Holley 600 on a 383

JakeBorg66 said:

I think you’re just noticing the better throttle response on a smaller carb, that’s probably too small for a 383. I’ve run different sizes carbs on a 440 in the dyno room and they make max hp with a surprisingly large carb, that 440 really liked an old BG CLAW 950 we had.

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RJRENTON said:

Most, but not all carbs, air fows are rated at a given pressure drop across the carb, such as 3.0" wg (some manufacturers use mm of mercury, some use 1.5" wg).

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BSB67 said:

What is 3" wg?

What carb companies don't use 1.5 inches of Hg for 4 bbls, and 3.0 inches of Hg for 2 bbls?

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peabodyracin said:

Absolutely. For street driving, I've consistently found a smaller carburetor to be more responsive overall. Let's face it, we're not into the secondaries hard all that often, and seldom see even 6000 RPM driving around town. It's much like the tendency to pick a cam shaft with a power band well outside of the street driving environment, and then wonder why the engine seem to be a pooch. There's something to be said for the spread bore concept, with smaller primaries.

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RJRENTON said:

The "secret" is the concept of mixture velocity. The smaller the carb or the spread bore design promotes high mixture velocities resulting in good throttle response and more even cylinder to cylinder fuel distribution due to the high velocities. Over carb-ing and over camming generally results in poor performance in day to day operation especially in low to mid RPM operation.
Operation with a "smaller" carb at HIGH RPMs will result in more pressure loss thru the carb. Most, but not all carbs, air fows are rated at a given pressure drop across the carb, such as 3.0" wg (some manufacturers use mm of mercury, some use 1.5" wg). The carb will flow more air but at a greater pressure, which ultimately result in horsepower being limited due to a reduction in air. This is reason for a spread bore (small primary large secondary) design....the "best of both worlds" so to speak. Just my opinion of course.
BOB RENTON

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RJRENTON said:

FYI...
WG is an abbreviation for water guage, where the unit of measure is inches of water. Doesn't Hollry use 3.0 inches or 3" wg ?
Found this info that may explain...
The basis for most cubic feet per minute (C.F.M.) air flow ratings such as those used by Holley and other carburetor manufacturers was established long ago by the Society of Automotive Engineers (S.A.E.).
Standard test vacuum was 1-1/2 inches of mercury for 4 barrel carburetors and 3 inches of mercury “Vacuum” for 1 barrel and 2 barrel carburetors. This was reasonable as it was about what a passenger car would develop. Of course, for the numbers to mean anything, tests would have to be run at a certain temperature and barometric pressure. Or be corrected to standard temperature and pressure. If not otherwise stated, ratings in cubic feet per minute are at standard temperature and pressure.
This system is confusing for racing applications for several reasons. Some racing engines actually develop much more or less vacuum than these ratings, so are difficult to compare. For example, a Nascar 390, 4 barrel equipped car might have 3 times the 1-1/2 inches of mercury standard or a very large 2 barrel equipped car might have only 1-1/2 inches of mercury vacuum.
Even more confusing is the fact that most flow benches in the racing industry measure dry air at low pressure drops, commonly 10 inches of water column (W.C.), one bore or venturi at a time. This result is multiplied by a factor to arrive at what that bore would flow at a higher vacuum (i.e. 1-1/2 or 3 inches of mercury), then that figure is multiplied (by four, in the case of a 4 barrel carburetor) to get the C.F.M. rating. This rating is usually much higher than it actually is.
Sorry if I did not explain it succinctly...there are also conversion tables to allow for comparison between various units of measure
BOB RENTON

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