A 427 engine is nearly as synonymous with the Corvette as fiberglass. Decades ago, a variety of 427s powered the most powerful models of the factory supercar era, while in the past decade or so, the 427-cubic-inch LS7 helped bestow instant legend status on the C6 Z06.
Chevrolet no longer offers a 427 in the Corvette, as even the latest, greatest Z06 uses a forced-induction 376-cube version—dubbed LT4—of the Gen V small-block family. And while there’re irrefutable thrills that come with a supercharged engine, the adage that there’s no replacement for displacement is equally valid.
In fact, for owners likely to put their Corvette through its paces in track day events or even on a more structured racing schedule, a naturally aspirated engine is often preferred over forced induction. That’s because the intrinsic heat associated with supercharging or turbocharging can affect the consistency of performance.
Longtime Corvette racing and street-performance engine builder Katech has had more than a little experience developing track-focused engines, particularly those based on the LS7. They’ve recently transferred that knowledge to the C7’s naturally aspirated LT1 engine, stretching its capability figuratively and literally. And with right around 700 horsepower, it exceeds the LT4 in maximum output.
“A supercharger is a great tool for building power and the LT4 in the Corvette Z06 does it beautifully, but this engine is designed for those who want a high-output naturally aspirated engine,” says Jason Harding, Katech’s Director of Aftermarket Operations. “As we continue to explore the capabilities of the LT1 engine, there’s more we’re getting out of it.”
Katech has been grinding away on LT1 performance since the engine’s introduction, including an early iteration of the 427 combination. They’ve evolved the design, first and foremost by changing the bore and stroke dimensions. Previously, Katech had dropped a 4.100-inch stroker crank into a slightly honed stock block, but the result was pistons dropping a little below the bottom of the cylinders, which contributed to more blow-by than Katech was comfortable with. The solution was changing to the LS7’s 4.125-inch bore and 4.000-inch stroke dimensions, which shortened the pistons’ respective journeys.
The rub in that plan was the fact that the diameter of the iron cylinder liners within the LT1’s aluminum block was only 4.06 inches. Stretching the bores as far as needed required cutting out the stock sleeves, boring out the block slightly and re-sleeving it. A time-consuming and admittedly costly procedure, but Harding says the results were worth it.
“The 4.000-inch stroke allows for a better piston design and reduced blow-by,” he says. “Our first the 4.070-inch x 4.100-inch engine didn’t meet Katech’s blow-by standards, so we went back to the drawing board.”
Katech has also swapped the stock intake for an MSD Atomic AirForce manifold and capped it with one of their new 103mm throttle bodies.
“The new intake-and-throttle body combination gave us the airflow we needed to push over 700 horsepower,” says Harding. “It’s a must to make this kind of naturally aspirated power.”
What Katech didn’t mess with was the general configuration of the combustion system. They used ported heads, but the combustion chambers and the unique topography of the factory piston head design are left essentially untouched to ensure compatibility with the direct-injection fuel system. It relies on the shape of the piston head to direct the fuel spray from the injector. GM spent zillions of hours getting it just right and altering it can cause significant performance and driveability issues.
The engine also retains cam phasing, but it’s modified via tuning and a physical limiter to work with Katech’s proprietary cam specs; the numbers of which we couldn’t pry out of them. Nevertheless, the dyno performance and overall tractability of the engine suggested they nailed the grind just right. Harding did tell us that the lobe profile was very similar to their popular K501 for the LS7, which is very driveable on the street. Katech expects comparable or better driveability on the LT1 due to direct injection.
The 427 LT1 also features Katech’s new dry-sump oil pan made exclusively for them by ARE, which you can see in the inset photo on the lead page. Note the secondary scavenge pump mounted on the engine. The stock internal scavenge and pressure pump remains, but the secondary external dry-sump pump pulls oil out of the crankcase and into the dry-sump tank very quickly. The external pump is also more powerful than the production gerotor (Gerotor is a positive displacement pump derived from “Generated Rotor.”) scavenge pump and it’s able to pull a significant vacuum on the crankcase, which helps ring sealing and oil control in a track-focused engine. Katech will sell the 427 LT1 with the factory dry-sump system for street cars and with the enhanced dry-sump system for track cars.
And while dropping this engine in a C7 Stingray is a no-brainer, it also sounds like an even better choice for the new Grand Sport. Its Z06-derived chassis package and this 700hp naturally aspirated 427 sound like the ingredients for road course, or freeway, supremacy.
Just as it has always been when mixing 427s and Corvettes.
01. The Katech 427 LT1 starts with the stock block as its foundation, but its iron cylinder sleeves stretch to only 4.06 inches in diameter. To accommodate the 4.125x4.000 LS7-spec bore and stroke dimensions, those sleeves are cut out of the block and the block itself is bored slightly to accommodate larger-diameter sleeves.
02. Made of centrifugally cast ductile iron, the replacement sleeves offer a tensile strength of around 120 ksi versus the approximately 30 ksi tensile strength of the conventional gray cast iron used in most OE engines. Ductile iron is also relatively plastic, meaning it flexes more than comparatively brittle cast iron, which helps prevent cracking of the sleeves.
03. All engines in the “LT” family feature standard oil-spray piston cooling that drenches the underside of the pistons and cylinder walls to help maintain optimal combustion temperatures and increase durability. Unfortunately, the stock oil jet design (top) doesn’t mesh with Katech’s stroker package so they developed custom jets based on versions from their high-performance LS7 engines.
04. Katech uses a rigid Callies 4340-forged steel crankshaft with a 4.000-inch stroke to complement the bigger bores to achieve the 427-cubic-inch displacement. The slightly shorter stroke than the company’s previous 427 package helps enhance refinement and reduce blow-by by keeping more of the piston within the cylinders at the bottom of the stroke.
05. The cross-bolted nodular iron main caps are tightened down with factory torque-to-yield fasteners that provide excellent clamping strength and location accuracy.
06. Thanks to the standard variable valve timing system, achieving significant power gains in an LT1 engine is much tougher with a basic camshaft swap. Katech uses one based on their street/track “K501” grind, but would rather keep mum on its specs. It’s used with a physical phasing limiter in addition to boundaries dialed in via tuning.
07. The pistons are an exclusive forged aluminum design made for Katech by Diamond Pistons. They retain the unique head topography of the originals—note the unique “scoop” in the center—because it’s essential for optimal combustion with the direct injection fuel system. The compression ratio for the stroker 427, however, is raised from the stock 11.5:1 to 12.5:1.
08. Forged Callies Compstar H-beam connecting rods provide greater strength than the factory rods and feature the stock 6.125-inch length, giving the engine a rod/stroke ratio of 1.53:1—a more optimal ratio than the previous 427 combination’s 1.49:1.
09. Compared to previous LS engines, the LT1 has a unique windage tray design with an oil scraper designed to improve oil flow control and bay-to-bay breathing. It requires a bit of clearance enhancement to prevent interference with the connecting rods.
10. Camshaft degreeing is vitally important in an engine with variable valve timing, and Katech uses this super-trick, electronically measuring apparatus to do the job. Katech says they spent countless hours playing with the camshaft phasing system to learn what the engine likes the best—and they’re keeping their findings close to their chest.
11. Cam phasing is retained on the 427 LT1 but the standard Active Fuel Management (cylinder deactivation) system is not, requiring blocking off the oil feed provisions for the stock “collapsible” hydraulic lifters and replacing the lifters with conventional hydraulic versions.
12. Katech has also developed a new valley block-off plate to replace the stock version, which has provisions for Active Fuel Management. The new aluminum plate also serves as the mounting pad for the direct injection system’s camshaft-driven high-pressure fuel pump.
13. The LT1 cylinder head design represents another significant departure from the LS architecture. Katech says their testing shows it generally flows better than an LS3 head, but not quite as well as an LS7 head. The rectangular intake ports are somewhat straighter than an LS3’s ports, but with a slight twist to enhance mixture motion.
14. Katech CNC-ports the intake and exhaust ports in-house to enhance airflow for the greater needs of the larger-displacement 427. It is a delicate operation because it is easy to break through the walls of the intake ports into the mounting holes of rocker arms.
15. Compared to the LS3 cylinder head design, the LT1 head features a smaller, 59cc combustion chamber designed to complement the volume of the piston’s dish—and consequently, machining the chambers is not a good idea. Doing so jacks with the direct injection system’s finely tuned atomization, which could severely affect performance.
16. On go the heads. Katech uses ARP head studs in place of the factory cylinder head bolts. Note the raised position of the intake ports, compared to an LS head, and the holes below the lower-right corner of the ports. They aren’t mounting pads for the intake manifold, but the locations for the fuel injectors, which protrude directly into the combustion chamber.
17. Katech upgrades the stock beehive springs to lightweight PSI LS1511 384-lb./in. springs to accommodate the new camshaft. They’re used with the stock rocker arms. Because of the reversed position of the valves on an LT1 engine, compared to the LS family, there are no longer offset intake-side rockers.
18. The stock fuel rails are installed next and connected to the engine-driven high-pressure fuel pump, which sends fuel to the injectors at around 2,175 psi. The fuel pressure for a conventional port-injected engine is around 60 psi and a carburetor is about 8 psi. The direct injection’s engine-mounted pump supports the traditional in-tank fuel pump.
19. Katech modifies the stock LT1 injector, achieving a 25 percent greater flow rate to feed the big-inch LT1. The direct injection injectors are like the Piezo-type injectors used in modern diesel engines for incredibly precise fuel control.
20. The stock intake manifold is replaced with MSD’s new Atomic AirForce two-piece manifold, which Katech says has proven to be worth up to 30 hp at high rpm when used with a high-flow throttle body. The two-piece design allows the internal runners to be modified.
21. Katech’s new 103mm throttle body complements the high-flow MSD intake manifold, offering the airflow capability required to feed the large-displacement LT1 at high rpm. Its opening is nearly 20 percent larger than the stock 87mm throttle body.
22. Dressing up the final engine assembly is Katech’s LT-family valve covers and coil relocation brackets. It’s a combination that looks light years better than the stock valve covers and coils (as shown in the dyno test image).
23. The completed engine was tuned with the E92 controller for a Corvette Stingray. It has provided another learning curve for the performance industry over previous LS engine performance because it represents a Bosch platform and more engine systems, such as oiling, are directed by it.
24. On the dyno, the Katech 427 LT1 delivered 701 horsepower at 6,800 rpm and 625 lb-ft of torque at 5,200 rpm. The high-rpm capability is astounding, but just as important is torque production. From 3,000 rpm until the tachometer needle breaks off, the engine produces no less than 520 lb-ft—grunt that will quickly erase any second thoughts about not selecting a supercharger.
Katech Track Attack: Indulging the Breeding
Border collies and other herding breeds tend to get a little crazy when they’re cooped up. It’s in their DNA to work and many owners let them exercise herding sheep on the weekend, relieving pent-up desires that are simply in their blood.
It’s pretty much the same thing with Corvettes, and once a year Katech lets owners indulge themselves and make the most of their cars’ capabilities. It’s called Track Attack, and for the 2016 edition the event moved to the new M1 Concourse (www.m1concourse.com), along famed Woodward Avenue, north of Detroit. It’s a “car condo” complex, providing specially designed storage facilities for auto storage. There is also a 1.5-mile road course snaking around the property.
“It’s a great facility to put your Corvette through its paces,” said Katech’s Jason Harding. “These cars were designed for this very environment and you come away with a deeper respect for what they can do—and how fun they are to drive.”
For novices and moderately experienced drivers, instructors are on hand to teach the basics of vehicle control, track etiquette and more. There are typically a few Corvette luminaries walking the pits and making laps, too. This year, we spotted former GM/Corvette engineer and champion racer John Heinricy and Corvette Racing driver Andy Pilgrim. In fact, you can check out a video of Pilgrim ripping around the M1 track in a C7 Stingray powered by the very 427 engine in our primary story here: www.youtube.com/watch?v=a7iEbrKx_qY.
Track Attack and other similar high-performance track day events allow owners to exercise their Corvettes properly. It’s what they were bred to do.