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C3 Driveability Improvement - DIY EFI

How one novice tuner improved the driveability of his C3 for less than $1,000

Barry Kluczyk May 29, 2012
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Nothing beats the prospect of a long, enjoyable drive in your Corvette--except, perhaps, an out-of-tune engine that stumbles, stalls, or generally falls short in the driveability department. We're talking mostly about the smog-laden carbureted C3 models of the mid-to-late '70s. These Quadrajet-fed small-blocks perform well enough when tuned properly and driven regularly, but if your Corvette is a fair-weather cruiser and sits for several months during the winter, that Q-jet can become a source of frustration and unreliability. Such was the case with a friend's '77 Corvette that was driven infrequently, mostly because of a gummed-up carb that made even quick drives a testy, smelly affair. That friend--Leonard Slebodnik--thought about having the Quadrajet rebuilt, but admitted he'd really prefer the driveability advantage of electronic fuel injection. "I really wanted the assurance that when I jumped behind the wheel, the engine would fire up and idle correctly every time," he told us. "A new or rebuilt carb only seemed like a temporary fix for me."

Vemp 1208 C3 Driveability Improvement Diy Efi 000 2/23

And while there are various injection conversion systems on the market, Slebodnik is a do-it-yourself kind of guy, and he wanted to see if he could build his own setup from scratch for less than $1,500. (A Q-jet rebuild is typically $400-$500, by the way.) It would involve some parts scrounging, an electronic controller for the system, and a bit of welding. Slebodnik is in no way a professional tuner--and this project represented his first time ever tackling air/fuel tables on a laptop--but he is blessed with a diagnostically inclined brain, which proved to be a big help.

"If I could do it, just about anyone could," he says. "The physical conversion of the parts is pretty straightforward, but you have to be really careful and diligent with setting up the tables with the controller software--and you should have a basic understanding of what the engine needs for proper operation."

The homemade system is directed by the venerable MegaSquirt EFI controller, which we've seen on everything from budget conversions, such as Slebodnik's, to mega-dollar drag-race combinations. It's intended for stand-alone systems, driving only the engine's injection system. It's also designed to be used with a speed-density air-metering system, so it should work with the GM TBI unit Slebodnik planned to use.

And here's the kicker of the project: It was done for only about $900. Seriously.

Collecting parts and getting dirty

Before diving into this project, Slebodnik spent considerable time on the Internet researching it. He found sites and forums--specifically and others who had attempted the same basic conversion offered advice on parts, installation, and tuning. With a materials list in hand, he started scouring auto parts stores, the Web, and a few salvage yards for the components. The basics included:

MegaSquirt controller, relay board, pre-built cable and labeled wiring from

- Throttle-body fuel injection from an '88-'93 GM truck

- Adapter plate for the intake manifold

- High-pressure external fuel pump and fuel filter from a late-'80s Ford F-250

- Fuel lines and clamps rated for high- pressure fuel injection

- LC1 wide-band oxygen sensor from Innovate Motorsports

Removing the original Q-jet and replacing it with the throttle-body apparatus is a comparative snap. The project gets more involved when it comes to routing and connecting the necessary fuel-return line from the throttle-body system to the fuel tank. Although the tank itself doesn't require modification, the removable section that contains the fuel-fill inlet and sending unit does, including a bit of welding. It also means removing the fuel tank, as well as bolting the new high-pressure fuel pump beneath it after reinstallation. Finally, a bung for the wide-band oxygen sensor will need to be welded into the exhaust system.

As for the MegaSquirt controller, it found a home beneath the forward portion of the center console, which provided reasonably easy access for plugging in a laptop for tuning. Room under the hood must also be made for the relay box that supports the injectors and fuel pump.

Tuning trials and tribulations

While Slebodnik had never attempted tuning a car before, he's an IT guy by profession, so he knows his way around a keyboard.

"Having never attempted anything like tuning for fuel injection before, it was a challenge," he says. "Even though I knew how an engine worked, and the basics of air/fuel ratios, and so on, I was still starting from zero when it came to getting the car started."

Slebodnik points to the Website and associated forums as an invaluable resource. They provide the programming basics needed to get the engine started and idling. After that, the trial-and-error fine-tuning process begins.

"One of the hardest things to learn at the beginning was setting the 'constants'--things like the required fuel, injector flow rate, required fuel under acceleration, and the targeted stoichiometric values [optimal air/fuel ratio]," he says. "With those set, you can get the car running, but the tuning variables are almost infinite, so for the first-timer, there's a lot of trial and error. Fortunately, you can save a 'tune,' and if it doesn't work as well as a previous one, it's easy to switch back."

As we were heading toward the deadline on this story, Slebodnik admitted that the cold-start enrichment tuning--which ensures that the engine has more fuel at start-up, until it reaches operating temperature--was still an issue to overcome.

"Yeah, it's still a bit cold-blooded at start-up, but I'm working on it," he says. "Once that's nailed, it will all be good. As it stands, it already runs much better than with the old carburetor."

Greater power and fuel mileage, right? Well…

Right off the bat, we'll address a couple of common misperceptions about converting to EFI. Generally speaking, don't expect huge increases in either performance or fuel economy on an otherwise stock engine. The injection system may foster more-efficient combustion, but that doesn't necessarily translate into noticeably more horsepower. You're looking at maybe a 5 percent increase in output, tops.

There are a couple reasons for this, the most notable being the fact that changing to fuel injection doesn't alter the airflow aspects of the intake manifold or cylinder heads. Those were pretty low-flow elements on '70s-era GM small-blocks, as engineers tried to squeeze more mileage and emissions efficiency out of them. So, electronically controlled fuel delivery or not, sticking with the stock intake and heads compromises performance. It's also one of the reasons this project uses a "two-barrel"-style throttle-body injection system rather than a "four-barrel"-style. There's simply no need for the latter on an engine rated at only 180 hp.

Of course, fuel-injected performance took a large leap in 1985, when the Corvette debuted Tuned Port Injection. That swap is entirely possible on a C3, too, but it's a more costly endeavor and drastically alters the underhood appearance. Slebodnik wanted to retain the look of a carbureted engine, and the throttle-body injection system he selected tucks nicely beneath a conventional air cleaner.

As for the fuel economy, again, the car's stock configuration is the compromise--this time in the guise of its three-speed automatic transmission. Sure, there were a few manual-transmission models sold in the later C3 years, but even they didn't offer the advantage of overdrive. Be it a three-speed slushbox or the Borg Warner T10-based four-speed, the top-gear ratio was 1.00:1. Combine that with the typical 3.55 rear axle of the day, and highway cruising can yield 2,500 rpm or greater. Regardless of whether the engine is fuel injected, it's still going to suck gas pretty fiercely on the open road.

The bottom line

Although Slebodnik is still revising his tune to nail down the cold-start performance, he's done spending money for parts on the system. The total--itemized in the accompanying chart--came to $904. That, of course, doesn't include the sweat equity of doing the installation and tuning himself, but the satisfaction of the project suggests the labor wasn't much of a factor for this project.

"There are a couple things I'd do differently, but I'd definitely do this again on another vehicle," he says. "Even with a couple of tuning issues to work on still, the driveability of the car is already much better than with the carburetor. I wouldn't go back to that again for anything."

Slebodnik says the experience has also given him a new appreciation and greater understanding of the complexities of modern engines, and why tuning is such a painstaking, specialized process.

Spoken like a true convert.


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