Although electronic fuel injection has dominated the street and racing scene for three decades the old reliable four-throat atomizer remains as popular as ever with hard core performance buffs. The beauty of carburetion is its wonderful simplicity – the mixing of air and fuel along with the modulating of throttle plates and accelerator pumps. No wading through a sea of confusing electronics. What’s more, few things are quite like the thrill of goosing the throttle and hearing an engine respond. Try that with your late model, fuel-injected, drive by wire ride.
These folks take serviceability even further with extraordinary attention to detail – lightweight billet aluminum construction, CNC machining technology, swappable idle air bleeds, easy to service metering blocks performance-tuned for street use, and four-corner idle adjustment on mechanical secondary carburetors to help fine tune the idle when you have an aggressive camshaft profile. Swappable idle feed restrictions further expand the idle adjustment capabilities.
Size Matters, Bigger Is Not Always Better
All that said, how do you choose just the right size carburetor for your performance project? We spoke with Booth Platt of Proform to get valuable insight into how to properly size a carburetor for your particular application. Viau Motorsports‘ American Nostalgia West AFX race class project is being put together, and part of that project includes a pair of Proform Race Series carburetors. Carburetor sizing boils down to matching carb size to an engine’s VE or Volumetric Efficiency.
The math works like this:
This formula gets you started, but it does not completely confirm proper carburetor sizing. The first order of business in carburetor sizing is what you want the engine to do. What rpm range do you expect the engine to operate? How much horsepower and torque do you want? Daily commute or weekend fun? Street or strip? Drag racing or road racing? At the end of the day, carburetor sizing boils down to what you want your engine to do, and when.
Of course there’s more to choosing a carburetor than just carburetor size. Cam selection, cylinder heads, intake manifold, compression ratio, exhaust system, and even driveline composites play into power just as much as carburetor sizing. What is your axle ratio? If you have an automatic transmission what is the torque converter stall speed? What do you have for gearing? Carburetor sizing is just as critical as all these elements because engine and driveline packaging is a team effort. Heads, cam, compression, and exhaust must all work together as a team for you to get the power desired.
Platt comments on street and race series carburetors, “It all starts with getting the right sized (CFM) carburetor to begin with because that part of the carburetor is not adjustable. We can change CFM only by changing the carburetor. Obviously the common misconception out there is that folks think that more CFM translates into more power. This story has been told a million times, but it’s always worth saying again: more CFM doesn’t mean your engine will make more power, nor make your car go faster. It’s kind of like buying shoes…too big and too small don’t fit well. With the bigger shoe you’re getting more material, but that doesn’t mean they’ll fit better.”
“We try to steer our customers into the correct CFM size based on our technical department’s knowledge of vehicles and how Proform carbs perform,” Platt tells us. “Once the right CFM is assessed we then look at the art of tuning. At Proform, we’ve made that pretty easy because most of our applications won’t need to be adjusted at all due to the popular settings they are tuned and tested with.”
We try to steer our customers into the correct CFM size based on our technical department’s knowledge of vehicles and how Proform carbs perform. -Booth Platt
“Beyond adjustability,” Platt adds, “there are premium components that make up our Proform carburetors, which include billet aluminum metering blocks and throttle plates as well as die-cast aluminum fuel bowls with sight glass windows. All this great stuff comes standard in Proform carburetors and at a very competitive price.”
“This year, Proform added a new line of Race and Street Series carburetors that feature a rich Black Diamond finish on the fuel bowls and main carburetor body. In addition to looking awesome with the purple billet components such as the metering block, the finish has some fuel temperature reducing properties that make it more than just a finish. Cooler fuel is a good thing. It helps your engine make more power,” Platt tells us.
You will also find the popular billet throttle plate employs superior strength, coupled with precision throttle shafts to ensure long term performance. All Proform carburetors have easy access adjustable secondary air flow. A Ford kick down is now included with the carburetor. Some carburetors require special tools or removal of the carburetor to accomplish this simple adjustment. Proform makes tuning and adjustment a cakewalk.
The Proform carburetor package is complete with high quality light weight die-cast aluminum fuel bowls with fuel level windows for quick visual reference. Float level changes do not require removal of a sight plug with the risk of spilling fuel. Proform’s bowls come equipped with 3/8-inch inverted flare fuel inlet fittings and will accept all conventional dual feed fuel lines compatible with Holley four barrel carburetors. No hunting all over the Web for just the right fittings and dual feeds.
Another great premium with the Proform is parts availability. If you cannot source Proform parts, parts for these modular carburetors are universal. Power valves, jets, boosters, metering blocks, and other items are interchangeable across brand names. You’re never out in the cold if a problem arises.
Once you’ve selected the right sized Proform Street/Race carburetor for your application, then what? Most Proform carburetors will perform handsomely out of the box. However, what if you live at 5,000 feet? Or what if out-of-the-box tuning isn’t a perfect fit for your engine?
Let’s talk about Proform tuning. Before you can properly jet a Proform carburetor all basic adjustments must be made. Float level should come to the base of the sight glass, no higher nor lower. Idle mixture adjustment should be performed with a vacuum gauge. Turn each idle mixture screw in until the engine begins to falter, then, back out slowly until the highest possible vacuum reading is achieved, and do the same with the other idle mixture screws. Keep in mind engine idle should be stable and smooth in addition to the vacuum gauge reading.
If you experience a rough idle, it’s suggested to check for vacuum leaks, which will cause a rough idle and make it nearly impossible to get the idle speed where it belongs. A vacuum leak will cause a fast idle in addition to a rough idle. Vacuum leaks can originate in many places: vacuum port caps missing, intake manifold or carburetor gaskets not sealing, even your vehicle’s air conditioning vacuum servos can be a source of vacuum leaks.
If you cannot ascertain the source of a vacuum leak, try spraying carburetor or brake cleaner around the intake manifold and carburetor base gaskets to see if engine speed fluctuates. If the engine surges, you’ve found your vacuum leak. Vacuum leaks can also occur underneath the manifold in the lifter valley if the manifold gasket is compromised in that location. There’s typically oil ingestion when a vacuum leak is in the valley.
Carburetor jetting can be a tricky science if you don’t know what you’re doing. Proform carburetors are jetted for most applications, however, there are always exceptions to out-of-the-box carb jetting. If you live in a high elevation or a place with extraordinary atmospheric conditions such as the tropics, extreme cold, or extreme desert heat you may have to jet for the environment. This has to be taken on a case by case basis.
The reason we have to jet up or down has to do with how much oxygen and atmospheric pressure we have in a given location. As vehicle elevation increases your engine is getting less air and atmospheric pressure. This means the engine would be getting too much fuel in higher, thinner air.
With each 2,000 foot increase or thereabouts, we have to go down one jet size in order to keep the engine happy. If your engine is content in Indianapolis with 0.80 jets, it’s going to be running fat at 5,000 feet. By the same token, if your carburetor is jetted for 5,000 feet you’re going to be running lean in Philly.
A good rule to follow on jet sizing is one jet size down for every 2,000 feet of elevation gained. If your Proform is fitted with 0.80 jets for sea level and 70-degrees F, which is the norm, jet down one size to 0.79, for starters, if you’re operating 2,000 feet above sea level. Are you going racing in Denver at 5,000 feet and you live on the flatlands of the American prairie? You will need to jet down two to three sizes and see how your engine performs.
Although this is our formula for jet sizing it doesn’t always go true-to-mark in every situation. Because engines vary a great deal when it comes to heads, cams, induction, and exhaust one size up or down based on elevation is a good baseline, but not always the rule. Another tuning consideration is power valves and accelerator pumps – both determine if you launch, or stay on the porch.
Accelerator pump cams and nozzles determine what you will have for throttle response. Cams are available in a variety of ramp rates, which are identified by color. Accelerator pump cavity/diaphragm sizing is yet another consideration. Power valves always stump performance buffs. However, they’re not hard to understand. If you have strictly a primary metering block you will have one power valve. If you have primary and secondary metering blocks you will likely have two power valves.
Power valves are rated at inches of intake manifold vacuum. Choice depends upon when you want the power valve to operate. Selection depends on where your engine lives rpm wise. Never fall into the mistake notion you don’t need a power valve. Most applications, without exception, must have a power valve. Power valve selection depends on what intake manifold vacuum is at idle. Most street engines idle with 12-22-inches of manifold vacuum. This means you will want, on average, a 6.5 power valve. You will want 50-percent of your engine’s idle speed manifold vacuum. Then, road test and see what the result is.
Proform believes an educated consumer is its best marketing tool. Performance enthusiasts do their homework in the forums, at the cruising spots, on the racetrack, and online. Word of mouth either sells the product or leads to its doom. This is why Proform doesn’t cut corners in its research and development efforts. By the time a carburetor makes it to market extensive research has been performed on cars and in dyno rooms. By the time it lands on top of your engine you can be confident it is right.
To see the complete line of the new Race and Street Series carburetors, visit the Proform Parts website, and be sure to check with its knowledgeable staff to make sure you get the right carburetor for your application.