An intake manifold is about as simple as it gets, right? They have no moving parts and merely have to connect the carburetor or throttle body with the intake ports. Do that in a non-restrictive manner, and you have a good intake manifold. What could be easier?
As it turns out, there’s much more happening with intake manifolds than merely connecting the dots. The physics of pressure excursions, finite pressure waves, and reversion pulses are just some of what occurs inside an intake manifold. The results of these actions are powerful, complex, intriguing, frustrating, and fun, all wrapped up in an aluminum spider-like package of runner length, plenum volume, approach angles, port cross-sectional areas, and a host of other actors. It’s kinda like a good Tom Clancy novel with protagonists, antagonists, plot twists, conflict, unexpected endings, intrigue, and a rousing good time.
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There’s that word again. We always start here because it’s important to know the why before we can get into how manifolds do what they do. Another good reason to start with theory is that this information applies to all engines, regardless of displacement or whose name is stamped into the valve covers.
We will concentrate our discussion on carbureted intake manifolds, but we’ll also touch on a few fuel-injected varieties as well. There are basically three types of intake manifolds — individual runner, single plane, and dual plane. Individual runner manifolds are essentially like the old Hilborn or Enderle mechanical fuel injection manifolds. These manifolds do not employ a common plenum between each intake runner, choosing instead to allow each port to operate independently and without interference from the other intake ports.
Dual-plane, or divided-plenum manifolds do employ a plenum, or common area between the ports. They use a divider that splits the plenum and the and right. The two left-side barrels feed one half of the engine while the carb’s opposite half feeds the remaining cylinders. It’s not a left vs. right bank thing either. The main reason for dual-plane intakes is to create 180-degree phasing, giving the plenum time to rest between pulses. If you look carefully at a typical dual-plane intake, the runners end up feeding cylinders on both banks. This also increases the number of twists and turns the runner must make, which is not the best thing when working with air and fuel, but is a necessary evil that can’t be avoided.
The third type of intake is what is called a single-plane or open-plenum manifold. In this configuration, all eight intake port runners feed into one large area directly underneath the carburetor, called the plenum. This arrangement reduces the length of each runner and by design, some runners end up longer than others. This has important ramifications that we will discover later. Generally, this manifold configuration also allows the designer to increase the cross-sectional size of each of the runners.
It is possible to build a single-plane intake, for example, that offers the additional runner length of a dual plane. But in order to achieve the length without the bends, this intake manifold must become very tall. This is how the tunnel ram came about. For street engines, especially in later model cars with low hood lines, this is impractical for all but the most outlandish Pro-Street-style machines. It is possible to create a longer runner length manifold in a twin four-barrel crossram configuration, but this too is often impractical. Here is where the practical limitations of the vehicle also play a part in designing manifolds.
This leads us into the meat of basic manifold design. One of the critical elements in determining how a manifold works is the runner length. Ideally, we need to take into account the entire length of the port from the centerline of the intake valve all the way up to the radius entry into the manifold plenum. Length is critical in helping shape the engine’s power curve. It’s best to think of the air entering into the cylinder as like a column of air, or perhaps even an invisible freight train carrying the air into the cylinder. When the intake valve opens as the piston is moving downward in the cylinder, there is a pressure differential created between the atmospheric pressure on top of the carburetor and the pressure inside the cylinder. This vacuum in the cylinder is actually very low pressure created by the increasing volume, which means the greater pressure on top of the carburetor pushes the air through the carb and manifold and into the cylinder. This doesn’t happen instantaneously. It takes time for that column of air to get moving, just like a heavy freight train takes time to gain speed.
Once our freight train of air begins to move, however, it gains velocity and picks up momentum. Even though air is very light, it also has fuel in with it and together, this column of air and fuel has a mass. With a longer port, this column of air becomes longer with additional momentum, much like a freight train with additional cars attached increases its momentum. But this greater column of air requires more time to get moving, basically because it is being pushed into the cylinder by atmospheric pressure above the carburetor. This column must start at the intake valve and then extend all the way up the port to the carburetor. This requires time, just like a long freight train, which accelerates very slowly at first.
Given this, longer runners tend to increase cylinder filling at lower engine speeds when there is sufficient time for this column of air to get moving and to fill the cylinder. Peak torque is the RPM at which the engine is most efficient, when there is sufficient time to fill the cylinder before the intake valve closes. Obviously, cam timing plays a big part in this as well. Below peak torque, the column of air and fuel has not achieved the ideal air speed to move quickly into the cylinder. Above peak torque, the inlet column of air and fuel has insufficient time to completely fill the cylinder because as RPM increases, the intake valve is open for a shorter period of actual time.
If the engine is tuned properly with intake, cylinder-head ports, cam timing, and exhaust components that work well together, it is entirely possible to achieve more than 100 percent volumetric efficiency (VE). This means that the column of air can actually fill the cylinder to more than the amount of air it would contain with standard pressure. This occurs because our freight train of air has momentum and can actually overfill the cylinder before the intake valve closes. We’ve seen highly-tuned, normally aspirated race engines achieve VE numbers of 110 to 115 percent. We’ve seen numbers approaching 130 percent with four-valve engines, but these are very peaky engines that only make power in a very narrow RPM band.
It would seem that if eliminating restrictions in the induction system is beneficial, then a very large port would be the best way to make power. While that seems logical, the internal combustion engine does not always act in what would appear to be a rational manner. Inlet air velocity plays a big part in engine operation. The speed at which the inlet gas travels is critical. If the inlet gas speed is too slow as a result of a port that is too large, the incoming column of air and fuel will not have the momentum necessary to overcome the increasing pressure in the cylinder. When this happens, increasing in-cylinder pressure will force the air and fuel back into the port before the intake valve closes. In addition, the pressure waves created within the engine will be reduced due to the larger cross-sectional area of the port. This reduces the intensity of the waves and therefore particle flow will be reduced. Conversely, a port that is too small is simply a restriction that reduces cylinder filling at higher engine speeds, killing power. So much like the fairy tale with Goldilocks and the three bears, the porridge has to be just right, or the engine just won’t be happy.
The best dual planes right now seem to be the intakes stamped with the big red E for Edelbrock. The top dog for most of our testing is the Performer RPM Air Gap. While the initial thought was that the Air Gap series was just the RPM manifold with a gap under the floor of the manifold, comparison testing quickly revealed that Edelbrock redesigned the Air Gap series adding about 10 ft-lb and 5 to 10 hp to the manifold over its regular RPM cousins. Thankfully, Edelbrock also deemed it necessary to build the Air Gap for the 8- bolt Vortec intake bolt pattern as well as the standard 12-bolt small-block intake pattern. This is an outstanding manifold for medium displacement small blocks looking for a great compromise between torque and horsepower.
We’ve even seen dyno comparisons pitting the RPM Air Gap against the much-vaunted Edelbrock Super Victor single plane. In this test, the 383ci smallblock was fitted with an excellent set of Dart Pro1 CNC heads and making over 500 hp. The 2925 Super Victor made the most peak power, but the Performer RPM Air Gap generated significantly more torque throughout most of the power curve. With a typical street car with a limited gear ratio of 3.55 or lower gears, this combination would be quicker in the quarter, especially with a heavier car that needs torque to help it accelerate. The 2925 manifold would be best in a lighter car with a 4 or 5-speed and deeper gears that could take advantage of the upper-end horsepower created by the single plane.
This is not always the case. For larger displacement small blocks, like those above 420 cubic inches, for example, would usually benefit from a good single- plane intake. While shorter runners reduce lower-end torque, in the case of a 427 to 454ci small block, these engines make so much torque anyway that you can afford to sacrifice some low-end torque in order to make more top-end power. The key would be to choose a high-flow intake with relatively long runners. Here is where an electronic fuel injection intake could be of benefit since this may open up intake runner options.
Another excellent dual plane is the Professional Products Cross Wind intake, which can generally stick with the Edelbrock Performer RPM Air Gap. These manifolds have not received a lot of attention, but recent tests seem to indicate they can make excellent power and are generally priced a little better than the Edelbrock, mainly because they are made in China. Despite some bias against their origins, these appear to be high-quality intakes that fit right. Both Dart and Brodix also offer dual-plane intakes that perform well and are worth it if you can get a deal on them. Because they don’t enjoy the popularity of the Edelbrock intakes, they are slightly more expensive, due to their lower production volume. If you are considering a dual plane, look for the tallest manifold possible. This indicates not only larger runners, but also larger plenum volumes that can help the transition from the plenum to the runners. This can create hood clearance problems, so be aware of that before making your final decision.
Here is the major market for the big cubic-inch small-block. There are numerous players that can all get the job done, so you have dozens of options. Among single planes, cross-sectional area of the ports and runner length make up the two biggest variables. Large plenum areas tend to make the manifold lazy at lower engine speeds, but they do help at higher RPM. Manifold height is another important criteria since this tends to stand the runners up, giving them a straighter shot at the port. There are also manifolds designed specifically for use with tall-deck engines. Let’s take a look at some of the better single planes.
Ironically, one of the better single plane intakes that we’ve run across is from a company that you don’t normally think of as an induction manifold source. ACCEL built a single plane intake manifold a couple of years ago for its ACCEL/DFI fuel-injection program. This manifold is fitted with fuel injector bungs and is part of ACCEL’s new GEN VII EFI program as a package complete with single plane intake, throttle body, injectors, and fuel rail. We’ve tested this intake against a couple of the better single planes out there and we’ve yet to find an out-of-the-box single-plane intake that beats this casting. According to ACCEL, this manifold was designed by John Lingenfelter, which would explain its excellent mixture distribution and combined torque and horsepower capabilities. You don’t even have to buy the entire fuel injection system in order to get this intake. You can get the bare manifold separately under part number (PN) 74140 or with fuel rails under (PN) 74139. This is not a budget intake, but if you were looking for serious power, this would be an excellent choice.
The intake manifold pages in the Brodix catalog are filled with 40 different single and dual-plane intakes, but most are manifolds intended for specific race cylinder head applications. Perhaps the most common Brodix single plane is the HV1000 for 4150 style carburetors. Like most single planes, th manifold comes with four-corner water outlets as well as dual distributor holddowns. The carb mounting pad is located 6.5 inches off the china wall, giving carb position an excellent shot at the ports. The china wall is the vertical portion of the lifter valley at the front and rear of the block that seals the manifold. The china wall gets its name because the curved shape looks like the Great Wall of China. This manifold is intended for use with -8, -10, -11 and Track 1 heads.
Dart actually builds 13 different single planes designed for either the 4150 or 4500 Dominator style carburetors. Theyalso make the same pieces for tall-deck engines. The ports are raised up above the manifold base to direct cool air underneath the plenum to help minimize manifold heating. One area to concentrate on when looking at any single plane is the divider wall between the ports. Since the top of the port is longer than the bottom, often the divider walls will be tapered upward so that this tends to increase the length of the port at the bottom, making the port appear straighter to the air. Dart also offers small-block spacers that allow you to use 18 or 23-degree manifolds on a talldeck block.
The most popular and versatile Edelbrock single plane has to be the Victor Jr. This manifold is available for both the standard small block and the Vortec head intake pattern and also comes in a couple of carburetor height configurations. The 2975 is the low profile manifold and can be optioned with a 1 or 2-inch carb spacer if height permits. There’s even a CNC port-matched manifold (PN 2900) that is blended out to the Fel-Pro PN 1205 intake gasket size. The Victor 4+4 PN2976 intake is an interesting manifold that utilizes two port cross-sectional areas that tends to spread the torque curve out over a longer RPM band. This intake doesn’t get much attention, but offers an interesting idea, especially for street engines.
The Super Victor (PN 2925) is one of the newest single planes. It features a 2.80 square-inch cross-sectional port area and is designed to be used with the new flat floor 23-degree heads. This manifold also sports a one-inch taller carb height, which should improve topend power. This Super Victor is also available for the Vortec style bolt pattern and sports a slightly smaller 2.60 square inch port cross-section. There are also Victor High-Port intakes for the raised runner Chevrolet heads as well as Victor 18-degree manifolds with huge 3.2 square-inch cross-sectional areas that would be mostly intended for high- RPM race engines.
Perhaps the most successful Holley single-plane, small-block manifold is the Strip Dominator. It enjoys continued success to this day. This tall manifold offers excellent runner length that works especially good with automatic transmission equipped cars to spread out the torque curve. Lately, the Holley lineup has been bolstered by the addition of a series of Keith Dorton-designed single planes that are intended for either standard or raised port heads. The 300-110 is intended for classes with unported heads, but also offers excellent velocity along with good mixture distribution with a 2.4 square inch cross-sectional area. The raised runner version employs a slightly greater 2.5 square inch area aimed at higher-RPM operation.
We mentioned this company in the dual-plane section and they also offer a couple of single-plane intakes that also perform well. In fact, we recently looked over the results of an intake shootout that pitted the Power+Plus Hurricane single plane against four of the better single planes on the market. When pitted against a Victor Jr., a Weiand, and a Bow Tie single plane intake, the Hurricane not only made the most horsepower, but it generated excellent overall power as well. The Hurricane showed exceptional promise and is designed around the Fel- Pro 1205 intake gasket.
Not only is this intake a great power producer but it also is a decent deal for the money. It should go for around $150 in the satin finish but is also available in a polished version for about $50 more. Professional Products has also recently released a Vortec head version of this intake as well. Both manifolds offers dual distributor hold-downs, four-corner water outlets, and even nitrous bosses if you want to do a little squeezing.
The other half of the induction side of the engine is the carburetor. Its job is also simple — throttle the air into the engine and mix fuel with the air in the right proportions. While carburetors have mystified enthusiasts for generations, it’s relatively simple if you learn how all the circuits work. There are plenty of books on this subject and we’re not going to add to that impressive list of books with this chapter. Instead, we’ll touch on some of the highlights of different carburetors, their good and bad points, and the fuel mixers we’ve had the best luck with. This should point you in the right direction and save time, money, and grief.
Let’s just touch on some basics and then we’ll get into a few specific evaluations. All carburetors are rated in cubic feet per minute of airflow (cfm). However, not everyone rates its carburetors in the same fashion. Holley, for example, rates their carburetors at 1.5 inches of mercury (Hg) in wet flow. This means that fuel is introduced through the carburetor while it is being measured. Edelbrock rates its carburetors in a dry flow condition, which makes them appear larger in flow capacity because there is no fuel in the air stream to displace air. Barry Grant rates his carburetors in a wet flow cfm condition, which like Holley, is a more conservative and perhaps accurate way to measure actual airflow.
While it may not appear to be that important, the difference between dual feed and single fuel inlet carburetors can make a big difference in power production. Most needle-and-seat assemblies are sized around 0.110-inch. With engine power on the rise, it becomes difficult for a carburetor with only a single inlet, like a typical Carter, Edelbrock, Quadrajet, or Holley single inlet to feed upwards of 400 horsepower, but only the Q-jet is a true single needleand- seat design. The others all have a single-feed inlet but with twin needles and seats and dual float bowl areas. Edelbrock has just released the Performer,dual-inlet, 800 cfm carburetor that addresses that point and may work well. But for those other single-inlet carbs, the best plan is to limit their use to 425 or less horsepower, since it is difficult for the carburetor to bring sufficient fuel in, especially with a limit of around 4 to 5 psi of fuel pressure.
The aforementioned Q-jet and Carter-style carburetors do offer excellent part-throttle mixture metering characteristics, however, which make them an excellent choice for a mild street motor. This is based on the Q-jetand Carter method of using a metering rod placed inside a jet. At light throttle, the tapered metering rod displaces a majority of area of the jet, creating minimal fuel flow. As the throttle opens and more fuel is required, the metering rod is pulled out of the jet, creating greater fuel flow. The position, taper, and overall size of the jet and the metering rod create a broad spectrum of opportunities for part-throttle metering. The Holley metering system (which the Barry Grant carburetors also emulate) is a much more simplified system using merely a fixed jet. Partthrottle metering is limited by using a power valve, which introduces more fuel into the main metering circuit under high-load conditions, which is also adjustable by changing the power valve.
The success and popularity of the Holley and later with the Barry Grant carburetors is their simplicity and modular design. Remove the four bowl screws, drain the fuel, and you can access the main jets in a less than 90 seconds. Swap them out and you can be up and running in just a few minutes. Improvements in booster design have led to the use of increasingly larger carburetors on the street without serious drivability problems. If the user does a little intelligent tuning, he can create an exceptionally responsive carburetor that gets decent gas mileage yet can still deliver amazing power. All of this can be done with a simple carburetor, but that rarely occurs without some tuning skills and a willingness to experiment.
For years, the performance magazines have preached conservative carburetor sizing for street cars for good reason. The majority of enthusiasts drive mild 350ci displacement engines that rarely make more than 300 to 350 hp. A 600 cfm Holley or even a 750 cfm Q-jet could actually work very well on these engines delivering excellent throttle response and fuel mileage. But the problem is that most of these same car guys believe their engine makes more like 450 hp and they have to have a 750 cfm or, better yet, an 850 cfm double pumper.
When it comes to big-inch small blocks, this is one time where a larger carburetor is probably a good idea. The very common 0-1850 600 cfm single inlet Holley vacuum secondary carburetor is just out-classed when it comes to feeding a 450 hp 383 with a roller cam and a single-plane intake. For most large-displacement small blocks, the 750 cfm Holley or equivalent is the entry level carburetor with sizing increasing when we get to engines displacing more than 420ci. But there’s a bunch more to this story than just bolting a 750 cfm carburetor on top of your engine and calling it a day. Let’s take a look at what Holley offers and then we’ll get into the fuel mixers from Barry Grant.
Before we get into specifics, let’s run through a few basics with Holley’s numerical references just so we’re all on the same page. The standard square flange Holley four-barrel carburetor is referred to in several different forms. The standard performance style is the 4150-style carburetor that is equipped with dual-feed fuel inlets and a metering block with removable jets on both the primary and secondary sides. A less expensive version is referred to as a 4160-style carburetor. This can be either a single or dual-inlet carb and uses a metering plate instead of a metering block on the secondary side. The entire plate must be replaced to change the secondary metering on these carburetors. This plate is also thinner, which shortens the length between the fuel bowls for a dual inlet fuel line. Of course, the 4160 can be converted to 4150 status by adding a secondary metering block that accepts removable jets.
There is also a 4500 series of carburetors known as the Dominator series. These carburetors were originally designed strictly for high-RPM, drag, and circle-track competition where the engine spends very little time at idle and part throttle. These carburetors use a much larger throttle body bolt pattern that require their own specific carb mounting pad on the manifold. Holley does make a 750 cfm Dominator carburetor, but the majority of these Dominators are either 1050 or 1150 cfm monsters that are not really intended for street use. While it is certainly possible to make these massive carburetors work on a street engine, there’s very little reason except for rare, high-end street machines to do so.
Within the 4150/4160-style, square flange carburetors, these fuel mixers are also differentiated by secondary actuation. The mechanical secondary or double pumper carburetors are the older and more sought after carburetors. Here, the secondaries are actuated after the primary throttle achieves roughly half open. This is called a staged throttle opening, and is necessary to produce some semblance of fuel economy on the street. There is also a vacuum secondary carburetor that uses air velocity through the primary side of the carburetor to signal when the secondaries should start to open using a vacuum diaphragm located on the passenger side rear of the carburetor. Unfortunately, these vacuum secondary style carbs don’t always fully open, which can cost power. For this reason, most serious street runners run mechanical secondary carburetors almost exclusively. Certainly, you can make a vacuum secondary carb very well if you’re willing to get into it and optimize it for your application.
The most popular 750 cfm Holley carburetor has to be the 0-3310. There are at least seven different versions of this vacuum secondary carburetor currently listed in Holley’s numerical carburetor listing. Most of these carburetors are 4160-style carbs and can do a decent job, especially if converted over to 4150- style with a secondary metering block. Stock primary jetting for these carbs is usually a 72 jet. Earlier 3310’s used a dog-leg style booster that droops down into the venturi and offers slightly better fuel distribution and throttle response. Newer 3310’s now use a straight leg booster that is not quite as responsive.
The double-pumper version of the 750 cfm Holley is most often referred to as a 0-4779 and there have been 9 different versions of this configuration. Despite the different models, little has changed with this carburetor and it remains one of the classic square-bore performance carburetors. The latest versions now come with 4-corner idle circuits and built-in blowout protection for the power valves that have also been seriously redesigned to make them more durable. Standard jetting for this carburetor is 70 or 71 (primary) and an 80 secondary jet, unless the carb came with a rear power valve, in which case the jet dropped to a 73. If you get confused with jetting your combination, a smart move is to always return to the stock jetting and retest. Often that works the best.
Beyond the 0-4779, Holley has produced several variations on the basic 4150-style carburetor. The most popular of these is the HP series of carbs. The HP series offers a high-flow version of this same venturi size, using a specific radius air inlet design that eliminates the choke horn and improves airflow into the carburetor. These carbs also feature more accurately located boosters for more even fuel flow between all four boosters, as well as adjustable idle and high-speed air bleeds and notched floats with jet extensions in the secondary side. These carburetors tend to offer a much more even fuel flow throughout the carburetor’s entire flow curve and can often be found to improve power over a standard 4150-style Holley, while also using less fuel. There is power in these carburetors over a standard 4150 and the price reflects this ability. However, these are still affordable carburetors and would work very well on a large cubic-inch small-block street engine. These carbs come in sizes ranging from 650 cfm through monster 950 cfm applications using Dominator fuel bowls. There are even methanol-calibrated HP carburetors for those crazies that choose to mess with alcohol.
Several years ago, Barry Grant decided to take a run at Holley and has parlayed that competition into a big time business success. Grant started out modifying Holley carburetors and decided it was cheaper and easier to build his own. The Demon line of carburetorsis modular like a Holley, but with his lines of Road Demon, Race Demon, and King Demon carbs, there is something for every application.
It really isn’t fair to Holley to compare the Race Demon line of general competition carburetors with the straight Holleys like the 750 cfm double pumper. The Race Demons are actually similar to Holley’s HP lineup, but offer some interesting upgrades. The base plate is a billet material rather than a casting and the metering blocks are constructed of a more accurate die-casting method rather than the older sand castings. Another important point about the Demon lineup is that the company spends a significant amount of time positioning the boosters in the venturi and ensuring that each of the boosters are matched. In many production-line carburetors, the boosters tend to migrate around inside the venturi, creating inconsistent signals and fuel flow from the four boosters. This is one point that the HP line of Holleys also addressed.
Each carburetor manufacturer matches the airflow through the carburetor with fuel flow for the mass flow of air through the carburetor. After experiencing dozens of dyno tests with these carburetors, it seems that the Demon line of carburetors does an excellent job of making great power with less fuel. This is created both by efficient boosters, and also excellent integration of the main fuel well and the air bleeds to create this fuel curve. We have seen several tests where a Demon carburetor made the same or more power than a competitive carburetor and usually used less fuel.
A great addition to an excellent single- plane intake like an Edelbrock Victor Jr. or a Professional Products Hurricane for a big-inch small block would be a 825 cfm Race Demon with a solid fuel delivery system to back it up. Or if a milder small-block like a 383 with a Performer RPM Air Gap is more your cup of hydrocarbons, then we’d top it off with a Holley 0-4779, or perhaps even a Road Demon of around 750 cfm.
Let’s get right into this. Everyone wants to know if electronic fuel injection (EFI) makes more horsepower than a carburetor. With everything else being equal, generally a carburetor will make a slight amount more power, usually due to what is called latent heat of vaporization. This means that when fuel is introduced into the air stream, the fuel begins the process of changing from a liquid to a gas. This vaporization of the fuel pulls heat out of the incoming air, making it more dense. Because a carburetor introduces fuel further upstream than a multi-point fuel injection system, it has a longer period of time to chill the air. This is usually why a carburetor can make more peak horsepower than EFI.
It should also come as no surprise that a even a simple single-plane intake manifold-type, multipoint fuel-injection system will cost roughly two to three times what you can expect to invest in a new intake, carburetor, and fuel delivery system. This certainly begs the question as to why you would want to go with EFI over a carburetor. We will venture the notion that the decision to run with EFI over a carburetor has less to do with horsepower and more to do with whether or not you are a believer in technology.
The carbureted guys will probably never buy into the aftermarket EFI, no matter how many bells and whistles are added to these stand-alone systems. We’re not going to waste anybody’s time here by trying to convince you one way or the other. But if you’re willing to look into the advantages of EFI, there is much to learn about the increased manipulation over the spark and fuel curves, which control the power output.
Perhaps one of the most salient points contributing to the success of EFI is its ability to handle otherwise rather balky, high-output street engines. Systems like the ACCEL/DFI version VII, the F.A.S.T. system, Electromotive’s TEC3, and Holly’s Commander 950 system offer significant advantages in partthrottle operation and ultimate control over fuel and spark that make these systems far superior to a carburetor.
The newer systems like the F.A.S.T. and ACCEL/DFI version VII systems also offer wide-band oxygen sensors that can give the user closed-loop feedback control over wide open throttle operation. You can even set the air-fuel ratio you want and then let the computer work with the oxygen sensor to maintain it. Not too long ago, this technology was virtually unheard of, and now you can get this kind of accurate control over the fuel with a street fuel injection system!
Want more? You can also step up from batch or bank-to-bank firing of the injectors to a more sophisticated sequential system. With sequential EFI, the injector fires only when the intake valve is open, offering yet even more specific control over the fuel. This has proven to be worth some power. This is especially important with thumper engines that demand very specific air-fuel requirements, especially at part throttle. Closed loop control, even with the factory-style oxygen sensors, allows the computer the ability to accurately maintain a 14.7:1 air fuel ratio at part throttle that not only reduces emissions but also produces fairly decent fuel mile age—much more than you could expect from a carburetor.
These EFI systems also control the spark side of the engine as well. This is really helpful at part throttle where a little bit of tuning can offer big drivability improvements. Where this electronic spark control is especially important if you decide to use nitrous. Most of the EFI systems (except for the Holley Commander system) offer one to three stage of nitrous control where both spark and fuel controls can be carefully maintained. The EFI system can trigger the nitrous based on various control parameters like throttle position, RPM, and also on a time delay. Once the nitrous is triggered, the EFI system will pull back the timing the exact amount specified as well as richen the mixture via the fuel injectors — you don’t even need a fuel solenoid. The EFI computer can also shut the nitrous off at the designated engine speed, ahead of the rev limiter so you don’t encounter problems. The one thing you never want to do is hit the rev limiter under nitrous. This will cause all kinds of expensive problems!
This has been just a rough overview of the advantages of EFI. There is a ton more information on these systems and we urge you to look more closely into the advantages of EFI before you just blow it off as an expensive gadget. EFI and a large cubic-inch small block could be a powerful combination.
Written by Graham Hansen and Posted with Permission of CarTechBooks