Few things confound Corvette buffs more than the mysteries of electronic fuel injection, how it works, how often it needs to be serviced, etc. Electronic fuel injection mirrors brake fluid, rear axle lube, and the servicing of door locks … we never think about it until it mandates service. Fuel injection is easily the most trouble-free element in our Corvettes because it operates flawlessly most of the time.
But what happens when it doesn’t?
Electronic fuel injection (EFI) on GM vehicles dates back to the mid-1970s and the port-injected intermediate-sized Cadillac Seville (Oldsmobile 350 V-8). EFI quickly made its way to Corvette as Cross-Fire injection in 1982 in light of tougher emissions and fuel economy standards. The Corvette’s Cross-Fire Rochester fuel injection was nothing like the Seville’s multi-port injection. In fact, the Seville’s multi-port injection was more sophisticated than the Corvette’s Cross-Fire injection and way ahead of its time with eight Bosch injectors on two fuel rails.
The Corvette’s Cross-Fire system was instead a pulsing throttle body injection (TBI) sporting two throttle bodies positioned cross-manifold style like the classic Z/28 cross-ram small-block Chevy manifold, which defined underhood cool. It was the most stylish high-performance intake manifold of its time. Chevrolet engineers conceived an electronic fuel injection package resembling the classic Z/28 cross-ram manifold. Cross-Fire was operated by GM’s Computer Command Control (CCC), which also was also applied to Rochester computer controlled carburetors of the period. CCC delivered pulses via solenoid-fired injectors.
You could ask why the Seville’s port injection system wasn’t adapted to Corvette. The answer then was cost. The Seville’s Olds 350 V-8 had twin fuel rails and eight fuel injectors with very involved electronics, which was decidedly expensive and outside the Corvette’s price range. At that, the Seville’s port injection had its own share of teething problems classic Seville owners still live with today. It would have undoubtedly been a disaster for Corvette buyers. Cross-Fire was more reliable, though it has its critics.
Although Cross-Fire gets rotten tomatoes from enthusiasts it is little more than a simple twin Rochester throttle body fuel injection system that has a lot in common with other GM TBI systems of the era. The Rochester throttle body system wound up in a variety of bread-and-butter GM applications in the 1980s before everything went port injection.
If properly maintained and serviced, Cross-Fire fuel injection is reliable and performs quite well. Cross-Fire’s reputation suffers from misunderstandings and the limitations of electronic fuel injection systems available in the early 1980s. It is more likely that enthusiasts who bad-mouth Cross-Fire also criticize the Rochester Quadrajet simply because they don’t understand it.
What makes the Cross-Fire injection-equipped L83 5.7L engine frustrating for some is its limitations. From an aftermarket performance standpoint Cross-Fire doesn’t like modifications like a hotter cam because it is programmed to live within a factory-programmed window of operation in terms of spark timing and fuel curve. Cross-Fire didn’t have the flexibility the TPI systems had later on. Cross-Fire is a system you learn how to live with as it came from the factory because there’s very little you can do with it in stock trim. Modifications can be made to both the manifold and the heads that yield improvement; however, you’re still limited by factory electronics.
Tuned Port Injection (TPI)
When you compare the Corvette’s Cross-Fire of 1982 and 1984 with the Tuned Port Injection (TPI) of 1985-up the two aren’t in the same universe. TPI was an incredible quantum leap in Corvette performance, not to mention other GM marques like the IROC-Z Camaro and Trans Am. TPI yielded roughly a 30 percent improvement in power and fuel economy. Like the Cadillac Seville from a decade earlier, TPI consisted of eight fuel injectors on two rails supported by high-tech electronics and a tubular intake manifold with tuned runners. It was the best fuel system ever offered in the Corvette at the time. However, even TPI is rather crude when compared to today’s EFI systems. TPI employed a batch fire system where all eight EV1 injectors fired as a group or could fire alternate banks. This didn’t help emissions, but it did a lot for power.
If you remember the high-tech 1990-’95 Corvette ZR-1s, you remember how advanced it was for its time. The ZR-1 was the first Corvette to have sequential port fuel injection where each injector fired as the intake valve opened. The ZR-1 wasn’t just another small-block Chevy-powered Corvette. It is among the most legendary Corvettes ever produced with its MerCruiser (Mercury Marine) built double overhead cam LT5 V-8.
TPI sports the same intake manifold on the 305 (Camaro) and 350 (Corvette). Where it differs is injector size: 19 lb/hr for the 305 (5.0L) and 22 lb/hr on the 350 (5.7L). Both engines employ the Bosch/Denso EV1 injectors, which are old-school today but high-tech for the time. The EV1 is a very common injector that enjoyed a long production run in OEM applications. It remains a very suitable injector for nearly any application though better injectors have come since. The EV1 yielded to better injectors like the EV6 and EV14, which offer greater precision and a vastly improved spray pattern.
How Fuel Injectors Work
Electronic fuel injection, as its name implies, is a means to controlling injector timing and pulse width (time period the injector is open and delivering fuel) as a means to controlling engine speed and power. A fuel injector is an electronically triggered electromagnet and needle/seat control valve actuated by your Corvette’s ECU, or electronic control unit. Some people also refer to the ECU as an ECM (Electronic Control Module) or PCM (Powertrain Control Module). The electromagnet or solenoid is electronically fired in time with the intake stroke on each cylinder, delivering a fuel spray into the intake port where it is mixed with air. The more the throttle is opened, the longer the fuel injector remains open.
An electronically controlled fuel injector consists of a filter where fuel enters the injector; a coil/solenoid; a fuel shaft; a pintle, ball, or disc; and a closure, or return, spring. The pintle design employs a shaft, needle valve and seat. This is the most common injector design. There’s also the ball-and-seat valve with a director plate underneath with a closure spring. The ball-and-seat valve tends to be self-cleaning and runs cleaner. And finally there’s the disc type, which runs cleaner and resists contaminants.
Fuel injectors must have a constant source of pressure courting 35-45 psi and timed pulses of electricity from the ECU to do their job. Chief areas of concern are Pulse Width, Duty Cycle, Flow Rate, Fuel Pressure, and both High and Low Impedance.
Pulse Width: The amount of time the injector’s needle is off its seat (open).
Duty Cycle: How long the signal is active versus how long it could be active.
Flow Rate: How many pounds per hour the injector is capable of flowing, measured as lb/hr.
Fuel Pressure: The maximum fuel pressure in the rail ahead of the pressure regulator at the injectors.
High Impedance: A high impedance injector has an impedance of 12 ohms at its solenoid or electromagnet coil, which means it operates at low current levels in order to keep circuit temperatures conservative (cool). Most OEM injectors are high impedance.
Low Impedance: A low impedance injector runs 2-3 ohms of resistance, but with a lot of current. Low impedance injectors are more of an aftermarket element used in racing applications.
Corvette Injector Sizes
1982, 1984: 67/68 lb/hr
1985: 24 lb/hr at 43.5 psi or 22.5 lb/hr at the factory 36-37 psi
1986-’93: 22 lb/hr at 43.5 psi
1994-’96: 24 lb/hr at 43.5 psi
1990-’92 ZR-1: 20.5 lb/hr both injectors at 43.5 psi
1993-’95 ZR1: 20.5 lb/hr both injectors at 43.5 psi (designed for ethanol)
1997-’98 LS1: 28 lb/hr at 53 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: 43 lb/hr
Injector Right Sizing
If you’re building an engine, or perhaps uprating an existing engine, it’s time to consider injector upsizing and engine tuning. RC Fuel Injection tells us an engine consumes 0.50 pounds per hour for each horsepower it produces. It calls that a Brake Specific Fuel Consumption (BFSC) of .50. If you’re going to turbocharge, that number goes up to .60.
Follow this formula to determine injector sizing: (Horsepower x BSFC) / (No. of injectors x .80) So, for a 400hp V-8 it would work out as such, (400 x .50) / (8 x 0.8) = 31.25 pound per hour injectors. If you want to convert to cc/min: 31.25 x 10.50 = 328cc per minute.
To put this into easy to understand terms, if you want to make 300 horsepower, a conservative number, you’re going to need 24 lb/hr injectors. Up the horsepower to 500 and your engine will need 42+ lb/hr. Remember, it is better to err on the side of bigger rather than smaller.
Injector Cleaning and Flow Testing
RC Fuel Injection tells us it performs a detailed cleaning and flow testing when it receives your fuel injectors. Your injectors are installed on RC Fuel Injection’s custom-designed-and-built, computer-controlled injector flow-testing equipment. A unique, non-flammable, gasoline-like test fluid is pumped at a precisely controlled pressure through the injectors while they are pulsed by injector drivers designed into the equipment. The volume of fuel that passes through the injector is accurately measured for a set amount of time. From this data, fuel flow is confirmed. During the flow test, injector spray patterns are visually inspected and analyzed by RC Fuel Injection’s technicians using both a bright room and strobe lights.
After that first flow check, injectors are transferred to an ultrasonic cleaning machine and again connected to injector drivers and submerged into a cleaning solution and pulsed while being subjected to ultrasonic waves that vibrate contaminants loose. The combination of the solution and the ultrasound aggressively clean the internal and external parts of the injectors. They come out of the solution hospital clean.
The cleaned injectors are again installed on the flow-tester for an aggressive run. At the conclusion of the test a printed report is produced. This report lists fuel flow rates (in pounds-per-hour and cubic centimeters per minute) before and after cleaning, and spray pattern assessments for each injector tested. Also included is the system balance number, which is the flow difference, expressed as a percentage, between the the highest and lowest flow rates amongst the injectors tested. The system’s total fuel delivery in lb/hr and cc/min along with a computation of potential horsepower at three different brake specific fuel consumption rates concludes the report. When you get your injectors back you then know their state of health. Cost at press time to get injectors flow tested, cleaned, and flow tested again is $24 to $28.50 per injector with a turnaround time is typically 24 hours, except for holidays.
RC’s “ProCal” Service
If you’re building an engine, you’re going to be interested in RC Fuel Injections “ProCal” fuel injection service. RC Fuel Injection’s Professional Injector Calibration (“ProCal”) service is aimed at engine builders doing fuel injection design or calibration for motorsports teams and those tuning high-performance street engines equipped with highly modified OEM electronic fuel injection (EFI) systems or custom-built EFI systems. The main difference between RC Fuel Injection’s standard and ProCal services are more complex calibration procedures and a more comprehensive test report.
If the customer wants RC Fuel Injection’s ProCal service, the injectors are tested using computer-controlled equipment over a wider range of pulse widths, usually from 0.5 milliseconds to continuous flow. ProCal tests injectors at pressures up to 125 psi. Additionally, if variable voltages are used, RC Fuel Injection will test injectors at any customer-specified voltage between 5 and 14 volts.
For most customers specifying ProCal, injectors will be calibrated at the pulse width necessary for wide-open-throttle and maximum rpm operation. From there, RC Fuel Injection will work down the powerband testing the injectors at a wide range of pulse widths and fuel pressures to ascertain function.
This intense calibration of injectors is valuable when you’re designing a computer-controlled fuel management system or making significant changes to an existing system. Injectors produced by different manufacturers, different injector drivers, varying fuel pressures, and changing voltages can make substantial changes in fuel delivery. Changing these parameters in combination without ProCal can leave a tuner with uncalibrated injectors in a state of confusion. Rates for ProCal are on an individual quote basis, depending upon the customer’s needs.
The 5.7L LT4 induction system of the 1990s is an evolution of the original GM TPI system introduced in 1985. Instead of EV1 injectors, the LT1 induction system had OEM Rochester fuel injectors. These are EV1 24 lb/hr injectors on an LT4 upgrade with an adjustable fuel pressure regulator. TPI became speed density instead of batch fire at the cusp of the 1990s.
Here’s an original LT1 engine with 22 lb/hr Rochester fuel injectors. EV1 injectors drop right in place of the OEM Rochesters.
From left to right are three generations of fuel injectors: Bosch/Denzo EV1, EV6, and the EV14. The EV1 employs a Jetronic two-pin plug. The EV6 and EV14 have a different UScar two-pin plug. Jetronic to UScar adapter harnesses are available to those who want to upgrade to EV6 or EV14.
RC Fuel Injection offers its own brand of injectors in virtually every type available. This is the EV1 to EV6 conversion, which enables you to upgrade from the vintage EV1 injectors to EV6.
The EV1 24 lb/hr injector (left) and the Rochester 22 lb/hr OEM injector (right) for the LT1.
This illustration shows the anatomy of a typical Bosch/Denzo EV1 injector. An electromagnetic coil is energized, which moves the needle valve off its seat allowing fuel to flow from the rail and atomize at the tip. A spring closes the valve when de-energized.
Here’s a closer look at a fuel injector’s electromagnetic coil, which levitates the needle valve off its seat inside the valve shell. This particular injector is for a throttle body injection system.
RC Fuel Injection begins fuel injector service by measuring coil resistance. This EV1’s coil resistance value is right where it should be, around 14.6 ohms. If there’s no reading the coil has an open circuit (inoperative) and should be discarded.
RC Fuel Injection tests a sample set of EV1 19 lb/hr injectors to study spray pattern. This particular set has more than 200,000 miles on them. Aside from traces of contaminants in these injectors they had an acceptable spray pattern. RC Fuel Injection will disassemble them, flow test and ultrasonic clean them, flow check them again, and fit them with new tips and seals.
This is a nice, uniform spray pattern, which is actually a non-flammable solvent on a par with gasoline in terms of density.
Look what happens when we ramp up injector cycling. This is approximately 7,000 rpm. Now that’s a lot of fuel.
The injectors are disassembled with filters, tips and seals removed for inspection, cleaning and flow testing. RC Fuel Injection tells us it is rare for injectors to have to be replaced. Periodic cleaning when you perform a tune-up (spark plugs and ignition service) is all they need.
This is a disassembled EV1. Within the shell is the electromagnetic coil. Shown are the return spring and the valve piston assembly. Within the valve piston is the needle and seat. Injectors become inefficient when they get dirty. Ultrasonic cleaning removes all debris and carbon deposits rendering the injector like new again.
An ultrasonic injector cleaning in solvent removes carbon deposits and other contaminants. RC Fuel Injection energizes the injector to cycle the needle valve allowing deposits to escape. “Any set of fuel injectors will benefit from RC Engineering’s clean and flow service,” RC Fuel Injection tells us. “This is even true of new injectors because the individual injector and system total fuel flows must be known for performance tuning purposes.” RC adds not all new injectors are up to published specifications.
Our EV1 injectors have been cleaned and are tested at this time from idle speed to high rpm.
Look at what ultrasonic cleaning did for the EV1 injectors. A nice spray fan ready for another 200,000 miles of service.
Each EV1 has this tiny filter, which is intended to keep debris out of the injector. These filters can become clogged and cause injector malfunction.
Two Bosch EV1 injectors side by side; with the one on the right assembled with a new tip and seals. Seals should be lubricated before installation.
The EV1 with seals and tip. Aside from professional testing and ultrasonic cleaning the injector service is quite simple. However, this is not something you can do in your garage.
When injectors are reassembled they get a final flow test to determine their condition and are carefully packaged for their return to the customer.
Look at this flow test at maximum flow from 19 lb/hr EV1 injectors.
Prior to filter installation and packaging, the injectors get a reverse blast of compressed air to remove any residual test fluid.
A pair of EV1 injectors side by side from two manufacturers; Bosch (left) and Denzo (right).
RC Fuel Injection offers not only fuel injection testing and cleaning, but an assortment of new injectors for virtually every Corvette application imaginable.