It took me a while, but I finally found out what size carb my '71 LT-1 has; at least according to Holley. I found a chart at the back of one of my carb manuals that indicated 780 cfm for List R4801A carb (4150). Usually, stats for the '71s only state that the carb for the LT-1 is a Holley, and no further details!

I was looking through my LT-1 notes and came across the carb specs for a 71 LT-1, though I own a 72 and can't remember why I wrote them down but here's what I got,    71 LT-1 Holley specs.......0-4801 or 0-4801-1.....# 3989021.....780 cfm..... rebuild kit # 37-1539, or poor mans kit #37-1544...jets  70's P 85's S....power valves 8.5 P&S....discharge nozzels .025...Vac secondary spring "yellow".  can't remember if kits are G.M. or Holley.,   Anips  anips2008-07-14 18:53:08

You can get Holley Power Valves at most good parts houses...
8.5 sounds aboot right to me...btw they are rated in inches of mercury(vacuum), not psi.
The rating is for when the valve opens, and allows more fuel to flow. An 8.5 rating means the valve will open at approx. 8.5 inches of vacuum. The valves are stamped on the metal part with the number. Most higher HP engines have cams that don't allow a lot of vacuum, so therefore they would use a lower number valve. If you have a cam with a lot of "lope" to it, and a lower numbered valve, the valve will just sit there and dump fuel all the time. IN that case, we would always just remove the power valve, plug the hole, and jet the carb up a few notches...not exactly a good set-up for a street car, but it works well on drag cars, or boats!
Hope that makes a little sense.

Thanks Anips and Joel..........just what I needed. Now - to work!