What’s a Hemi?

My very first car as a kid in England was a 1946 Riley RME. I thought it was cool because it had a chrome grille like a ’34 Ford and it had a race-developed, twin-cam HEMI—whatever that was. Back then, there was no internet to look things up but a trip to the library revealed that the word HEMI was an abbreviation for hemispherical combustion chambers—whatever that was.

Believe it or not, HEMI-heads are nothing new and their history can be traced back to the early 1900s when they could be found in a number of European cars including the 1904 Welch Tourist, the Belgian Pipe of 1905, the 1907 Italian Fiat Grand Prix car, the French Grand Prix Peugeot of 1912 and the Italian Grand Prix Alfa Romeo of 1914—race-bred alright. However, it was the Welch design that became the blueprint for the many successors that included numerous motorcycle engines.

Where the HEMI-head differs from other cylinder head designs such as the “flathead” Ford which is known as an “L” head design, is that their combustion chambers are hemispherical or half-bowl-shaped compared to most chambers that resemble a flattened, double egg. The chamber operates in a cross-flow configuration where the air-fuel mixture flows in one side; the more-or-less centrally located spark plug ignites the mixture and the exhaust gases exit on the opposite side from the inlet.

The use of a HEMI-head became prevalent in motorcycle engines because not only was it efficient, but it was not an overly complicated assembly in a single-cylinder application where the pushrods ran up the outside of the cylinder. Incidentally, a HEMI-head can be used with a pushrod, SOHC or DOHC valve train.

Believe it or not, HEMI-heads are nothing new and their history can be traced back to the early 1900s when they could be found in a number of European cars…

The concept even worked well in early air-cooled, radial airplane engines that are more-or-less a number of single cylinders arranged in a circle around a common crankshaft. In fact, by 1921 the U.S. Navy had announced it would only order aircraft fitted with air-cooled radials.

Obviously, World War II propelled engineering development, as it did with much technology, as speed and power became all-important. Chrysler worked with Continental on the development of a giant, 1,792 cubic-inch (ci) V-12 that would be used in the Patton tank. It produced 810 horsepower and 1,560 pounds-feet (lb-ft) of torque and enabled Chrysler’s engineers to gather some valuable information that they put to good use in their post-War automobiles.

In 1947, Zora Arkus-Duntov, the so-called “Father of the Corvette”, was commissioned by Ford Motor Company to improve the output of their aging flathead V8s. Zora, his brother Yuri and designer George Kudasch developed an overhead valve conversion (OHV) for the Ford V-8 that featured hemispherical combustion chambers. Tagged the “ARDUN”, which was a contraction of ARkus-DUNtov, their OHV heads looked great and increased the power, however, they were somewhat temperamental.

Only about 200 sets were made in the U.S. before Duntov moved to the U.K. to work with Sydney Allard where a few more sets were made for Allard’s J2 sports car. For many years, ARDUN heads were a much sought after hot rod accessory until the mid-90s when Don Orosco began to reproduce them. He made about 30 sets before the tooling was sold to Don Ferguson whose family continues to produce the heads albeit updated with some modern technology along with a compatible cast-aluminum block. Companies such as H&H Flatheads are known for building complete ARDUN engines.

While the Duntovs were working on the OHV Ford, Chrysler engineers John Platner, a graduate of the Chrysler Institute of Engineering, and William Drinkard, manager of the Engine Development department, got to work in 1948 downsizing that tank engine for use in an automobile.

The engine was tough and you could throw all kinds of power-enhancing devices from blowers to nitro and it thrived on it.

What they came up with was a 90-degree, 330 ci, cast-iron V8 engine with HEMI-heads. Code-named A-182, the “HEMI” was not quite ready for production and a lot of valve train development still needed to be done along with some ignition and crankshaft work.

Nevertheless, Chrysler debuted the HEMI V-8 for the 1951 model year as standard in the Imperial and New Yorker models and optional in the Saratoga. Initially, the “Fire Power” capacity was 331 ci due to an “oversquare” 3.81-inch bore and 3.63-inch stroke. With a 7:1 compression ratio (cr), it produced 180 hp and 312 lb-ft of torque but weighed a whopping 745 pounds—one head alone weighed almost 120 pounds and you’d better be wearing a belt when you lift one.

Chrysler’s DeSoto division came out with their 276-ci “Fire Dome” version in 1952 and Dodge followed suit with their 241 ci “Red Ram” in 1953. Although all three engines differed in detail, they shared the same basic architecture.

In 1955, Chrysler claimed a dual 4-barrel (bbl) Carter version the first production car to produce 300 hp. The displacement was increased in 1956 to 354 ci and the engine now produced as much as 355 hp and became the first American engine to produce 1 hp per cubic inch.

Two years later, the infamous 392 version was introduced and it was almost square having a 4-inch bore and a 3.906-inch stroke. It had a taller ‘raised deck’ compared to previous engines; however, the heads were cast with wider ports so that earlier manifolds could be used with the new heads on the new block. The following year, a single carb version with 9.25:1 cr was rated at 345 hp while a dual-carb version offered 375 hp.

The 392 is significant because it became the drag racer’s engine of choice, especially in the fuel ranks: Top Fuel, Funny Car, and Fuel Altered. The engine was tough and you could throw all kinds of power-enhancing devices from blowers to nitro and it thrived on it.

By 1958, the 392 was producing 380 hp but had reached the end of its production life. It wasn’t until 1964 that Chrysler re-introduced the engine and officially called it a HEMI. Nicknamed the “elephant engine,” because of its size and weight, the new Gen II HEMI displaced 426 ci. Not initially available to the public, it was used in NASCAR in ’64 but not in ’65 because it was not available in a production car and therefore could not be raced.

Not to be outdone, Ford also introduced a 427-ci HEMI in 1964. Nicknamed the “Cammer” because it had a single overhead cam (SOHC), engineers had worked hard to design a symmetrical combustion chamber with the plug located for maximum efficiency only to discover that the plug didn’t care where it was. The plugs were then located near the top of the cylinder for easy access. NASCAR wasn’t at all happy about these “special” racing engines, however, the “SOHC” motor (pronounced “sock”) remains a “halo” engine for Ford.

Chrysler fixed their NASCAR problem in 1966 by introducing the “street” HEMI with lower compression, a milder cam, cast instead of tube headers and two 4 bbl Carter AFB carbs. The Gen II HEMI was produced until 1971 and was rated at 425 hp at 5,000 rpm and 490 lb-ft of torque at 4,000 rpm.

Of course, this is only the American version of HEMI history. Across the pond, in the homeland of the HEMI, the Europeans never left the concept alone.

Incidentally, the 426 HEMI is a HEMI in name only. Rather than build the new 426 from the old architecture of the 392, Chrysler engineers chose to use the existing 440 Wedge-head big-block. That said, the 426 evidences many improvements over the Wedge and indeed the 392 and became the modern drag racer’s engine of choice and was known colloquially as the “late model” compared to the 392 “early model.”

As the factory HEMIs came to the end of their respective lives Ed Donovan of Donovan Engines introduced a cast-aluminum 417 ci aftermarket version in 1971 that was based on the 392. That was followed in 1974 by Keith Black’s 426 HEMI based on the factory 426. Versions up to 573 ci are now available as are heads and numerous other parts milled from billet aluminum from numerous aftermarket manufacturers such as Hot Hemi Heads.

In fact, we use a billet 417 ci Donovan block with billet heads from Hot Hemi Heads in Ron Hope’s Rat Trap AA/Fuel Altered that we race. With a billet BDS supercharger and 90-percent nitro, it produces some 3,000 hp. However, in current Top Fuel/Funny car racing they use architecturally similar 500 ci blocks milled from forged billet aluminum.

These proprietary blocks are produced in-house by Don Schumacher Racing and John Force Racing but similar blocks that can withstand in excess of 10,000 hp are available from companies such as Brad Anderson and Alan Johnson Performance—all based on that Chrysler HEMI.

Of course, this is only the American version of HEMI history. Across the pond, in the homeland of the HEMI, the Europeans never left the concept alone. For example, Daimler, using Triumph motorcycle architecture, developed two aluminum-headed HEMI engines of 2.5 and 4.5-liters.

Other British brands such as Aston Martin and Jaguar both employed hemispherical combustion chambers in the DOHC V-8s and straight 6s respectively. However, no doubt the most well-known use of their HEMI-head was by Porsche in many of their engines—particularly the flat-six boxer engines of the 1963-’99 911s.

What Is Autocross?

Autocross claimed to be the start for anyone looking to get into road racing. Though, when you look at it, it’s just a bunch of cones in a parking lot or a big patch of asphalt. What is autocross and why is it the place road racers should start?

When it comes down to dollar-to-seat time, it’s hard to beat the low cost of autocross. Well, normally low cost, we’ll touch on that later. However, in most cases, if you want the best environment to get a feel for your car and improve your driving skill behind the wheel, it’s going to be your cheapest and relatively safest bet. You don’t even have to have a special car to do it, either, as the car or truck you’re driving now can usually be used. You’ll see people show up in anything from Volkswagen Golfs to Corvettes to S10s. The only special thing you need is a helmet and many organizations will be happy to provide you a loaner one.

To define it, autocross is racing in the same way that time trial and time attack are. You’re not racing wheel-to-wheel but racing for the fastest time in your class and overall. You won’t even be on the course at the same time as another car like you would be on a big race track. You also won’t hit the same speeds, either. That’s why it’s looked at as a lower risk way to get into road racing and build up your skill set behind the wheel.

The course is laid out on a big patch of asphalt or concrete. Cones are set up and it can be a course that loops on itself or be straightforward with no confusing loops. Depends on how your course designer is feeling that weekend. A course can be open or very tight, depending on how fast the sanction has determined for the maximum speed of an autocross. Very rarely does a course allow you to hit speeds over 50-MPH. Most will only allow you to use up to second or third gear, depending on your transmission.

Those cones are also indicators. Two cones standing straight up are gates. Four cones in that same position indicate the start and finish. A cone laying down beside a gate indicates how many times you go through it. A single line of cones in a straight line is a slalom, but if a cone is laying down on either side, the pointed end indicates which side you enter it while no cones indicate you can enter either side.

If you want to be the fastest driver, however, you are going to need to start upgrading your vehicle. The first thing most will tell you is to upgrade to a set of ultra-high-performance tires like the Milestar MS932 XP+. It is the single best initial upgrade you can do to your autocross car because it improves traction, cornering, and braking in one go. From there, you go with your suspension, brakes, reducing weight, and everything else that your rulebook allows for. That’s why autocross starts out cheap but eventually becomes as expensive as any other form of racing, but that’s normal, too.

If you’re entering your first autocross, don’t worry with all of that. Just go, have fun, and learn. Though, if you have your own helmet, bring it. That way, you won’t accidentally bring home the one you borrowed.

What Is Drifting?

When it comes to motorsports, there is nothing quite like drifting. It’s not racing in that you’re trying to complete a course in the fastest time possible. So, what is drifting and what makes it a motorsport, then?

When you first get into racing, you’re taught that you only want a vehicle to exhibit neutral to just a little bit of oversteer. You want to use the throttle to drive you out of the corner and only to add more when you need just a little more rotation. Drifting throws all of that out of the window. Or, at least it seems to do it. What you have, instead, is equal parts skill of the driver and chassis setup prowess of the crew chief.

While the cars of drifting are in an extreme state of oversteer, they are also exhibiting a lot of grip. That may read counterintuitive from what you witness, but if you set up a car too loose (give it a chassis that drives with too much oversteer) you get a car that is absolutely undrivable. Many beginning drivers look for tires that don’t have enough grip or overinflate their tires to reduce grip because their chassis are set up with too much understeer from the factory. If they don’t go beyond the settings from the OEM, their cars won’t ever get the angle they really want. They will continue to fight the car until they change toe, camber, caster, spring rates, and even their dampening settings on their aftermarket shocks.

Once they do, drivers will want tires that have a lot of grip, like the Milestar MS932 XP+. Without that grip, the car will over-rotate and probably spin out. Once they get beyond that level, they will then start to drive in tandem with another car on the track. There are two goals in a drifting competition and it depends on if you’re leading or following.

All through a drifting competition, you are judged by three to five judges (depends on the sanctioning body). During qualifying, to place you in a Top 16 or Top 32 format, you will be judged on your line, angle, and style. Once placed and going into tandem, each driver is judged by that panel with two tandem runs. The drivers will swap from lead and follow on the two runs. For example, driver A will lead, and driver B will follow on run one. Once that run is done, they swap with B leading and A following.

If following (or chasing), your goal is to stay as close as possible to the lead driver while also mimicking that driver’s lines while drifting. If you’re leading, your goal is to drive with as much angle as possible while getting close to clipping points and zones without interfering with your line. If you hit the wall but your line stays the same, you won’t be judged against and the following car also must mimic that. If both drivers do too good (or both do equally bad), there will be a full run usually called a One More Time. Depending on the sanctioning body, you may have two, three, or as many as needed to determine a winner.

Speed is not always criteria but trying to finish the course in the fastest time isn’t the goal of drifting. Instead, the goal is to simply drive better than your competitor in the eyes of the judges. Therefore, big angle kits, sophisticated shock design, and tires with plenty of grip are all a big part of professional drifting. If you can drive your car with a bigger angle, on a better line, and gap your follow driver while also being able to keep up with your lead, you’re probably going to win. That’s not always the case, though, as with all this pushing to the limits, things will break. Even the best driver with all the money in the pits will lose thanks to a $0.30 part. However, you won’t know that until you go out and drive.

What Are Halogen Headlights?

It was the revolution that brought about the modern headlight. However, what is this mysterious thing called the Halogen Light Bulb?

Halogen lighting is the typical light you see on most vehicles that aren’t a premium brand. From headlights to fog lights to auxiliary and off-road lights; halogen is the inexpensive go-to for lighting on nearly anything with wheels. Essentially, the way it works is that there is a tungsten filament that heats up and burns to produce light. Normally, that filament would evaporate away until either the bulb was black or it broke. Halogen creates a reversible chemical reaction cycle with the evaporated tungsten and allows it to stay at the same output until it eventually burns out, usually after 250-hours.

Headlamps in the US were basically locked to the standard filament bulb from 1940 to about 1968 and the establishment of the National Highway Traffic Safety Administration (NHTSA) and the Federal Motor Vehicle Safety Standards (FMVSS). Europe, however, wasn’t locked to a standard and introduced the first halogen lamp for automotive use in 1962 with the H1 bulb. Even though it was proven to not only be a better light that lasted longer than the sealed beam, they were prohibited in the US. Once the fuel crisis hit, US lawmakers began to face pressure from the public and automobile manufacturers alike to finally allow new headlight standards and the “new” technology of the halogen bulb.

However, halogen lighting was limited to being stuck inside a sealed beam until the 1980s and the introduction of the 9004 bulb. The original H1 bulb, the one Europe had since the 60s, wasn’t approved for use in the US until 1997. Since then, we’ve had a slew of H-types used and approved in the US. What we’ve also gained are more aerodynamic front ends that allow for better fuel economy and performance.

Though, this also meant we lost the iconic pop-up headlamp in 2004 with the C4 Corvette and the Lotus Esprit ending production in that year. With smaller, slimmer shapes, the need for lowering the headlight to match the drastic angle of the front end was no longer required. Housing designs and better reflectors, along with the increased candela power of halogen bulbs, no longer mean we had to have a big bulky light on the nose of our cars and trucks.

Halogen bulbs are a great and inexpensive way to get lighting if all-out performance isn’t critical and you’re fine with changing a bulb. However, if you’re looking for more power and are approaching speeds of over 100-miles-per-hour, you really need a High-Intensity Discharge, or HID, light. That being said, the halogen light bulb probably isn’t going away for some time yet.

Rock Racing vs. Rock Crawling

These are two motorsports that are popular across the globe but started in the United States. On the surface, they seem similar, but what is the difference between rock crawling and rock racing?

Looking a rock crawler and racer, you’d think both rigs are the same. They climb over big boulders and go through stuff that would normally break a normal car. They feature a jungle gym of tubes and giant tires. They both feature selectable four-wheel-drive systems, big axles, and immense power. However, when you get down to it, these two vehicles aren’t even close to being the same in practice.

A rock crawler is designed to drive on a defined course, but not in the fastest time possible. They go through gates and take penalties for hitting specific cones, backing up, using their winch, and much more. It’s about pure finesse and driver pathing skill over finding the fastest line.

Rock racing, to contrast, is all about racing to the finish line faster than the person ahead or in front of you. While these rigs can race side by side, they aren’t a wheel-to-wheel race like you’d see in an oval race or at a road course. It’s the same as desert racing, but with the added difficulty of climbing rocks and boulders the size of those Milestar Patagonia MT Black Label tires you have on your own rig. You don’t get penalized for hitting a cone, backing up, using your winch, or anything other than short-cutting the course.

You’ll also see a difference in how they are built. A rock crawler will use a single air shock per wheel, feature four-wheel steering, and be built to be as light as possible. A rock racer, on the other hand, will feature two shocks per tire (in the unlimited class) with a coilover and a bypass, have only the front wheels turn, and built to be as strong as possible. Rock racing rigs are designed with jumps, compressions, and speed in mind – a rock crawler, not so much.

While the two disciplines started out with similar rigs, rock racing has evolved to become something that pushes the limits of what a rig can do. Not only does it have to be fast, but it must survive some of the harshest racing environments known and sometimes created by man. However, many of the rock racing parts have helped improve the parts you see on rock crawlers, too. Higher strength axles, gearboxes, and stronger, yet lighter components have all come from racing to the benefit of crawler and street rig alike.

What Is an Intake Manifold?

State of Speed Basics – The Manly Science of Automotive Knowledge

Spend any time hanging around with gearheads and you’ll hear the term “intake manifold” thrown around, usually in the context of a discussion about the merits and weaknesses of various designs. What does an intake manifold do? It’s simply the piece of plumbing that connects the intake ports on the cylinder head to the carburetor or throttle body, distributing fresh air (and sometimes fuel – more on that in a minute) evenly to each combustion chamber. The design of the intake manifold has a profound effect on the performance of the engine, helping to determine whether it’s happiest at high RPM or churning out torque down low, and with some clever engineering it can even broaden an engine’s powerband far beyond what a simple, single pipe connecting an individual throttle body to the cylinder head can provide. 

The intake manifold for a multi-cylinder carbureted engine, like a classic domestic V8, has a few main jobs to do. It has to evenly distribute air to each cylinder, do the same for the fuel atomized by the carburetor, and have enough internal volume to keep the carburetor happy since it works best when it feels a consistent ‘pull’ of vacuum drawn by the engine as it runs. Manifolds for carbureted engines are referred to as “wet” because the air flowing through them has fuel mixed in, and they have to be designed without sharp turns or twists that might cause fuel to separate out as it tries to make the corner along with the airflow. 

“Dry” manifolds for modern EFI or direct injection engines have an easier job; they only have to evenly flow air, and can be a little more convoluted without ill effects. But wet or dry, there are some design elements both share. Ideally, an intake manifold should provide as little resistance to airflow as possible at wide open throttle – imagine sucking air through a straw, compared to breathing through a paper towel tube. If that was the only consideration, you’d just make the runners as big in cross-section as possible, but moving air also has inertia and velocity, so the diameter of the runners has to be a compromise between being large enough to have low resistance under high demand, but small enough to keep that column of air moving as rapidly as possible to help push out exhaust gas and completely fill the cylinder during the intake stroke. 

Runner length is also important; all other things being equal, longer runners boost torque at low RPM, while shorter ones favor horsepower at high revs. Many factory intake manifold designs take advantage of this by having internal butterfly valves that allow the engine to switch between long and short runners, or even have separate runners for multi-valve heads that employ variable camshaft lift, duration, and timing. Finally, plenum volume is important – this is the “reservoir” that all the individual runners draw from, and changing the size of the plenum can also profoundly influence an engine’s powerband. 

Some modifications, like “port matching,” a process where an intake’s individual runners are hand-shaped to precisely conform to the cylinder head’s intake ports, will benefit any engine. More extreme changes to runner length and diameter, plenum volume, and even the interior finish of the manifold are most useful when they’re part of an overall plan for the engine build. A race manifold won’t help an otherwise-stock street engine – in fact, the wrong manifold will often kill performance. Before contemplating a change to your engine’s intake tract, make sure you understand what the effects will be, and how it will interact with all the other factors of your combination, including camshaft grind, displacement, desired peak RPM, and even transmission gearing. 

An intake manifold’s job is to connect the throttle body or carburetor to the intake ports on the cylinder head(s). Individual runners connect the plenum (the central, shared space) to the ports, and their length and cross-sectional size play an important role in determining the power band of the engine. This manifold is a Weiand “dual plane” design for V8 engines that is designed to provide very streetable power delivery characteristics. Notice the “waffle” pattern at the bottom of each plenum – this is an intentional feature that helps keep the fuel and air consistently mixed on their way to the cylinders.

This Weiand V8 manifold is a “single plane” design with short runners and all eight cylinders feeding from the common central plenum. These kinds of manifolds are optimized for maximum horsepower at high RPM, but sacrifice torque down low. 

This aftermarket Honda B series manifold from Edelbrock has relatively short runners connecting the intake ports to the plenum, optimized for free breathing at high RPM. Notice how close the injector bungs are to the flange that bolts to the cylinder head – fuel sprays directly at the back of the intake valves, and doesn’t have to travel any distance through the “dry” intake manifold runners. 

Edelbrock’s Honda D series intake manifold has longer runners designed to match the more torque-biased design of that engine family. Long, narrow runners will encourage airflow velocity, which helps the cylinders fill completely at lower RPM. 

Check out this aftermarket Honda manifold – here, the injectors are placed higher up in the runners, which allows a little bit more time and distance for fuel to atomize before flowing past the intake valve.

Many factory and even aftermarket intake manifolds are made of composite plastics rather than cast aluminum. Because they act as a kind of insulator, they can keep intake air cooler than a metal manifold for the same engine. This is a CAD illustration of FAST’s intake manifold for LS V8 engines, and it shows the unique three-piece design that splits into an upper and lower plenum housing that conceals individual intake runners. 

A look inside the FAST manifold with the top removed shows the individual runners. They all draw air from the same plenum, but FAST offers different runner lengths to fine-tune the manifold’s performance to match an engine’s power band. 

What Is EFI?

State of Speed Basics – The Manly Science of Automotive Knowledge

In order for an internal combustion engine to run, you need to have some way to mix fuel and air together in the right proportions – too much of either ingredient, and the engine runs badly if it even runs at all. To make things more complicated, the right mixture is constantly changing all the time, depending on load, engine RPM, and even the weather. For most of automotive history, carburetors took care of this essential task but as the world demanded cleaner and more fuel-efficient engines, those complicated mechanical air/fuel mixers just couldn’t provide the precision and flexibility required and electronic fuel injection came to the rescue. So, what is EFI and how does it work?

At its most basic, an EFI system simply measures or calculates the amount of air an engine breathes, applies some logic that tells it how much fuel needs to go with it in order to burn properly, then adds that fuel to the intake air. Some EFI systems directly meter all the air that flows past a sensor – these are often called “mass airflow” for obvious reasons. Others, called “speed-density,” calculate how much air is moving through the engine based on factors like engine RPM, air temperature, throttle position, and what’s known as “volumetric efficiency” – a measurement of what percentage of the theoretical maximum airflow is actually making its way into the cylinders at any given moment. 

No matter which strategy the EFI system uses to calculate airflow, an oxygen sensor in the exhaust pipe provides feedback to the computer to help refine the air/fuel mixture. This “closed loop” operation gives the EFI the chance to continuously correct itself to compensate for any disagreement between the programmed fuel supply and what reality actually demands. The tweaks the computer learns during closed loop operation often carry over to “open loop” situations, like when you are at wide open throttle and full power. This means that an EFI engine is constantly adjusting itself to match the conditions. 

In the hands of a skilled tuner with the right software, EFI provides the opportunity to change how an engine runs with the click of a mouse and the alteration of a few numbers in a table, but for most of us, it just means that our cars run better, get better gas mileage and make more power, and start on a cold day at the turn of a key. 

Back in 1957, Chevrolet introduced Rochester mechanical fuel injection as an option on the Corvette, but the system was so temperamental to tune and so few dealership mechanics were trained to take care of it that many owners scrapped the entire system and replaced it with a conventional carburetor. Today, original Rochester “fuelie” setups are worth thousands of dollars whether in working condition or not.

During the transition from carburetors to electronic fuel injection in the late 1980s, many manufacturers used “throttle body injection” where the computer controlled a few large-capacity injectors located where the carburetor would be in an older engine. This was a very practical way to gain most of the advantages of EFI with the fewest changes necessary to the existing engine design, and today, throttle body EFI conversions like this one from FiTech are a popular way to upgrade to electronic fuel injection. 

All modern factory electronic fuel injection systems use an oxygen sensor (and often more than one) to determine the engine’s actual air/fuel ratio by sampling the exhaust gasses. This allows the EFI to continuously correct the ratio to deliver exactly the right mix for the conditions. Factory EFI systems used to rely on ‘narrowband’ sensors that could only signal whether the exhaust was rich or lean of the 14.7:1 air to fuel mixture that represents complete combustion, but today’s OEM engines (as well as aftermarket EFI systems) use more sophisticated ‘wideband’ sensors that can accurately measure a much broader range of lean or rich ratios.

This high-performance aftermarket EFI system from Edelbrock features sequential port injection – there is one fuel injector per intake runner, and the computer activates it in time with the opening of the intake valve for maximum precision. Some dedicated racing port fuel injection systems even have more than one injector per cylinder, with a small-capacity one providing very precise control at part-throttle and a large capacity injector that takes over at wide open throttle.

Electronic fuel injectors come in several standard shapes and sizes, as well as different flow capacities to match engines ranging from motorcycles to turbocharged Pro Mod dragsters. They all have one thing in common, though – they act as electronically controlled valves that deliver a precise dose of fuel on command, supplied from a pressurized fuel rail. This Holley injector is an older factory-style design that is still popular with racers thanks to its durability and high maximum capacity.

What Is a Carburetor?

State of Speed Basics – The Manly Science of Automotive Knowledge

There are three things necessary for an internal combustion engine to operate – Fuel, air, and a source of ignition. For most of automotive history, one piece of hardware controlled two of the three required elements: the carburetor.

At its most basic, a carburetor is any device that combines fuel and air into a mixture that will support combustion. In Perfect Chemistry Land, gasoline wants 14.7 parts air to each part fuel for a complete burn, but because engines don’t visit Perfect Chemistry Land very often, a carburetor has to be able to deliver air/fuel mixtures that are both leaner (more air) and richer (more fuel) than this perfect “stoichiometric” 14.7:1 ratio, depending on many different factors.

Sometimes the ratio will need to change for better fuel economy, or for maximum power. It will have to be different when the engine is cold than it is when the engine is up to its normal operating temperature. And it will even need to change moment to moment as the throttle (the “butterfly” valve connected to the accelerator pedal that controls how much air passes through the carburetor and into the engine) changes position.

In order to do that smoothly and effectively, carburetors evolved from simple devices that vaporized fuel into what are essentially sophisticated analog computers. The main data inputs are the manifold vacuum and throttle position, which represent how much air the engine is trying to draw in, and how much air the driver is allowing it to have, respectively. Based on these two primary factors, a complex series of air passages, calibrated orifices called ‘jets’ or tapered metering rods, and any number of other clever mechanical devices controlling the flow of air and fuel work together to deliver the correct mixture to the engine.

Carburetors fell out of favor for factory vehicles in the late 1980s as electronic fuel injection became available (and less expensive), with the last carbureted vehicles sold in the US bowing out in the early ’90s. Manufacturers found that EFI made it easier to comply with tighter emissions requirements, but in the world of high performance, carburetors still enjoy a lot of popularity for both street and race vehicles.

The Holley 4150-style carburetor is easily the most well-known performance carb and can be found on countless different factory muscle cars and race vehicles.

Edelbrock’s AVS carburetors are an update of another classic 4-barrel carb design. This polished show-quality model features an electric choke – the black cylinder on the left side of the carburetor contains a mechanism that automatically opens and closes the choke based on the electrical signal from a sensor installed in the engine’s cooling system.

Holley’s ‘Dominator’ 4500-series carburetor is the big brother to the classic 4150, designed for engines that need more airflow than a smaller carburetor can provide.