This is what the RapidJet metering block and cartridge system look like. The cartridge sli
RapidJet Conversion
Over the years, there have been several companies that have offered quick-change jet conversion kits for the Holley carburetor. Recently, the RapidJet folks showed us a slick Holley conversion system the company has been working on for years. What intrigued us about the RapidJet metering block is that this system offers much more than the ability to change jets in the carburetor without removing the float bowls. The RapidJet system starts with a hard-anodized billet metering block (that comes in three different colors) that incorporates a cartridge that is easily accessible from the top of the carb once the air cleaner is removed. Once you've removed the cartridge, reinsert it upside down in the metering block, and the standard Holley jet can then be easily changed. The hard anodizing means the block is safe for all different types of fuels, including alcohol. All the passages in the block and cartridges are line of sight, which means there are no little hidden corners that can trap dirt.
The most interesting part of this kit is that all the metering orifices found in a typical Holley metering block are contained in the RapidJet cartridge. This includes the idle feed restrictor (IFR), the power valve channel restrictor (PVCR), as well as the multiple main well emulsion holes. This places all the fuel and air metering devices in one convenient location where they can be easily swapped to fine-tune the carburetor to your specific application.
The bottom hole (arrow 1) in the cartridge is the idle feed restrictor and is located on t
For our test, we decided to use the same Holley we had just converted into a 750 HP and then swap the primary and secondary metering blocks to the RapidJet system. To begin with, we carefully measured and then replicated as closely as possible the original Holley metering hole sizes. In discussing this, we anticipated that the first test with this conversion might go rich because the billet metering blocks utilize a significantly larger 0.213-inch-diameter main well compared with a typical Holley block's 0.140 inch. Because the main well is larger, it will tend to move fuel more easily.
The baseline Holley main metering blocks employ only two main well emulsion holes measuring 0.027 inch in diameter. In addition, the PVCR measured 0.039 inch in the primary and was blocked off in the secondary, while the IFR measured 0.035 inch. Once replicated, we ran the engine again to see if the fuel curve really changed.
While the air/fuel ratio did not indicate that the engine was rich, after a couple of minor changes, we concluded that the engine wanted less fuel, especially at lower engine speeds.
We leaned out the main jets, but this left the top of the curve too lean. Rather than messing with the high-speed air bleeds, we decided to block off one of the two emulsion holes, which would tend to make the engine run richer at the top end of the rpm scale. Usually, a jet change will help one spot in the curve but hurt the rest of the curve, so jetting changes become a matter of compromise. By using the RapidJet cartridge to help us tune more quickly, we were able to home in on an extremely accurate curve for this engine quickly. Much of this was because we didn't have to pull the fuel bowls off each time we wanted to make a change. This alone saved hours on the dyno.
As you can see from the results, all this effort paid off with a much more stable fuel curve, and the engine made more power. All told, our test was a rapid success.
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This is a stock Holley metering block with three emulsion holes in the main well. Some aft
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We converted our Holley 750 Street HP carburetor to RapidJet blocks and ran it on the dyno
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You can use a Holley bowl screw to thread into the cartridge to help remove it. Once the c
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