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How To Boost Reference your Holley Carburetor

Signal Strength: How to Make your Carb Blower Friendly

Evan Perkins Nov 9, 2015
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Adding a supercharger to an engine is one of the simplest ways to make more power. There is no complicated intake and exhaust piping required, such as with a turbocharger, or any of the associated plumbing requirements of nitrous.

The installation of a blower is usually a one-day – or quicker – job that can pay huge dividends in power and torque. With a fuel injected engine, optimized fueling is only a computer reflash away, but us old-school carburetor guys, specifically those with Holley 4150/4160 series carbs have a little bit more work to do before our engines will run correctly when boost hits.

The area in need of tweaking is the power valve. But, before we get into the mechanics of how to make the necessary changes, some background info on what the power valve does is required.

The power valve is a vacuum-operated fuel enrichment device that opens at low manifold vacuum (high engine load) conditions to direct more fuel to the carburetor’s main enrichment circuit (this is what ultimately flows out of the boosters) through the venturis and into the engine. At low engine load, such as idle or cruise, high vacuum in the manifold holds the power valve closed, keeping the mixture correctly lean. As the throttle blades open, allowing more air into the engine, the manifold vacuum drops, opening the power valve and providing the engine with the extra fuel it needs. All in all, it’s a very simple and effective system.

Here’s where things can get tricky: In a supercharged application, where the carburetor sits directly on top of the blower, the vacuum signal gets a bit muddled. As the engine accelerates and the blower begins to build boost in the manifold, the carburetor (specifically the power valve inside) can see additional vacuum. Because the blower is forcing larger quantities of air into the intake manifold than the engine could breathe on its own, there is a vacuum created at the base of the carburetor.

This can be a big problem, as the vacuum can be significant enough that the power valve will never open, starving the engine for fuel at high-load conditions (the worst time to run lean). A band-aid for the problem is to remove the power valve altogether and install a plug in its place. This requires larger (richening up) main jets (4-10 sizes) to compensate for the lack of a functional power valve, but almost always causes the engine to run very rich at part throttle.

A power valve plug can often work well in racing situations, where the engine only ever operates at wide-open throttle. However, it is a lousy compromise for street-driven engines that creates a less-than-ideal fuel curve.

The ideal solution to the problem is to “boost reference” the power valve. While that may sound intimidating, it is very straightforward modification. Rather than feeding the power valve vacuum from just below the throttle plates, where added vacuum from the blower can occur, its feed can be relocated to underneath the blower in the intake manifold. This ensures that the power valve sees accurate manifold vacuum and enriches the fuel circuit when the engine needs it most.

Check out the captions below to see how simple it is to boost reference any 4150/4160-style carb. Note: this modification does not work for blow-through carburetors, only draw-through setups, in which the carburetor sits on top of a roots-style blower.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 01 Before 2/22

1. Here is our specimen, a very typical — and quite grungy — Holley 4150 carburetor. While this carb is clearly in need of a rebuild and thorough cleaning, we grabbed it to detail the process modifying for draw-through applications. This task is the same for any 4150/4160-style carb.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 02 Remove Fuel Bowls 3/22

2. The first step is to remove the fuel bowls and metering block from the carburetor to gain access to the power valve well.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 03 Hole 4/22
Boost Referenced Carburetor Holley 4150 4160 Power Valve 04 5/22

3. With the fuel bowls removed, we can see the well where the power valve resides. The small hole (indicated) is the feed hole that directs vacuum to the power valve. This will need to be plugged.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 05 Valve 6/22
Boost Referenced Carburetor Holley 4150 4160 Power Valve 06 7/22

4. Here is a standard Holley power valve. Vacuum exerted on the diaphragm side holds the valve closed until it drops to a specified level (usually 6.5 inches of Hg) where the spring overpowers the vacuum, opening the valve.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 07 8/22

5. In order to provide the power valve with an accurate vacuum signal, we needed to route it underneath the blower. To do this, we drilled a small hole from the outside of the carb, into the power valve well, making sure we didn’t breech any internal passages and that the hole would not interfere with the power valve itself.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 08 9/22

6. Here is where we chose to drill our new feed hole (located on the throttle-side-primary of the carb).

Boost Referenced Carburetor Holley 4150 4160 Power Valve 09 10/22

7. Use a sharp drill bit and go slowly. Breaking the bit off in the carb could really ruin your day … and your carb.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 10 11/22

8. Success! Here you can see how the drill bit protrudes into the power valve well.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 11 12/22

9. Here’s another view of the completed power valve feed channel.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 12 13/22

10. A scrap piece of steel 3/16-inch steel brake line was used to complete the new power valve feed. We coated it in sealant and lightly tapped it into the hole we drilled.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 13 14/22

11. Here you can see just how the new vacuum port will feed the power valve

Boost Referenced Carburetor Holley 4150 4160 Power Valve 14 15/22

12. We decided to add a -3 AN fitting to the end of our new vacuum feed. This makes for a more precision look, though a bit of rubber hose would have accomplished the same task.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 15 16/22

13. This neat, AN flaring tool from Speedway Motors was used to ready the brake line for a -3 AN fitting.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 16 17/22

14. Our new power valve vacuum feed was done, but we still needed to plug the old feed. For that, we used some quick-curing gas tank epoxy. Two-part epoxies, such as JB Weld, will also get the job done, but we liked that this epoxy was designed with gasoline resistance in mind. There shouldn’t directly be gasoline in the chamber, but some vapor will inevitably be in the air.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 17 18/22

15. Here is a close-up of the power valve well with a dab of epoxy blocking the old power valve feed and a small donut of it adding additional sealing to the brake line we used for the new feed.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 18 19/22

16. We kept all of our bits and pieces in order to make reassembly quick and easy. This job doesn’t require too much of the carb be removed but it’s a good practice to keep.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 19 20/22

17. Here you can see the back of the metering block and how the power valve will sit in the modified well.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 20 21/22

18. We reassembled our Holley, though it will come apart for a full rebuild in the very near future.

Boost Referenced Carburetor Holley 4150 4160 Power Valve 21 22/22

19. Our finished carb is ready to top a blower. However, in order for this modification to work properly, the feed will need to be routed to underneath the blower via a vacuum port in the intake manifold. Forget to hook that up and the power valve will stay shut always, effectively voiding all your hard work!

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