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C3 Big-Block Engine Swap - Big-Block Party

Part 3: Prepping our '69 for the installation of a modern, EFI big-block

Paul Sisia Aug 21, 2011
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In our first installment, we took you through the process of removing our old, tired, and not terribly exciting small-block. We also showed you how we prepped our '69's engine bay for the installation of a new Tri-power EFI big-block.


Part 2 was a step-by-step build plan showing how, with the help of a select list of high-performance suppliers, we assembled this 21st century Rat motor.

This month, we'll install all the new upgrades we've chosen, along with the main ingredients of our project—specifically, the engine, a super-duty Gearstar 4L60E transmission, and a made-to-order, bulletproof rearend from Precision Corvette Differential.

There's a learning curve with every "first-ever" build, and it quickly became apparent that our C3's stock automatic transmission didn't have a chance of handling the new big-block's output. So we switched to a Gearstar Level 4 electronically controlled 4L60E/four-speed automatic. It has an 11-inch, custom-stalled, full-billet, lock-up torque converter, and it's built to handle up to 650 hp and 600 lb-ft of torque. Follow along now as we find out what it takes to get a transmission to Level 4 capability.

A Complete Trans-Formation
Gearstar Performance Transmissions says it "custom builds your transmission to order." Fortunately, that's a fact and not just a slogan. Much as you would when having a bespoke suit made, you'll speak directly to one master tech, who will then build your transmission—based on your specific requirements—from start to finish. The journey to building a Level 4/4L60E starts with qualifying the case, then high-pressure washing and bead-blasting it. It's then re-washed, re-inspected, and painted.

The reassembly process begins with a master overhaul kit with Kolene steels and Raybestos racing frictions. Gearstar uses a carbon- fiber, super-wide 2-4 band that utilizes 100 percent of the available surface area. Two valvebody-reprogramming kits are used to improve shift quality and durability, and all of the internal electrical parts are replaced with updated components. A complete bushing kit; new, heavy-duty Torrington bearings; and new thrust washers are also installed.


To handle the additional torque, Gearstar uses a high-capacity pump with a hardened rotor and vanes. The input drum is hardened and reinforced with a billet sleeve to greatly increase strength, and the clutch drum is also modified, allowing it to tolerate higher rpm levels. Additional clutches and steels are added, which boosts clutch capacities by 30 percent. The Level 4/4L60E also incorporates a fully rollerized planetary gearset that provides enhanced durability and greatly reduces power-robbing drag. The four-pinion OE planetaries are replaced with hardened five-pinion steel planetary gearsets.

This Level 4/4L60E is one of the most flexible fully electronic transmission packages available, allowing us to completely customize shift timing, shift feel, and lock-up application. The bottom line is that every part of the transmission is brought up to super-duty specs so it can handle the power and deliver years of trouble-free service. And Gearstar guarantees every transmission it sells. Welcome to Level 4.

Supporting Mods
In addition to the transmission, we needed to address a number of items that, upon closer inspection, didn't look up to the task at hand. The first thing we did was treat ourselves to the beauty and durability of powdercoating. There's a whole list of powdercoating shops in the Atlanta area where we live, but the name that kept coming up was Miller's Powder Coating. That's probably because Bill Miller is a true hot rod and motorcycle enthusiast and has done powdercoating work for virtually every car and bike club in the state.


Once we had our upper and lower control arms coated, we were hooked. We ended up powdercoating the entire front suspension, the trans crossmember, the rear-diff crossmember, and a few other odds and ends. Miller's can even powdercoat your entire chassis in virtually any color you'd like.

Now that our chassis was cleaned and painted, and our front suspension powdercoated, we were loath to reinstall the stock power-assist slave cylinder. It's always leaked, and we were told that a new stock-replacement unit would end up in the same condition in just a few months. The best remedy was to upgrade to a new Borgeson power-steering box. It offers better road feel, a quicker turning ratio, and eliminates the slave cylinder completely. The Borgeson kit is complete and includes the hoses, lines, and a new rag joint as well. The installation was straightforward and, best of all, uneventful.

It didn't make sense to reinstall old brake pads or shock absorbers, either. We chose to upgrade both the pads and rotors with new units from EBC Brakes. They're specifically designed to work with stock Corvette calipers, and their rotors are slotted and partially drilled to avoid cracking. We painted the calipers with caliper paint to complete the look.


For shock absorbers we were back on the phone with Summit Racing, whose techs recommended QA1 adjustable gas units. We also replaced the stock rear multi-spring setup with a single composite spring from Volunteer Vette. It'll deliver better ride quality and reduce a fair amount of weight as well. (Volunteer provided our front big-block springs, too.)

One of the biggest and most noticeable upgrades we made was to eliminate all the stock pulleys and belts and install a Vintage Air Front Runner serpentine belt system, which came complete with a new and more efficient A/C compressor, power-steering pump, and 140-amp alternator.

The Front Runner system is truly a sight to behold. Everything is polished or chromed, the mounts are all one-piece 6061-T6 billet aluminum, and the fasteners and bolts are from ARP. It includes a high-volume aluminum water pump, and the installation is painless. Our only mistake was thinking the job was going to be difficult, which led us to read the install instructions. Vintage Air has simplified this procedure to the point that you can save a lot of time by simply following the picture diagrams.

Prior to lowering the engine between the framerails, we removed the lower pulley for clearance, installed a set of Prothane urethane motor mounts, and then laid our Hedman ceramic-coated headers in place.


Once our big-block found its new home, it was apparent the lower pulley wasn't going to clear the front crossmember. We used a cutting wheel and notched a 1x4-inch section. Another necessary modification involved the power-steering pump. Since it's a universal pump, we needed to cut off the plastic return fitting and drill and tap the hole for a 90-degree fitting. This allowed it to clear the upper-control-arm cross-shaft.

Finally, it was time to bolt up the Gearstar 4L60E. But first, we needed to install the Denny's HD driveshaft into the rear-differential yoke. The front of the driveshaft slips into the back of the trans, and the trans is raised to finish the installation. Our new trans does have a slightly larger billet servo on the side, which initially made contact with the trans tunnel. Using a small Dremel attachment, we cut a 2x3-inch notch in the tunnel to accommodate it. We'll need to remove the carpet on the passenger side of the tunnel and re-glass this cut at some point, to keep road noise and heat from entering the interior.

With the engine, trans, and rear diff installations finished, we focused on all the little details necessary to make everything look neat, clean, and professional. We mounted the FAST computer on the inner fender well, and placed the transmission computer in the storage compartment behind the passenger seat. Both are now protected from road damage and rainwater, and still easily accessible if we need to plug in our laptop. The MSD 6 box and coil are mounted on a 1/4-inch aluminum plate and bolted to the firewall right behind the MSD distributor.


Next, we routed our 1/2-inch fuel line and 3/8-inch return line on the outside of the framerail, behind the rocker cover. Here, they're protected from undercar heat and road scrapes, and are completely hidden from view.

For a little more "bling"—and way better cooling—we installed a Be Cool all-aluminum radiator with twin electric fans.

Once we had everything mounted in the engine compartment, we covered our wiring harness with new GM harness covers. They perfectly match the covers on the FAST E-Z EFI harness and give it a clean, modern look. Last but not least, we bled the brakes, had a four-wheel alignment done, checked for leaks, and reinstalled the original hood, which now boasts a reproduction 427 emblem from Volunteer Vette.

Firing it Up
We quickly forgot about all the parts and the hours of labor that went into this build the second we turned the key and heard the engine explode to life. Yes, explode. Small-blocks start, and it's fairly uneventful when they do. When a big-block starts, it sounds like a bomb going off. I clearly remember the first time I heard an original 427/435 fire up. It had side pipes, and it scared the life out of me—the kind of scared when you can't stop smiling.


David Fulcher took the first spin…literally. As he dropped the shift lever into "D" and touched the gas pedal, the car spun the tires right there in the service bay.

Don't miss the next issue when, along with final photos, we'll have the complete report from Walden's dyno, a full road-test review, and our driving impressions—including braking, steering response, handling, acceleration, and even a fuel-mileage summary. See you then.

Bulletproofing Our IRS
Bulletproof and IRS are two words you don't normally see in the same sentence. The guys making 500-plus hp and more than 500 lb-ft of torque usually run it through a solid-axle rearend. But Corvettes since 1963 have been built with an independent rear suspension (IRS) for good reason. The ride and handling from an IRS are far superior to the antiquated design of any solid- axle rear. Unfortunately, your Vette's IRS was never designed to handle more power than the factory engine developed, so getting it to cope with our big-power big-block started with some very stout race-ready parts. The source—Tom's Differentials.


The parts list from Tom's is extensive, but cut corners here and you'll find yourself stranded on the side of the road. Tom's provided a 3.73 U.S. gearset, an oversized and hardened cross-shaft, a 31-spline spindle kit, a billet 1330 pinion flange, Eaton solid clutches, a hardened spider-gearset, and 30-spline inner stub axles. Throw in the fact that every nut, bolt, and shim are hardened, and you'll see we've basically replaced everything but the case. After all, to do it right, all the "weak links" in the system need to be removed from the equation.

While we were at it, we also tossed the original trailing arms and ordered new arms, bearing supports, and parking-brake components from Lonestar Caliper.


With parts in hand, it was time to move on to the machining, setup, and assembly of our rear diff. The experts at Tom's warned us that there are only a handful of machinists left who have the experience to build an IRS capable of handling this type of power. The reason? It's an incredibly time- and labor-intensive process, and everything inside the diff needs to be set to within thousandths of an inch. At the top of the "recommended" list was Mike Dyer, a 35-year veteran and the owner of Precision Corvette Differential.

Dyer starts by cleaning and media blasting everything, and then boring the new carrier to fit the oversized axles and cross-shaft. He machines a new flat for the cross-shaft lock bolt that is 0.060 deeper, then radiuses the edges and polishes the carrier to remove any casting imperfections. The clutches are pre-scuffed so they're already seated before you turn the first wheel, and the spider gears are shimmed to 0.002 backlash. The ring gear is installed with Loctite, and the axles are machined to 0.005 endplay. Even the tips are polished.

The ears on the case are also machined off for axle clearance, and the bearing-cap holes are drilled and tapped for 1/2-inch bolts. (Dyer machines these HD caps from raw blanks.)


The Timken bearings and races for the carrier and case are installed next, followed by the pinion and carrier themselves. The gears are then set up by pattern—and then the whole thing is disassembled and cleaned to surgical standards.

Finally, Dyer does it all over just to make sure it's perfect. The final assembly includes the installation of the new Torrington stub axle bearings, along with the pinion and crush sleeve; the pre-load is then set to 15 in-lbs. The carrier is then installed and set to 0.007 backlash. As with building a race engine, everything is checked and rechecked.

When we received the complete rear diff and new trailing arms (which Dyer also built), one spin of the axle stubs told us everything we needed to know. The fresh setup had that smooth, almost silky feel that only comes with a machined part that's perfectly on spec. Our project now truly has a Precision Corvette Differential.


While we were at it, we ordered larger-diameter, "nitrous-ready" halfshafts and a new HD main shaft from Denny's Driveshafts. (The Denny's units come complete with heavy-duty universal joints as well.) By comparison, our old half- and mainshafts looked like they came out of a Corolla. Denny's even used our old parts as a template for size, length, and correct yoke spline, to make sure everything will go back together just the way it came out.

So there you have it: big-block power, a super-duty transmission, a bulletproof rear diff, and a set of nitrous-ready halfshafts and mainshaft. Guess where the weak link is now? The rear tires. We're gonna shred 'em like mozzarella cheese.


Gearstar Performance Transmissions
Akron, 44308
Vintage Air
San Antonio, TX 78266
Summit Racing
Akron, OH
EBC Brakes
Sylmar, CA 91342
Borgeson Universal
Torrington, CT 06790
Denny's Driveshaft
Kenmore, NY 14217
Memphis, TN 38118
Volunteer Vette
Knoxville, TN 37914
Jim Ellis Chevrolet and Corvette Center
Atlanta, GA 30341



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