Video: On The Dyno With Edelbrock’s Gen-III Hemi Cylinder Heads

When it comes to engine swapping, one of the lesser-seen modern powerplants is the Gen-III Hemi. A lot of us have knocked around the idea of playing with the “new” Hemi engine, but it has really come down to the fact that besides the insane availability of other platforms, there is a pretty sizeable cost difference with the Hemi. On the scale of cost, the Hemi leans more towards the Coyote than the LS.

However, Prestige Motorsports has decided that the Gen-III Hemi is going to be its next big endeavor. No stranger to the Gen-II Hemi platform, Prestige has a wide variety of early-Hemi experience under its belt, and is now moving into the modern version. Luckily for us, our friend Jeff Huneycutt of The Horsepower Monster was on hand to record this first round of testing.

The factory 5.7-liter Gen-III Hemi block was opened up .010 inch, but the Eagle crankshaft added over half-an-inch of stroke to bring displacement up to 392 cubes (or 6.4-liter).

To start with, Prestige took an older 5.7-liter Hemi and remachined the entire block. In that machining, the bores were opened up .010 inch to 3.927 inches from stock. Then, an Eagle 4.050-inch stroker crankshaft was added for a little over half an inch of extra stroke in the combination, for a total displacement of 392 cubic inches. While it shares it’s displacement with the factory 6.4-liter Gen-III Hemi, they are achieved via different methods.

Attached to the crankshaft are a set of Eagle ESP 6.125-inch forged H-beam rods with a set of custom forged DSS Racing pistons hanging off the ends. They feature an 8.0cc dome and a slipper-skirt design to shed weight from the normally heavy design.

For the camshaft, Prestige chose a mild cam that you would likely find on the street since that’s the market they are looking to enter. It’s a COMP Cams hydraulic-roller with 230 degrees intake and 240 degrees of exhaust duration at .050-inch, and a 113-degree lobe separation angle. With the factory rocker ratio of 1.64:1, the cam has .603 inch of lift on the intake side and .597 on the exhaust.

Forged H-beam Eagle ESP rods combined with forged DSS pistons make up the bulk of the rotating assembly. The DSS pistons feature an 8cc dome to bring compression up to 10.9:1, while weighing only a fraction of the OEM slugs.

Topping the short-block off for the first round of tests are a set of completely factory 5.7-liter Hemi cylinder heads (not the Eagle head). The factory 2.00-inch intake valve and 1.55-inch exhaust valves sit in the factory 83cc chambers, making for a 10.9:1 compression ratio. The factory intake and exhaust runners are completely untouched. New beehive valvesprings were added to the factory heads in order to accommodate the increased valve lift in the camshaft.

The heads are fed by a Holley Sniper EFI sheetmetal intake manifold with a 92mm Sniper EFI throttle body. The ignition coils have been upgraded to the MSD units, and 42 lb/hr fuel injectors fitted into the ports, both controlled by a Holley Terminator-X ECU. Hooker 1-7/8-inch primary headers will be used for both sets of cylinder heads.

On the baseline sweep with the stock cylinder heads, the engine was run from 3,500 rpm to 6,300 rpm. Peak numbers were 496.5 horsepower at 6,000 rpm and 463.3lb-ft of torque at 4,600 rpm – both of which are the same RPM peaks as the stock 5.7. There was also an airflow monitor on the intake pipe, which registered a peak ingestion of 687.4 cfm.

The Edelbrock cylinder heads used in the test feature CNC machined combustion chambers which measure the same 83cc as stock (with smaller options available) and house significantly larger 2.165-inch intake and 1.650-inch exhaust valves. The intake port is significantly larger than stock at 202cc and is as-cast except for the initial feed in.

At that point, the stock heads were pulled off and the new units fitted. The all-new castings from Edelbrock are in the RPM line of heads. The CNC machined chambers are 83cc, just like the stockers, so there will be no change in compression, However, the chambers are much more oval than the factory pieces and the valves are significantly larger, using a 2.165-inch intake and 1.650-inch exhaust valve, along with a 202cc intake runner. The springs that come with the head were swapped for a set of dual valvesprings in order to handle a little more RPM.

The second round of testing saw a slight increase in the RPM of the pull, starting at the same 3,500 rpm but pulling to 6,600 rpm this time. With the graphs on top of one another, there was a clear gain above 4,700 rpm, but surprisingly there was a slight loss of power below that point. However, the horsepower peak increased significantly climbing to 556.3 horsepower at 6,400 rpm. The torque peak also occurred significantly further out at 5,600 rpm and picked up 16.7 lb-ft for a peak of 479.6 lb-ft.

The airflow curve matches the power curve, with the slight dip prior to 4,700 rpm, which Huneycutt hypothesizes is the smaller ports are maintaining velocity in the lower RPM range before the sheer flow of the larger ports in the Edelbrock head starts outperforming the stock head. It’s not hard to make the small sacrifice on the bottom for the huge benefit up top, which will only be further increased with a more aggressive camshaft or forced-induction.

As you can see from the graphs, even with the small losses below 4,600 rpm, the torque curve extends significantly beyond the factory cylinder head, and the horsepower curve really outperforms the stock heads. As you can see from the airflow graphs, the Edelbrock pieces flow significantly more air.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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