In terms of today’s highly technical world, carburetors are the simplest, but often most misunderstood, means of fuel delivery to an internal combustion engine. From the first gasoline powered automobile in 1885 to the late 1980’s, carburetors were the primary method of fuel delivery for auto makers.
When fuel injection became the standard, many assumed that the death bell had sounded for the typical carburetor system. In fact, nothing could be further from the truth. Carb companies have continued to refine their system of fuel delivery and in many cases modern carburetors can even make more power than alternative systems without all the complicated sensors, computers and electrical wiring.
The selection in carburetors has become more intense as the range of models has grown. In the muscle car heyday of the late 60’s, carburetor choices were limited primarily to Rochester, Carter, Weber and Holley. Today’s modern technology and manufacturing processes have opened up the doors to a lot of choices in carburetors although the number of manufacturers has dwindled significantly.
With this amazing range of options comes the ability to select a carb that can be easily tuned for your application and provide the best power potential for your engine. One of the most difficult pitfalls to avoid when selecting a carb for your beast is “the bigger is better syndrome.” A 4500 series Dominator carb sounds mighty impressive, but how much do you really gain or lose when selecting a race carb over a street carb?
We enlisted the help of Proform Parts Product Development Manager Rick Hobbs, to help us find the answer. Rick has several years of experience in finding what parts that performance guys are looking for, then developing a high quality package for consumers that is affordable and adjustable.
We also brought in Zach Baker, Technical Manager at Quick Fuel Technology. Zach has years of experience with carburetors, and his knowledge of street and race carbs was essential in getting a well rounded view of carburetors for each application.
Our final expert, and longtime contributor to our stable of magazines, is John Dickey. Dickey is the founder and owner of AED Performance in Richmond, Virginia, which provides performance parts to the racing community. Dickey’s company also runs racing schools that operate on tracks in Nashville, Tennessee and Dover, Delaware.
About ProForm Parts’ Carburetors
Proform’s Rick Hobbs explained that the company started out with performance main bodies for carburetors. These main bodies were direct replacements for many of the popular carburetors on the market and were manufactured by Specialty Auto Parts, the parent company of Proform. Not only were they direct replacements for the stock parts, but they were totally new, not rebuilt, with performance upgrades built in. “We started off with the performance main body, then teamed up with Quick Fuel Technology for the assembly. We had this race body, so we thought that if we could get them assembled, we could offer a full carburetor to our customers,” said Hobbs.
Hobbs also defined the goal when designing the Proform line of carburetors, “affordability was a key in designing these units. It had to be very cost affordable with all the features that high performance guys expect. What we ended up with was a very tunable carburetor that has so many extra features that most guys wont ever use like adjustable air bleeds. The customer may never need or use these features but they like knowing that they have the option.”
There were some additional features that Proform includes on most of their carburetors that would be considered extras. Take for example the jet extensions on the secondary side of the main body. “The jet extensions on the secondaries keep the jets covered with fuel under hard launch conditions. It was something that we wanted to include to keep with the performance concept and it didn’t raise the price of the carburetor for the consumer,” Hobbs explained.
The Challenge of Carb Selection
On the surface, the “bigger is better thought process” seems to make sense. A larger 950 cfm race carburetor is bigger and therefore should pump more fuel and air to the engine and make more power.
Quick Fuel’s carburetor technical manager Zach Baker dispelled that myth by saying, “The reality is that there is a huge separation between the efficiency of a street motor and the efficiency of a race motor.
A carburetor is specifically designed to do one or the other. A race carburetor isn’t designed to be efficient at lower rpms. A street engine spends 85% of its life between idle and four thousand rpms. A race engine will spend most of its time from forty-five hundred rpms and up. The fuel curves are almost backwards.”
Understanding the differences in fuel curves has allowed Proform to tailor a line of carburetors for the street and street/strip market, and a completely different line for the race only crowd. As long as the user knows what the goal of the engine is, carburetor sizing becomes the most important issue.
Baker dispelled another misconception about carburetor sizing by saying, “A lot of people think that a bigger carburetor will run richer. A bigger carb actually runs leaner. This happens because the motor doesn’t have enough airspeed to pull fuel out of the carb.” Baker explained that you can make a carb that is too large for the engine work, “but it requires a lot of work and adjustments like larger squirters to provide the required amount of fuel.”
John Dickey of AED Performance echoed the same sentiment on carb sizing, “It’s all about selecting the right size carburetor, that is tuned properly for your combination. If you do, I feel that you will have a performance advantage.” According to Dickey, “most of the time we are talking about the parameters for race carb selection, but the same holds true for street carburetors also. The big three parameters that need to be considered are cubic inches, rpm range, both minimum and maximum rpm range and power level.”
Understanding that bigger is not better as far as carb selection brings us to the automatic question; How do you choose the correct size carb for your engine? Baker uses a tried and true, age old formula to calculate the carb cfm size: Multiply the engine’s cubic inch displacement by the Max RPM of the engine and divide by 3456. In the case of our test engine, we would multiply the CID of 440 by Max intended RPM, which for our dyno testing will top out at 6500 RPM and divide by 3456.
Baker’s advice when there is a choice between two carburetors is to “go with the smaller one.” The primary reason for selecting a carb with less cfm rating even after calculating the cfm with the aforementioned formula is that the calculation represents an engine with 100% volumetric efficiency. As Baker noted earlier, most street engines operate in the 70% – 90% efficiency range. A fully modified race engine usually operates in the 95% range. “Volumetric efficiency is not constant throughout the RPM range but is usually most efficient at the engine speed where the most torque is produced. Selecting a smaller cfm carb will match the engine’s efficiency more closely,” he says.
Dickey clarified his selection process for street and street/strip applications,”The difference in street/strip selection falls under the “rpm range” process. According to Dickey, using the standard CFM=CID X RPM X VE ÷ 3456 will give you an approximate size for race carbs.
“Street cars are nearly the same except they have a 3,000 stall converter as opposed to the race car with a 5,200 stall converter. In this case we would use a smaller carburetor as we have to accelerate from a lower RPM.” He went on to say, “In a race car we can use a larger carburetor because it will be accelerating from a much higher rpm. Even though the engine will not consume all of the available cfm, there will be reduced pumping losses getting air into the engine with a larger carburetor.”
As you can see, it isn’t easy determining the correct carburetor selection and there’s no real formula that dictates the absolute correct carb for every application. “It’s based on part science, part knowledge and part experience,” says Dickey.
Both John Dickey and Zach Baker agree that the variables with today’s aftermarket parts like cylinder heads and manifolds make it even tougher to pick the right carb size. Both agree that getting good knowledgeable help from carburetor professionals is essential to selecting a baseline carburetor. Doing it right the first time will also save some serious money and aggravation.
Sizing a Carb for Our Dyno Mule
As luck would have it, our friends at Dart Machinery were building us a dyno mule engine for future testing. While we were sitting in the office with our feet on the editorial desks, we wound up discussing cool article ideas for carbs when the machinists at Dart called in with an update on the engine build. A light bulb switched on and we asked if they would mind doing a couple of dyno runs with different carbs when the engine was completed. “It’s gonna cost you big time,” replied Dart’s Advertising Director, Jack McInnis.
Dart Machinery had taken on the task of building us a “large and in charge” small block Chevy dyno mule engine platform, specifically for the purpose of doing dyno testing on various components and chassis. Power and reliability combined with real world performance were our only requirements of the Dart engine building crew. What they delivered was a bullet proof foundation that we could use over and over on the dyno with no problems. Dart’s SHP block bored out to 4.185″ with a Lunati Pro Series forged steel stroker crankshaft and Lunati I-beam connecting rods with forged pistons comprised the heart of this beast.
Capped off with Dart cylinder heads, Lunati valve train and a high lift, long duration Lunati camshaft. Lots of other goodies made this a dyno mule a true one-of-a-kind performance engine. Stuffed with ARP bolts, Fel-Pro gaskets and a Total Seal ring package, our mule was every bit comparable to a full on race engine instead of a street machine. We did this to ensure that any dyno testing would cover a more complete range with repeatable and dependable results.
We called our friends at Proform Parts and managed to get our hands on a great four barrel street carb with 750cfm rating and vacuum secondaries and a top of the line four barrel race carb with a 850 cfm rating and mechanical secondaries for a dyno shoot out of street vs race carbs. We were going to get our first test on the Dart dyno mule engine before it even left the Dart machine shop.
Our Test Carbs
Our goal was to use a couple of budget friendly carburetors that represented a familiar cross section of what gear heads commonly use at the track and on the street, Proform’s Product Development Manager Rick Hobbs had this to say about the Proform Carburetors that we selected,”The biggest advantage of Proform Carburetors is the great price for all the features that you get.” This gave us a clear idea that these carbs were probably very common in the weekend warrior type classes. The exact audience that we were shooting for.
To keep our testing in the realm of “fair and balanced” we chose to pit the two race carbs against each other on the dyno. As far as street carb vs. race carb, we want to take a look at the features of each and discuss what makes those features work for their specific application.
Differences between Street Carbs and Race Carbs
Typically, in the most generic terms, race carbs have been routinely more “tuneable” than their street duty counterparts. By tuneable, we mean that in the past, race carbs featured removable air bleeds, emulsion tubes and orifices fitted with changeable jets while the street carbs had fixed components for the application that it was originally intended for. For improved airflow, race carbs typically don’t have concessions for street comforts like chokes. Choke towers are milled down or not even cast into the body of a race carburetor. No choke tower means no choke plate to restrict airflow.
For the most part, street rodders have considered carbs with vacuum secondaries more of a streetable choice and mechanical secondary equipped carbs more for racing applications. AED’s John Dickey expressed why he believed this common theory has lived on for so long, and why it may not be accurate; “I personally feel that many people recommend a vacuum secondary carburetor because of them having a wider selection window. A vacuum secondary carburetor is more forgiving than a manual secondary carb because you can use a larger vacuum secondary carburetor without getting into trouble because of the progressive secondary opening rate.”
According to Dickey, “You can use a vacuum secondary carburetor on a mild 350 engine all the way to a moderate 500 inch engine. However, this could end up being too much of a compromise to make the most of what is available. I would rather select the right mechanical secondary carburetor for your application.”
What’s the Advantage of a Mechanical Secondary?
Like the name implies, mechanical secondaries are controlled by stepped linkage from the primaries whereas vacuum secondaries rely on manifold vacuum for actuation.
Because the vacuum type secondary carburetor has a more controlled actuation rate, the pressure drop below the throttle plates would not be as great as that of a mechanical secondary carb. The greater the pressure drop below the throttle plates, the more fuel that is pulled from the circuits and the fuel/air charge to the engine will be more dense providing more power.
Many amateur carburetor “experts” will say the reverse is true and that is why carburetors with mechanical secondaries need the second accelerator pump. This is partially correct because the “double pump” mechanical secondary carburetors initially depend on the accelerator pumps to provide adequate fuel. When the airflow is established enough that fuel begins to pull from the main circuit then the mechanical secondaries generate the pressure drop below the throttle plate.
Dickey explained in more detail by saying, “The advantage comes in having a secondary accelerator pump for the launch and also having the secondary throttle plate totally open at wide open throttle (WOT) aiding in air/fuel distribution.”
The downside to a mechanical secondary for street use is drivability and economy. The extra pump shot can make the mechanical secondary type carb a bit unruly for the street. The problem is compounded when too large of a carb is used and the pump shot is consumed before the airflow to the main system starts pulling adequate fuel. The engine will stumble under this condition and is commonly referred to as “bogging down.”
Race Carb versus Street Carb Summary
Unless you plan on taking the time to calculate the flow requirements and test a few different carbs, a street carburetor with vacuum secondaries will work perfectly for heavy duty street use. For the occasional street/strip guys, a properly selected street carb with vacuum secondaries will handle anything that you throw at it and still have good manners for city driving. Race carburetors can play a significant role in street/strip applications where the user is on the track often and willing to give up some fuel economy and city street civility. Keep in mind that the ProForm carburetors have a serious level of tunability, so if you have the knowledge and experience, either line of ProForm’s Carbs can be dialed in to the application.
Street Carb Versus Race Carb on the Dyno
We wanted to test out the claims that our experts had made on carb selection, so we pitted two very different carburetors of different sizes on our Dart Machinery 440 ci dyno mule engine.