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Atomizing Fuel: Single-Point Injection

November 18, 2011 by Matt

Holley Throttle Body Injection Pro-Jection

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:

Filed under: Atomizing Fuel, Technical

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