Lean condition under light acceleration
My suggestion: Tune for best performance. In other words don't worry about the AFR number, much less aim for any AFR number. Use the AFR to see the effect of changes to carb under various conditions.
This is a cut and past but worth reposting -
If you're just going for engine efficiency the general trend of AFR can be described as follows:
"The [relationship] is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or" somewhat more. Cruising, especially at lower speeds, requires little throttle and puts relatively little load on the engine. Half throttle or mid-load would be going up hills or part throttle acceleration. "The thing to understand is most engines respond to being leaner...at part-throttle."
In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown below, or as high as 90%. Load relates to manifold vacuum, and therefore vacuum is used to signal when enrichment is needed. This enrichment is created by the "step up" lifting the rods on a Carter type carburetor and the 'power valve' opening on a Holley type.
F is Fuel/Air ratio for gasoline. Invert the numbers to convert to Air/Fuel ratio.
0.08 = 12.5 AFR
0.667 = 14.7 AFR
0.06 = 16.6 AFR
Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be cruising 60 mph on flat or steady 40 mph up a long grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.
The acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.
"This ... graph is from Walter B. Larew, Carburetors and Carburetion. At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967. He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.
This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."
quotes and graph from Tuner on Innovate Motorsports Forum and full text now reposted at RFS
note. If you follow the link to RFS and tapatalk requests you join, just click the x, and ignore the request. You do not have to join to read the forum.
My suggestion: Tune for best performance. In other words don't worry about the AFR number, much less aim for any AFR number. Use the AFR to see the effect of changes to carb under various conditions.
This is a cut and past but worth reposting -
If you're just going for engine efficiency the general trend of AFR can be described as follows:
"The [relationship] is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or" somewhat more. Cruising, especially at lower speeds, requires little throttle and puts relatively little load on the engine. Half throttle or mid-load would be going up hills or part throttle acceleration. "The thing to understand is most engines respond to being leaner...at part-throttle."
In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown below, or as high as 90%. Load relates to manifold vacuum, and therefore vacuum is used to signal when enrichment is needed. This enrichment is created by the "step up" lifting the rods on a Carter type carburetor and the 'power valve' opening on a Holley type.
F is Fuel/Air ratio for gasoline. Invert the numbers to convert to Air/Fuel ratio.
0.08 = 12.5 AFR
0.667 = 14.7 AFR
0.06 = 16.6 AFR
Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be cruising 60 mph on flat or steady 40 mph up a long grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.
The acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.
"This ... graph is from Walter B. Larew, Carburetors and Carburetion. At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967. He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.
This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."
quotes and graph from Tuner on Innovate Motorsports Forum and full text now reposted at RFS
note. If you follow the link to RFS and tapatalk requests you join, just click the x, and ignore the request. You do not have to join to read the forum.
