Your question makes the point for me - you can use an indexed gear set to alter the timing (in the case of mine) in 2 degree increments over a 16 degree spread. But unless you check it with indicator and degree wheel, you're assuming your cam manufacturer and timing set manufacturer are accurate with their products. They are not always. Lots of stories out there of people with - you pick 'em - Crane, Ford Racing, TFS, even Comp Cams installing them 'dot to dot' = straight up = O degrees advance/retard, and later for other reasons, checking them to find out they were 2, 3, 4 degrees off. In other words, if the cam card said the intake was supposed to open .004" lift at 28 BTDC, they found out when it was installed dot to dot that it was actually opening at 24 BTDC or 30 BTDC.
Buddy Rawls who handled my custom for me, told me that he's gotten custom grinds from CompCams as much as 4 degrees off. He knows it 's the cam because he's got a timing set which he's checked and knows to be accurate. Buddy always mocks them up before he sends them out; and if you send him your timing set, he'll mark it up for you with the cam so you know you've got a proper install - esentially he's doing the degreeing for you in that case. The lift, LSA, and durations were all accurate on mine, but according to his mockup, the cam was ground 1 degree advanced - that is, instead of the .050" lift occurring at 5 degrees ATDC as he specified when he ordered it, it was occurring one degree early at 4 degrees ATDC -- when he mocked it up 'dot to dot'. Since my index set allows changes in 2 degree increments I had a choice. I could set it at the zero dot to dot mark - and I measured the same 4 degrees ATDC at .050" lift that Buddy did, which would mean the cam was actually installed 1 degree advanced; or I could set it at at the 2R mark (2 degrees retarded according to the gear set), and when I measured that, the intake was opening 2 degrees later at 6 degrees ATDC -- which is equal to an actual 1 degree retarded set up. I chose the former - I put it in dot to dot which in my case is actually 1 degree advanced. Given the rotation of the engine, as mileage goes up and the chain stretches, installing a degree or 2 advanced will allow actual timing to be just about right.
When your specs say the cam is ground with 4 degrees advance, hopefully you can see now what that means. If you were to put the indexed setting at 4A, theoretically you would have added an additional 4 degrees advance for a total of 8. If you put it at 4R, theoretically you would have 'undone' the manufactured 4 degree advance, returning it to a 0degree install. I say theoretically because - you know what's coming - unless you check it with degree wheel and indicator - you're assuming they manufactured it accurately.
