can. You’re kidding, right? It doesn’t matter what the fuel’s stoichiometric value is, the measurement is of excess or shortage of oxygen from stoichiometric. Worrying about converting Lambda to AFR just needlessly complicates things.

There are many more technical complications regarding the accuracy and usage of oxygen sensors but that is a long complex tale.

Yes, there are and here is some of it.
http://wbo2.com/lsu/lsuworks.htm If this is too long and complex feel free to ask questions or search, there’s lots more information on the WWW

[David Redszus]

[

quote="Tuner"]

An oxygen sensor cannot provide air/fuel ratio.

Yes, it can. You’re kidding, right? It doesn’t matter what the fuel’s stoichiometric value is, the measurement is of excess or shortage of oxygen from stoichiometric.

No I'm not kidding at all. The sensor doesn't know anything about the fuel that is being used. It only knows which side of the set point it is seeing.

Worrying about converting Lambda to AFR just needlessly complicates things.

Now we agree. A sensor displays a voltage based on partial pressure of oxygen in the exhaust compared to the partial pressure of oxygen outside the exhaust. The output voltage is supposed to correspond to the Nernst Equation but typically does not due to temperature related resistance values. It is then calibrated against a known exhaust gas. As the partial pressure departs from the known calibration value the output voltage varies as a log function. It is accurate only within a very small window.

The window of accuracy can be increased by the use of a broadband which uses a pumping cell to maintain a constant value and then measuring the current draw necessary to maintain equilibrium.

A lambda value will tell us the departure from the calibration set point (Lambda One) but has no relation to the amount of fuel being used.

So if we say the engine is running at a Lambda .95 that's OK, but that has nothing to due with air/fuel ratio. Unless we very arbitrarily assume that Lambda One corresponds to a stoic value of 14.7-1, and then .95 would be an air/fuel ratio of 13.965-1. But if we do not know the true stoich value of the fuel being used, we cannot make the above assumption. I don't know of a single fuel with a stoich value of 14.7-1; its a nominal value.

[Troy Patterson]

Back to the original posters question, or statement;

30 degrees advance at idle is a major reason for the engine to idle like crap. Adding more fuel or providing an excessively rich a/f ratio hampers the combustion process - apparently enough to mask over the excess initial timing - assuming the engine hasn't got something wrong with it.

Starting with a reasonable amount of initial timing - to determine maximum initial timing, you can slowly advance the distributor a few degrees at a time. Take note how the engine is running. When you've reached over advance, the engine takes on a nervous idle quality. Back it off a couple of degrees until the engine is smooth again - that's it. Now, adjust centrifugal advance and you're on your way.

Never, tune by a number, tune by what makes the engine idle or run the best.

As others have said, depending on the flow value of the power valve channel restrictions, idle air bleed and jet or restriction, amount of transition slot exposed at idle and so forth - the power valve being open (or plugged) increases 'well head' pressure in the main well - where the idle circuit draws from. Increasing 'well head' pressure has a very similar effect as increasing jet area - richening the mixture.

We've argued long and hard about O2's here. They do not measure engine output as a basis for determining optimal a/f ratios so they are largely junk - okay a bit overstated - but you get the idea. They are helpful - if you require an aid - to help you understand the effect of changing the various elements of the tune or calibration in the carburetor of fi - assuming you can keep your eyes on it and now kill yourself on test drives. Data acquisition is good here or a camera filing your foot and gauges.

If you tune to a theoretical optimal number - you're missing the promise land. It's too late to go any further - and we've hashed it around so anyone desiring can do a search.

Troy Patterson TMPCarbs.net TMP Carbs

[RayJE Carburetors]

i disagree about the power valve opening or closing changing the idle mixture AFR or even jet size for that matter...... the thing that we are forgeting is that the idle well has to pull up over a hump and this is vented via idle airbleed at the top of the idle well... if this wasnt the case then the idle well would syphon and we would all have a cylinder full of fuel when we wake up in the morning..... ...if the well head effected IDle fueldelivery then just raising or lowering float level would be easiest way to do you experiment.. rather than a power valve...

the fuel level in main well at idle will not go higher just because the jet and PVCR are bigger .. it defies physics...... the fuel level at idle wil be the same as the fuel level in bowl...on some situations it will be slightly higher because the booster is pulling the fuel upthe well even at idle...

if well head effected it then the placement of IFR would effect AFR ......some blocks they are down low ...some blocks they are up high

if u have 2 containers conected via a pipe .. it doesnt matter what diameter pipe connects the 2 .... as long as both are at same pressure then the fluid in each will be the same

[k-star]

Thanks for all the reply's, even tho we got a bit off track..

The new LM1 will be ordered Friday..

Thanks a bunch to Dennis (AFD) for taking the time to explane the wide band deal to me.


I will up-date my results as soon as i get the LM1 all hooked up..

Keith

[Doug Schriefer]

Also remember that with ANY O2 system you can get false readings. The score board or stop watch is still the best answer. Was working on PS car a few weeks ago that the O2 system kept saying the engine was too rich, yet the plugs, and other factors showed the opposite. Added fuel, and hey went quicker... These are all great tools, but remember all of the other stuff as well.

[David Redszus]

Also remember that with ANY O2 system you can get false readings.

Bang. Right on. Of almost equal importance to the air/fuel ratio is the mixture preparation. A mixture that is not fully evaporated can easily show a lean condition since that is what available to the engine for actual combustion. Mixture prep is greatly influenced by the distillation curve of the fuel being used.