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Fitted to the first- and second-generation (C1 and C2) Corvette V8s, Rochester fuel injection was GM’s first foray beyond carburetion.

Hard on the heels of Mercedes’ fuel injection experiments across the Atlantic in the mid-’50s, the Rochester mechanical fuel injection system was less a counterpart to the German automaker’s racing-focused program and more an effort to enhance low-speed driveability in their high-performance Corvette sports car.

One of the primary issues was hood height. Perched atop the intake manifold surmounting a tall small-block Chevy V8, a carburetor and its air cleaner assembly would fit under the low, sloping hood of a sports car only if the intake runners were relatively short. And while shorter, wider runners are ideal for airflow at high rpm, they’re not best for engine efficiency and cylinder filling at low engine speeds, compromising the low-rpm, off-the-line “punch” of the Corvette’s V8. Longer, narrower runners (and a consequently taller intake manifold) are desirable during the engine’s warm-up period too, when the manifold heats unevenly and has the potential to develop “hot spots” that affect individual cylinder operation.

GM’s solution to these conflicting requirements was to task one of their prime subcontactors—Rochester—with the development of a mechanical fuel injection system. First fitted to a ‘Vette in 1957, the resultant system wasn’t nearly as dependent a carb on intake runner length and profile. It solved all the issues, giving the engine a smooth warm-up sequence, muscular low-rpm behavior, a decent top end and allowed the use of a shorter intake manifold for low hood height.

Reminiscent of Bosch’s later K-Jetronic system, the Rochester’s injectors sprayed continuously into the intake ports upstream of the valves. The higher fuel pressure required over a carb was delivered by a cable-driven mechanical pump, and metering was controlled by a movable plate situated in the intake manifold whose position varied with airflow, and thus engine load. The plate’s movement sent a vacuum signal to the fuel pressure regulator, which controlled the amount of fuel flowing to the injectors.

It was a remarkably simple system, with only a few downsides, among them cost, as the lower production volumes of the Rochester fuel injection system couldn’t benefit from economies of scale. Also, fuel economy compared to contemporary carbs was poor (not that it really mattered much in the ’50s and ’60s) and all the components had to be adjusted with precision for the system to function properly, so it required some specialized knowledge and tools. Aftermarket hot-rodders found its ultimate performance potential poor as well, seeing as how there was an accepted body of knowledge about how to tune carbs for performance, and aftermarket injectors and high-flow intakes were unheard of during the Rochester’s heyday.

Overall, though, it’s a neat system, and its adoption way back in the late ’50s just goes to show that GM is frequently much more forward-thinking than I give them credit for.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

• Continuous Injection (K-Jetronic)
• Single-Point Injection
• The Dell’Orto carb
• The SU carb

Bosch’s proprietary continuous injection system (CIS), also known as K-Jetronic or K-Jet, is an interesting hybrid of mechanical and modern fuel injection.

First fitted to the early ’70s Porsche 911, CIS was eventually adopted by a whole host of European automakers, from Audi and VW to Volvo, Ferrari and Lotus. It functions exactly as its name would suggest: Fuel is pressurized by the pump and metered continuously to injectors near the engine’s intake ports, the flow rate controlled by a movable circular plate mounted in the intake stream, attached to the fuel distribution unit. CIS resembles mechanical fuel injection in that there’s a direct relationship between the position of the plate and the flow of fuel, but does allow for some electronic control and closed-loop O2 sensor feedback. It straddles the two methodologies, developed before mass production of fully digital fuel injection was realistic, but band-aided in its later years as a less expensive stopgap system while Bosch’s much more advanced digital Motronic system came to market.

Advantages? It’s cheap, and once properly dialed-in, very reliable. CIS’s rudimentary nature (read: lack of sensors) eliminates many potential failure points, and the basic components used to deliver fuel—the injectors and fuel distributor / air flow plate assembly—are quite robust. Compared to the carbureted systems it replaced, CIS offers the ability to meet a broader envelope of engine fueling requirements, was considerably more efficient while still being emissions-compliant, and isn’t nearly as affected by weather vagaries or other environmental factors.

Downsides? Fuel metering, while more precise than most carburetors, still isn’t as accurate as sequential common-rail port injection. Additionally, the presence of the air flow plate and fuel distribution constrain the intake path considerably. Air has to flow up through the plate and then embark on whatever twists and turns it must make in order to reach the cylinders. By contrast, the metering unit of fuel injection with a flapper-door AFM or MAF sensor can be positioned wherever it needs to be to optimize the intake path. And finally, CIS is constrained by its semi-mechanical nature, lacking flexibility in the face of ever-changing emissions and efficiency requirements.

I drove a CIS-equipped car for the better part of 3 years. After an initial pig-rich condition was sorted out by a local specialist, the car’s fuel injection system ran perfectly and required absolutely nothing of me for the remainder of our time together. CIS is unique and certainly doesn’t readily surrender its secrets, but I grew to respect and appreciate the durable nature of the system Bosch developed.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

• Rochester Fuel Injection
• Single-Point Injection
• The Dell’Orto carb
• The SU carb

This may come as a bit of a surprise to those of you used to seeing fuel injectors neatly arrayed under a fuel rail, or at least poking out of individual runners, but that arrangement certainly wasn’t the first fuel injection design. No, it was arrived at after years of development of much more rudimentary systems, such as the one featured today: Single-point injection (SPI), also known as throttle body or central fuel injection.

In the period from the early ’70s to the late ’80s, when carburetors were on the wane, done in by draconian emissions regulations as well as the relentless march of technology, engineers came up with a dozen different ways of injecting fuel into the cylinders. SPI is arguably the simplest method that emerged from that time frame—essentially nothing more than a gutted two-barrel carb body, the innards replaced with a pair of injectors. The electronics were analog more often than not, controlling the fuel rate in a very crude manner in response to demand from the engine.

The advantages are obvious: The switch from carburetor to fuel injection was made with the absolute minimum amount of disruption to the engine peripherals. Elements such as the air cleaner, fuel lines, intake manifold and even the fuel pump could remain unchanged. Engineers got a slightly simpler and more flexible—albeit unfamiliar—fuel delivery method to hone in pursuit of fuel economy, clean emissions and ultimately power. The marketing guys, for their part, got to brag that their new models were fuel injected, and the accounting department was pleased at the minimal development cost.

To anyone familiar with modern multi-point fuel injection, the downsides of SPI are plain: The manifold design was compromised in the sense that all the runners had to converge in one location, fuel metering could not be timed to meet the intake strokes of individual cylinders, and atomization, owing to the low fuel pressure, was poor, hampering economy.

In the end, SPI was just a stopgap. Multi-point fuel injection was the next wave of progress, and would replace SPI entirely by the early-mid ’90s.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

• Rochester Fuel Injection
• Continuous Injection (K-Jetronic)
• The Dell’Orto carb
• The SU carb

The Dell’Orto carburetor never achieved the popularity it deserved. Imagine a dual-barrel sidedraft carb that flows almost as well as the industry performance standard Weber, but is much more flexible and docile in real-world driving, and you have the Dell’Orto.

So why didn’t it catch on? The consensus seems to be a matter of parts availability. Dell’Orto (sometimes spelled without the apostrophe: “Dellorto”) is a small Italian outfit, and it seems they never really had the financial ambition or production acumen of the better-known Weber, and as a result, parts are more difficult (though not impossible) to come by, in contrast the Weber’s almost Holley-like myriad of configurations and applications.

That said, the Dell’Orto was spec’d for a number of factory (not just aftermarket or racing) applications, among them the twin blow-through turbocharged setup for the first-generation Lotus Esprit Turbo, shown at top. They were also used on several Alfa Romeos, and the carb was always praised for its excellent fuel atomization and efficiency, such as it was. It’s a popular aftermarket carb for applications as diverse as Mazda rotaries, Porsche flat sixes or Nissan L-series engines.

And what makes it so much better-behaved than the temperamental Weber? The main reason is a larger accelerator pump. To cover the lean condition and avoid the potential engine stumble when the throttle is quickly opened, the carb provides a quick shot of fuel via the accelerator pump. Nearly all carbs have them (SUs being a notable exception); however, the Weber’s are proportionately small for the carb, requiring bigger main jets to provide fuel to cover for the transition from coasting/idle to the main circuit. The result is lack of metering precision and efficiency for the Weber compared to the accurate, miserly Dell’Orto, although the Weber’s ultimate fuel delivery potential surpasses its rival, generally-speaking. Given the choice, for the kind of driving I do, I’ll take the Dell’Orto hands down.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

• Rochester Fuel Injection
• Continuous Injection (K-Jetronic)
• Single-Point Injection
• The SU carb

Today we begin a new series upholding the rapidly-fading knowledge of an obsolescent technology: the carburetor. Most modern tuners would say I’m wasting my time, but I maintain there’s a certain je ne sais quoi to carburetion, a patina, a mixture of art and science, that we’ve all but lost in our transition to computerized, sterile, efficient fuel injection. The latter may be superior in every respect, but as with record turntables, Polaroid cameras or arcade games, we do lose something in the progression to new technology: the attendant experience. There’s value in that.

Engineers have devised innumerable methods of combining air and fuel via engine vacuum, the principle behind carburetion. My Datsun 240Z project car features a pair of SU carbs, so it’s obviously the particular design I’m most familiar with. I could get James May-ish and wade deeply into the technical particulars of the design, its operation and the like, but I’ll spare you and summarize up the pros and cons of the fabled Skinner’s Union carburetor, exhibited by many vintage British, Japanese and Swedish cars. The highlights:

• Simple design. In contrast to most other carb designs, with their multiple barrels, venturis, circuits and jets, the SU has exactly one barrel, one venturi, one float bowl and one jet. As an engineering solution to the problem of providing a correct volume of fuel and air throughout an engine’s rpm and load range, it’s uniquely elegant. Instead of optimizing a carb for one particular set of conditions and band-aiding the rest of the operating range, the inventors of the SU looked at the big picture of the engine’s requirements and worked from there. The result is a simple yet effective design that excites my inner engineer.
• Fuel metering precision rivaling fuel injection. Another consequence of the developers’ design approach is the incomparable precision of the fuel metering. The SU carb constantly adapts to whatever conditions the engine is experiencing seamlessly, whereas other carbs have discrete circuits for different sets of parameters, and transitioning between them can be less than smooth. So whatever demands are puts on the engine, the SU accommodates them perfectly and delivers just the right amount of fuel.
• Easy to tune. A side effect of the carb’s simplicity is its ease of tuning—if you know what you’re doing. Armed with a basic understanding of its principles of operation and the knowledge of a few of its quirks, a tune-up is a quick and easy affair, often no more complicated than a few readings with a synchrotester and a few screw turns. No swapping out jets, no delving into the carb’s innards. Simple.

And the drawbacks of the design:

• Airflow. The secret to the SU’s adaptability and precision is the variable venturi created by the moving piston housed in its signature dome. Unfortunately, a cylinder isn’t the most aerodynamic shape, so the airflow entering the SU invariably strikes a vertical wall before accomplishing anything else for the engine. The air loses energy it could otherwise use to draw fuel from the jet, and the engine expends power overcoming the drag.
• Sidedraft only. Another downside to the presence of the piston is that it limits the SU to a sidedraft orientation only. Unlike the rival Weber carb, available in sidedraft (DCOE) or downdraft (IDA) configurations, the piston of the SU requires gravity to operate, and thus airflow through the carb is limited to the horizontal. For some engines, say, a Porsche flat-6, this limitation renders the SU all but unusable.
• Limited aftermarket support. Within the corners of the automotive world where the SU was offered from the factory, there is a great deal of tuning lore, secrets, techniques and the like. But the inherent limitations of the carb (airflow, etc) for performance applications means the SU never really caught on in the wider performance scene. So tuning support lags far behind, say, Holley. It’s a shame, really; as lovely a design as the SU is, it deserves wider recognition.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

• Rochester Fuel Injection
• Continuous Injection (K-Jetronic)
• Single-Point Injection
• The Dell’Orto carb