My generalized rule for normally aspirated engines is for an increase in exhaust duration where mufflers are being used.  If the engine is using a decent set of headers that scavenge well and are not connected to an exhaust system, then a camshaft with symetrical lobes on both the intake and exhausts works fine where normal head porting has been performed.  As Frank mentions, a 4°-6° split is a good place to start if wanting to make the camshaft non-symetrical in nature.

Lorena, Texas (South of Waco)

Royce,

        I like 'em. I run about 6 degrees more exhaust than intake with the blower. Opinions will vary on this. Probably if you clean up the exhaust ports and run headers you wouldn't need to. All depends on the combination.

Frank/Rebop

Bristol, In ( by Elkhart) 

John or Ted, or anyone else for that matter, I would like to hear your opinion about any benefits of a dual pattern cam on a Y vs a single pattern for a hot street engine. Is the exhaust good enough that you don't need the extra duration vs the intake?