With the C6 ZR1, a new threshold of factory performance was achieved with its supercharged LS9 engine. Rated at 638 horsepower, it was the highest-output engine in a regular-production Corvette until the C7 Z06 was introduced with its 650-horsepower LT4 engine.
Both engines displace the same 6.2 liters, but the LT4 makes just that much more power with a smaller supercharger, an Eaton TVS-based 1.7L compressor (the LS9 uses a TVS-type 2.3L blower). Direct injection is the crucial distinguisher and enabler for the LT4’s higher output.
Direct injection enables more precise fuel control, because rather than introducing fuel in the intake manifold, where some of it washes against the intake runners, it’s introduced directly in the combustion chamber. And with carefully shaped piston domes that complement the shapes in the combustion chamber, a cleaner, leaner and optimized combustion of the mixture is the result.
On the green side of things, direct injection fosters greater fuel efficiency and lower emissions. On the performance side, it allows for a higher compression ratio, which boost horsepower. The LT4 boasts a high 10:1 compression ratio, which is nearly a full point higher than the LS9’s 9.1:1.
Tuning tricks and a conversion to E85 can help push the LT4 beyond 800 horsepower, but as Katech’s Jason Harding points out, that’s about the limit with the factory blower.
“It simply runs out of air,” he says. “The stock 1.7L supercharger can’t push enough air to safely support more than about 860 horses.”
The solution is comparatively simple: a bigger blower. Magnuson offers a TVS-based 2.3L supercharger that bolts right up to GM’s LT engines, including the LT4. It will push the engine to 900 horsepower, but for that final shove over the cliff, Katech can stretch the LT4’s displacement from 6.2L to 7.0L, or 427 cubic inches. With an E85 conversion and a set of ported heads, it’s a combo that gets real close to the magical 1,000-horsepower threshold with more than 1,000 lb-ft of torque.
But, there’s a catch … fueling the monster. It’s a bit trickier with direct injection, particularly with E85.
“It’s a balancing act,” says Harding. “You can’t just insert much larger injectors to compensate like you would with a port-injected engine.”
In the C7 ZR1’s LT5 engine, Chevrolet inserted a set of supplemental port injectors in the supercharger manifold. Upon demand, they kick in to keep the fuel curve fat, happy and detonation free. There’s an additional controller for them, too.
That’s not an option on the LT4 and its factory control system. To optimize the airflow capability of the Magnuson 2300 supercharger on the 7.0L engine, Katech teamed with upstart manufacturer Xtreme DI, which has developed add-on high-pressure fuel pumps to boost the delivery capability of the factory fuel system. For this combination, they increased it to about 3,600 psi (250 bar).
Crucially, the system adds fuel volume, thanks to Katech’s 30-percent higher-flowing injectors, while maintaining fuel pressure across the rpm band. That’s the biggie here, because direct injection engines require a lot of fuel pressure, something around 2,900 psi (200 bar) for the LT4 in stock form (compared to about 60 psi (4.1 bar) in a port-injected engine), and, generally speaking, fuel pressure tends to decrease as fuel volume is increased. It’s an even larger concern in forced-inducted engines because fuel demands increase dramatically as boost climbs and the fuel system has to keep up.
For this combination, they increased it to about 3,600 psi (250 bar). Along with Katech’s uprated injectors and the E85 conversion, they’ve taken the combination to an impressive 984 horsepower and 1,022 lb-ft of torque on an engine dyno.
“Maintaining the pressure differential for the injectors is critical to ensuring the necessary volume is delivered for each duty cycle,” says Xtreme DI’s founder and former Bosch engineer, Uwe Ostmann. “The misconception is that higher pressure will deliver more fuel. It won’t and that’s what’s important with this system, because it delivers more volume while simultaneously maintaining the necessary pressure.”
In many port-injected applications, tuners will boost-reference the fuel system 1:1, increasing fuel pressure one pound for every pound of boost. That doesn’t work in a direct injected engine because the fuel is being introduced with combustion pressure is very high and the fuel pressure has to overcome it.
There’s an additional challenge, too. The engine-mounted, engine-driven high-pressure direct injection fuel pumps on the LT engines are positive-displacement pumps, not electric pumps. That means their maximum output ramps up with engine speed. However, the TVS-type blowers are so good at building power at low rpm, a modified engine can quickly run out of fuel before the direct injection pump is up to max capacity.
With the already leaner-burning, high-compression design of the direct injection combustion system, inadequate pressure could quickly turn into detonation or burning a hole in a piston, which will happen much quicker under boost.
In a nutshell, the Xtreme DI supplemental pump is, like the factory direct injection pump, driven by the engine and is plumbed into one of the engine’s fuel rails. With Katech’s injectors, the fuel volume increases significantly, all without a drop-off in fuel pressure as engine speed increases and boost maxes out. And because the fuel pressure remains strong across the board, even at low rpm, where the blower is already shoveling in the air, there’s no need to limit boost at the peak torque level to prevent rotating assembly catastrophe.
An additional advantage here, too, is adaptability. There’s barely room under the hood of a C7 for an extra layer of Armor All, let alone a stand-alone auxiliary fuel pump, especially with the larger Magnuson blower in place. The Katech/Xtreme DI system tucks into the factory accessory drive system, taking up no extra space on the chassis.
“It’s perfect in that regard,” says Ostmann. “It’s totally integrated with the stock fuel rails and doesn’t intrude on other items in the engine compartment.”
We can’t, in good conscious, wrap this up without a word about the catch to the solution for the catch … and that’s the cost. As shown in our story, the fuel pump kit is around $5,300, while the set of upgraded injectors is nearly $3,500. Plus, you’ll need a few more items, including a serpentine belt (inexpensive) and an additional auxiliary pump to bolster the conventional in-tank fuel pump (expensive). All told, the fuel system enhancements will ring up more than $10,000 and that’s without the supercharger.
So, cheap it ain’t, but if you’re serious about building big LT power and doing it safely, this is the way to do it. As this engine combination has demonstrated, the payoff with direct injection is as great as the challenges surrounding the latest advancement in modern performance.
Technology finds a way. In fact, we’ve seen an even bolder LT engine experiment from Katech and Xtreme DI. Think LT5 and twin turbos.
But that’s another story. Vette
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