Engine-pedia: all you need to know about the 1064hp LT7

1064hp. 5.5-litres. 4-cams. 8000rpm. The LT7 may not have a wildly inspiring name like “FIRETURKEY FREEDOM MISSILE” but it might be as good as it gets for raw V8 power from an American automobile manufacturer.

While it shares the familiar Chevy small-block (and LS and LT) bore spacing of 4.4in, this new LT7 V8 is a galaxy away from the GM pushrod engines we’re used to. It displaces 5.5-litres, runs DOHC heads and an 8000rpm redline, sports two turbochargers and one flat-plane crank, and is the most powerful production engine in America with 1064hp@7000rpm, and torque of 1355Nm@6000rpm.

The LT7 was actually developed as part of the Gemini programme meaning the LT7 is closely related to the 670hp LT6 used in the C8 Corvette Z06, which is the world’s most powerful NA V8. As they were both designed at the same time as part of the Gemini programme, the NA LT6 and TT LT7 share alloy block design, though the latter has extra machining to support turbo cooling and oiling. With a 104.25mm bore and a short 80mm stroke, the LT7 runs the same style of flat-plane crank as the LT6, but with machining on the counterweights to suit the new design of con rod used in the ZR1.

High-RPM performance and engine stability were key aspects of both Gemini engines. The engineers believe the fast-revving, high-RPM capabilities of a flat-plane crank engine give a much better performance driving experience than a cross-plane engine.

The crank counterweights were modified due to the LT7’s unique dished piston and shorter con rods, which also feature thicker gudgeon pins and fatter short-ends respectively. GM also dropped the compression on the LT7 from 12.5:1 down to a boost-friendly 9.8:1, while the engineers also pulled the redline back from the LT6’s 8400rpm down to 8000rpm. The dry sump oiling system is carried over from the LT6, but with seven scavenging stages as the low-mount turbos needed a scavenge port to ensure a constant supply of cooling oil.

Supplying the Go Sauce, the LT7 relies on PDI (as did the C7 ZR1). This is a combination of both Direct Injection as well as traditional port-EFI, which means there are 16 fuel injectors at play during wide-open throttle as the fuelling demands are far above what Direct Injection can handle.

Because DI systems aren’t easily upgraded to handle this type of power, the simplest solution is to plug an extra eight injectors into the traditional spot on the intake manifold. The PDI system idles on port injectors, brings the DI system on-board while driving, and then uses both when going flat-out, in similar style to the C7 ZR1’s supercharged LT5 V8.

GM engineers drew up new cylinder heads with larger, CNC-machined combustion chambers (59cc) and a fresh port design, while the valvetrain (including camshafts) was also redesigned with a boosted application in mind. Along with changing to 45mm titanium intake valves and 35mm Nimonic exhaust valves, the LT6’s radical intake was binned in favour of a more traditional plenum with twin 65mm DBW throttles, as the boosted application simply didn’t need the airflow trickery of the highly strung NA engine.

Though the first C4 ZR1 used a DOHC 5.7-litre LT5 and the C6 and C7 ZR1s featured supercharged V8s, GM’s King of the Hill has never run turbochargers thanks to packaging issues with the front-engine layout. This all changed with the mid-engined C8 platform.

Twin 76mm BorgWarner ball-bearing turbochargers feature electronically-operated wastegates and are mounted on integrated exhaust manifolds, as also found on Nissan’s R35 GT-R to improve boost response. The big snails feature “port-shrouding” which has been used on aftermarket turbos for years to improve boost response and stability, while the ZR1 also has “intelligent anti-lag” on the ECU tune that is meant to adapt to a driver’s style.

They push 20psi worth of atmosphere into the spicy 5.5-litre V8, but can push up to 24psi when heat loads rise, countering the effects of poor air density. Intake charge is cooled by a pair of water:air intercoolers mounted on top of the engine, which is also painted in a unique Edge Blue.

Initially Gemini engineers “only” wanted the LT7 to make 800hp but the very first dyno runs bested that figure with the wastegates held wide open, limiting boost to its lowest possible figure. While all LT7s will be hand-assembled by master engine builders at the Performance Build Centre in Bowling Green, Kentucky, the 1064hp small-block has passed GM’s 240,000km street-driven durability testing and carries a warranty.

To put the LT7’s performance into perspective, it bests the 1990 C4 ZR1’s 375hp at 2800rpm and is making more than the 2009 C6 ZR1’s 638hp by 4100rpm, and more than the C7 ZR1’s 755hp LT5 by 4750rpm. It makes more power than the Dodge Demon 170, but also the Bugatti Veyron, Ferrari LaFerrari, Koenigsegg Agera, Porsche 918, Mercedes-AMG ONE and McLaren Speedtail hypercars.

There is no word yet on whether the LT7 will end up as part of Chevrolet Performance’s range of crate engines, but previous ZR1 powerplants like the C6 LS9 and C7 LT5 have both been available aftermarket. The good news is LT6 and LT7 share their bellhousing pattern with other pushrod LT engines, which will greatly help with fitting a transmission suited to front-engine layouts.

CRANK IT UP

GM BRASS have repeatedly claimed the decision to equip the LT6 and LT7 engines with a flat-plane crankshaft down to wanting high-RPM fun. But I feel there is a lot more to it than just that.

Flat-plane cranks offer two great benefits to sports cars, by removing the need for counterweights, and fire cylinders in sequence on opposing banks. With a large bore and short-stroke this adds up to a high-RPM screamer of an engine, which is exactly what you want on a twisty road or race track.

Chevrolet’s decision to move the Corvette to a mid-engined platform was claimed to be because they’d done all they could with a front-engined platform. What wasn’t said was GM wanted to push the price tag for Corvettes up to help recoup the ever-increasing development costs, and this pushed the car into genuine supercar territory.

Moving to the mid-engined platform meant Chevrolet could then explore proper sports cars traits like a high-RPM, overhead cam, turbocharged drivetrain package, which wouldn’t have been as attractive to traditional Corvette buyers when the car had its pushrod V8 mounted next to the steering wheels.

The flat-plane set-up lend an aura of supercar credibility as Ferrari has used flat-plane V8s for decades, and the fast C8 ‘Vettes are gunning precisely for those cars. Chevrolet knows it needs a slice of the European supercar market to keep the Corvette alive and profitable.

 Glossary:

PDI: the combination of both port EFI and Direct Injection on one motor

Port-shroud turbos: precision holes in the compressor housing to allow overboost to bleed out, preventing turbo-damaging compressor surge

Gemini: GM’s internal code for the development of LT6 and LT7 engines, referencing the Gemini space program

Dry sump: high-end oil system which moves most of the oil to a remote-mounted tank, ensuring consistent oil pressure in a high-performance engine. Also requires several scavenging zones to pull oil out of the engine efficiently

Intercooler: a way to cool charge air to improve density and performance. Water:air intercooling uses coolant to chill the charge air, with the coolant pumped through cooling cores to reduce heat build-up. More complex than air:air intercooling, but is considered to offer superior resistance to heat soak

Charge air: intake air which has already been compressed, but hasn’t travelled past the throttle bodies

Wastegate: a valve on the turbine side of the turbocharger designed to bleed off excess boost pressure. Traditionally operated by a spring and engine vacuum, electronic versions use servos operated by the ECU

Nimonic: a superalloy high in nickel content used to make the exhaust valves. It was chosen due to its ability to withstand extreme temperatures the exhaust side of the motor will see