best air fuel ratio

[1972ho]

Now that's a mouth full,Sounds like a lot of test and tune days or track rental and seeing the nearest track in southern Cali
is 200 plus miles away that may not happen.I'll more than likely keep making minor changes at the division races.

[john hennessy]

the problem with jetting and to some extent ignition timing without a computer controlled engine is building a 3D map by trial and error.

[1972ho]

Hey one more question about this set up i ran this car at pomona in feb.13 and the condition weather wise was 29.05 barometer temp.'s in the 80's DA range anywhere from 1800 to 2500 ft and i was running a different holley 780 and the rpm was about 500 to 600 rpm lower in las vegas with the air at 27.5 to 28.05 DA from 2500 to 3950 ft 65 to 80 temp.in vegas would that lose in rpm be due to the QF carburetor or just the conditions, i only changed the carburetor everything else remained the same.Also ET and MPH were lower by about 3 tenth's and 3.5 mph.

[fdicrasto]

If I changed a carb ONLY and was down 3.5 mph, before even screwing around with that carb, I would have put the old carb back on,made a pass and then started analyzing the numbers and making choices based on the logical and the obvious. Phil D.

[Truckedup]

Lots of possibilities here. What is max power producing in the am may not be in the pm. Run an engine in Denver one day and the next in Chicago and get different results. Carbs need constant tuning. To be safe,(intimidated) I had an air fuel meter wired into my streetrod. 502BB with Accel 7 EFI. I prefer to let people like John Meaney keep my 'stuff' right. I fought carbs for decades and they won. Track tuning is surely different than street tuning. Our author is track focused and I'm not qualified with racing carbs. Sorry.

A well tuned, efficient combination will not need nearly as much jetting change for altitude as most think, sometimes none at all - just a gear change. I've seen guys overcompensate for it many times.

How much altitude change are you talking about,2000 feet?

[treyrags]

Most extreme example 3600' - Houston to Lubbock. Same jets, same timing, same headers. Only changed ring and pinion from 6.0 to 6.2. New national record, number 1 qualifier. Most others were leaning jets, changing timing, changing headers, heavier flywheel, etc.

[banjo]

I think engines that need a jet changes with altitude changes have other issues with the vaporization of the fuel mixture. When you make significant jumps in altitude, the atmospheric pressure that the engine sees changes. This changes the dynamic pressures within the engines. Compression is one of the ways that you can improve combustion efficency and if you are on the line where your at and then you go to a higher altitude location,you lose a bit of compression which could cause issues with the efficency of the engine. I have seen this with my own car. When I first built my motor it had around 13.7 compression. I race at a variety of different tracks that see -DA to 9000 feet of air.It the cooler months the car would run very good and consistant, but when the summer months hit with the higher DA, It would slow more than I would expect and not be as consistant. I freshen up my motor and bump the compression to 14.5 and the car definitely runs alot better and is more consistant now. It didn't run any better at the lower altitudes, but it definitely picked up in the higher altitude/hot months.

Another thing, the smoothness of the 02 trace is more important than what the number is. If your trace is jagged and not flat, chances are your not getting a consistant mixture within the chamber. I definitely saw a difference before and after compression bump in the heat. Once you get the mixture smooth and flat, than you can really start optimizing your jetting, and that goes back to giving it what it wants and not tuning to a specific ratio. If its not smooth and flat, jet changes are a compromise.

Now when I go from high altitude to low altitude, I don't touch the motor, I do however have a different converter and gears that I run in the summer vs the winter months.

[RAS]

To answer the original post I'm going to say between 12.6:1 and 12.7:1 will be the reading on your best run. Please let us know how this comes out. Curious.

[Warp Speed]

Lots of of factors to consider when trying to find optimum AFR for a given combo/system.

It all starts with the fuel being used.
The 02 system being used and how it is calibrated.
02 sensor location.
Acceleration rate.

All of the above have either an effect on target, measured reading, or sensitivity to optimum.

What's weird is Big Joe mentions it being related to HP/ci. Our stuff is over 2hp/ci, and with our fuel and sensor locations optimum power is .91L/ or 12.5-1 AFR.
About as controlled data as you can get..........

[treyrags]

Lots of of factors to consider when trying to find optimum AFR for a given combo/system.

It all starts with the fuel being used.
The 02 system being used and how it is calibrated.
02 sensor location.
Acceleration rate.

All of the above have either an effect on target, measured reading, or sensitivity to optimum.

What's weird is Big Joe mentions it being related to HP/ci. Our stuff is over 2hp/ci, and with our fuel and sensor locations optimum power is .91L/ or 12.5-1 AFR.
About as controlled data as you can get..........

Yeah even at 2+ HP/IN things like 2x4 vs 1x4 carb, plenum height, individual cylinder timing, etc. all have an influence on the "correct" afr.

[Warp Speed]

Yeah even at 2+ HP/IN things like 2x4 vs 1x4 carb, plenum height, individual cylinder timing, etc. all have an influence on the "correct" afr.

Above mentioned are on the bottom of the list..........if at all!

IMO...........

[treyrags]

That's where the "etc" comes in

[1972ho]

I'm pretty sure most of you will say it may not mean much,But if you look at the qual sheets for the Las Vegas race and the Pomona race for pro stock and stock most all if not all had a performance gain in mph and et from in pro stock at least 4 to 6 mph and et .10 to .18 from qual 1 to qual 16 and in stock most car had et gains of .15 to .45 gain at mph of up to 5 mph I'm not say I have a pro stocker are anything just that the weather condition are maybe the main factor in the gains/loss from Pomona to Las Vegas

[rfoll]

Where I race the temp swings faster than I can change jets, make a pass, and analyze the results. The temps at 5:00 can be 75 degrees, and be at 50 less than 2 hours later, with similar humidity and barometer swings along with it. One night during eliminations, the dew fell and increased my et 1/4 second. There was actually a layer of fog just below the hood at the far end of the track.

[headman]

To really put a spin on this...
Isn't "air/fuel" in most conversations, really just a measurement of heat left in the exhaust, and an assumtion that this heat is due only to unburned fuel???

Should we not be interested in the CO% at best power as a better read into actual air/fuel?

when making best power, you may find the so called air/fuel all over the map (12.5:1 or 14:1)
You may find the CO % is a much more stable indicator, across load and rpm ranges.