Looking at specs on a 450hp (basically the 500hp except 454 not 502). Single plane with 800 holley. The jetting specs were 81 75 primary, 89 93 secondary. Any idea why the fuel distribution would be so inconsistent? I would think, especially with a single plane that it would be more even.

Tons of reasons !

THe biggest reason is because it's an OE manufacturer. They seem to like to go that far with tuning.

Speaking of which, GM's LS6 had a dual plane intake and it's carburetor was staggered + not all same boosters per throttle. Some had little tabs on them.

SB,
I understand what your saying. But why would a single plane need a 75 on one side and an 81 on the other? That's 6 jet sizes!! I am helping a friend who built basically a mock hp450 and I am wondering where to start with jetting. The 800 holley comes with 72 primary and 87 secondary jets.

It's a different carburator but same cfm. Stagger wouldn't end up being the same + you don't have the tuning tools and eqiupment OE has.

Treat it as any other 'Universally Calibrated Performance' Holley carb. Keep primaries same as each other and secondaries same as each other. Jet for best performance or maybe 1 size larger than best performance for safety sake.

It is a mechanical secondary. Will keep jets the same side to side as you said. Thanks

Staggered jetting is can be very effective in an automotive application; however, Mercury's did some staggered jetting with the carb'ed intakes. You can bet that Mercury's testing was extensive and they set up the AFR rich for low octane on the water fuel availability. IMO, for carbureted high preformance marine use, it is important you are never lean. A somewhat rich mixture and 93-94 octane is low cost compared to the potential for detonation. Spark plugs are cheap....let'er eat!