Holley Jetting HP500/Dart

[RickR]

We had a post last week about staggered jetting with an open plenum intake (Dart) and a Holley 800CFM used on their carb HP 500.

I contacted Merc High Performance and verified that the jetting for the Carb HP 500 is
Front Port 81
Front starboard 75
Aft port 89
Aft starboard 93

Sounds to me like they had some lean cylinders and used the staggered jetting to balance them out. And they want to run rich on the secondaries. A 93 jet flows a lot of fuel :eek:

[rayjay]

Sounds to me like they had some lean cylinders and used the staggered jetting to balance them out. And they want to run rich on the secondaries. A 93 jet flows a lot of fuel :eek:

Or doesn't get good signal. I'm far from a carb expert, in fact if I think it's running rich it'll probably be lean, but fortunately I have some friends that are very good at carbs. But one thing I have noticed after porting heads and running things on the dyno is that the really good working cylinders sometimes have far smaller jets than you would think because the vacuum signal or pulse from the cylinder is so strong it doesn't need as big a jet to get enough gas. On most of the bikes I worked on were we just ported the head and did not do anything else like change the cam (or cam timing) or raise the compression, we actually had to reduce the size of each main jet. Some BBC heads, including the stockers, have two problem (longer and more convoluted) intake ports per side because of the design of the head. If I remember correctly it is 1,4,5, and 8. Manifolds usually need some help on those cylinders to get the flow right, especially wet flow like that from a carb. Turtles, cones and such are used to create vortexes to help the fuel stay in suspension. Remember the stagger height fuel injection stacks on the BBC engines used in road racing like the old McLaren Can-Am cars. They were not to give a broader torque range as some people thought, but were just to get the intake ports to breath the same. The new 496 and Pontiac Big Chief heads overcome this by using staggered ports that are all the same. Also work by a good porter and / or starting with one of the good aftermarket heads can fix this.

rayjay