The fuel system used on the Ford FE engines was generally conventional throughout its years of production. An eccentric bolted to the front of the camshaft actuates a mechanical fuel pump, which is mounted to the driver side of the timing cover, and this supplies fuel to the carb. The vast majority of FE engines use a single carburetor (2- or 4-barrel) mounted to a cast-iron intake manifold.
Although produced in a limited number each year, the factory high performance engines had a lot more variety in fuel delivery. Most of the high-performance engines used aluminum intakes (see Chapter 10).With rare exception, all the factory and aftermarket single 4-barrel intakes are designed to accommodate the Holley 4150 mounting pattern. Holleys were offered in a single 4-barrel, two 4-barrels, or three 2-barrels. The two 4-barrel engines used a pair of 4160 model Holley carbs mounted in line (more details later).
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Ford carburetor packages (aftermarket and non-production that were available only through the parts department) were similar in configuration but also with a few notable exceptions. Those were the offsetmounted dual-quad setups used on the Edelbrock and Mickey Thompson cross ram intakes, the early multiple 2-barrel intakes, and the Weber carburetor systems.
Mechanical fuel-injection systems, featuring eight individual runner stacks, have long been available for the FE in a variety of configurations. But these range in popularity from simply very unusual to extraordinarily rare. Current aftermarket electronic fuel-management systems are bringing this technology to the FE engine builder. Edelbrock now offers converted carburetorstyle intakes with the fuel-rail mountings and injector bungs already in place on the Victor intake casting.
I’ll touch on most of these, but the primary focus of this chapter is on readily available 4-barrel carburetor systems for street and mild race applications.
4-Barrel Carburetor Options
The vast majority of engine projects are topped off with either a single or a pair of 4-barrel carburetors. While this is not intended to be a carburetor book, I do need to cover the subject in some detail so you can get the most out of any engine build.
The non-performance engines from Ford often had an Autolite 4100-series carburetor. A surprisingly advanced design in a number of ways, the Autolite carb never developed strong following as a highperformance part because, in part, Ford chose to run Holley carbs on all of the factory-issued high-powered combinations. That being the case, the Autolite has dual integral fuel bowls, easily changeable jetting, low-leakage-potential bathtub design with very few gaskets below the fuel level, and annular boosters. While this carburetor is best used in a restoration-style project, where appearance outweighs power, it can still be tuned to run quite well. Ironically, they were the obvious design inspiration for the Holley 4010 and 4011 carbs of the 1990s, which have recently been re-released as Summitbranded carburetors.
Holley 4-Barrels: Basic Design Elements
The ubiquitous Holley 4-barrel is the indisputable carburetor of choice for the average FE builder. Holley 4-barrels come in a few different configurations, such as 4150, 4160, etc. Note that all of the technical information presented here also applies to the other carburetors using similar Holley-derived modular architecture.
Holley identifies its carburetors by model and then historically by airflow and design features. Holley carburetors and components have an engineering number cast or stamped in numerous locations. These numbers include an “R,” such as 12R1234-5. The engineering numbers are useful for component identification, but the carburetor assembly’s true part number is a “0-xxxx” stamped into the leading edge of the choke horn.
The standard-mounting-flange Holley carb is model 4160, with a metering block on the primary side and a metering plate on the secondary. The primary metering block has replaceable jets and a power valve. The metering plate is thin and has only drilled holes for fuel metering with no adjustability.
The Model 4150’s carburetors have adjustable/replaceable jets and a metering block on both the primary and secondary side. They share the same mounting pattern as the less-expensive 4160, but are more readily tunable for various combinations.
Holley carbs, whether 4150 or 4160, can be either single or dual feed. A single-feed carburetor has a fuel inlet on the linkage side of the primary bowl, and a transfer tube running back to the secondary bowl on the opposite side. A dual-feed carburetor has so called “cathedral” fuel bowls that have an inverted “V” appearance as viewed from the end. They have a separate fuel line feeding each bowl—often joined together at a fuel fitting outside of the carb.
Model 4160 carburetors are almost always vacuum secondary designs, but manifold vacuum does not actuate the vacuum secondaries. They use a large diaphragm mounted to the side of the main body to open the rear barrels by sensing airflow through a bleed feeding into the primary venturi—assisted in some cases by another airflow-sensing feed on the secondary side. The signal generated by airflow past the bleed(s) is balanced against a spring in the diaphragm housing, which can be altered/selected to speed or slow the secondary opening rate.
Model 4150 carbs can be either vacuum secondary or have mechanical secondary actuation. Only the dual-feed, mechanical-secondary carburetors are properly referred to as “double pumpers,” which is a descriptive name taken from the necessary accelerator pumps on both primary and secondary sides of these carbs.
Dominator 4500-series carburetors are a completely different package because the main bodies and linkage are unique. However, they share external components, such as bowls with the traditional Holley 4150/4160 series. The Dominator was designed for racing from the very start and has an integral main and throttle body, along with replaceable boosters. The 4500 Dominator has a unique and large mounting flange and large throttle blades, and they are all “double pumpers.” Most of them carry airflow ratings of 1,050 or 1,150 cfm.
Carburetor selection is most often based on potential airflow measured in cubic feet per minute. Subjecting each carburetor configuration to a test procedure as specified by the Society of Automotive Engineers (SAE) determined the airflow ratings. Unfortunately, the last decade has seen a drift away from this scientific process, however flawed, to a marketingdriven naming program in which the carb’s assigned “SAE airflow number” now has no real value.
The SAE procedure rated 4-barrel carbs at 1.5 inches of vacuum drop. This meant that, while on the test fixture, cfm was measured at the point where airflow was high enough to generate 1.5 inches of vacuum below the carb’s open throttle plates. At the rated test flow, the carburetor represents a restriction in airflow, which is not ideal from a performance perspective, but a fixed restriction value is necessary so the volume of air various carbs can flow is quantified. Hence, this provides a basis of comparison between carburetors. In essence, this means that the test is valid for comparison, but not definitive for selection by the rated cfm—a key flaw found in most of the common carburetor-selection formulas.
In addition, proper carburetor airflow testing was done on a “wet” bench. On the bench, a fluid with specified characteristics similar to gasoline was running through the carb’s boosters and circuitry. Since fuel has both volume and mass, this has a significant impact on the amount of air that can flow through a given venturi.
Vendors of modified custom carburetors began to aggressively market their products, and it became common practice to test carbs with tweaked parts on low-capacity, dryflow benches. These gave artificially inflated airflow values, which some consumers took to heart—making purchases without real knowledge of the actual gains achieved, if any. Eventually, Holley followed the marketing trend and assigned equally arbitrary numbers to its HP-series carbs. A Holley HP 950 is essentially a 750 main body with an 850 base plate, along with a host of other useful upgrades, but it is decidedly not a 900-cfm carb. A 1050 Dominator outflows a so-named 1,000-cfm 4150-style carb by a wide margin.
At this time, it is best to make your sizing decision based on throttle bore and venturi bore diameters. Discuss your project parameters with your chosen supplier, and listen carefully to their recommendations.
In addition to the general airflow ratings procedure, there are a few other items that merit discussion before going into the selection process. First among these are boosters. The carb boosters are those small venturi rings that are sticking out into the airstream. The mounting legs are swedged/spun into position in the main body of the carb, and the small venturis serve to both increase the “signal” and to deliver the maincircuit fuel into the airstream.
Holley-style carburetors have one of three booster configurations, each with advantages. The least expensive is a simple straight-leg booster, which looks like a simple ring with a straight piece of tubing coming out of one side. The next design has a downleg booster. As the name implies, the downleg design “droops” from its mounting position and places the ring lower into the main venturi. They are more difficult (hence expensive) to manufacture, but also work better in most 4150/4160 applications—delivering more fuel with less of a flow restriction. The third is the annular booster; this design offers a larger cross section and an array of fueldelivery holes, which are placed all around its diameter, as opposed to the single delivery orifice of the other two. Annular boosters are regarded as being more restrictive, but as starting full flow earlier in the RPM band and in a more evenly dispersed pattern.
Picking the Right Size Carb
There are a lot of various formulas floating around that use RPM, engine size, and assumed volumetric efficiency to select the right carburetor. As noted above, most of them are flawed because the carburetor airflowrating process assumes a restriction.
I normally guess large on carbs, compared to most folks. I can tune good driving into a large carburetor but cannot make a small carb flow more air. On the 390-based 445-ci stroker motors, I’ve seen an increase of 20 hp on the dyno by going from a basic vacuum-secondary 750 carb to a downleg-booster double pumper with the same airflow rating. I’ve also seen a comparable gain going from that 750 double pumper to a larger 850. Although I am sure that the smaller carb might drive nicer, the bigger one always makes more power.
On the larger 482-inch FE stroker engines, I almost always use either a 2×4 setup or a single 4500 Dominator. The Dominator has proven to be a stronger package than any of the single 4150-style carbs I’ve tried, and the dual quads are better yet.
When choosing dual-quad carbs, there are a couple things to be aware of. On the factory-style FE 2×4 medium-riser setup, the two vacuum secondary 4160 carbs are mounted backward on the intake. The “secondary” goes toward the front of the engine and the linkage is on the passenger-side of the car. Model 4150 carbs do not fit. This unusual arrangement is for distributor clearance.
During dyno testing, I observed that carbs with the secondary barrels and light springs in the diaphragm housings open pretty slowly, even on large engines. So I size the carbs as if they were a single double pumper and essentially pretend that the rear barrels are not even there—putting a pair of 750s on a street engine works just fine from a tuning and driving standpoint. If the engine requires the added airflow, the back barrels come in, and they can be tuned to some extent with lighter springs.
The power valves are vacuumoperated fuel-delivery devices that work in concert with the main circuit of the carburetor to deliver the full amount of fuel necessary at wide-open throttle. The power valves are found on the primary metering block of almost all Holley carbs, and on the secondary block in many. Often improperly blamed for drivability issues, the power valves are an important factor in the total tuning package.
Power valves are normally held closed by manifold vacuum. They open and allow fuel to flow when vacuum drops below a predetermined level. The opening point for each power valve is marked on the body or washer face of the valve. A 6.5 valve is designed to open up at 6.5 inches of vacuum.
The power valves deliver fuel through two orifices or channels in the metering block called power valve channel restrictions (PVCRs). Fuel delivered through the PVCRs do not go through, nor is it affected by the main jets. Depending on the carb model, these channels measure from .032 to .093 inch in diameter and make up a considerable percentage of the overall fuel delivered. Power valves for Dominators may be different, though. Higher-flow models are colored gold and are needed with the larger-diameter channel restrictions.
A properly selected power valve and channel restriction combination allows leaner, and thus “clean and crisp” part-throttle operation while providing the correct fuel delivery for WOT power.
A case can be made for removing the power valve from the secondary side of carbs, but you are going to have opened secondaries with manifold vacuum in many circumstances. Removing the power valve from the primary side is rarely the correct answer on anything other than a drag race-only application, which runs at WOT only. In any case, removal of the power valve requires an increase in mainjet area that is commensurate with the area of the now non-functional PVCRs. You cannot simply assume some arbitrary jet-number increase. Each carb model has had a different amount of fuel flowing though the power valves, and on some Dominator models it approaches 50 percent by area.
Blown power valves are not nearly as common as you might think. When bowl screws have been over tightened, the main body gets pulled in at the corners over time. The now-reduced clamping pressure at the center of the metering block gasket causes it to leak after a backfire— pulling fuel from the centered accelerator pump and power valve passages right into the vacuum cavity below them. The owner replaces the so-called blown power valve, sticks a nice new gasket in there, and fixes the problem until the next backfire.
The correct power valve is one that stays closed during light, partthrottle operation but opens when the throttle is “crowded” or under a fairly heavy load, such as when the vehicle climbs a hill. For example, if you feel a flat spot or get a surge under that driving condition with a 6.5-inch valve, you need a power valve that opens just a bit earlier with a higher vacuum number— maybe try a 7.5-inch valve. You also want a valve that stays closed at idle or at least is not pulsating at idle. On an automatic, you check vacuum in gear and then go an inch lower.
Floats, Needles and Seats
Holley carbs have a float bowl at each end of the carburetor. Although they at first look the same, the front and rear bowls are different. The float level on a Holley is externally adjustable, and verifiable with the brass sight plug on the side. The normally accepted standard is to have the fuel just at the bottom of the sight plug, but I prefer to set the level with the bowl removed and inverted, and the float parallel with the top of the bowl. I have seen cases where high fuel pressures allowed the fuel to appear okay at rest, but a low bowl setting lets the engine lean out at higher RPM due to inadequate float drop.
The plastic see-through sight plugs seem like a good idea, but I’ve seen them become hard and break.
The needle and seat have a pair of O-rings that must be in good shape. A cut ring either leaks externally or allows fuel to bypass the needle causing flooding. Old fuel or race gas with odd additives can cause the viton tip on the needle to erode prematurely.
There is a bumper spring that needs to be below the float arm when assembled—helping to pick it up.
Jets, Bleeds and Tuning
Tuning a carburetor seems pretty easy at first. Then it gets progressively more challenging as you learn more about the interrelationships between the circuits. This book is not intended to be a comprehensive tuning guide, but rather a series of discussion points to help you find your way. As a general rule, if you have to make big changes to get a carb to respond—like more than four jet sizes—you should likely have things checked out carefully before moving forward.
The best tuning tools are a good set of ears and eyes, followed closely by data in the form of oxygen sensors and brake specific fuel consumption (BSFC) numbers. In the first part of the tuning process, control is defined by a linear fuel curve. This provides air/fuel ratios that are reasonably smooth and respond predictably to changes. The BSFC numbers should track with the oxygen values, which means both show richer or leaner values together as changes are made. With control established, you can then make changes to see what the engine “wants” for peak performance at any given point in the RPM band or driving situation, and those bleed changes alter the power curve. If you see smoke or hurt plugs, or hear knocks or odd sounds, always recognize these problems and recheck the sensors and data before going further.
Main metering jets are easy to change and are the most frequently replaced tuning components in a carburetor. At least half of the time, they are changed for the wrong reasons, or are expected to cover for other issues.
Main jets work in concert with the fuel level, the emulsion bleeds in the metering block, the power valve channel restrictions, the boosters, and the main air bleeds. The overall fuel curve relies on the relationship between all of these, and a radical change in jet size alone can have some unpredicted results.
As an example, main jets are always “on,” while the power valve circuit is only active at wider throttle openings. If you have good light throttle or cruising driving characteristics but are too lean at wide open, you should consider increasing the size of the PVCRs, rather than the jets. Increasing jet size fattens your part-throttle fuel more by percentages than it does the wide-open, and the PVCRs make up a good portion of wide-open fuel delivery.
The main air bleeds are at the top of the carburetor alongside the choke horn. These are sized to work with the booster design and the jetting/ emulsion package to tailor the fueldelivery curve and the onset of maincircuit fuel delivery. Changing the bleed diameter affects the starting point for the main circuit, the amount of fuel delivered by a given jet size, and the amount of air introduced into the main fuel stream. These changes can be pretty dramatic and a touch unpredictable at times due to the aeration factor. If you choose to do your tuning with bleeds, keep good notes, start with a small incremental change, and watch for trends before making any “big” changes.
Emulsion bleeds are the series of small holes drilled or installed vertically in the metering block main wells. The holes below the float level serve as added fuel feeds; those above act as additional air bleeds, and those in the middle as a combination of the two, which produces aerated fuel. As the engine RPM goes up, the fuel level normally drops and the overall bleed package changes. As with the main bleed, small changes can have a noticeable impact on fuel delivery. Plugging a bleed, or changing its size or position alters the overall fuel-delivery curve.
Holley-style carbs have a great deal of adjustment potential in the pump circuit. The squirter is the outlet for the pump, and it’s found at the top of the venturi section. These are usually the first part changed, but they should be considered as part of a system rather than as a single item. Squirters are marked for flow, referencing the diameter of the outlet holes in them. Most carbs start off with something around .025 inch, but sizes of up to .037 inch are common for tuning.
The pump itself is below the fuel bowl, and can be either a 30- or 50-cc piece. The volume measurement referenced is actually an average of 10 strokes. The fuel from the pump goes from the bowl past an inlet check, and into the pump cavity. From there, the pump stroke pushes it through an upward-angled passage in the metering block, into the main body, and upward to the squirter mounting. There is a discharge check valve just below the squirter to prevent backflow and control fuel pullover through the squirter. The screw that holds the squirter in place needs to be a drilled piece for higher flow requirements of .035 inch or more.
A plastic cam mounted to the throttle lever operates the pump. The cam can be changed (many are available) to alter both the timing and volume of the pump shot. By changing the location of the cam relative to the throttle position, you can delay the onset of pump action, and this is useful for a drag car that leaves the starting line at an alreadyhigh throttle position.
The size of the pump cavity and the amount of stroke provided by the pump cam determine the amount of fuel available through the acceleratorpump circuit. Increasing the size of the squirter delivers the available fuel more quickly, and decreasing it slows fuel delivery. In vehicle testing, this is the only way to determine the correct amount of and timing of the pump circuit. A short hiccup followed by solid acceleration is a common sign of inadequate pump shot.
Fuel Injection Overview
“FE” and “fuel injection” are terms not often used together in the same sentence, but that is changing. The early mechanical fuel-injection systems that are found on eBay.com and in swap meets have a lot of cosmetic appeal, but they are not very street friendly when in good condition. And few if any of them remain in that shape.
On the other hand, I am now seeing an increase in the number of engines being retrofitted with electronic fuel-management systems— blending the cool cosmetics with modern functionality. These are most often using a carburetor-style intake manifold with injector bungs and fuel rails mounted above each runner. The throttle body takes the place of the centrally mounted carburetor, and incorporates a throttle position sensor (TPS) along with an idle air control (IAC). A 1-bar manifold air pressure (MAP) sensor is mounted in the engine compartment, while an inlet air temperature and a coolant temperature sensor are installed on the engine. One or two oxygen sensors are mounted into the exhaust system. The distributor, cam, and crank triggers are connected as well, to complete the connections required for the management system.
Engine management systems from F.A.S.T., Big Stuff 3, DFI, Holley, Edelbrock, and others are now available to control all parameters of engine operation. Each system has differing capabilities and requirements, but the similarities are enough to cover them as a group for this discussion. They generally control and monitor all fuel and ignition adjustments and have datalogging capability. Once everything is installed, connected, and programmed with the basic engine parameters, you should be able to fire the package up.
Using a laptop computer, you can set virtually any variable from timing and idle speed to fuel mixture without opening the hood. A pair of tables is used to define the basic setup. One reflects comparative efficiency of the engine at a particular load and RPM, and the other targets air/fuel ratio at those same points. It is a far quicker process to establish control of fuel delivery with the EFI system than it is with a carburetor. A tuner manipulates the efficiency tables until the corrections to a target air/fuel ratio are minimized. Once achieved, the same tuning process of finding what the engine “wants” is followed to get the desired results.
Choosing the proper injector size is important. Too large an injector has a slow duty cycle and does not deliver good low-RPM performance or idle quality. Two main reasons to go EFI in the first place is superior idle and low- RPM performance. A smaller-thanoptimal njector can be crutched to an extent by using higher fuel pressure, and runs better on the street. A common rule is to target an 80-percent duty cycle at peak power, and therefore, 65-pound injectors support more than 800 hp at 45-psi fuel pressure—a set of 36-psi injectors supports more than 450 hp at that same pressure.
The benefits of an EFI package include better starting, driving behavior, and idle quality with wilder cams, because fuel delivery with EFI is not vacuum dependent. You can also get bettercontrol of fuel delivery in a wider range of operating conditions, such as under boost—a condition where a carburetor would have trouble compensating. The only real downside is the initial cost, which runs well into the thousands of dollars. Of course, race-quality carburetors are far from cheap.
Written by Barry Robotnik and Republished with Permission of CarTech Inc
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