There have been a lot of advancements in the world of carburetors, even since the advent of electronic fuel injection. Whether due to competition or not, there’s no denying that today’s enthusiast has a dizzying array of options when it comes to choosing the right carburetor for their application. Thanks to advancements by companies such as Quick Fuel Technology, carburetors are much more adjustable and easier to modify than ever before.
Race Or Street?
Years ago, if you were racing or dealt with traffic on a “red light to red light basis”, you chose a carburetor specifically designed for “race” applications. If you spent more time in stop-and-go traffic, a street carburetor was most specifically in your future. Today, the lines have blurred considerably in what folks consider a “street car”, and also, what constitutes a “race carburetor”.
Horsepower has exceeded what was once reserved for only the highest levels of competition, and cars equipped with power steering, overdrives, street tires, and thousands of miles on their odometers have eclipsed well past what once had the fans standing on their feet at the largest drag racing events in the nation. Likewise, some of the most trick features once only found on the premier race-only carbs have now worked their way into carburetors with vacuum-secondaries and, egad, choke valves!
To illustrate how far we’ve come, let’s take a look at Quick Fuel Technology’s Street-Q series street carburetor and a similar example from their race-ready Q series line. Quick Fuel has an extensive array of carburetors ranging from wild to mild, and these two carburetors represent the middle-ground, where the finish line tends to intersect with the double-yellow lines that weave their way across the nation.
While both carbs have no provision for a choke, which may remove any street cred for some folks, the Street-Q is clearly geared toward the serious street enthusiast. Major differences between the Street-Q and the race-bred Q series is the use of cast metering blocks and a cast throttle body on the street version, in place of fully-machined billet components on the race model.
All of the critical functionality is retained with two-circuit metering, down-leg boosters, changeable idle and high-speed air bleeds, and four-corner idle tunamility. All are items once reserved for full-race fuel mixers. Another benefit of the Street-Q carburetor is a universal throttle lever design that accommodates 700R4 bracket mounting holes, Ford automatic kickdowns, and the ability to mount linkages for Chrysler vehicles.
This means all of the tunability available to those looking to eke out every last pound-foot of torque on the track is also available to those requiring license plates and registration. And before you crawl all over us asking why would we want that on the street, remember that proper tuning also allows for better drivability and fuel economy, and that is something that any street driver can appreciate. Another added benefit is that you get all this with the Street-Q, without having to shell out the extra coin for expensive billet components.
QFT’s race series metering blocks and throttle bodies are CNC’d from billet aluminum and come packed with features not possible with cast parts. Billet offers the ability to deliver close tolerances, extremely flat surfaces, and zero porosity. QFT billet metering blocks also utilize screw-in air bleeds to determine orifice sizes whereas cast parts require drill bits and more press-in restrictions to get the calibration right.
Choosing the right carburetor to begin with is as much part of the tuning process as swapping jets, pumps, and power valves. The difference is, if you start with the closest base carb to suit your needs, you limit the amount of tuning you’ll need to make the selected carburetor perfect for your application. Can a larger carb be made to work? Sure, but if a smaller carb takes less “work” to get right, why not start there? But, where do you start?
There is a basic formula that has been around for decades, and well, since the same math that built the pyramids is in use today, we can trust it to get us close. Simply multiply the engine’s cubic-inch displacement by the maximum RPM of the engine and divide by 3,456. That means your 350 to 360 cubic-inch small-block turning at 6,200 rpm would need a 637 cfm carburetor to feed it. Many companies offer 650 cfm carburetors, but many times, we enthusiasts shop for carbs like we shop for cams – thinking bigger is always better! In reality, if your engine is stock, gravitate toward a lower CFM rating, if you have a larger camshaft and better flowing heads, then leaning toward a higher CFM could better utilize your set up.
Remember, engines feed on fuel, not air. Being able to ingest copious amounts of air does no good if the vacuum signal isn’t sufficient to draw the appropriate amount of fuel into the intake. Likewise, keep in mind that a street engine typically maxes out in the 70- to 90-percent efficiency range, while race engines will get closer to 95-percent volumetric efficiency.
That means that unless your engine somehow reaches 100-percent efficiency, the math is already skewed higher than the engine will ever need. The reality of this could be 20-percent or more! If you know your engine’s volumetric efficiency, you can include it into the formula to get closer to your engine’s exact needs, simply multiply CID x RPM x VE ÷ 3,456 = CFM.
Vacuum Or Mechanical
Once you get the right-sized carburetor for your engine, then comes the question of vacuum or mechanical secondaries? Factors such as vehicle weight, gearing, operating rpm, type of transmission, and even the choice of engine components all factor in to getting that perfect air/fuel ratio. While telling your buddies you installed a “double-pumper” might get you a second glance at the drive-in, if your car is too heavy, geared too high or has a tight converter, you may wind up bogging your way to the finish line.
There is a reason that many folks default to using a vacuum secondary carb on the street; they are more forgiving, because the secondary circuit is regulated by the needs of the engine, not by mechanical actuation. Not to say that a mechanical secondary can’t be made to work, but it will require more tuning to get it just right. Thankfully, Quick Fuel offers many of their carb series’ with either vacuum or mechanical secondaries.
Thanks to decades of racer ingenuity influencing modern carburetor design, making adjustments can become part of the process without becoming a challenge. Both the Race-Q series and Street-Q series carburetors enjoy the benfits of tunability, although the end result may vary.
Being a race-focused carb, the original Q series’ fuel curve is weighted much more toward WOT operation than low-end transition and drivability or cold start capability.
I would tend to spend more time tuning part-throttle cruise on a street car than on a dedicated race car by changing idle air bleeds and power valves. WOT tuning is beneficial for both applications for not only the most power, but also to ensure it is running at a safe and efficient AFR. – Keith Jessee, Holley/Quick Fuel
Many of the tuning capabilities between the two carburetors is quite similar, although the initial out-of-the-box tuning of the carburetor may be better suited to either street or track operation. Likewise, differences between the billet metering block and the cast units used on the street version also include adjustable emulsion circuits. The billet units have threaded orifices for adjustable restrictions, whereas the street version utilizes drilled orifices. Many things go into tuning the emulsion of a carburetor, which include the high-speed air bleeds (which supplies air to the emulsion orifices), the emulsion orifice size, number of orifices, and even the float level.
This is advanced tuning of the carburetor, and availability to make adjustments to this circuit is reserved for those most focused on WOT operation in the Q-series carbs as an added benefit of having billet components. This doesn’t mean it’s a race-only consideration. According to Keith Jessee at Holley/Quick Fuel, “Part throttle cruise is a big focus for street carburetors, because that’s what the majority of driving consists of. Changing the bleed package or emulsion package is one of the main things we do to tailor similar carburetors for either street or race use.”
While mechanical secondaries have always enjoyed tunability of operation due to their indifference to an engine’s airflow, even vacuum-operated secondaries are now seeing more refinement to cater to an engine’s particular needs.
As air is introduced by the air jet at the top of the chamber, tuning the emulsion jets will tailor the fuel curve precisely to the engine’s needs. If the air/fuel ratio is not linear in that at mid-range, the engine tends to go lean while the low and top-end are fine. The emulsion passages can be modified to adjust the air/fuel appropriately.
The goal of tuning the emulsion restrictions is to achieve the proper air/fuel ratio for every operating range of the engine. This requires persistence and knowledge of how these systems work.
The Bottom Line
The envelope of enthusiast car’s operation has broadened greatly over the past couple of decades, and what was once considered “race-car-only” performance can now be found on new car dealer’s lots. Granted, you won’t find any carburetors under those hoods, but thanks to companies such as Quick Fuel Technology, the lowly carb that has served us so well for so long hasn’t been relegated to the pages of history. In fact, with the availability and tunability that we enjoy from modern carburetors – like these from Quick Fuel Technologies, they’ll be living a life of power, performance, and drivability, both on the street and the track!