max AIR/Fuel graph?

[rdane]

Anyone care to explain to me what I should be looking for on this bit of dyno info?

No idea what the air Fuel ratio should be doing. But I have a chron sheet that runs one from 2.500rpm to 6500rpm with almost a flat line at 13.5

On another it starts at 15 and drops in another flat line to 11.

One from 13 dips to 10.5 and back up to 12.

Then another starting at 3K rpm and 14 running a smooth curve to 12.75 and back up in an even longer curve to 13.

What is good and what is not?
TIA

[350HP930]

14 to 15 at low power outputs are acceptable but 12 to 13 is better at WOT conditions.

For this reason your samples sound accetable but do vary a bit. What kind of fuel system do you have?

[tobluforu]

Do you know at what rpm your car produces the most hp? If so this is the point at which you want your car to be set around 12.5-13. Lower means to much fuel and visa versa. I have never done my porsche, but I have done my turbo saab which puts out 212 h.p at 5500 w\an a\f ratio of 12.4. It did not start at this, but the tech tuned the amm, timing, water inj. and RRFPR to achieve this. The first run I was only at 201 w\an a\f ratio of 11.9-way to much fuel, timing, not enough water. Lean is mean!
My 2 cents.

[rdane]

Four different cars:

almost a flat line at 13.5
' 83 CIS peaked at 13.25 with another '81 CIS peaking at 13.25 also

On another it starts at 15 and drops in another flat line to 11.
'87 Carrera peak hp came at 11.75 Can I assume his engine is being choked off?

One from 13 dips to 10.5 and back up to 12.
'79 CIS with peak HP @11.8 on airfuel

Then another '79 starting at 3K rpm and 14 running a smooth curve down to 12.75 and back up in an even longer curve to 13.CIS with peak HP @13.35 on airfuel

Thanks gentlemen

[island911]

"Carrera peak hp came at 11.75 Can I assume his engine is being choked off?" I would speculate that the Porsche engineers where balancing/trading-off power for fuel econ & emissions. At the higher rev's piston speed is up, meaning going leaner (and a faster flamefront)is okay.* [edit]woops, had that backwards; thought you were saying fuel was choked off. . .not@11.75)[/edit]

What I remember, yors had a dip in the midrange. . .I can't think of why. CIS is usually pretty steady; as you have seen.

*lots of non-linear variables in basic combustion.

The basic stoichiometric air/fuel ratio is 14.7:1. (thats the ideal ratio of air & fuel for complete combustion (meaning no gasoline or oxygen is left as it passes the sensors in the tail-pipe). -right-

In the world of chemical kinetics reation rates are dependent on heat and pressure. Like the powder behind a slug builds pressure fast enough that everything goes supersonic. . .though if you light a sprinkled a trail of powder on the ground the pressure rise just isnt there, so you just get fizzle.

So in the combtion chamber. . .If you run slightly rich you will slow down the chemical kinetics. If you run slightly lean, the whole thing can get snow-balling (detonate) more easily.

. . okay, Im just rambling now, must stop, and go back to work.

[rdane]

Don't run off just yet....

"What I remember, yours had a dip in the midrange. . .I can't think of why. CIS is usually pretty steady; as you have seen."

Actually the only CIS that ran a reasonably straight line was Jim's.

Mark had a huge full point + dip at mid range. Mine had a similar dip but more shallow and broader. Yours a down hill line from 14+ to 13 but at least fairly steady. The Carrera looked like a winding river course running down hill.

I was curious what the max a/f meant too. The guy running the dyno said my car was running rich, but if what toblu says is true, I'm running a tad lean with about 12% at 5500 rpm. Comments?

[Superman]

The dyno guy said my A/F was perfect, FWIW.

[island911]

I took a look at the A/F graphs. The later SC's (small runners) were about the same(to each other). . . .and the earlier cars had the same shape, but MW's was shifted down (rich -- just a crank of the adjustment screw will change that.)

I find it interesting that the earlier SCs had the same pronounced dip (going richer) around 4000rpm. It appears to be on purpose, as most every little detail of an SC is.

Lets see, lower compression, bigger runners, more gas, and the same HP (at that rpm) comp'd to the later SC's. (things that make you go hmmm)

My guess (for now) is the 'big runners' engines where (maybe) having a combustion chamber mixing problem at that rpm.

Wheres Roland when you need him?

[rdane]

Originally posted by Superman
The dyno guy said my A/F was perfect, FWIW.

OK, smart guy Now tell us how you got it there please?

[AES]

There was a very enlightening discussion about this in this thread

[ttweed]

Originally posted by Mark Wilson
The guy running the dyno said my car was running rich, but if what toblu says is true, I'm running a tad lean with about 12% at 5500 rpm. Comments?

Mark-

That 12 number is a ratio, not a percentage, so you have it backwards. If your Air-to-Fuel ratio is 12 and around 12.5-13 is ideal, that means you need a bit more air and are running a tad rich, not lean. Look again at what tobluforu said- a lower number means too much fuel, a higher number means too much air.

TT

[island911]

Originally posted by AES
There was a very enlightening discussion about this in this thread

Thanks AES.

IMO, the very "good read" part of that thread is:

Originally posted by Bill Verburg
Doug straight from the Bosch manuals;

The effects of excess-air factor and ignition timing on specific fuel consumption and exhaust emissions are illustrated in Figures 4 and 5. Initially, specific fuel consumption drops in response to increases in the excess-air factor; it starts to rise again from l = 1.1 - 1.2 onward. As the excess-air factor increases, so does the optimum ignition advance angle that is, the amount of advance for the lowest specific levels of fuel consumption. The relationship between specific fuel consumption and the excess-air factor (assuming optimum ignition timing) is explained as follows: the air deficiency encountered in the fuel-rich range leads to incomplete combustion, while delayed combustion and combustion miss occur as the operating limit is approached in the lean range, resulting in higher levels of specific fuel consumption. The optimum ignition-advance angle increases at higher excess-air ratios due to the accompanying delays in flame propagation, and the ignition timing must be advanced to compensate for these delays.

HC emissions, with a minimum at l = 1.1, display a similar response pattern. The initial rise within the lean range can be attributed to cooling on the walls of the combustion chamber; the flame is extinguished by the cooling effect exerted by these surfaces. Extremely lean mixtures lead to delayed combustion and failure to ignite, phenomena that occur with increasing frequency as the operation limit (lean misfire limit/LML) approaches. Below l = 1.2, greater ignition advance will produce higher HC emissions, but it will also displace the operation limit further into the lean range. This is why more generous ignition advance will produce lower levels of HC emissions in the lean range beyond l = 1.25.

The nitrous oxide (NOx) emissions pre-sent a completely different pattern. They rise in response to higher oxygen (0₂) concentrations and higher peak combustion temperatures. The result is the characteristic bell-shaped curve for NO_x emissions: Increase up to l = 1.05 due to higher 0₂ concentrations and higher maximum temperatures, with a subsequent sharp drop in the lean range, a result of the rapid decrease in peak temperatures that accompanies the higher levels of mixture dilution. This response pattern also explains the considerable influence exercised by ignition timing. NO_x emissions escalate sharply as the advance angle increases.

Attempts to comply with emissions limits by operating the engine in the l = 1.2 ...1.4 range place substantially greater demands upon the ignition timing. Because the three-way catalytic converter requires mixture control to l = 1 to function properly, the ignition advance angle is the only optimization criterion that remains available with this kind of emissions-control strategy.

[Superman]

Dane, I had doubts about whether my mixture was okay, but it seems that it's just fine. John Walker set it, months ago.

[Steve W]

In general, from Charles Probst book:



When trying to derive the point of maximum power, usually there are other factors that need to be considered, including fuel octane used and the mixture one wants to factor in to provide a three point margin against predetonation. Richer allows the mixture to run cooler in the combustion chamber, but too rich and you loose some hp. But you can make up for that with increased boost, as in a turbo, or a small increase in ignition timing.

[350HP930]

In the case of a high output turbo running a bar of boost or more running 12:1 can be advantageous for peak tuning.

I have been looking at a lot of dyno sheets with wideband O2 data lately and have yet to see a souped up turbo running anything less at full boost and WOT.

!2:1 is a bit rich for NA motors but its some juicy stuff for 15+ PSI of boost.