Holley Tri Power Series Part 2

Wayne Scraba

24 Nov 2015

Last issue, we previewed the intake portion of Holley’s brand new three-deuce setup for the small block Chevy. If you recall, the intake is a 180-degree dual plane Weiand that is setup for street use. It accepts conventional cathedral bowl Holley two barrel carburetors with a maximum throttle bore size of 1.50-inches. Those Holley carbs are the focus in this issue of “The Burnout”.

The manifold is engineered to accept a 325 CFM Holley carb in the middle, accompanied by a pair of 350 CFM carburetors. That’s a lot of CFM – 1,025 in total, and certainly too big for a street-driven small block. But there’s a catch here (and one that a big print competitor of ours failed to notice in their review of the system): Two-barrel carburetors have their CFM ratings performed at a different pressure drop than similar four-barrel carburetors. Here’s how it works:

A two-barrel carb is tested (for flow) at 3″ Hg depression. On the other hand, four-barrel carburetors are flow rated at 1.5″ Hg depression. These are the industry standards for carburetor airflow testing.

As you can see, the two-barrel test depression is double the four-barrel test depression, however flow varies with the square of depression. That means that it’s easy to convert from one test depression to the other. Here’s the formula:

Divide the cfm by the square root of the pressure it was flowed at, and multiply that by the square root of the pressure you want to calculate for.

500cfm / (sqrt of 3) = 288.675134595

288.675134595 x (sqrt of 1.5) = 353.55339059

That’s the hard part. There’s a simpler way (but perhaps not quite as accurate) to accomplish the conversion:

CFM divided by 1.414

500 divided by 1.414 equals 353.6

Using our tri-power setup as the example, then:

1025 divided by 1.414 equals 724.9 or, 725 CFM.

725 CFM is pretty much right on target for a street driven small block, particularly when you consider the fact the secondary’s can be staged. Internally, the carburetors are very much standard issue Holley (which is a very, very good thing!). That means tuning and replacement parts are readily available. It also means the carbs are simple to work on with a straightforward design.

In this issue, we’ll look at the outside features found on the tri-power carbs. Next issue, we’ll open them and have a look inside. Meanwhile, here are the photos of the exterior features of the carburetors:

The center carb has a two-barrel CFM rating of 325. As you can see, it is equipped with an electric choke. More later.

The pair of end carburetors are not equipped with choke assemblies. They have a two-barrel rating of 350 CFM. See the text for more info on flow numbers and CFM calculations.

All three of the carburetors are equipped with idle mixture adjustment screws on either side of each fuel-metering block.

Float level on all three carbs is set the same: By way of the adjustable needle and seat assembly at the top of the bowl, and viewed through the bowl window. This is common high performance Holley fare.

All three carbs incorporate replaceable accelerator pump shooters.

Additionally, all three carbs are setup with accelerator pumps (unlike some of the drone carbs found on musclecars). They all have 30-cc pumps and pink pump cams.

Idle speed on each carb is set just like any other high performance Holley carb – by way of a screw on the baseplate.

We pointed this out previously: The center carb uses an easy to hook up electric choke.

The center carb is also fitted with a vacuum port (if required).

Finally, the center carb is equipped with this simple linkage arrangement. Down the road we’ll show you how it easily hooks up.

Check Out – Part 1 – Part 3 – Part 4