Choosing between mechanical- or vacuum-operated secondaries on a Holley four-barrel carburetor really comes down to not only understanding the engine’s performance requirements but also what the driver wants to feel through the bottom of the seat.
The underlying beauty of a four-barrel carburetor designed for street performance is that the front half, or forward two barrels, handles the brunt of the work—that is, cold-start, idle, law-abiding acceleration from stopped and routine, low-load cruising. Everyday driving on just two barrels improves drivability and fuel economy. When extra power is essential for freeway-ramp acceleration, passing or Saturday night passes at the drag strip, then the rear two barrels will open as needed to feed the extra fuel and air to the engine and increase power.
The basic operational theory is the same for mechanical and vacuum secondaries, but the execution is quite different—and that’s where understanding the engine’s demands is critical to making the right choice. The end user must consider the type of vehicle and weight, transmission choice, gearing, camshaft timing, intake manifold style and intended use.
“These are the types of questions you’ve got to ask yourself,” stresses Holley engineer Laura Shehan. “Another thing is, what do I want to feel?”
There are enthusiasts, mostly with manual transmissions or high-stall torque converters, who seek the neck-snapping rush of quick-opening secondaries and need a double-pumper carb with mechanical secondaries, while others focus more on cruising and showing. Those users will benefit more with vacuum secondaries.
Not being honest about the engine and vehicle setup along with the intended use can get a user in trouble. That’s because mechanical-secondary carbs, when not properly tuned, or used in the wrong application, can be temperamental.
“With a vacuum-secondary the universal application theory usually applies,” says Shehan. “With a mechanical, if your application doesn’t need the size or the response that carb is designed for, then tailoring accelerator pumps and tuning are more critical.”
How Do They Work?
Opening the secondaries is first dependent on the velocity of air flowing through the primaries. As the primary side opens, the air velocity flowing into the intake manifold increases with the engine rpm. This surging airflow provides a stronger and stronger vacuum signal to the diaphragm that controls the secondaries. As such, the secondaries open progressively as the engine requires more air and fuel to keep up with the throttle demand.
As the secondary throttle blades open and air starts flowing, an additional vacuum signal is generated to the diaphragm to keep the secondaries open as needed. However, the initial opening rate is controlled by a single diaphragm spring. The stiffer the spring, the slower the rate of the secondaries opening.
Holley offers a tuning kit with seven color-coded springs so that the user can adjust the secondary opening rate to suit the needs of the engine and vehicle. Black is the stiffest, followed in order by brown, plain, purple, yellow, orange and white, which is the lightest.
The plain or silver spring is most common in production carbs. Some carbs come with black and yellow springs for tuning options out of the box. Other Holley carbs, like those intended for off-roading, come with black springs to offer the user the safest starting point. There are application-specific carbs that will have a more appropriate spring for the intended use. Check the product details and installation instructions for more specific information on springs selection from the factory.
Holley offers tuning kits for the vacuum secondary system that includes springs of different weights. These range from the black spring, which is the stiffest, to the white spring, which is the lightest. Be sure to verify which spring is used in the original application for your carburetor...while the silver spring is most common, some carburetors utilize specific springs for the intended use. You can order a vacuum secondary diaphragm spring kit with p/n 20-13.
There are two types of diaphragm housings on Holley carbs. Changing springs in the traditional housing requires the removal of the choke housing as well as removing the diaphragm housing itself to access the spring. Holley does offer a quick-change conversion that doesn’t require the removal of the two housings, effectively cutting the installation time in half. On a side note: Quick Fuel carburetors use a vacuum secondary housing with an adjustment screw to control the vacuum signal pulling on the diaphragm, and therefore controls the rate of the secondaries opening.
Some long-time racers and enthusiasts have avoided the kit and tried cutting the spring already installed to reduce some of the stiffness. This is not a good idea.
“Everybody wants to apply their own black magic to a carburetor. People have their own theories about power valves, squirters, how to set the accelerator pump,” says Shehan. “The vacuum spring is another one of those things where certain people, in certain applications, are going to do what they would normally do, regardless of trying to influence them otherwise.”
In order to remove the vacuum diaphragm, you will first have to remove the choke housing from the carburetor's main body so that you can access all of the diaphragm's mounting screws. Be sure to be very careful when removing the small E-clip off of the diaphragm's linkage...it's tiny! If it does manage to escape, you can order a new one (p/n 36-1QFT).
The irony is that cutting a coil off the spring and stretching it out to make up the difference in length actually increases the effective spring rate of the spring and may not provide the desired effect.
The goal of changing springs is to eliminate any bogs, stumbles or hesitations during acceleration.
“You want to put the lightest spring that's going to suit your application,” advises Shehan. “You want your secondaries to open up, but not too soon.”
In other words, if the vehicle accelerates smoothly with no flat spots, or perhaps you’re not even feeling the secondaries open, then a lighter spring can be tested. However, if there is a bog in the 2,000 to 3,000 rpm range when the secondaries would normally open, then a heavier spring will likely solve that problem. If there’s a hesitation at the initial throttle opening or off-idle, then the accelerator pump may be to blame.
When changing springs, some users may remove the small check ball that’s located in the vacuum passage. Not all carbs are designed with a check ball; but if so equipped, the ball is there to help avoid sudden or unwanted opening of the secondaries. Not returning the ball to its seat when changing springs may invalidate any tuning improvements the user is trying to make.
Once you remove the four screws from the vacuum housing's cap, here is what you will have: the spring will fit onto the stud on the underside of the lid, while the diaphragm itself will be in the bowl. Note the complete hole in the diaphragm rubber: this is for the pressed in restriction, or in older carburetors, the check ball location.
“Also, when you're tuning your springs for your vacuum secondary, make sure that the air cleaner is installed,” notes Shehan. “If you don’t have the air cleaner on, that changes the dynamics of the pressure drops and that changes the vacuum signal.”
Don’t judge the secondary opening by revving up the engine in the garage. Vacuum secondaries open under engine load, so if they do open by winging the throttle then they will most likely bog the car when it’s accelerating under load. Also, do not try to run without a spring in the diaphragm housing. This could lead to very erratic idle performance.
One final tip: don’t toss the diaphragm and improvise some crude method of opening the secondaries manually. A four-barrel carburetor designed with manual secondaries will have a second accelerator-pump nozzle or squirter that adds additional fuel when the secondary throttle blades open. This design is often referred to as a double-pumper carb because there are accelerator-pump fuel nozzles for both the primaries and secondaries.
“You want that extra accelerator pump shot in place to overcome some of the bog or hesitation that you might have by not having enough fuel when those secondary plates open,” says Shehan.
This small cork gasket on the diaphragm housing is for the secondary venturi pick-up port. If this gasket is damaged in any way, replace it with a new gasket (p/n 8-9QFT).
Because they work by sensing engine load, vacuum-secondary carbs are very forgiving when compared to their mechanical secondary counterparts. That’s why an engine that requires a 650-cfm carburetor can operate reasonably well with a larger 750 cfm—because the engine will draw only as much air as it needs. But installing a mechanical secondary carb requires a more precise match to the engine. In other words, over- or under-carbureting will definitely impede performance.
“That’s why we offer double pumpers in 50-cfm increments,” notes Shehan.
Correctly sizing a double-pumper is very critical to optimum engine performance, especially on a full-race configuration with a radical cam that limits manifold vacuum. If the carb is oversized for the engine, then that extra shot of fuel may overwhelm the engine before airflow through the carb is controlling the main fuel feeds in a normal manner.
If you have a need to tune the vacuum system often or just want an easier way to access the spring, a quick change vacuum secondary housing cover (p/n 20-59 or 38-1000QFT) will get the job done. A plate covers the diaphragm, while two screws will provide access to the vacuum spring without removing the diaphragm itself.
Few double-pumper carburetors open the secondaries at a one-to-one or on an equal percentage rate as the primaries. Those carbs are generally reserved for all-out drag cars that run WOT throughout the run. Most double-pumper carbs are equipped with a progressive linkage that starts opening the secondaries when the primaries are about 40 percent open. There are kits that allow a quicker or later opening for fine tuning a road-race or dirt track car that is often on and off the throttle.
These tips apply to Holley four-barrel carburetors. There are other styles of carbs, such as the Street Demon, that uses an air valve or auxiliary throttle valve to control the secondaries.
A properly tuned carburetor with vacuum secondaries will provide excellent drivability and fuel economy when compared to a mechanical-secondary carb. It will also be more forgiving in terms of size selection and tuning flexibility. A mechanical-secondary carb will provide better throttle response but must be tuned and sized correctly or it can be fussy to live with on a street machine.
“The vacuum-secondary carburetor is going to take into account the engine needs, much more so than a mechanical-secondary carb,” sums up Shehan. “A vacuum carb responds to the engine’s actual needs as opposed to a simple mechanical linkage.”
How to Custom Tune a Holley Vacuum Secondary Carb
The reason that Holley four-barrel carburetors are so popular is because they're very easy to work on. Even if you've never worked on a carburetor before, Holley's modular design makes it easy to disassemble, tune, and reassemble. In a matter of a few screwdriver minutes, you can feel comfortable working on one of these fuel mixers.
In this session, we decided to tackle how to adjust and tune secondary opening rate for a vacuum-operated Holley. Vacuum secondary Holleys are easy to identify because of the large diaphragm housing positioned on the passenger side of the carburetor that connects to the secondary throttle shaft. These carburetors use air velocity through the primary venturis to create a low pressure (vacuum) signal channeled to the top of the vacuum diaphragm housing.
As this low-pressure signal increases, atmospheric pressure pushes upward on the bottom side of the diaphragm, working against a small spring that sits on top of the diaphragm. The tension rating of this spring determines the point where the secondary throttles begin to open as well as the rpm when the throttle is completely open.
We've reproduced a chart from the Holley catalog that offers some insight into both opening points and fully open engine speeds. At first glance it would appear that for either a 350 or 402ci engine that the lightest spring would be the best choice. But there are real world inputs like vehicle weight and gearing that directly affect these functions.
The point in engine rpm that the secondary throttle begins to open is determined by the load created by the spring. Holley equips all vacuum secondary carburetors from the factory with the heaviest (black) spring, which opens the secondary the slowest at the highest engine rpm. Holley does this, because there's no way to know the specific engine combination. So the stiff vacuum secondary spring ensures there'll be no hesitation that can occur if the secondary opens too quickly.
Vacuum secondary carburetors aren't equipped with a secondary accelerator pump circuit like there is on double-pumper mechanical secondary Holleys. This makes the initial opening point and rate crucial for vacuum secondary carburetors. Vacuum secondary carbs demand sufficient air velocity through the secondary side as the throttle begins to open in order to stimulate fuel flow from the secondary boosters to prevent hesitation.
Holley offers an inexpensive kit with seven springs that offer a broad range of opening points. The lighter springs in this kit can open the secondary too quickly, which will create that hesitation. Some enthusiasts think that this jolt is the signal the secondaries are opening correctly. This is actually incorrect. If vehicle acceleration feels jerky during or after wide-open throttle (WOT), this indicates a problem. The ideal situation is to open the secondary quickly but also as smoothly as possible to maximize performance.
Changing vacuum secondary springs used to be a hassle. The first step depends upon whether your carb is equipped with an electric choke. If so, the choke housing must be removed to allow access to the secondary diaphragm housing. The housing must be removed from the carburetor before all four cover screws can be accessed. Once the housing is unbolted from the carburetor and disassembled, it's very easy to tear that thin rubber diaphragm in the threads of the four lid screws during reassembly.
In the captions, we show you how to do this job properly, but the best advice is to convert the stock vacuum secondary lid to a Holley Quick-Change kit. The kit comes with a two-piece metal lid. The main lid seals the diaphragm to the main body like normal but offers a second, quick-access lid sealed with an O-ring. In the days before this Quick-Change lid, we used to store several housings each fitted with a different spring for fast changes.
If you study the Holley chart, it might appear that a very weak spring is the best because most of the springs on both the 350 and 402ci engine don't fully open the throttle until late in the rpm curve. What isn't indicated on the chart is that the throttle was probably at 90 percent open very early in the rpm, so opening further really doesn't hurt power because this isn't a major restriction to airflow. The most important point when choosing a spring is that the secondary doesn't open too soon causing a hesitation.
After you've modified the secondary spring, there's also a simple bench test to make sure the system works properly. With the carb sitting on the bench, open the primary throttle linkage and with an air nozzle, shoot compressed air past the small nozzle in the primary venturi wall. This will create a vacuum signal in the diaphragm, and the secondary should open. This way you know the system works. This can also be performed on the car if you like, with the engine off.
Vacuum secondary spring tuning is very much a cut-and-try system, but we'd suggest initially replacing the heavy black spring with the purple version. This will definitely open the secondary throttle sooner and is a great place to start your testing. If that spring works well, you can then try something lighter like the light yellow one. Each engine is different, and it's possible that the purple spring might even be too light for some applications.
Another old trick to avoid is to not place a screw (or a zip tie) in the secondary throttle linkage slot on the driver side of the carburetor. This will mechanically force the secondary open sooner, but it always creates a bog. So don't bother with this little effort, unless you want to experience what a real hesitation feels like.
Experimenting with vacuum secondary springs is fun and will almost always result in a quicker opening secondary throttle when converting from the stock black spring. The job is simple; it can be done on the car, and you should be able to instantly feel the results. This is one time when a swift kick in the pants is a good thing!
Secondary Throttle Opening Ranges
|Color||Load||350ci - LightLoad||350ci Heavy Load||402ci LightLoad||402ci Heavy Load|
|Black||Heavy||2,720||Not FullyOpen||2,390||Not Fully Open|
|Holley Secondary Diaphragm kit||20-13||Summit Racing|
|Holley Quick-Change housing kit||20-59||Summit Racing|
|Holley Quick-Change 2x4 housing kit||20-73||Summit Racing|
|Holley replacement vacuum diaphragm||135-4||Summit Racing|
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Images via Holley
The carburetor is a wondrous but sometimes-mysterious device mounted on top of an engine that makes it run. The way it works is by drawing fuel and air into the engine by way of a vacuum that is created while the engine is running. Regulating the airflow into the carburetor are throttle blades that are connected to the throttle pedal. Some engines can make do with two small blades (two barrel), but on a performance engine, a lot more air and fuel needs to be fed into the engine. These engines typically have four throttle blades (four barrel). Depending on the carburetor, a four-barrel’s secondary plates will open at different times, and can be actuated by different means.
When it comes to the secondary system of a Holley four-barrel carburetor, the opening of the secondaries can be done by either a vacuum signal, or by way of mechanical operation. The rate at which the secondaries of a mechanical-secondary carburetor (double pumper) are actuated is predetermined by the configuration of the secondary linkage (some secondary linkage systems are easily adjustable, but that’s another story). The linkage controlling the secondary throttle plates is designed to begin opening the secondaries once the primary throttle plates have opened roughly 40 degrees.
It is common practice to use mechanical-secondary carburetors on racecars along with many high performance applications. They are able to react quicker than a traditional vacuum-secondary carburetor. That being said, it is possible to use a properly tuned vacuum secondary carburetor on a street driven car, and make it perform equally well.
A vacuum-secondary carburetor is well suited for use on mid to heavyweight street-driven cars that are backed by an automatic transmission. A vacuum secondary carburetor tends to be more forgiving than a double pumper, since they do operate by sensing engine load. This is because the secondaries open in relation to the vacuum signal developed by the engine. When a predetermined amount of vacuum is sensed, the vacuum diaphragm pushes against a spring. When the spring’s resistance pressure is overcome by engine vacuum, the diaphragm is able to pull the secondary throttle blades open.
The rate at which the vacuum secondaries open can be manipulated by changing the springs in the secondary diaphragm. If the secondaries open too early (the spring is too light), you will experience a hesitation, because there will not be an adequate vacuum signal to pull enough fuel into the venturi; especially since a vacuum secondary carburetor has no secondary accelerator pump to cover this transition. When they are opened too late (the spring is too heavy), you might not run into any drivability issues, but performance will be hampered, and the engine will feel a little sluggish.
Speaking of sluggish: The ignition-timing curve should be optimized prior to tuning the secondary opening. If the timing curve is not optimized, your engine will feel lazy, no matter what adjustments you make to the carburetor.
Back to tuning the vacuum secondary system: According to Holley: “If we could make our vacuum operated secondary carburetors perform better by opening the secondaries mechanically, it would be in our advantage to do so since all that vacuum actuating hardware is expensive and requires much time and money to calibrate. Our mechanical secondary carburetors all utilize a pump shot to prevent bogging when the secondaries are opened. Those who feel a kick in the pants when the secondaries “kick-in” are actually feeling a flat spot during initial acceleration because the secondaries have already begun to open and have weakened the fuel delivery signal to the primary boosters. The engine struggles to increase speed and what they actually feel are the secondary nozzles “crashing in” as the engine finally reaches the speed where it provides the proper fuel delivery signal to the primary and secondary venturi. Opening the secondaries early causes the situation described above. The secondaries must not open until the engine requires the additional air. This allows torque to increase along the peak torque curve. Performance is compromised less by holding the secondaries closed a little too long than by opening them a little boo soon. If the opening rate of the vacuum operated secondaries is properly calibrated there should not be a “kick-in-the-pants”; only a smooth increase in power should be felt.”
Due to the fact that no two cars (and no two carburetors) are tuned exactly alike, Holley offers a kit under part number 20-13 that is designed to fine-tune the secondary operation for your car. This kit contains several different secondary diaphragm springs. What’s the difference between them? Basically, the larger diameter the wire, the stiffer the spring and the later the secondaries will open. Essentially, the springs all have a different “rate”.
______________________________________________________________ Holley springs from light to heavy: White – Lightest Yellow (Short Spring) Yellow Purple Plain (Steel grey) Brown Black – Heaviest _______________________________________________________________
Typically, heavier cars require stiffer secondary diaphragm springs than lighter cars. The configuration of the air cleaner along with its restriction plays a critical role in spring selection. Because of this, it’s important to tune with an air cleaner installed if that’s the configuration you run the car in (obviously, mandatory for a street-driven car or truck). If testing in your garage or driveway, never “wing” the throttle and expect to witness the secondaries open. If the secondaries open, that usually means they are opening too soon. Secondaries should only open when the engine is under load (more below). Never trim a spring in an effort to make a spring lighter so that the secondaries will open sooner. Contrary to what you might first think, clipping springs actually increases spring rate which, will in turn delay opening.
Secondary spring tuning isn’t an exact science. Honestly it’s a game of trial and error. If the car in question has a medium-heavy or heavy spring in the secondary side (for example, brown or black), the secondaries might not open, even at very high RPM. A lightweight 350-cubic inch, street driven small block hot rod might not open a black spring secondary at an engine speed of 8,000 RPM! The tuning solution is actually extremely simple: Install a very soft spring and then work your way back (with progressively stiffer springs) to a point where the car is both driveable and responsive. Keep in mind that vacuum secondaries should not hammer open. The ideal situation is one where the spring opens the secondary butterflies gently.
Sounds simple enough, but there are so many variables, where do you begin? Holley notes that the idea is to tune the secondary spring while the car is in high gear. This tends to maximize the load the engine sees. If there is no stumble in high gear, Holley tuning experts point out there won’t be a stumble in the lower gears either. If you tune the other way around (using first or second gear as the base line), additional torque multiplication from the gearing exists. This tends to disguise overall performance, particularly when the car is in high gear and you hit the throttle hard.
Check out the accompanying slideshow for more info:
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