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.
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.
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.
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.
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 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.
“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.”
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.
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.