General AFR Guidance (Holley Tuning)
Is there a rule of thumb for optimal AFR for making power?
Basically its what crackedback wrote: Best WOT is usually in the 12 to 13:1 range. But regardless of load, the air:fuel ratios will depend on the fuel and the engine. The AFR should be a straight line at WOT in top gear (max load). If it is, then just adjust the jetting up or down for max power. The mph at the end of the 1/4 mile is a good indicator of power if everything else is consistant.
In the past I've tuned idle mixture based on the highest vacuum reading
That is still a good way to tune idle. Just be aware that with the transmission in neutral the engine can be made to run so lean it will not have enough power when put in gear to keep the rpms up. For an automatic transmission, tune it for highest vac in Drive at idle.
When a cam has a lot of overlap, the engine usually wants a richer idle than a typical stock cam.
Cruise needs will vary with load and throttle position. Test somewhere around 30-40 mph for light load, low throttle, lower rpm. Test again at highway speeds where there is more resistance and (assuming a 3 speed) the engine is turning mid rpm and on hot rodded engine may be more efficient than it had been at lower rpm. Another reason to test at two different cruise speeds is that at low throttle, the carburetor is supplying fuel through the "idle circuits".
There are some published tests which can be used as general guidelines, but basically the engine will want leaner fuel mixture as the load increases (more throttle). But as it gets close to max load, it will then want richer.
Chrysler described it simply here:
Carburetion Fundamentals & Facts (Session 273) from the Master Technician's Service Conference
More in depth description and graphs are here:
http://racingfuelsystems.myfunforum...racteristics_At_Different_Load_about1350.html
