Inside the New Edelbrock VRS-4150 Four-Circuit Carburetor

Inside the New Edelbrock VRS-4150 Four-Circuit Carburetor

Who would have thought that more than two decades into the 21st century, we would be getting excited about a brand new high-performance carburetor? Yet here we are, blessed with technology like direct electronic injection and electronic throttle control and yet, we’re still geeking over a 4150-style fuel mixer of the old-school variety.

But let us not be misled. This new Edelbrock VRS-4150 carburetor is not just a rehash of the classic design. While at first glance it looks like a traditional 4150, there are sufficient improvements to warrant a closer look. Edelbrock was kind enough to send us a 750cfm version that we could take apart and see what makes it tick — or more accurately how it meters fuel and air.

Edelbrock VRS-4150

The Edelbrock VRS-4150 brings a new level of sophistication to the traditional line of modular carburetors. The main body is a full half-inch taller than a standard 4150 which is noticeable in the distance between the top of the metering block and the bottom of the air cleaner base. Also of note is the air bypass idle-air speed hex adjuster located in the center of the main body. There are two of these, one on each side.

Feature Rich Design

The tremendous adjustability built into this carburetor may intimidate some people since there are dozens of restrictors and jets to manipulate. Edelbrock calls this a four-circuit carburetor and we’ll dive into what that means in a moment. While all this adjustability points toward dedicated competition engine applications, in the right hands this carb would be an outstanding high-end street carburetor.

According to Wesley Cameron, Edelbrock’s product engineer for the VRS-4150, “This carb is our attempt at the best of both worlds. It is very race-oriented with no choke and a taller overall height to make the venturi design better, but it also has all the common vacuum ports and a provision for a throttle position sensor for the street customer. Also, the large annular boosters generate much more signal than a typical small down-leg, which makes the reaction time quicker for fuel to start flowing out of the booster.”

We’ll start with a few overview features and then get into the details that could make this carburetor a go-to piece for those racers and enthusiasts who can appreciate the advantages of a better mousetrap.

Edelbrock VRS-4150

This overhead view of the carburetor immediately reveals the annular boosters as well as the four-circuit air bleeds for each of the four venturi.

The carburetors will come in 650, 750, 850, and 950cfm sizes, and the mounting plate is drilled so that it will fit both 4150- and 4500-pattern intake manifolds. The photos may not reveal that the carburetor’s main body is a half-inch taller, which offers advantages for mixture distribution. Those of you who have changed a few jets in your lifetime will appreciate the quick-drain fuel bowls — with a larger capacity — that can feed from either side. There are clear sight glasses on both sides of each bowl as well, along with four-corner idle mixture adjustability, large vacuum hose ports front and rear, and perhaps less obvious, the proper TV-cable connections on the throttle linkage.

Carburetor cognoscenti will immediately note the annular boosters, which are nicely streamlined into the venturis. Another obvious addition are the four air bleeds atop each venturi. We’ll detail those in a moment. Along those same lines, it will quickly become apparent that if a bleed or restrictor could be added to adjust the fuel curve to improve power, this carburetor probably has it.

Edelbrock VRS-4150

If you remove both bowls and metering blocks, it becomes apparent the main body of the carburetor and base plate are integrated — this means no screws to fall out and this integration also eliminates mismatching between the base plate and venturi. Also, note the power valves on both primary and secondary metering blocks.

Adjustable To The Nines

Speaking of adjustability, carburetors employed on big engines with big camshafts often require drilling the throttle blades in order to properly place them relative to the transition slot. A better way to accomplish this is by adding a separate, adjustable air bypass circuit. The Edelbrock VRS-4150 offers this adjustability with two separate hex-head bypass screws located on each side of the main body, just below the air cleaner mounting flange. It is these screws that you can use to adjust idle speed, allowing the throttle blades to remain in their proper position relative to the idle transition slot.

Another added feature is the built-in throttle position sensor (TPS) fixture on the driver side of the carburetor. This is a nice addition for cars that might need a TPS reading for properly controlling an electronic transmission like a 4L80E, or it can also be used for data logging during competition activities.

Edelbrock VRS-4150

Edelbrock also offers three different boosters from drop-leg to annular in varying feed sizes. Boosters are easy to change by removing the booster stake tubes (arrows). Plus there are three feed sizes of these tubes from .140, .150, to .160-inch.

Now let’s get into the finer points that you’re waiting for. Earlier, we mentioned that Edelbrock calls this a four-circuit carburetor. Those circuits are broken down into 1) idle circuit, 2) intermediate circuit, 3) low-speed main metering circuit, and 4) high-speed main metering circuit.

Four Circuits Are Better Than Two

A typical street carburetor can be described as a two-circuit, with its idle circuit and the main metering circuit. Some race-only 4500-series carbs add an intermediate circuit that makes it a three-circuit carb. The intermediate circuit is useful for sophisticated competition and racing engines for applications where subtle changes to the main metering circuit can be more finely tuned.

Edelbrock VRS-4150

This billet aluminum metering block is crammed with circuits. Below are the main restrictors located on the main body side of the metering block. A – Idle and transfer slot jet. B – Low-speed main metering bleeds. C – High-speed main metering bleeds. D – Power valve channel jets

Edelbrock calls the VRS-4150 a four-circuit carb because in addition to the idle and intermediate circuits, engineers split the main metering circuit into two separate channels on the metering block for low- and high-speed tuning at wide-open-throttle (WOT). This creates four screw-in air bleeds on top of each venturi for each of these circuits.

Initially, this may seem complex, but in reality, what this offers is more finite control over WOT tuning. Cameron described the intermediate circuit this way: “The intermediate circuit tunes in a little different with our carb [compared to] the usual tuning on 4500-series carbs. What I have seen on the dyno is the intermediate circuit changes the slope of the full-throttle air-fuel graph [versus] RPM.

Edelbrock VRS-4150

On the fuel bowl side of this primary metering block are A) the main jets, B) the intermediate circuit jets, and C) an additional idle and transition slot fueling jet to add more fuel for big-inch engines with a monster cam with extremely soft idle vacuum. These idle jets are blocked off in all carbs but could be opened up for additional fuel.

“For example, let’s assume I have the 750 carb on a 540-horsepower LS and we start the pull at 3,000 rpm and end at 7,500 rpm. If the AFR (air-fuel ratio) is 13.0:1 at 3,000 and goes to 13.6:1 AFR at 7,500, I would change the intermediate feed jet a few sizes bigger. What that would do is change the 3,000 rpm range to roughly 12.7:1 but it would make the 7,500 rpm (point) go to 12.7:1 or 12.8:1. It is highly dependent on airspeed through the carb so as airspeed increases, the fueling does as well.”

This Edelbrock map reveals the location of the air bleeds for each of the four main circuits of idle and transition, intermediate, high-speed outer, and high-speed inner. The high-speed outer controls the lower WOT RPM band while the inner bleed controls the high rpm side circuit.

Let’s say you have a bracket car with a throttle stop and a Powerglide. The combination of an intermediate circuit allows the ability to manage the top-end of the fuel curve while leaning out the low- and high-speed emulsion jets to perhaps add or subtract fuel in the lower engine speeds separately with more finite control.

(Left) This inside view of the secondary fuel bowl shows the cutouts for the jet extensions as well as how the float has been radiused on the ends to be less sensitive to fuel sloshing to one side during high lateral G-loads in cornering. Just below the float you can see the “S”-shaped moustache which is another innovation to minimize fuel slosh or aeration of the fuel. (Right) With the throttle blades open, you can see the location of the intermediate circuit discharge tube. This indicates it is a completely separate circuit from the main metering circuit and therefore can be tuned with its separate feed restrictor and air bleed independently from the main metering circuit.

Just within the WOT side for one venturi you have four air bleeds, five emulsion circuit bleeds, a fuel jet for the intermediate circuit along with the power valve channel restrictors, and of course, the main jets. Yes, this is complex because several circuits overlap, but the opportunities for WOT fuel curve tuning should satisfy even the most ardent racer.

There’s much more to this carburetor than we have the space and attention span to go into with more detail, but suffice to say that there’s a book’s worth of tuning procedures that would be worth diving into if there’s enough reader interest. Carburetors are a long way from extinction and this new Edelbrock VRS-4150 ardently underscores that statement.

(Left) A really nice feature is the idle bypass hex adjuster that should be used to set idle speed on engines with really big camshafts. Using this idle bypass to set the idle speed allows the throttle blades to remain in their proper position relative to the idle transfer slot. (Right) Edelbrock also takes the time to pin both the primary and secondary throttle shafts to prevent lateral movement. This offers much smoother throttle movement and prevents the blades from rubbing on the side of the bore which can cause wear.

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About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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