Best AFR for consistency

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Consider the weather stations that predict your next run simply based off of weather change. Good historic data in your log book would help you do the same. Running your incrementals when you cut someone in will help your next round dial too. Trying to re-jet will probably bounce you around since you can’t pick jet size by tenths. Do your best to keep engine and trans temps the same round to round…sometimes I drive through the pits or stall up on the converter in the staging lanes after a long delay. If your car isn’t very consistent in the first place it probably isn’t the AFR. All that being said, you gotta find the AFR that your particular combination runs best on…mine likes 13.2 at WOT in high gear. I run sniper injection with learn and compensation turned off (just have the fuel table locked in), basically a smarter-ish carburetor. And I just dial based on conditions (like all the folks are saying).

I use these two calculators a lot:
Calculate Air Density Index- Wallace Racing
Calculate ET Change From HPC Change- Wallace Racing
 
You guys go ahead and chase your carb, I’ll sit down, check my notes and dial accordingly. No point overthinking it.
 
Once you have your carb dialed in it may require one jet change per season, March to November race season
 
The A/F ratio should remain flat with respect to RPM. That is, you want 12.7 ish from the stall RPM of your converter to the shift point or trap RPM at WOT. Carburetors are proportional fuel air mixers, so they want to do this, and it would be a challenge to make a properly sized carb do any different. A carburetor can compensate for changes in air density whether changed by barometric pressure (altitude) or temperature up to a point. If you race at sea level, you might find that a jet change might help in Denver, but that's a 5000 foot difference (on the other hand, the car might perform similarly on a cold day in Denver and a scorcher in San Diego). If you are looking to run competitively at your local track, FIND THE JET COMBO THAT MAKES YOUR CAR FASTEST ON A TYPICAL DAY (70 degrees or so), don't get too hung up on A/F numbers. The car will go a little slower if it's hotter, and pick up a bit if it's cooler. This is where keeping notes helps. Most tracks have a weather station that reports temperature and barometer values if not density altitude directly (or call ATIS at the nearest airport). It's the density altitude that matters, that's literally the amount of air available to the engine. Note this on your time slips and you can build a database which will predict how the car will perform at any given DA. The more data points, the more accurately you can predict. But if you change the jets, you have to start over with the notes, because it's literally a different car. Most bracket racers I know make engine and chassis changes in the off season and try to keep the car exactly the same once points start counting. Build a car, any car, and learn how to drive it the same each run, and know what it'll do based on weather. A 17 second car with a driver that cuts a consistent light can and commonly does beat a 9 second car a driver can't drive the same way twice.
 
dusterbing the peace said:
Consider the weather stations that predict your next run simply based off of weather change. Good historic data in your log book would help you do the same. Running your incrementals when you cut someone in will help your next round dial too. Trying to re-jet will probably bounce you around since you can’t pick jet size by tenths. Do your best to keep engine and trans temps the same round to round…sometimes I drive through the pits or stall up on the converter in the staging lanes after a long delay. If your car isn’t very consistent in the first place it probably isn’t the AFR. All that being said, you gotta find the AFR that your particular combination runs best on…mine likes 13.2 at WOT in high gear. I run sniper injection with learn and compensation turned off (just have the fuel table locked in), basically a smarter-ish carburetor. And I just dial based on conditions (like all the folks are saying).

I use these two calculators a lot:
Calculate Air Density Index- Wallace Racing
Calculate ET Change From HPC Change- Wallace Racing
Click to expand...
I am not under the impression a given A/F will get me away from adjusting my dial with the weather. I am asking to see if and at what A/F people have seen the changes with weather effect ET less.

Thanks for the calculator suggestions, I really appreciate it.
 
Wouldn't it come down to jetting in the end? I mean you want the best AF ratio at WOT or when you are blasting down the track, setting it up at idle only tunes the idle circuit...
 
I think it was said before, but just tune it well for the 70* day and it’ll be really good most of the time. If your a track guy looking for every last available edge, it’s a round by round tuning adjustment.
 
In my experience fat tunes show more swing in et than lean tunes do. Not talking piston burning lean but on the lean side of the safe range, I don't think anyone can tell you what afr is best for your combo, that'll take some testing.

This and $1.75 will buy you a cup of coffee in some places.
 
C130 Chief said:
The A/F ratio should remain flat with respect to RPM. That is, you want 12.7 ish from the stall RPM of your converter to the shift point or trap RPM at WOT. Carburetors are proportional fuel air mixers, so they want to do this, and it would be a challenge to make a properly sized carb do any different. A carburetor can compensate for changes in air density whether changed by barometric pressure (altitude) or temperature up to a point. If you race at sea level, you might find that a jet change might help in Denver, but that's a 5000 foot difference (on the other hand, the car might perform similarly on a cold day in Denver and a scorcher in San Diego). If you are looking to run competitively at your local track, FIND THE JET COMBO THAT MAKES YOUR CAR FASTEST ON A TYPICAL DAY (70 degrees or so), don't get too hung up on A/F numbers. The car will go a little slower if it's hotter, and pick up a bit if it's cooler. This is where keeping notes helps. Most tracks have a weather station that reports temperature and barometer values if not density altitude directly (or call ATIS at the nearest airport). It's the density altitude that matters, that's literally the amount of air available to the engine. Note this on your time slips and you can build a database which will predict how the car will perform at any given DA. The more data points, the more accurately you can predict. But if you change the jets, you have to start over with the notes, because it's literally a different car. Most bracket racers I know make engine and chassis changes in the off season and try to keep the car exactly the same once points start counting. Build a car, any car, and learn how to drive it the same each run, and know what it'll do based on weather. A 17 second car with a driver that cuts a consistent light can and commonly does beat a 9 second car a driver can't drive the same way twice.
Click to expand...
This is well written Chief, I got me many a mile in a C130! Thanks for the post!
 
Way back in the day when i had only my et slips from today and yesterday and simple dial gauge, showing air pressure.
I found jetting my car on the lean side keep my et much closer from the heat of the day to the cool nights.

For two year i took Bracket Racing real seriously! First year i took 17 place and the 2nt year i took forth. I was in 2nt but chocked. (A story for another time)
How did i do this. got the car dialed in the best that i could and left it the Fing Alone. put a lock on my tool box, made notes on my et slips.

As far as were to start out with a afr... 11s! If you have one cyl that is way leaner then the rest. (actually vary Probable) then the AVERAGE of 11s will keep you save as you sneak up on "YOUR" best AFR.(ET)
 
Rat Bastid said:
Unless you lose 1500 feet of air you shouldn’t need to change anything.

Warren Johnson said a couple of decades ago when he went to Denver he didn’t change a jet. That should speak volumes.
Click to expand...
When he quit racing he sold 14 sets of carburetors. Different carbs for different conditions. So, he did not change jets, he changed to his Denver carbs.

Years ago Ron Krisher told me that the carbs were so different for Denver that he had a set that got used once a year. At Denver.
 
Gomangocuda said:
When he quit racing he sold 14 sets of carburetors. Different carbs for different conditions. So, he did not change jets, he changed to his Denver carbs.

Years ago Ron Krisher told me that the carbs were so different for Denver that he had a set that got used once a year. At Denver.
Click to expand...


That’s not what he said at all. I know how many carbs and intakes these guys had.
 
The guys I race with generally go on the lean side, in their experience the leaner side of the range tends not to "chase the air" as much as carbs running on the rich side, and yields a narrower range of times. Several of them dial based on measured weather changes, a few have their own weather stations and at least one takes real time readings which are then displayed on a screen along with predicted E.T. to dial.

I installed an a/f gauge in my car years ago, tuning has aimed at high-12s at WOT and I haven't changed jets in several years in either my TQ or Mighty Demon. When we raced in Denver back in 2014 I drove the car there with the TQ as it would have been easy to lean it out driving up in altitude if needed. Approaching via I76 I saw the a/f ratio get richer but it wasn't a significant change that day. Home track is at 950' altitude.

I race in Sportsman class with a limit of 12.0 in the quarter mile, 7.50 in the eighth, I know racers who tune to get as close as possible to those numbers to have a better chance of surviving dual red light launches. Other than those going to the lean side of tuning I don't know of any particular tricks they use to stay consistent. The nature of tricks, I suppose!

Have recently taken steps toward data recording my A/F numbers. Hoping to find a relationship between performance and whatever level of detail I can get from the electronics. Dialing based on weather seems pretty linear, at least the most common tool racers use to predict the next round's run, based on past performance.
 
BelvedereII said:
The guys I race with generally go on the lean side, in their experience the leaner side of the range tends not to "chase the air" as much as carbs running on the rich side, and yields a narrower range of times. Several of them dial based on measured weather changes, a few have their own weather stations and at least one takes real time readings which are then displayed on a screen along with predicted E.T. to dial.

I installed an a/f gauge in my car years ago, tuning has aimed at high-12s at WOT and I haven't changed jets in several years in either my TQ or Mighty Demon. When we raced in Denver back in 2014 I drove the car there with the TQ as it would have been easy to lean it out driving up in altitude if needed. Approaching via I76 I saw the a/f ratio get richer but it wasn't a significant change that day. Home track is at 950' altitude.

I race in Sportsman class with a limit of 12.0 in the quarter mile, 7.50 in the eighth, I know racers who tune to get as close as possible to those numbers to have a better chance of surviving dual red light launches. Other than those going to the lean side of tuning I don't know of any particular tricks they use to stay consistent. The nature of tricks, I suppose!

Have recently taken steps toward data recording my A/F numbers. Hoping to find a relationship between performance and whatever level of detail I can get from the electronics. Dialing based on weather seems pretty linear, at least the most common tool racers use to predict the next round's run, based on past performance.
Click to expand...

Did you get the adapter cable and drivers figured out yet?
 
No, just hoping the PL-1 formatted the disc correctly, waiting on the cable so results may take a week or so.
 
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