Power Valve Selection: The Definitive Answer
* Part throttle cruise AFR is vague. At least to us on this side of the screen. Load is unknown, throttle opening is unknown, circuits supplying fuel are therefore somewhat unknown. Somewhat because if changing jet causes lean surge, we can conclude the primary main circuit is involved. Although throttle position is helpful, my experience has been that a steady throttle cruise condition at 30-45 mph is probably on the transition slots, and at 65 or 70 mph almost certainly on the main circuit alone.
* Aiming for cruise AFR numbers we're aiming at the wrong target. Those things off the widebands are just references numbers so we know more about the change made. From what you've written, you are probably correct to conclude the jetting for cruise is as lean as it will tolerate at the moment. If the e-holes and MAB are changed (for example to get a flatter AFR) then the jetting may change.
*To test for best fuel restriction at WOT its neccessary to get the engine at the maximum load it will see and give it and the fuel system time to stabilize for the that load and then go from air flow at that rpm through the rpm of maximum horsepower.
* Aiming for WOT AFR numbers aiming at the wrong target. The target is max torque and power, best measured at 1/4 mile track.
When comparing WOT jetting to cruise jetting, its important to see how flat the AFR is at 100% load. In other words, high gear through max hp.
And if its not flat, then the emulsion or airbleeds need to be worked with.
Just look at the trends shown here. Because even in 3rd or 4th gear, AFR from a WBO2 is going to jump around depending on the sampling frequency and speed of the system. (If its really jumpy, that could be timing or the logger wiring etc.)
The jetting shown above is probably close, but once a flat AFR is established, then the jets may have to be changed until we get out best ET and MPH. Then it wouldn't hurt to check if the AFR is still flat. The more drastic the fuel restriction changes are, the more likely air bleed sizes may need some adjusting.
Above was a dyno run.
Below is short test on the road with rolling start. First gear is useless for establishing jetting/PVCR.
Second gear is flat and suggests, but doesn't guarentee, that the air bleeds may provide a flat AFR delivery through 6000 rpm.
It does provide some confidence the e-holes are about correct since they tend to most effect the begining of the fuel curve.
Finally, remember the WBO2 interpretation is just that. Change the timing and the AFR may shift. Well of course it probably hasn't. A carb delivers whatever the engine's airflow demands. Its just the burn has changed such that the left over O2 has changed.
Issues here with the test method which can give misleading results.
* Part throttle cruise AFR is vague. At least to us on this side of the screen. Load is unknown, throttle opening is unknown, circuits supplying fuel are therefore somewhat unknown. Somewhat because if changing jet causes lean surge, we can conclude the primary main circuit is involved. Although throttle position is helpful, my experience has been that a steady throttle cruise condition at 30-45 mph is probably on the transition slots, and at 65 or 70 mph almost certainly on the main circuit alone.
* Aiming for cruise AFR numbers we're aiming at the wrong target. Those things off the widebands are just references numbers so we know more about the change made. From what you've written, you are probably correct to conclude the jetting for cruise is as lean as it will tolerate at the moment. If the e-holes and MAB are changed (for example to get a flatter AFR) then the jetting may change.
*To test for best fuel restriction at WOT its neccessary to get the engine at the maximum load it will see and give it and the fuel system time to stabilize for the that load and then go from air flow at that rpm through the rpm of maximum horsepower.
* Aiming for WOT AFR numbers aiming at the wrong target. The target is max torque and power, best measured at 1/4 mile track.
When comparing WOT jetting to cruise jetting, its important to see how flat the AFR is at 100% load. In other words, high gear through max hp.
And if its not flat, then the emulsion or airbleeds need to be worked with.
Just look at the trends shown here. Because even in 3rd or 4th gear, AFR from a WBO2 is going to jump around depending on the sampling frequency and speed of the system. (If its really jumpy, that could be timing or the logger wiring etc.)
The jetting shown above is probably close, but once a flat AFR is established, then the jets may have to be changed until we get out best ET and MPH. Then it wouldn't hurt to check if the AFR is still flat. The more drastic the fuel restriction changes are, the more likely air bleed sizes may need some adjusting.
Above was a dyno run.
Below is short test on the road with rolling start. First gear is useless for establishing jetting/PVCR.
Second gear is flat and suggests, but doesn't guarentee, that the air bleeds may provide a flat AFR delivery through 6000 rpm.
It does provide some confidence the e-holes are about correct since they tend to most effect the begining of the fuel curve.
Finally, remember the WBO2 interpretation is just that. Change the timing and the AFR may shift. Well of course it probably hasn't. A carb delivers whatever the engine's airflow demands. Its just the burn has changed such that the left over O2 has changed.
