C3 Big-Block Engine Swap - Big-Block Party

Installing an injected Rat motor for improved performance, fuel mileage, and driveability

Paul Sisia Aug 1, 2011 0 Comment(s)
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If you're reading this headline, especially the part about better fuel mileage, and shaking your head, you're not alone. Virtually everyone we spoke to said it just wasn't possible. More than one even advised against wasting our time. So the real question is, why would we even try?

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Well, like every owner of a small-block- powered C3 Corvette, we're just sick and tired of hearing the same annoying question. At every local car show it's, "Does it have a big-block?" At the cruise-in, "Does it have a big-block?" Heck, at just about every stoplight, "Nice Vette...does it have a big-block?"

Let's face it: big-block Corvettes are legendary, both on the street and on the track. They have the mystique, the power, and a sound that just can't be duplicated by a small-displacement powerplant. Plus, that look is unmistakable. If you want to instill a sense of awe when you open your hood, nothing beats those wide valve covers--and, if you're lucky, a triangle air cleaner sitting atop a trio of two-barrel Holley carbs, the automotive trifecta.

The Plan
Our aim is to show you how to remove your small-block and then build and install a big-block that looks virtually identical to the 427/435 engine we should have ordered back in 1969. Only this time, there will be no leaking carbs, no difficult cold and hot starts, no fouled spark plugs, and no 6 mpg like the original delivered. No, this will be a modern, aluminum-headed engine with a unique triple-throttle-body fuel-injection system--triangle air cleaner and all. It'll be mated to a super-duty Gearstar 4L60E transmission that can handle the power and help deliver both impressive fuel mileage and lower-rpm highway cruising.

The Vehicle
We'll be using a beautiful Daytona Yellow '69 convertible running a non-original 406ci small-block. It's a great motor, but it's inherently compromised by the previous owner's desire for top-end power and a good quarter-mile sprint time. It has a single-plane intake, a big-jet Holley four-barrel, and a dual-point ignition system without vacuum advance. That's all fine for top-end power but doesn't help with low-to-mid-range performance or driveability--and it's certainly not set up to deliver maximum fuel mileage.

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As a baseline, this engine was averaging only 10 mpg. Simply switching out the big Holley carb for a smaller unit improved fuel mileage 40 percent. That's a nice increase, but it's still hard to jump up and down about 14 mpg.

Our 406 also suffers from some troublesome oil leaks from the typical spots: valve covers, oil pan, rear main seal, and even around some of the intake-manifold bolts. We've put off pulling the motor, resealing it with new gaskets and replacing the carb, intake, and ignition because, well, we just couldn't stop dreaming about how great the car would be with a big-block. Let's stop dreaming and get started.

Removing the Engine and Transmission
We'll begin with a stern admonition: Do not attempt this job by yourself. There's no better way to injure yourself than to try to remove an engine and trans without assistance. Because of the weight and bulk of the parts involved, this operation requires a hoist and several sets of hands to guide them out without damaging your vehicle. That's especially true when your vehicle is made of fiberglass.

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If you're searching for a shop to do the work for you, the first thing to do is contact your local Corvette Clubs for recommendations. For this particular project, look for a shop that is experienced with older Vettes, and with big-blocks in particular. During our search, we were surprised to hear recommendations for the Jim Ellis Corvette Center, part of a large, family-owned Chevrolet franchise here in the Atlanta area. This particular dealership's lead tech, David Fulcher, is a Master Certified Corvette Technician. He built and autocrossed his own big-block C3 years ago. And when we asked, "How long have you worked on Corvettes?" his reply was a reassuring, "Every day for the past 30 years."

It's also quite comforting to drive your old Vette into a brand-new, 80,000-square-foot service facility with a huge Chevrolet sign on top and more Corvettes on the premises than you can count.

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