21st Century ZL1

A Tribute Build of the Legendary All-Aluminum Big-Block Delivers Big Torque and Greater Street Performance

By Barry Kluczyk – Photography by the Author

It’s been said that legends rarely live up to their hype, but the original, all-aluminum ZL1 big-block did just that.

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Originally conceived as a lightweight racing engine for the Can-Am road racing series (Group 7), the original, solid-lifter 427ci ZL1 engine was never intended for production vehicles, but Chevrolet’s motorsports engineers never counted on an enterprising and forward-thinking Illinois dealer named Fred Gibb. He used Chevrolet’s COPO (Central Office Production Order) special-order system to produce the legendary Camaro ZL1.

01 ZL1 Crate Engine - GM Performance
There’s nothing quite like the sight of a new, aluminum ZL1 block in its crate to light a project’s fire. Available straight out of the Chevrolet Performance catalog (PN 12370850), it’s a modern casting based on the original design, with features including splayed four-bolt mains and screw-in galley plugs.

Even though the engine made its mark earlier and continued to be used in racing into the ’70s, it was in those 69 ’69 Camaros—and a pair of Corvettes—that grew the myth of the ZL1. Over the years, Chevrolet stoked the embers, including a reissue of the aluminum cylinder block, which spawned a Ram Jet-topped ZL1 crate engine and an anniversary 427 crate engine.

In the production cars, the engine was officially rated at 430 hp, but true output was closer to 525-530 horses, which was no small feat for a naturally aspirated engine in its day. The basic recipe included a 12:1 compression ratio, a mechanical cam with 0.560/0.600-inch lift, and open-chamber heads based on the L-88, but with slightly larger exhaust ports. It was all topped with an 850-cfm Holley double-pumper. Serious stuff back then.

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02 ZL1 Engine Head
The block comes with a standard 9.800-inch deck height and centrifugally spun cast-iron cylinder sleeves, with a 4.240-inch bore diameter. Valley Performance finish-honed the sleeves to 4.250 inches just like the original ZL1.

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More than 50 years later, it’s a performance threshold that’s still admirably lofty. Consider the 6.2L LT2 in the C8 Corvette. It’s 12 percent smaller in displacement than the ZL1, but with a comparable compression ratio of 11.5:1, and it’s rated at 490 hp. Assuming the ZL1’s true output was closer to 530 hp, but we shave off 12 percent to equalize the displacement with the LT2, the rating comes down to 467. Considering the LT2 employs advanced technologies, such as cam phasing and direct injection, it’s an awfully close comparison to the capabilities of a single-carburetor-fed big-block from more than half a century ago.

03 ZL1 Sleaves
After comparing the 4.250-inch bores against the cylinder heads’ combustion chambers, Valley Performance deemed it prudent to machine reliefs into the top edges of the cylinder sleeves to help unshroud the valves and maximize airflow through the engine.

It’s worth noting that the LT2 breathes through exhaust headers that look like they were scavenged from an offshore racing boat, while the ZL1 wheezed through restrictive cast-iron manifolds. A set of headers on the old ZL1 would have easily closed the 23hp gap between the engines.

In other words, it really was true that there was no replacement for displacement. And a big, solid-lifter cam. And high compression.

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04 ZL1 Crank
A 4340-forged crankshaft with a 4.000-inch stroke gives the engine its 454ci displacement. The original ZL1’s stroke was 3.760 inches.

A Modern Tribute

In Michigan, Valley Performance set out to build a contemporized ZL1 as a more streetable tribute that would accommodate modern pump gas. They started with a modern ZL1 block and oval port heads from Chevrolet Performance, but the goal of greater streetability would ultimately mean a lower compression ratio and a “smaller” camshaft than the original engine—parameters that would give builders Jack Barna and John Lohone a challenge.

“We dropped the compression ratio down to about 10.4:1 and used a cam that would produce good manifold vacuum for an engine that was going to spend almost all of its time on the street,” Barna says. “The original ZL1, of course, was really a racing engine. It wasn’t designed for cruising around town. We weren’t building a racing engine.”

05 ZL1 Rods
Chamfering or “tear dropping” the edge of the oil feed passage holes in the crankshaft journals reduces the shearing of the oil flow as it enters the passage, promoting enhanced oil flow for the crank journals and rod bearings, which helps build up the oil-film cushion more quickly and ensures adequate oil flow to the rod bearings.

To overcome the deficit induced by the lower compression, Barna and Lohone took the same course of action Chevrolet did, starting in 1970, when the company knew it was faced with looming compression ratio cuts to satisfy forthcoming federal emissions regulations and unleaded fuel: They increased displacement. Chevy upped the big-block’s inches from 427 to 454 and so did Valley Performance. Heck, Chevy even did it in the early ’00s with their Ram Jet ZL1 crate engine. It was really a 454.

Sacrilege? It depends on how you look at it—and when you look at it, you can’t discern the displacement, while Valley Performance’s solution was the most pragmatic for achieving the engine’s performance goals, without the benefit of high compression.

06 ZL1 internals
Also in the interest of greater oil control, a lead-in groove was carved into the coated lower main bearing for the main oil feed to the main bearing, which enhances the time pressurized oil is flowing to the rod bearings by at least 10 percent, according to builder John Lohone.

“No, it’s not a 427, but there was more to the ZL1 than the displacement,” Barna says. “It was the aluminum block and heads and we’ve got that, along with induction from a single Holley four-barrel carburetor. The cubic inches may vary, but the spirit is the same.”

Another deviation from the original specs was the use of a hydraulic roller camshaft in place of the old-school mechanical flat-tappet design. Again, practicality won the argument, even at the expense of that unmistakably hostile, aggressive clatter of the solid lifters.

07 ZL1 torque Spec
The splayed four-bolt main caps are located with ARP studs and fasteners. The 16-degree splay of the outer fasteners is a design difference with the original ZL1 block, which had four-bolt mains but non-splayed. Also, the original block used iron caps, while the modern block uses billet steel caps.

“To be honest, there’s not really much point in building a street engine today with a mechanical flat-tappet camshaft,” says engine builder John Lohone. “Even if you set aside the greater driveability aspects of a hydraulic roller, it’s much easier to wipe out a solid-lifter flat-tappet camshaft these days. The hydraulic roller is absolutely the way to go.”

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The project engine’s cam specs include 0.578/0.617-inch lift, which compares very closely to the original ZL1’s 0.560/0.600-inch lift, while the 230/234 degrees duration at 0.050-inch is a less than the original’s estimated 250 degrees at 0.050-inch. That means a less overlap than the original ZL1’s camshaft, which would enhance low-end power production in the project engine, particularly torque.

08 ZL1 Clearance
To make sure every last component was in optimal operating condition, Valley Performance disassembled the engine’s brand-new oil pump and blueprinted it, confirming and adjusting clearances, as necessary, to ensure all of the tolerances were spot-on.

“Again, we were aiming for a strong street engine, first and foremost,” Lohone says. “The comparatively smaller camshaft was the just-right size for the rpm range at which this engine would spend most of its time.”

The dyno results, at Apex Competition Engines, confirmed as much. The engine made 500 lb-ft by 3,000 rpm and never dipped below that threshold until 5,500 rpm, peaking at 557 lb-ft at 4,700 rpm. As for horsepower, it matched the capability of the original ZL1, peaking at 527 hp at 5,100 rpm.

09 ZL1 Pistons
For the rods and pistons, it was Diamond forged slugs and Molnar H-beam rods. The rods are 6.405 inches long, or 0.270-inch longer than the standard Chevy big-block rod length. That extra length delivers a zero deck height with the head gaskets, while also improving the rod/stroke ratio to 1.60:1, which reduces side loading on the pistons.

“The important takeaway from the results is this engine makes the same power as the original, but with a vastly more streetable demeanor,” Lohone says. “It’s a pump-gas engine that’s happy at low speeds and there is no need for periodic lash adjustments. Just fire it up and hit the road.”

So, a little less compression and a little less duration, paired with a few more cubic inches and a contemporary valvetrain delivers a street-friendly, modern tribute to one of the most legendary engines of the muscle car era. Those solid lifters will be missed, but it’s difficult to argue with the results achieved with Valley Performance’s formula. ACP

10 ZL1 Piston Install
Along with the better rod/stroke ratio, the longer connecting rods push the wristpins higher on the pistons, which means the pistons themselves are a little shorter and consequently lighter—a combination that makes for a lighter reciprocating assembly with less friction, enabling the engine to rev a little quicker.
11 ZL1 Camshaft
In addition to the displacement difference, this modern ZL1 varies the most from the original with a hydraulic roller camshaft and complementing valvetrain. It’s a Howard’s camshaft, with 0.578/0.617-inch lift, 230/234 degrees duration, and a 110-degree lobe separation angle on a 108.5-degree centerline. It’s a comparatively mild cam, but one that still contributes to exceptional low-rpm torque production. The roller lifters are from Gaterman.
12 ZL1 timing chain
The camshaft and crankshaft are linked with a double-row timing chain.
13 ZL1 Head install
Next came the cylinder heads. They’re oval-port aluminum castings from Chevrolet Performance, with 110cc chambers and relatively large 290cc intake runners, but still not as big as the big rectangular ports on the original ZL1.
14 ZL1 Valvues
The heads came with good-sized 2.19/1.88-inch valves. Valley Performance honed the valve guides to spec (0.0016-inch intake and 0.0020-inch exhaust), performed a wet-flow valve job and some bowl work, port-matching the intake manifold and header ports—tasks designed to optimize airflow.
15 ZL1 coolant ports
To ensure good airflow velocity at lower engine speed, the intake runners were not enlarged, but the openings were cleaned up a bit to optimize the transition between the heads and intake manifold. After the valve job and other porting work the heads were bench-flowed at 0.600-inch lift to the tune of 325 cfm on the intake side and 241 cfm on the exhaust side. Not bad at all.
16 ZL1 Lifter Springs
Valvespring details include 1.540-inch-diameter, dual-coil springs, which were included with the assembled heads from Chevrolet. The pressure is 128 pounds at a 2.000-inch installed height and 347 pounds open at 1.382 inches.
17 ZL1 intake Gasket
After slipping in the Gaterman roller lifters, a set of 3/8-inch-diameter pushrods was installed, measuring 7.550-inch-long on the intake side and 8.660-inch-long on the exhaust side, each sized to optimize the rocker geometry.
18 ZL1 Rocker Arms
Rounding out the valvetrain is a set of Harland Sharp 1.7-ratio aluminum roller rocker arms. Unlike the original ZL1’s mechanical valvetrain, these will never need periodic lash adjustments.
19 ZL1 Intake Mani
Topping off the engine is another aluminum component from Chevrolet: a dual-plane intake manifold. It was selected for its street-driving advantages, with better off-idle performance and low-speed power production, compared to an open-plenum design that would have been used for early, full-race ZL1 engines.
20 ZL1 Carb
The carburetor is a Quick Fuel-modified Holley rated at 950 cfm. The Quick Fuel claim to fame is annular fuel boosters, which feature a number of very small discharge holes designed to optimize atomization, which can enhance low-speed responsiveness and performance.
21 ZL1 Dyno
On the dyno at Apex Competition Engines, a 20-minute warm-up with a light load helped break in the rings and identify tuning needs. The carb was very close right out of the box, requiring only one quick jet change. Timing was initially set at a conservative 32 degrees, but with the heads’ big combustion chambers, the engine welcomed much more aggressive timing.
22 ZL1 Custom
With the air/fuel ratio in check, total timing was finally bumped to 39 degrees and the engine responded with its best performance: 527 hp at 5,100 rpm and 557 lb-ft of torque at 4,700 rpm. That’s comparable to what the original ZL1 was capable of and, better still, the torque was strong all the way across the rpm band, with more than 500 lb-ft from 3,000-5,400 rpm.
3,000 502 286
3,100 509 300
3,200 516 314
3,300 523 328
3,400 528 342
3,500 531 354
3,600 532 364
3,700 530 374
3,800 530 383
3,900 531 394
4,000 533 406
4,100 537 419
4,200 542 434
4,300 547 448
4,400 551 461
4,500 554 475
4,600 556 487
4,700 557 498
4,800 556 508
4,900 554 517
5,000 549 523
5,100 542 527
5,200 532 527
5,300 520 525
5,400 503 517
5,500 483 506

Peak numbers in bold.

Apex Competition Engines
(989) 248-5005

Automotive Racing Products

Valley Performance
(616) 522-9710

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