Re: 2bbl vs 4bbl
I could not state a percentage of increase but I think it would be significant. With stock compression, you would be best served sticking to a 500 cfm carb as opposed to any 600 cfm. Unfortunately your only choice in a Marine 500 cfm is the Holley and they are hard to come by used. When I did mine, with all the other changes did, I used a 600 cfm Holley.
When I went to a 4bbl on my 302 it was in combination with raising the compression from 8.0:1 to about 9.0:1, using Fords latest greatest breathing GT40P heads and putting in a modified cam so it is hard to be sure how much extra power came from what change. I am sure that the carb was a good part of it. At this point I have still not run mine at WOT. The thing is a real monster and I may have to reprop it.
If you do it, you also would need to learn to keep your 'foot' out of the secondaries if you would expect to still get reasonable mileage. The first time I ran mine I was very throttle happy and it drained the tank pretty fast. I was down at Shasta when gas was selling for $6.00/gal. Ouch! I learned to not be so 'happy' very rapidly. Running on the primaries only, you could expect the same mileage as the two barrel, but you would have the extra power when you need it.
I have a friend who is just finishing rebuilding his 898 305 cheb. He also pulled the dished pistons and replaced them with flat tops to increase his compression to just over 9.0:1. He kept the stock cam and will intitiallly stick with the stock 2 bbl, so we will soon see how much that 1+ point will do all by itself.
