Last month, we published Part one of a series covering the history of the late-’60s’ legendary 427 cubic-inch big-block, and compared them with the modern day ZZ427. But a brief history lesson isn’t enough to satisfy the cravings for the “Holy Grail” of 427 ci engines.
The ZL1 is still one of the most coveted engines for Chevrolet enthusiasts, but the chances of getting your hands on a factory ZL1 is slim. Unless luck is on your side or you are willing to re-mortgage your home, building a replica is going to be your best bet. But,what will it take to replicate the legendary rat?
The original ZL1 big-block is still the Holy Grail of the big-block muscle car era. – Bill Martens, Chevrolet Performance
Although the majority of parts for the legendary ZL1 are no longer available from Chevrolet Performance, a modern version can be built using a combination of aftermarket components. For this article, genuine Chevrolet components will be listed when possible, but there are several companies that offer components suitable to replace the originals. Ultimately, components that are listed as direct replacements for the originals were selected as first priority, with availability factoring in as well. Please note that there are several great companies that can provide quality parts that are not listed. The purpose of this article is not to provide an end-all-be-all list, but more of a general guide to building a twenty-first-century ZL1 engine.
A Strong Foundation
Even though most components are no longer available for the ZL1 through Chevrolet Performance, perhaps the most important component still owns a full-page spot in its catalog. The block, made from 356-T6M aluminum with cast-iron cylinder sleeves, comes in the standard 9.800-inch deck height with 4.240-inch bores. Four-bolt steel main caps are used with the three center caps featuring splayed bolts on the outer holes. Water and oil holes are capped with AN style O-ring plugs.
The ZL1 aluminum block is still available from Chevrolet Performance.
The original engine came with a mechanical flat-tappet camshaft. However, the block has been updated to include provisions for installing a hydraulic-roller camshaft. It also will accept up to a 4.375-inch stroker crankshaft. Keep in mind that the block has been tested for applications up to 650-horsepower, which could easily be exceeded with a roller cam and long stroke.
For the purpose of recreating the original 427 ci engine from 1969, the block will be plenty stout to handle our 550 to 600 horsepower. However, the bore and deck height dimensions will need to be modified. The standard bore for the 427 ci engine is 4.250 inches, while the new block comes with a finished bore of 4.240-inches. Sportsman Racing Products (SRP) offers a shelf-stock piston that comes in .030 and .060-inch overbore sizes. To avoid the time and extra costs of special order pistons, consider having the block finish honed to 4.280-inches. This will ultimately up the cubic-inches of the engine to 433, but we don’t know of anyone who has ever complained about a few extra cubes.
With a 3.766-inch stroke, 6.135-inch connecting rod, and a 1.765-inch compression height, the assembly height works out to 9.783 inches (1/2 stroke + rod length + compression height). This will place the piston .017-inch in the hole and set the compression ratio to 11.6:1 with a .040-inch (compressed thickness) head gasket. Since the goal is 12:1, a thin head gasket must be used. A quality multi-layered steel (MLS) head gasket like one from Cometic with a .027-inch compressed thickness will provide just enough volume to compensate for the .030-inch overbore.
King Bearings have an aluminum-based bearing material that is strengthened with silicon, and these bearings have excellent wear resistance, embedability, conformability, and anti-seizure properties. The SI series will be a perfect fit for this performance engine.
A forged crankshaft like one offered from Scat Crankshafts, will make a good replacement for the extinct original. Today’s forged crankshafts are made from 4340 steel, and can handle a tremendous amount of torque. Paired with a set of bushed H-beam connecting rods, the rotating assembly is ready to turn some serious RPM.
Scat offers a forged-steel crankshaft and H-beam connecting rod that makes for a solid rotating combo.
The ZL1 cylinder heads utilized an open chamber, and since a 12:1 compression ratio is the target, a large dome is needed on the piston. SRP’s shelf stock 427 ci open-chamber piston fits the bill precisely. The pop-up piston takes up 48 cc of chamber space, making the TDC volume 1/12 of BDC. Optimum flame travel is achieved by CNC machining the piston domes with radiused valve reliefs. Compression ring grooves are 1/16-inch, while the oil groove is 3/16-inch. A set of file-fit rings is available from SRP and must be purchased separately.
A 44 cc dome quenches the air/fuel mixture, creating a 12:1 compression ratio. Can you smell the race fuel yet?
The entire rotating assembly should be balanced. Typically, a 427 ci engine is internally balanced. There is no additional weight added to the flywheel or harmonic balancer to create an imbalance. Be sure to double check with the machine shop.
Camshaft and Valvetrain
The last major component of the short block is the camshaft. A mechanical flat-tappet camshaft was stock, and Comp Cams has created a Nostalgia Plus series of camshafts. The ZL1 version uses modern lobe technology that Comp says makes it better than the original. The advertised duration is 299/309 degrees on the new version, as compared to 322/334 degrees on the old. The lift is higher, with .581-inch on the intake and .622-inch for the exhaust. The more aggressive lobe design allows for shorter duration and ultimately better street drivability.
Comp’s Nostalgia cam in a kit includes lifters and a dual-roller timing set. The cams are usually ground with four degrees of advance, so installing it straight-up should yield a 106-degree intake centerline. Verify the grind and the install by using a degree wheel or, if you don’t have one, the TDC-method.
Using a K-kit from Comp Cams makes the part selection process much easier.
Seal the timing components with a stock timing chain cover, flip the engine over and install an oil pump and drive rod, and a stock replacement pan from Canton, which provides all the factory dimensions and capacity. With a fresh shot of black paint, the pan looks original. The pick-up tube is a standard press-in style, so having it tack-welded onto the pump is always a good idea.
SFI-approved balancers are required if the engine will see any track time.
The harmonic balancer is the final component needed to finish off our modern-day short-block. Professional Products offers a quality balancer for this project. If the engine will ever see the track, it is best to go with an SFI-approved component to avoid breaking any rules. If Chevrolet parts are your top priority, the balancer used on the 572 ci crate engine can be utilized. However, be prepared to shell out roughly $600 for this one component.
The best time to install the balancer is before the cylinder heads are installed. This allows the number-one piston to be positioned exactly at TDC, and then an adjustable timing pointer can be properly indexed to provide a repeatable timing reference for the life of the engine.
Topping Off the Build
Now that the lower half is built, the focus can be moved to the top end. Cylinder heads have come a long way since 1969, and today’s as-cast heads are better right out of the box than a hand-worked set of heads from the past. Modern day manufacturing processes have also brought the price point down to a reasonable level as well. Therefore, there are a lot of options out there when choosing cylinder heads.
Although the original L88 cylinder head that was installed on the ZL1 is no longer available, Chevrolet Performance does have an updated casting that is a direct replacement. The heads feature the same combustion chamber and valve size as the original, and they are also NHRA approved. But, the intake runners are a whopping 315 cc, making for sluggish air velocity in the lower RPM range. A smaller runner will give better drivability for the street. If the engine is going to be street-driven, something like Edelbrock’s 290 cc Performer RPM head would be a better choice.
The original L88 heads used in 1969 are no longer available. However, Chevrolet Performance offers an NHRA-approved version, and other replacements are available from manufacturers like Edelbrock.
Either way, starting with a bare head will allow the use of the recommended valve springs for the Nostalgia cam from Comp. Their K-kit provides the springs, retainers, keepers, and valve seals needed. Going with a kit ensures you have the correct spring for your application. The only components left to finish assembling the heads are a set of valves from Manley and the appropriate spring locators and shims to achieve the proper installed height.
A head bolt kit from ARP will provide the needed clamping force to clamp the Fel-Pro head gaskets, keeping the cylinders sealed. Since the engine is running a high compression ratio, a set of MLS gaskets are a nice upgrade. To keep the compression ratio in check, a .040-inch compression thickness and 4.310-inch bore gasket should be used.
Finish off the valvetrain with pushrods, guide plates, and rocker arms from Comp Cams.
Investing in Horsepower
In 1969 the cost of having a ZL1 installed in your factory Corvette was $4,718.35. Although the sticker shock was enough to scare most away, the investment would have been well worth it today.
Putting one together today using aftermarket components will cost you an additional $8,000. With a 550 horsepower rating, that is more than $23 per horsepower.
7/16-inch screw-in rocker arm studs and guide plates can be installed after the heads are torqued down, and 3/8-inch hardened pushrods will provide a solid connection between the lifter and rocker arms. With the rocker arms installed, cold lash needs to be set.
There is an important note to consider with the Nostalgia camshaft. The hot-lash setting is listed as .010-inch for the intake and .012-inch on the exhaust. Because this engine uses an aluminum block and heads, the lash will grow significantly from cold to hot. (It is my personal experience as a professional engine builder that the lash on an all aluminum engine will grow .020-inch from cold to hot.) After talking with Comp’s tech line, they recommend setting the cold lash at .004- and .006-inch. After the engine reaches operating temperature, check the hot lash to see how much expansion took place. It is likely that the hot lash will need to be set closer to .024- and .026-inch to avoid hanging a valve open on a cold engine. Comp also stated that the hot lash setting listed on the cam card is a suggested lash and can be adjusted to accommodate a specific application.
Chrome valve covers are available from Chevrolet Performance to cover the valve train. Finally, the rectangular port, dual-plane manifold straddles the two cylinder heads connecting them to the carburetor. A Holley 4150 series, mechanical secondary carburetor feeds the beast, while an HEI distributor is an easy replacement for any old-school rat.
21st Century ZL1 Big-Block Parts List:
12370850: Chevrolet Performance aluminum ZL1 Block – $5,265.95
3704817: Chevrolet Performance fuel pump pushrod – $10.73
1514: Mr. Gasket fuel pump mounting plate – $11.19
535-9801: ARP engine bolt kit – $123.93
23-7001: ARP oil pump stud kit – $5.71
135-2501: ARP balancer bolt kit – $25.32
Approximate machining costs – $500
TOTAL: $12,728.69 – 12,863.52
Still to Come: Chevrolet Performance ZZ427
The third and final installment of this series on 427 big-blocks will cover the current engine available from Chevrolet Performance: the ZZ427. This modern version of the iron-block L88 offers a complete performance package for today’s enthusiast with street performance in mind. The pump-gas friendly crate engine has many similar traits to its predecessor, with some modern day updates. Stay tuned.