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Everything You Need to Know About Injectors

Further fueling your fuelie knowledge

Rick Jensen Nov 2, 2015 0 Comment(s)
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Imagine for a minute that the amazing, over-caffeinated meatbag you call your body actually isn’t so amazing. That every energy-expending action you make—from the tiniest facial expression to a full-on sprint—requires instantaneous and precise fuel intake. Aside from holding really still and eating a lot, you’d probably spend lots of time deciding how to break your food down into portions that would correspond with each movement’s energy requirement. After all, if you ate too little for a task you’d feel weak (or pass out), and if you ate too much you’d feel sluggish (or pass out). Throw in the catch-22 of needing to eat while prepping your food, as well as the serious logistical issues arising out of having sexytime, and this doesn’t sound like much fun.

Thankfully, humans are built to automatically store energy and use it when needed, and our mouths do the rest. However, your engine doesn’t have it so easy. Cold and hot startup, idle, WOT and every rpm and load in between requires an instantaneous, precise amount of fuel to perform flawlessly. Your ECM gets the credit for these complex calculations—we’ll be discussing the brains of your Corvette’s operation in a future article. But your fuel injectors do the actual fueling, and there’s a lot more to them than meets the eye. Here’s how they work.

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Operation

The term fuel injector is a misnomer: as your high-pressure fuel pump keeps feeding your fuel system, without injectors stopping the flow your engine would be instantly overwhelmed by fuel. So these little cylinders actually prevent fuel from being constantly injected. How? Fuel injectors are electromagnetic solenoids that are controlled by a vehicle’s ECU. The ECU takes sensor readings, and instantly crunches numbers for how much fuel is needed for the desired air/fuel ratio. Milliseconds later, it grounds the injector, the injector opens, and the exact amount of fuel is pulsed.

The amount of fuel an injector releases depends on three factors. The first is pulse width, or the amount of time that the injector is open. (This should not be confused with duty cycle, which is the ratio of how long the signal is active compared to how long it could be active.)

The second is flow rate, or how much fuel can flow in a predetermined time—pounds per hour is the domestic standard, and CCs per minute can also be used.

And the third is the fuel pressure in the rail feeding the injector.

There are two distinct types of fuel injectors: high impedance units, and low impedance units. A high-impedance injector (aka saturated) uses a high resistance value of around 12 ohms across its coil, but runs at low current levels to keep the circuitry cool. However, running at low current also reduces the injector’s response time and dynamic range. Typically, production vehicles use high-impedance injectors.

A low-impedance injector (aka peak-and-hold) uses low-resistance coils that only need in the ballpark of 2-3 ohms, but needs higher current levels to operate. If these injectors constantly ran at the required amp rating, it would overheat and damage the drivers. So a switching mechanism is used in the circuit: it uses high current to open the injector, and then once it’s spraying, reduces current to keep it open for the duration of the pulse width. Typically, aftermarket ECUs and race applications utilize low-impedance injectors.

While some ECUs can handle both high- and low-impedance injectors, it’s best to stick with whatever type of injector your ride came with.

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History

Corvettes adopted fuel injection over 50 years ago. In 1957, Rochester Ramjet mechanical fuelie systems created 1 hp-per-cube 283s, and this technology peaked in 1965 with 375-horse 327s. For more info, check out this fascinating piece on Corvette mechanical fuel injection. But for this story, we’ll be focusing on electronic fuel injection.

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Corvette EFI’s humble beginnings began in 1982, when Cross-Fire Injection debuted on the L82 engine. An electronic/mechanical hybrid, the heart of this system was two large fuel injectors sitting on a modified racing intake manifold. Their centrally located fueling method was not unlike a carburetor’s (and, in fact, this system’s fuel pressure was low like a carb setup, too). However, its Computer Command Control ECU used sensor input to command injector pulses, which were actuated by solenoids atop the injectors. The part mechanical, part electronic Cross-Fire made 200 horsepower and 285 lb-ft of torque—10 horses and 5 lb-ft more than the previous, carb’d 1981 Corvette—and provided superior drivability, throttle response, and fuel economy. But due to the absence of a 1983 Corvette, Cross-Fire Injection only lasted two model years, and was shelved after the 205-horse, 290 lb-ft L83 in the 1984 model.

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A brand-new EFI system called Tuned Port Injection bowed in 1985. Sitting atop the L98 350, TPI’s signature long intake runners got most of the attention, but this system’s true power was its multi-port fuel injection, or MPFI. In stark contrast to the Cross-Fire system, MPFI gave each cylinder a dedicated fuel injector that worked with a high-pressure fuel system. This allowed much more precise fueling, which resulted in more power and even better drivability and economy.

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Tuned Port Injection used batch-fire operation: this method can fire all eight injectors at once, or alternately fire the left and right bank of injectors, and doesn’t require any cam or crank position sensors to operate. As such, it was both a technical leap forward and fairly affordable, especially for the mid-1980s. However, as batch fire injected fuel while intake valves were both open and closed, fuel atomization and emissions weren’t optimal. But batch fire injection helped 1985 L98s make 230 horses and 330 lb-ft, which eventually topped out at 250 hp/350 lb-ft in the 1991 model. In fact, batch fire was used on the 1992-’93 LT1s as well, which produced a heady 300 horses and 330/340 lb-ft of torque.

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It’s fitting that the first Corvettes to use sequential fuel injection were the nasty 1990-’95 ZR-1s, which used two fuel injectors per cylinder! Instead of firing the injectors indiscriminately during the combustion cycle, sequential injection fired each cylinder independently, just as its intake valve opened, for highly precise fueling. While these systems require additional cam and crank sensors to work, they greatly improve idle, drivability, mileage, and emissions. Sequential fuel injection started on base Corvettes in 1994 and continued through the LS series of engines, a mainstay of Corvette fuel injection for 20 years.

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Corvette fuel injection systems are always changing, and nowhere is that more evident than in the new 2014-up Stingray’s LT1. Back when Vette EFI was in its infancy, the Cross-Fire system only ran 13 psi of fuel pressure. That increased greatly with the first-year TPIs, which ran 37 psi of fuel pressure. It increased again to around 43 psi for later TPIs and LT1s, and finally peaked with 58 psi in LS engines.

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But even 58 psi can’t hold a candle to the new LT1/LT4 direct injection systems. The LT1’s DI injectors are fed by an ultra-high pressure, 150-bar fuel pump. That gives them over 1,400 psi of fuel pressure, and combined with a combustion-chamber location and precise spray pattern, DI offers superior fuel atomization and a more complete burn. This maximizes cooling, allows a higher compression ratio, and results in higher power, better economy, and fewer emissions compared to a port-injection system. Oh, and they flow an insane 125 pounds per hour, too!

Construction

There are three main types of port fuel injectors: pintle, ball and seat, and disc. All three share common components, like a body, electrical connector, O-rings, filter, coil assembly, and nozzle/tip. And all are also unique, specifically regarding the way that they release fuel. So we’ll cover common parts first, then move on to individual nozzle/tip designs.

External Components

The body contains all of the fuel injector’s internal components, and it can be metal, plastic, or a combination of both. Interestingly, in the 19 years between TPI and LS1/LS6 engines, fuel injector length (measured between the O-rings) stayed around 2.5 inches. It wasn’t until 2005’s LS2 engine that they started getting shorter, at 2 inches. And the LS3/LS7/LS9 injectors were smaller still, at only 1.5 inches.

The electrical connector is where the engine’s injector wires connect. This connector is made up of two terminals: a 12-volt power terminal and an ECM-controlled ground terminal. The vehicle’s ECM turns on an injector by activating this ground, which pulses the injector and releases fuel. Note that there are different connector types: TPIs, LT1s, and LS1s used the earlier EV1 connector, which is smaller and more rectangular. LS2 and later injectors used the EV6 connector, which is a larger, rounded square shape.

An injector’s O-rings keep high-pressure fuel from leaking out onto your engine. There are typically two on each injector: the top O-ring seals it to the fuel rail and the bottom O-ring seals it to the intake manifold. Modern fuel injector O-rings are made of Viton, a fluoroelastomer (synthetic rubber) that’s highly resistant to extreme heat and cold, as well as chemicals and fluids like gasoline.

Internal Components

A fuel inlet area at the top of the injector incorporates a fuel filter, which is the last line of defense for any contaminants that have made it past the fuel tank’s or fuel line’s filters.

Fuel flows through a centrally located fuel shaft inside the injector, which leads to the injector seat.

An injector’s coil assembly is energized when current is applied, which in turn activates the solenoid and moves the pintle, ball, or disc to allow fuel to flow. The coil assembly is made up of wire wrapped around a plastic bobbin that encircles the injector’s fuel shaft. This coil wire can be made of brass or copper, and its terminal type and length can vary—these variables determine its resistance, measured in ohms.

Crucial to an injector’s design is the internal spring, which is right above the armature apparatus. This spring could loosely be compared to a cylinder head’s valvespring: both the spring and the fuel’s hydraulic pressure keep the injector closed, but when it opens on the coil and solenoid’s command, this spring ensures that it quickly closes, stopping the flow of fuel.

10 Injectors Pintle Inj Courtesy Tomco 10/28

Armature Designs

A pintle design uses a shaft, needle valve, and nozzle with a tapered seat. When the coil assembly and solenoid is energized, this valve lifts up from its seat, allowing fuel to flow out the nozzle. Pintle-type injectors have been factory-installed for years; however, since the pintle assembly is right on the end of the injector, they’re more prone to carbon contamination compared to the other designs.

11 Injectors Ball Seat Inj Courtesy Tomco 11/28

A ball and seat design uses a ball and seat valve, and a director plate directly underneath. When the injector is energized, the ball raises off of its seat, allowing fuel to flow through tiny holes in the director plate, then out of the spray tip. Because of the director plate’s design and these components’ distance from the nozzle, the ball and seat design is less susceptible to fouling.

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And a disc design uses a disc, a disc seat, and a shim. When energized, the disc raises up off the disc seat and contacts the shim. And fuel flows through the disc holes and out the nozzle. The disc design, like the ball design, is set higher up in the injector body, and resists carbon deposit formation.

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Stock Corvette Injector Sizes

Here are the injector sizes for each EFI-equipped Corvette year.

1982-’84: 67/68 lb/hr
1985: 24 lb/hr at 43.5 psi (22 lb/hr at the ’85’s 36-37 psi rail pressure)
1986-’93: 22 lb/hr (43.5 psi)
1994-’96: 24 lb/hr (43.5 psi)
1990-’92 ZR-1: 20 lb/hr primary, 20 lb/hr secondary (2 per cylinder) (43.5 psi)
1993-’95 ZR-1: 20 lb/hr primary, 20 lb/hr secondary (2 per cylinder) (43.5 psi) (redesigned for ethanol resistance)
1997-’98 LS1: 28 lb/hr (58 psi)
1999-’00 LS1: 26 lb/hr
2001-’04 LS1/LS6: 28 lb/hr
2005-up LS2: 33 lb/hr
2006-up LS7: 39 lb/hr
2008-up LS3: 41 lb/hr
2014-up Stingray: 125 lb/hr (direct injection)

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Should You Service Or Replace Injectors?

Unless your factory fuel injectors are internally damaged or extremely high mileage, they’re still usable. However, a loss of performance or a drop in fuel economy can signal that those OEM squirters need attention. So if you’re the kind of enthusiast who wants to keep your Corvette factory stock, or you’ve picked up a set of injectors and aren’t sure of their condition, consider professional injector servicing.

Companies like DeatschWerks offer comprehensive injector cleaning and servicing procedures. Their Dynamic Injector Service includes injector disassembly and inspection, ultrasonic cleaning to remove deposits, and replacement of all O-rings, seals, and filters. Next, coil, leak, flow, and rpm ramp/pulse width ramp tests are performed. Finally, system, fuel pressure, and performance calculations are done, the injectors are ID-etched, and lubed and bagged. DeatschWerks even offers pre-test flow rates for before-and-after comparison, and advanced testing to discover linearity and battery offset information.

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Injector Buying Tips

Injector sizing: You buy injectors for two reasons: either to replace OEM injectors that are worn out or damaged or to add more fuel for engine upgrades.

If you plan on keeping your Vette stock, many Corvette vendors sell replacement injectors with the factory flow rates. Some are a better design than the factory ones (cough…Multecs) and some have been updated to fix early shortcomings (i.e., alcohol-friendly 1993-’95 ZR-1 injectors). Once installed you won’t have to make any other changes like retuning, and if your old injectors weren’t up to par, your new squirters may reward you with restored power that you didn’t know you lost.

If you’re buying higher-flow injectors to complement engine mods, you’ll need to find a set that’s compatible with your engine, and also choose the right flow rate to conservatively cover your power goals. Remember, most injector companies recommend that you keep maximum injector duty cycle at 80 percent. You’ll need a new PROM or revised tune anyway, so always go up a flow rate to be safe. (Plus, when you finally decide to blow past that 80 percent duty cycle with more mods, you may get lucky and not have to change injectors again.)

Flow-matched sets: Fuel flow varies from injector to injector, and installing a set with differing flow rates results in sub-optimal performance. These days, many companies sell flow-matched injector sets. The percentage of variance is very low, and some high-end injectors can be matched to under 1 percent. And going a step further, big-dollar injectors can be grouped on their lag times—the amount of time between when the injector is energized, and when it is fully open. Most street Vettes wouldn’t need such precise calibration, but high-end, big-power builds can definitely benefit.

Impedance: Normally you should stay with the high-impedance units that came on your car—unless you’ve gone to an aftermarket ECU that allows low-impedance injectors.

Fuelie Formulas

Injector Flow Rate

For years, the standard injector sizing formula took horsepower, number of injectors, Brake Specific Fuel Consumption, and injector duty cycle to determine the best injector flow rate. The formula looked like this:

(HP / # of injectors) x (BSFC / duty cycle) = flow rate

Let’s use a 1997 Vette with an LS1, and factor in an 80 percent duty cycle and a .50 BSFC for naturally aspirated engines:

(345 / 8) x (.50 / .80) = 26.95 lb/hr

So according to this formula, 27 lb/hr injectors were needed to keep the ’97 LS1 safe and happy. The Corvette team used 28-pound squirters that year, so it’s in the ballpark.

Precise Injector Sizing

High-end injector company Injector Dynamics (ID) has a different flow calculator on its site. While it won’t give specific pound per hour numbers, ID claims it provides a more detailed estimation of injector need. Why? Injector Dynamics states that few people have access to an engine’s actual BSFC, and that determining every engine’s actual BSFC is difficult. As BSFC is a measure of efficiency, different engines with different timing settings (and therefore different power levels) will all have different BSFCs.

So ID’s calculator takes into account engine displacement, the number of cylinders and valves, boost, lambda at idle and WOT, fuel ethanol percentage, fuel pressure, cam aggressiveness, and fuel system type, and then crunches the numbers for you. The result is a graph showing each Injector Dynamics injector performance, from the 725 to the 2000. We chose the 850 injectors since they’re a direct replacement drop-in for Gen IV mills, and on the smaller side flow-wise.

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For the first formula, we used a naturally aspirated LS3 with an aggressive street cam and valvetrain. At 7,000 rpm, the 850s were flowing around 300 liters an hour, with a low 36 percent duty cycle. Would numbers change based on higher 4 bar LS fuel pressure? 885 cc at 3 bar. Around 955cc? at 4 bar = 91 lb/hr.

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For the second formula, we used a forced-induction LS3 with an aggressive street cam and valvetrain, and 10 psi of boost. At 7,000 rpm, the 850s were flowing about 575 liters an hour, with a 60 percent duty cycle. Would numbers change based on higher 4 bar LS fuel pressure? 885cc at 3 bar. Around 955cc? at 4 bar = 91 lb/hr.

Check out ID’s calculator here.


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Fuel Injector Guide

The following parts are great options for any Corvette owner who needs replacement or performance fuel injectors.

Note that the prices are either provided by the company, or taken from an online store.

Holley EFI

Holley has been the undisputed leader in fuel systems for over 100 years. And now, Holley EFI is dominating the performance world as our with fuel-injection products for GM’s LS engine. Choose from their wide selection of injectors, flow rates, and power levels, for both high-impedance factory applications and low-impedance racing applications.

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Company: Holley, www.holley.com
Injectors: Holley EFI 24 lb/hr
Impedance: High
Rated To: 385 hp
Part Number: 522248
Price: $343.95 (set of 8)

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Company: Holley, www.holley.com
Injectors: Holley EFI 36 lb/hr
Impedance: High
Rated To: 575 hp
Part Number: 522368
Price: $343.95 (set of 8)
Notes: Holley high-impedance injector sets are also available in 30 lb/hr (480 hp), 42 lb/hr (670 hp), and 48 lb/hr (765 hp)

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Company: Holley, www.holley.com
Injectors: Holley EFI 66 lb/hr
Impedance: Low
Rated To: 1,050 hp
Part Number: 522668
Price: $391.95 (set of 8)

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Company: Holley, www.holley.com
Injectors: Holley EFI 160 lb/hr
Impedance: Low
Rated To: 2,525 hp
Part Number: 522168
Price: $577.95 (set of 8)
Notes: Holley low-impedance injector sets are also available in 83 lb/hr (1,325 hp), and 120 lb/hr (1,900 hp)


Injector Dynamics

At Injector Dynamics, we are proud of our position as the technological leader, and proud of our ability to function at the same high level as the auto manufacturers. We are the only motorsport injector supplier providing emissions compliant high flow injectors to OE manufacturers. We are also the only supplier to have earned the title of authorized technical partner with Bosch, the largest automotive supplier in the world. We work at the highest level of automotive technology, and we produce products like the ID1300 for the motorsport community, which has set the standard for high flow fuel injectors.

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Company: Injector Dynamics, www.injectordynamics.com
Injectors: ID1700 160 lb/hr
Impedance: High
Rated To: 2,300 hp
Part Number: 1700.34.14.15.8 (set of 8)
Price: $1,984
Notes: C6 Corvette application

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Company: Injector Dynamics, www.injectordynamics.com
Injectors: ID1700 160 lb/hr
Impedance: High
Rated To: 2,300 hp
Part Number: 1700.60.14.14.8 (set of 8)
Price: $2,000
Notes: C4 and C5 Corvette application


FAST

FAST Precision-Flow Fuel Injectors deliver the outstanding idle quality, unparalleled flow consistency, and durability essential for high-performance EFI engine applications. Engineered to yield faster throttle response, reduced clogging, and superior fuel atomization, Precision-Flow Fuel Injectors are manufactured to the tightest tolerances in the industry. They feature state-of-the-art magnetics to improve injector opening times for faster throttle response, a precision-ground pintle and wide spray angle for superior fuel atomization, and a redesigned valve body that resists clogging and improves hot starting.

25 Injectors Fast Ls3 Ls7 L99 L76 50lb Highimp 30507 25/28

Company: FAST, www.fuelairspark.com
Injectors: LS3/L99/L76/LS7 50 lb/hr
Impedance: High
Rated To: 750 hp
Part Number: 30507-8
Price: $412.45 (set of 8)
Notes:

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Company: FAST, www.fuelairspark.com
Injectors: LS2 46 lb/hr
Impedance: High
Rated To: 680 hp
Part Number: 30462-8
Price: $412.45 (set of 8)
Notes:

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Company: FAST, www.fuelairspark.com
Injectors: LT1/TPI 36 lb/hr
Impedance: High
Rated To: 550 hp
Part Number: 303608
Price: $331.45 (set of 8)
Notes:


Racetronix

Racetronix has been dedicated to the design and manufacture of high-quality automotive performance parts since 1999. The affordable Continental Deka series 4 and 5 are a popular injector for high-performance enthusiasts. They offer fast, linear response, good atomization for drivability and big power, E85 compatibility, EV1 and EV6 connectors, and all necessary adapters. Available in 42, 46.5, 53, 63, and 80 lb/hr flow rates.

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Company: Racetronix, www.racetronix.biz
Injectors: 42-80 lb/hr
Impedance: High
Rated To: N/A
Part Number: See website
Price: See website
Notes: Available in short, mid-length, and standard heights, with EV1 or EV6 connectors, and with O-rings and height/electrical adapters to fit various intakes and fuel rails.

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