cam advance or retard
i know it advances the timing 4 degrees but what does it actually do
pros and cons
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Changing ignition timing thru the tune, though unrelated, can change the idle quality, just like your example with the distributor in an SBC.
You can't change the overlap without grinding a new cam, but you can advance an existing cam to shift the powerband down a touch, or extend it towards the top end a bit by retarding the valve events. If you find you need to move things more than 4 degrees in either direction, its generally accepted that the next cam size should be selected - smaller if you are advancing, larger if you are retarding.
Last thing, with a big cam, advance or retard can effect PTV clearance on both sides. Advancing a given cam will open the intake valve sooner, opening the valve further in relation to TDC and reducing your clearance there, while giving a little more clearance on the exhaust. The exact opposite occurs when you retard, which holds the exhaust valve open later, reducing its clearance at TDC.



