Cam timing changes should be small, as only a couple degrees can make dramatic performance changes.
My experience with race cars and some HP street cars, has been that the range from 4 degrees advance (over stock) to 2 degrees retarded is about all that should be done.
Overly advanced cam timing kills both top end power and gas mileage (per Competition Cams tech man to me years ago). Retarded cam timing really hurts bottom end power and throttle response.
It's important to know the stock cam timing specs so a considered decision can be made on changes.
When cam timing is checked, those results should be compared to the factory specs, and if the cams are retarded from that stock spec, better overall performance can almost always be had by bringing the specs up to stock. Advancing from there might bring even better performance, but will probably take the engine out of smog spec.
Remember that all of these changes are more complex with a twin cam engine than it would be with something like an old 350 Chevy/360 Mopar/351 Ford single cam engine.
To check stock cam specs, one will need to attach a degree wheel to the crankshaft end, install a pointer for that degree wheel (and justify top dead center), get a dial indicator mounted to contact a valve spring retainer, and then record readings at whatever lift the factory uses for its specs - maybe .020" of lift, or possibly the race industry standard of .050" lift, the so-called 'duration @ .500'.
Yes, the task is somewhat complicated, but the results are worth the hard work is takes both to learn how to do it, and then to actually do it.
