Are Carbs Making You Look Bad? - The Best Engine Build Means Nothing If It Doesn’t Run Right - Engine Builder Magazine

Are Carbs Making You Look Bad? – The Best Engine Build Means Nothing If It Doesn’t Run Right

It’s hard to believe that we’re still talking about carburetors in 2017, especially since the last vehicle sold in America with a carburetor was more than 25 years ago. This seemingly rudimentary device has been fueling the internal combustion engine since its inception more than 100 years ago.

It’s hard to believe that we’re still talking about carburetors in 2017, especially since the last vehicle sold in America with a carburetor was more than 25 years ago. This seemingly rudimentary device has been fueling the internal combustion engine since its inception more than 100 years ago. Even NASCAR Sprint Cup and NHRA Pro Stock finally succumbed to EFI. So why are so many of your performance engine customers clinging to their carbs?

The answer is quite simple…they offer an incredible ”bang-for-the-buck!” A well-tuned carburetor or set of carburetors will make as much or more peak horsepower than fuel injection – even today. It took the Cup and Pro Stock teams months of development to climb back to the power levels they were at with carburetors when they switched to EFI. Why? The simple answer is fuel atomization – a carburetor mixes fuel and air better than either mechanical or electronic fuel injection simply “shooting” fuel through a nozzle. A carburetor venturi can produce a smaller fuel droplet size than a fuel injection nozzle (excepting direct injection that uses extreme fuel pressures to ensure atomization).  A well-atomized homogeneous mixture burns quicker and more completely, extracting more power from a given amount of fuel. However, this is a true statement, if and only if, a carburetor is tuned properly.

This 427 SOHC-powered ’67 Fairlane designed and built by Steve Strope’s Pure Vision, is a perfect reason that carbs won’t be disappearing any time soon. This Cammer features a rare cross-ram manifold with four two-barrel Holley carbs fitted with unique “Le Mans” fuel bowls. This one-off engine is not only a head-turner, it thunders the ground.

Another reason why your performance customers have not eagerly adopted fuel injection is that factory systems are very restrictive from a fuel and airflow perspective and until recently, aftermarket fuel injection systems have been expensive and difficult for the DIYer to install. All of that is quickly changing with the new throttle body retrofit EFI systems from Holley and FiTech that retail for less than $1,000 and will probably rival the price of premium high performance carburetors by the end of the year. These systems bolt directly in place of a four-barrel carburetor and are becoming a viable alternative to a new carb purchase. Just about anyone can tune one of these “self-learn” EFI systems, while there is but a handful of people in the entire country capable of custom calibrating a carburetor’s fuel curve. EFI installation has become more “driveway friendly” too with bolt-on oxygen sensors and return-less fuel systems (no welding and fuel tank hassels).

However, many of your customers may not want to switch to EFI, or the systems discussed earlier just won’t fit their application. These loyal carb customers can have a wide range of needs: The restoration guy wants a numbers-matching carb or at least one that resembles the size and model that came on their particular car. The street/drag/bracket race guy wants the carb that will simply perform best on his engine, and the “class” racer wants his carb to provide an advantage over everyone else in his race series.

As an engine builder you have a few basic options: 1. Let the customer deal with his own carb – only to have him inform everyone on social media how badly your engine build sucks. 2. If you must reuse a carburetor, you can send it to a reputable carb shop for rebuilding and recalibrating to the new engine specs. 3. You can select and purchase the best new carb available for the new engine. 4. You can take the time to properly rebuild and calibrate the existing carb.

Carb Restoration

The path you take really depends on your shop’s business model and the level of customer satisfaction you are trying to achieve. Being much older and hopefully wiser than most of the EB readership, this is how I would approach the carb issue if I were running a small- to medium-size custom engine rebuild shop. For the guy that wants to spend the least money possible, I would simply install a rebuild kit, adjust the carb to factory specs and send him down the road. For high-end restorations I would make certain that I had the correct carb (year, model, and tag number, etc.) by casting numbers and send it to qualified restoration center such as Jet Performance, Sean Murphy Induction, The Carb Shop, Quick Fuel Technology’s Custom Shop or many other local restorers. They completely clean and re-dye the castings, re-plate the metal components, re-bush the throttle shafts and install all new original parts and hardware. The finished product is essentially a brand new carb in every way, plus they retain all of the original I.D. tags that are so important and valuable to a restorer and car collector.

For all other customers I would select the best new carburetor that best fit the intended application, and the choices these days are virtually endless. Four-barrel carbs are readily available in production sizes from 450cfm to 1400cfm; economy or premium models; square bore or spread bore; vacuum or mechanical secondaries; with or without chokes; polished, coated or natural finishes; die-cast or billet components; straight, down-leg or annular boosters; blow-through or draw-through supercharger/turbo; gas, methanol or E85 fuels and street, drag, circle track or road race models. With so many choices, how do you know which carb will be the right carb for your engine?

Here are a few guidelines to help you with your selection:

Street & Daily Drivers

If the vehicle is simply a daily driver application there are several great choices from all the major carb manufacturers such as Edelbrock, Holley, Demon, Quick Fuel, ProForm and others. They are available in 450 – 800 cfm sizes and many with mechanical or electric chokes. Stick with vacuum actuated secondary models and electric chokes for ease of cold weather operation and optimum driveability. The only reason to use a mechanical choke is if you are working on something special like replicating a Mopar Max Wedge cross-ram setup that originally came with manual chokes. Also, vacuum secondaries are the best choice for vehicles with stock torque converters and tall highway gears. The secondaries open only when, and as much as needed, boosting throttle response and fuel economy.

Spread-Bore Carbs

Most of the new carbs available are square bore models. But if you have a spread-bore manifold your choices are limited to using a square bore carb adapter (if you have adequate hood clearance), getting your Q-jet rebuilt or restored, or purchasing a new Street Demon from Holley that fits both square- and spread-bore manifolds. Another advantage of the new Street Demons is that they come with throttle linkage configured for the popular overdrive automatic transmission TV cables. You don’t have to purchase an add-on linkage bracket separately. Getting the transmission’s internal pressure correct with the proper linkage geometry could mean the difference between a satisfied customer and a costly comeback (if you warranty the engine installation). Many new carbs now come with this correctly designed TV cable hook-up.

The idle airscrews on a modular style carb are located on the sides of the metering blocks. Standard carbs have a pair on the primary side and the newer performance carbs have “four-corner idle”, both primary and secondary. Ideally, all screws should be adjusted the same. Start at 1.25 turns and adjust for the highest idle rpm from there.

Street/Track Carbs

For multi-purpose street/track vehicles, an engine builder really has his work cut out in selecting the right carb. But the following is a fairly reliable approach: First, determine how much airflow the engine needs to support the projected horsepower.  To help you with this there are CFM calculators and charts scattered across the internet. Just search for CFM Calculators and see what pops up. Also, the carburetor companies have very dedicated technical support teams that can help you with this.

Choke or No Choke?

Next up is the choke issue. It is my opinion (and I live in mostly sunny LA) that every street vehicle requires a choke, period! Nothing is more maddening than starting a car, putting it in gear and it stalls, or having to sit there and warm up the engine for 10 minutes before driving away – not happening!

Vacuum or Mechanical Secondaries?

This choice only applies to Holley/Demon/QFT/ProForm modular-style carburetors because most of the others have some style of integrated vacuum secondaries inherit in their designs. This choice is primarily decided by the level of engine build (mild or wild), and drivetrain modifications such as lower gears and hi-stall torque converters or manual transmissions. If the engine has a big cam, elevated power band and the drivetrain to match, mechanical secondaries are a good choice. For vehicles with a mild cam, stock torque converter or highway gearing, vacuum secondaries are much better. Application is a factor too. You cannot replace the direct, instant response of a mechanical secondary carburetor intended for an autocross or road course with a vacuum secondary carburetor. When you need snappy response off the corner you cannot be waiting for the secondaries to open.

How Much Is Enough?

When you look and see how many variations of a 750cfm street carb an individual company sells, it becomes evident that they are building carburetors to meet price points in the marketplace. An interesting phenomenon is that many companies have a new low-cost line of carbs that seem to have many or all of the features of their premium carbs. They are constructed out of lightweight die-cast aluminum (heavy zinc carbs are only appropriate for restos), feature replaceable air bleeds and power valve channel restrictions, modern fuel bowl and float design and four-corner idle adjustment. Plus they have the correct overdrive transmission TV cable linkage. What more do you need?

One of the most often overlooked tuning tips is to select the correct power valve for your engine. Most carbs come with power valves that open below 6.5˝ of manifold vacuum, but the best way to determine the correct power valve is to check manifold vacuum at idle and in gear with your foot on the brake. Take that reading and divide by two. So if your engine has 7˝ of manifold vacuum it needs a 3.5˝ power valve.

These “commodity performance” carbs are extremely affordable and will cover about 80% of the needs of your customers. However, there will be those customers sold on the look and the romance of anodized billet components found in the premium carbs, and there is no denying the strength advantage of a billet throttle plate or durability of a billet metering block. But these billet components are best utilized by racers who are regularly changing jets, power valves and subjecting their carbs to the abuse of drag racing throttle stops. A good rule of thumb is to “price” the carb to the engine build. If you are creating a $15-$20K street/track engine, buy the best carb that fits the engine’s requirements. If you are building a $3,500 multi-purpose engine, one of the new commodity performance carbs will work just fine.

Specialty Carbs

You’ve probably heard the expression “always use the right tool for the job.” It holds true for carb choice too. Companies like Edelbrock have a Performer series of carbs for off-road with “sprung” needles and seats to help fuel control in rough terrain. QFT builds circle track carbs with wedge-style floats to control fuel in hard left-hand corners. Holley and QFT specifically build different styles of carbs for blow-through and draw-through supercharging – each with its own fuel calibration and booster style. Really big (1050-1400crm) 4500 Dominator drag race carbs are available with down-leg or annual boosters and two or three-circuit fuel metering (better ask the factory techs which one you need). The list goes on with methanol carbs and E85 carbs (the most affordable race gas on the planet). The point is, you no longer have to modify a carb for a specific application – it already exists and will run better than something that was “adapted” for the application.

Carb Finishes

Whether you select a new carburetor with a bright, natural or black finish is pretty much personal choice and what looks best with the vehicle and engine compartment. However, some of the new coatings such as QFT’s Black Diamond finish will lower fuel temperature in the carb by up to 6% and is extremely resistant to fuel stains and the corrosive nature of the alcohol blended fuels.

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