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Hi Jack;
Pretty good discussion going already I think. Paul has pointed out the lack of a clearance ramp on the Volvo cams, and that would tend to increase the valve timing dramatically with lash. A change of lash of perhaps .006 would then increase lift by that amount, which is below negligible, but that same .006 could increase duration a substantial number of degrees. Passini's book states as best I remember that such an increase in duration will affect cylinder pressure but not gas flow past the valves in or out. As Paul points out, a tight exhaust valve will blow down pressure early, but not add substantially to the flow in total.
As much as I don't understand why Volvo recommended lash as big as .022 on some B20's, I also know that the lash specs are not arbitrary. There must have been a reason. From my own experimenting, I have found that .016 on both with a stock cam reduces idle quality, and noise to an extent. Customers don't like noise, and they don't like rough idle quality. What to do about that over the years has resulted in my personal preference for .018 on both. Still idles well, keeps the noise down.
As to compression even with our (Canada) locally available 94 octane (RON+MON over 2)I find the B20s will ping above about 10.25 or more. I try to run stock centrifugal timing. I remove the vacuum retard unit and weld the two plates together to improve point gap stability. Seems to work quite well.
It might be worth doing a cranking compression test at .020 inch, and one at .014 inch on the same cylinder and seeing if there is an appreciable difference.
Last my understanding of flat tappet cams is the the base circle transfers to the acceleration ramps through a clearance ramp, which also varies according to hydraulic or solid lifters. Volvo doesn't use a clearance ramp, but that transition must still occur, just with a bit more violence.
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