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Advancing the cam timing makes to valve openings occur sooner and the cam behave like it has less duration at lower engine speeds.Retarding the cam timing delays the valve openings making the cam behave like it has more duration making more topend power at the expence of low speed performance.Not that it makes any difference on an LS1 motor,but on distributer equipped engines the ignition timing is changed a similar amount and must be corrected.
The duration (214/220) and LSA will stay the same no matter what you do, they are ground in when the cam is made.
The cam spins and forces up the lifters, pushing up pushrods, which push on the rockers, who force the valves open. The cam spins one time per two revolutions of the crank. That relationship is fixed by gears on the crank and cam, connected by the timing chain.
OK, so the valves are opening and closing when the cam spins. Advancing the cam is when you use an adjustable chain and make the valves open and close sooner. Retarding is the opposite - opening and closing later in relation to the crank turning. This lets you have a little control on the powerband, as earlier closing of the intake valve builds better pressure at low rpms and makes more torque. Closing later allows more air to fill the chambers at high rpms, making more power up there. So advancing shifts power lower at the expense of peak upper rpm power. Retarding gives up some low end for more rpms and peak power.
So advancing doesnt change the duration or lift of the cam, it changes where the valves open and close. Most aftermarket cams you see have either +2 or +4 advance already ground into the cam - the lobes were moved 4* forward during the grind. Thats it to allow for slop in the stock timing chain and aid the mid range a little.
Personally, I'm tempted to go with a bigger cam and advance it a little more and see the results.