392 HEMI®

If you thought big horsepower was going away, think again. Mopar® showed a 392 cubic inch 470 horsepower HEMI® V-8 at the recent SEMA Show. It was in the special-edition Dodge Challenger SRT8® 392. This 392 keeps the legendary engine displacement of Chrysler’s original FirePower 392 V-8 built during the 1957–58 model years. The engine is very different from the current Mopar 392 HEMI Crate Engine—beyond having a different bore and stroke.

Both the block and heads have significant differences from the crate, the 5.7L and the 6.1L engines—and the new 392 has variable cam timing, Multi-Displacement (MDS) and a new active intake manifold.

“It’s one of our most robust engines ever,” says SRT® engine design supervisor Kraig Courtney. “It’s pretty slick. Some of the things it’s using are directly from the 6.1, such as the exhaust manifolds, but we’ve done a lot of things internally to make it even more robust.”

The block has better bay-to-bay breathing to reduce pumping losses generated by piston movement. The block’s water jackets are larger and were designed to improve circulation around the bores and give better water delivery through the heads for better heat rejection. New oil squirters and new oil cooler reduce piston temperature, which allows for more aggressive calibration settings without causing abnormal combustion. 

The cylinder heads have a thicker deck for structure and larger water jackets that help keep the engine cool for lower stress, which improves component durability. The cylinder heads use a smooth core technology like that of the Viper. Casting cores use the same smooth FS90 sand as the big snake. The larger diameter valves have tried and true cooling technology: the exhausts are sodium-filled for temperature transfer.

The valves are controlled with ovate-wire beehive valve springs. “The “biggest baddest valve spring in production today”, says Courtney. “The loads we have to deal with are pretty big, so it’s a pretty monstrous spring. We were able to do that and keep away from the damper because dampers always have issues.

The underhood view is very different from that of the 6.1L engine. The first thing you see is the new active intake manifold. It’s a dark composite material—and there’s a new cold-air-induction system that takes air from down low.

The active intake manifold and the variable camshaft are the next steps in technology for the 392 HEMI. These two features allow Chrysler’s engineers to tailor the engine for a much-improved seat-of-the-pants driving experience. “Feel the torque, feel the power,” might be the slogan to describe the way this engine works.

The first 392 test engines had big power, “We were pretty confident that we could get 500 hp,” Courtney continued. “But the engine didn’t feel all that better than the 6.1 in the car. The protype 392 was strong up high, but as you accelerated it didn’t give you that kick in the butt we expected—a kick in the butt approaching that of the muscle cars of old. It didn’t come on until the RPMs came up and then the power was there. The first 6.4 (392) just wasn’t as strong feeling as the numbers said it should be.

“So we took a look at the intake that’s being used in our trucks right now, it’s an SRV dual-runner intake that we can run short runner or long runner. Obviously you run short runner for power and long runner for torque. So we made up some manifolds and ran them in a car and it felt a LOT better. Not as much horsepower, but the area under the torque curve was far bigger. So this car comes out of the hole really strong. Its launch is impressive. It’s got power on tap all the way up through the full range.

“It does 0-60 times with authority. We can go out and, quite honestly, take on some of the competition’s cars that are more powerful according to their numbers. We had a 6.1 that was modified by an aftermarket tuner and were running it off against the 392 and there was no real difference—we were staying right with them with our naturally aspirated 6.4 car.”

The 392 has the best horsepower-per-liter numbers of any cam-in-block two-valve out there, and this includes the LS-7. This is all done with fuel efficiency in mind. The combination of variable camshaft timing (VCD) and the MDS help the 392 perform at this 
new benchmark.

The VCD system allows camshaft movement of 37 crankshaft degrees to optimize power and torque dependent on the driver’s request as seen by the accelerator position. The camshaft is adjusted along with the intake runner length and the MDS system to give the optimum result.

“We can actually run on four cylinders for a lot of our highway mileage,” says Courtney. “That’s another reason for the SRV and the phasing cam. As we pick up torque, it can interestingly stay in 4-cylinder mode for a long time. So, once your car is moving and you are cruising down the highway it’s running on four cylinders, so you are getting some pretty astounding fuel economy.”

The engine is currently called a “392” in SRT applications—the 2011 Dodge Challenger, Dodge Charger and the Jeep® Grand Cherokee, but it might be seen as the 6.4 in other possible high-end applications. Nothing on the latter is known at present.

And all the improvements in block and head strength, and cooling suggest that this robust engine might be a candidate for supercharging. Courtney had no comment on that, but nevertheless an interesting thought.

This table provides a detailed comparison among the various HEMI engines—including the original 1957–58 392 Gen I.

Roger Meiners is a veteran automotive historian, writer and photographer with 35-years experience in the automotive industry.