GM's 1064hp Vette

[Wild one]

What Makes the New Corvette ZR1's Engine So Powerful? An Engineer Explains​

We ask an automotive engine calibration engineer how Chevrolet’s new 5.5-liter LT7 V-8 makes more than 1,000 hp.
Greg BanishWriterChevroletPhotographerAug 14, 2024


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GM recently dropped the details on the new 2025 C8 ZR1. As an engineer who has worked in both OEM development and aftermarket performance, I’m nothing short of impressed. I spent the last quarter-century bouncing between walking the aisles of the Performance Racing Industry show and releasing engine calibrations that have been driven by millions of unsuspecting users around the world. I have seen lots of 1,000-hp engines. I have had lots of engines go to production after passing emissions, durability, NVH, and safety standards. I haven’t seen very many that did both. Sure, there’s the Bugatti ($3 million and 1,200 hp), the Aston Martin Valkyrie ($3 million and 1,000 hp), and the Ferrari SF90 ($540,000 and 986 hp, combined), but all of these are about as available as unicorn tears to mere mortals. Engines like the Hellephant make over 1,000 hp but have zero chance of passing emissions, so they’ll never be offered in legal roadgoing cars. What we have in the ZR1 is a real accomplishment, and a point where my two worlds seem to have collided.



I instantly got a ton of questions from friends on both sides of the industry. Most were along the lines of, “How did they do that?” To me, it’s not the engineering behind it that’s so surprising, but rather the management greenlighting such an impressive engine package. Let’s walk through some of the features to clear things up.


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1,064 HP—How Did We Get Here? ​

The LT6 engine in the C8 Z06 already makes 670 hp in naturally aspirated trim; the LT7 is a derivative of that engine, with boost added. We’ve done this a ton in the aftermarket, with both superchargers and turbos. My own LS3 (ancient technology in comparison) picked up about 200 hp over the base 390 hp when I added 7 psi from a pair of turbos. So, roughly half an atmosphere added 50 percent more power. Apply that math to the LT6, and you can see where I’m going here. Granted, GM reduced the static compression ratio from 12.5:1 down to 9.8:1, so we lose a little efficiency but gain knock resistance and reliability. Rumor has it that the first tests without the wastegates closed (meaning they would have only been running a few psi of boost) yielded about 830 hp without breaking a sweat. Knowing that they somehow got away with running 20 psi makes the 1,064 hp number no surprise at all.
We see the same split port (PFI) plus direct injection (DI) scheme used on the LT5 to get all the necessary fuel in there. These systems, made possible by the modern Bosch ECU family, have become common, and make it easy to support big power while still having precise in-cylinder injector control. There’s just enough DI contribution to get the knock suppression benefit needed to run pump gas with good combustion control while the PFI injectors jump in to deliver the required fuel mass at the highest loads. Ford does the same thing on many of the EcoBoost and Coyote engines as well. Having worked on emissions-legal boost with these, I’m definitely a fan of the solution.

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Getting the tailpipe emissions that clean requires a fundamentally clean-burning engine that can stay very close to the stoichiometric air-fuel balance, and a catalyst capable of reacting off any leftovers before they exit. Getting that catalyst to work requires that it be warmed up above 300 degrees Celsius very quickly to minimize cold start emissions. This is usually done by creating lots of exhaust gas heat immediately upon startup and directing it toward the catalyst brick.
The integral exhaust manifold and turbine housings are the first step here, reducing the amount of metal in contact with the exhaust gases so this heat can be passed along downstream rather than absorbed by the metal. The electronic wastegates can be commanded full-open at startup to provide an easier path to the catalyst rather than going through the turbines. After the turbines, GM used dual-wall construction on the downpipe leading to the catalyst inlet again. The thinner inside pipe again absorbs less heat during the critical startup phase while the thicker outside pipe carries the weight of the assembly.

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The selection of 67mm (huge, in OEM terms) turbine wheels is also a benefit here for emissions. Most smaller-displacement turbo engines on the market use relatively small turbines to help spool quickly, preventing lag and delivering low-end torque on demand. The downside to running small turbines is that they often start harnessing the power of the exhaust gases at very low engine speeds, stripping enthalpy (internal energy and pressure) from the gases to perform work on the compressor shaft. A lot of these small-engine programs have trouble keeping their catalysts warm at low speed because of the work being done by the turbos so early. With a healthy 5.5-liter engine, this really isn’t needed anymore, and the larger turbines that allow enough top end flow for 1,064 hp can be used. With over 400 lb-ft on tap before 2,000 rpm, nobody will miss the boost here. The side effect is that the gases going through the turbines aren’t forced to do as much work near idle, so they don’t lose as much heat or energy on their way to the catalyst. This lets the bricks warm up much quicker, like they would on an engine without the turbos, reducing cold start emissions.
Finally, the exceptionally good power-to-weight ratio means that the ZR1 engine doesn’t have to work very hard to meet the docile acceleration requirements of the emissions test traces. Such a favorable ratio means that emissions can remain the priority (rather than power enrichment or component protection) during the majority of driving conditions without the customer noticing.

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How Can the Catalysts Live at 1,000-Plus HP? ​

Read the internet forums and Facebook discussions and you’ll find no shortage of “experts” proclaiming that cats just won’t live with big horsepower. GM already knew how to make cats live at 755 hp on the C7 ZR1, and the new system really isn’t much different. GM still must control the temperature of the bricks (which often requires a significant amount of added fuel), but they get a helping hand from the turbos again. That drop in exhaust enthalpy we talked about earlier comes in on the helpful side here. Gases exiting the turbines are often about 300 degrees cooler after performing work on the turbine blades. This radical drop in temperatures makes life a lot easier on the catalysts at wide-open throttle (WOT). Most of the WOT fueling is there to control inlet temps to the turbine rather than the catalyst. GM’s use of exotic MAR alloy lets them tolerate about 1,040 degrees Celsius of inlet temperature without failing, where most turbo alloys and catalysts are limited to 950 degrees before failing. Dropping that 1,040-degree inlet temp down to around 800 degrees after the turbines goes a long way toward keeping the catalysts from melting. With a supercharged engine, the bricks see whatever gas temp would have been entering the turbines.

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See All 8 Photos

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[Wild one]

1,064 HP—How Did We Get Here? ​

The LT6 engine in the C8 Z06 already makes 670 hp in naturally aspirated trim; the LT7 is a derivative of that engine, with boost added. We’ve done this a ton in the aftermarket, with both superchargers and turbos. My own LS3 (ancient technology in comparison) picked up about 200 hp over the base 390 hp when I added 7 psi from a pair of turbos. So, roughly half an atmosphere added 50 percent more power. Apply that math to the LT6, and you can see where I’m going here. Granted, GM reduced the static compression ratio from 12.5:1 down to 9.8:1, so we lose a little efficiency but gain knock resistance and reliability. Rumor has it that the first tests without the wastegates closed (meaning they would have only been running a few psi of boost) yielded about 830 hp without breaking a sweat. Knowing that they somehow got away with running 20 psi makes the 1,064 hp number no surprise at all.

We see the same split port (PFI) plus direct injection (DI) scheme used on the LT5 to get all the necessary fuel in there. These systems, made possible by the modern Bosch ECU family, have become common, and make it easy to support big power while still having precise in-cylinder injector control. There’s just enough DI contribution to get the knock suppression benefit needed to run pump gas with good combustion control while the PFI injectors jump in to deliver the required fuel mass at the highest loads. Ford does the same thing on many of the EcoBoost and Coyote engines as well. Having worked on emissions-legal boost with these, I’m definitely a fan of the solution.

Did They Say 20 PSI or 24 PSI of Boost? ​

Both. At normal temperatures and barometric pressure, 1,064 hp can be had at 20 psi. If a customer runs the car in a warmer climate, or at higher altitude with lower inlet air density, the turbos have enough headroom to just spin harder and deliver the same air mass flow rate to overcome the inlet conditions. Running robust water-to-air charge coolers means that GM should also have consistent control of the final air inlet temperature to the intake valves.


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Why Turbos Instead of a Supercharger Like the LT4 and LS9 Use? ​

Any engineer who paid attention in his junior year should remember a class called Thermodynamics. It’s all about how much energy is available at certain pressures and temperatures. Exhaust gases have lots of both, and we waste most of it. Putting a turbine in the exhaust harnesses this unused energy to drive the compressor. Yes, the turbine adds some backpressure and pumping losses to the engine, but this still works out to be a much smaller number than the accessory load of your typical supercharger at full load. When we are talking triple-digit horsepower to drive a supercharger, moving to much less costly turbocharging looks tasty if you have the room for the hardware.


Earlier Corvettes just didn’t have the space for GM to package turbochargers close to the engine and still meet all their other engineering standards for clearance, heat control, and service. At the time, a tidy supercharger system nestled in the vee of the engine solved the problem with an acceptable amount of drive losses at WOT. I have installed turbo kits on older Corvettes. GM would cringe if they saw how tight things get. I was also left wanting for larger turbines as I hit the limits of the smaller turbines. With the engine moved to the rear, where there’s tons of room, packaging the hot turbos becomes an option without the usual vetoes from the vehicle team. Making boost using exhaust energy helps increase the net horsepower when all is said and done. Doing that with 67mm turbine wheels instead of the 56–60 mm wheels I had to use previously also helps extend the top end significantly.


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Why 'Only' 828 Lb-Ft of Torque? ​

You see that pool table of a torque curve from 3,000 and 7,000 rpm? That’s not an accident. This is almost always driven by component limits, usually from the piston manufacturer. The crowns and ring lands are specified to withstand a target cylinder pressure, usually something on the order of 120 bar (1700-plus psi) for the typical production turbo engine, before they start denying warranty claims from overloading. In the press release, GM stated 11 megapascals (MPa, or around 110 bar), so this is right in line with expectations and leaves some safety margin. Racing pistons routinely see higher loads, but over a much shorter lifespan. The height of this torque table is likely a direct representation of how much cylinder pressure GM is willing to tolerate inside the LT7. They’re just able to generate and hold that pressure across a wide rpm range. If we were to watch a trace of manifold pressure across the rpm, we’d likely see how GM is modulating the boost to get a consistent cylinder fill that results in this flat torque delivery. This is common practice in lots of other turbocharged engines, albeit at much lower levels.


Sure, they could make more torque in the midrange, but with 828 lb-ft going into the transmission, we begin to worry about input shafts, gears, output shafts, ring-and-pinion sets, halfshafts, and, oh yeah, the poor tires. The transmission control module (TCM) can change gears at lightning speed, so it can select a different ratio with more gear multiplication of the input torque in the blink of an eye, if we haven’t somehow already overwhelmed the available traction.

On the low end, the torque curve is also tied to compressor capability. GM wants to avoid compressor surge, where the turbos try to make too much boost at low engine speed, because this can damage the compressor wheels over time. They ride the “surge line” of the compressor map until total flow is high enough to deliver the desired boost pressure. That said, it still offers 400 lb-ft before 2,000 rpm, which is not bad.


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How Can This Pass Emissions? ​

None of this horsepower matters in a production environment if the engine (and vehicle) can’t pass emissions. With the new LEV4 emissions standard requirements on our doorstep, the ZR1 must fit into that envelope somewhere. This means it must at least be under 0.070 grams of combined NMOG and NOx emissions on the familiar FTP75 urban drive cycle, among other requirements. This is no small feat, as this concentration is cleaner than the background samples present in some big cities.

Getting the tailpipe emissions that clean requires a fundamentally clean-burning engine that can stay very close to the stoichiometric air-fuel balance, and a catalyst capable of reacting off any leftovers before they exit. Getting that catalyst to work requires that it be warmed up above 300 degrees Celsius very quickly to minimize cold start emissions. This is usually done by creating lots of exhaust gas heat immediately upon startup and directing it toward the catalyst brick.

The integral exhaust manifold and turbine housings are the first step here, reducing the amount of metal in contact with the exhaust gases so this heat can be passed along downstream rather than absorbed by the metal. The electronic wastegates can be commanded full-open at startup to provide an easier path to the catalyst rather than going through the turbines. After the turbines, GM used dual-wall construction on the downpipe leading to the catalyst inlet again. The thinner inside pipe again absorbs less heat during the critical startup phase while the thicker outside pipe carries the weight of the assembly.


See All 8 Photos



See All 8 Photos

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[Wild one]

The selection of 67mm (huge, in OEM terms) turbine wheels is also a benefit here for emissions. Most smaller-displacement turbo engines on the market use relatively small turbines to help spool quickly, preventing lag and delivering low-end torque on demand. The downside to running small turbines is that they often start harnessing the power of the exhaust gases at very low engine speeds, stripping enthalpy (internal energy and pressure) from the gases to perform work on the compressor shaft. A lot of these small-engine programs have trouble keeping their catalysts warm at low speed because of the work being done by the turbos so early. With a healthy 5.5-liter engine, this really isn’t needed anymore, and the larger turbines that allow enough top end flow for 1,064 hp can be used. With over 400 lb-ft on tap before 2,000 rpm, nobody will miss the boost here. The side effect is that the gases going through the turbines aren’t forced to do as much work near idle, so they don’t lose as much heat or energy on their way to the catalyst. This lets the bricks warm up much quicker, like they would on an engine without the turbos, reducing cold start emissions.

Finally, the exceptionally good power-to-weight ratio means that the ZR1 engine doesn’t have to work very hard to meet the docile acceleration requirements of the emissions test traces. Such a favorable ratio means that emissions can remain the priority (rather than power enrichment or component protection) during the majority of driving conditions without the customer noticing.


See All 8 Photos

How Can the Catalysts Live at 1,000-Plus HP? ​

Read the internet forums and Facebook discussions and you’ll find no shortage of “experts” proclaiming that cats just won’t live with big horsepower. GM already knew how to make cats live at 755 hp on the C7 ZR1, and the new system really isn’t much different. GM still must control the temperature of the bricks (which often requires a significant amount of added fuel), but they get a helping hand from the turbos again. That drop in exhaust enthalpy we talked about earlier comes in on the helpful side here. Gases exiting the turbines are often about 300 degrees cooler after performing work on the turbine blades. This radical drop in temperatures makes life a lot easier on the catalysts at wide-open throttle (WOT). Most of the WOT fueling is there to control inlet temps to the turbine rather than the catalyst. GM’s use of exotic MAR alloy lets them tolerate about 1,040 degrees Celsius of inlet temperature without failing, where most turbo alloys and catalysts are limited to 950 degrees before failing. Dropping that 1,040-degree inlet temp down to around 800 degrees after the turbines goes a long way toward keeping the catalysts from melting. With a supercharged engine, the bricks see whatever gas temp would have been entering the turbines.

How Do You Make a Car Like This Faster? ​

Let’s be honest—the tires are the limiting factor on available acceleration until well past “go-to-jail speed.” This car could make 2,000 hp and it wouldn’t be much faster to 60 mph with the same two tires pushing. It’s the traction control system that is really in charge here, not the available input power to the transmission now. Adding slicks or drag radials will almost certainly speed things up, with the risk shifting to the transmission and clutch. Accelerating quicker below 100 mph leaves us with either shedding weight (good luck—GM already used lots of carbon fiber) or utilizing the front tires too. We already saw how quickly the E-Ray launches, and we see a big, empty spot up front in the ZR1. It’s the worst-kept secret where GM is going on the next step.


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What Would I Change First? ​

This is the fun part. After decades of getting paid to pump up the performance of so many vehicles, I've finally found one that makes me question the need. Instead of raising the maximum possible power, I would look at ways to keep the power at the optimum without falling off as things heat up.

It remains to be seen how much cooling capacity there is in those two water-air intercoolers and their front heat exchangers. Every water-air intercooler system I have worked on seemed to benefit from efforts to reduce the water temp. More exotic front heat exchangers or an A/C chiller retrofit would be my choice here. Then you can just let the factory ECU see the cooler temps and adjust accordingly.

Sure, it’s calibrated to run on premium pump gas, but I can’t help but wonder what happens if you accidentally put some ethanol in the tank. My testing has shown that most modern vehicles can run about 35 to 40 percent ethanol without tripping fuel trim errors in the ECU. This extra ethanol really helps with both knock-suppression and cooling. At 20 psi, I’d wager there’s a benefit here.

If drag racing is your thing, sticky tires will almost certainly help too. Just don’t be surprised when 1,064 hp helps find the limits of other driveline parts when you dead hook. Given time, the aftermarket will respond here too.

 

[BossHogg]

I'm wondering how the driveline is going to survive. Also, how do they plan to hook the tires in with that much power? Torque management I suppose.