gen III hemi build


The Grumpy Grease Monkey mechanical engineer.
Staff member

Gen III Hemi – Modern Power Maker
Written by Robert McGaffin on February 6, 2015
Steve Dulcich - writer; Johnny Hunkins - photographer;

Get 600-plus streetable horsepower from a stroked Gen III Hemi 5.7L truck motor.

Hemi has always been synonymous with Mopar power, and fortunately the legacy built with the early Hemi and the 426 lives on in today’s Gen III. While the first generation of Chrysler Hemi was reserved for higher-end applications, and the 426 served in only the most exotic of muscle machines, the Gen III breaks the mold as everyman’s Hemi. Trucks to regular passenger cars, the Gen III is a volume piece that takes the Hemi out of the elitists’ realm. Now for the upshot—for brutal street power this latest Hemi is arguably the best of them all.

The massive power potential is inherent in the Hemi’s opposed-valve design, allowing space for large, free-breathing ports…

The massive power potential is inherent in the Hemi’s opposed-valve design, allowing space for large, free-breathing ports with a direct shot into the cylinder. The production iron 426 Hemi heads were cavernous, and delivered approximately 300 cfm of intake airflow—a stout number for that era. The latest third-generation Hemi heads easily eclipse the stock 426 Hemi’s airflow. On top of the airflow, these modern heads come stock with fast-burning twin-plug chambers, a shallower valve angle for greatly reduced combustion chamber volume, and twin active quench pads adjacent to the valves. The engineers did their homework here, taking the two-valve Hemi layout to what may be its optimal form.

Where the Gen III really breaks new ground is in availability. You won’t find a 426 at the local boneyard, but regular mom and pop trucks and passenger cars have been built with the Gen III for over a decade now, making the used pipeline flush with Hemi power. Showing the potential for Hemi power is this 402-cube stroked example built by Daniel Boshears. Starting as nothing more exotic than a common, early-version 5.7-liter pickup truck engine, and using production cylinder head castings, this street beast delivers 570 lb-ft of tire melting torque and 613 horsepower. Here is what went into making it happen.

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…regular mom and pop trucks and passenger cars have been built with the Gen III for over a decade now, making the used pipeline flush with Hemi power.

Bottom End Build

This project actually started with a couple of different complete 5.7 engine cores, a 2006 and a 2010. As Daniel tells us, “I was going to build the 2010 [Eagle] engine because it was a very low mileage salvage yard engine, but the variable valve timing set up made it difficult to get a cam core for this application. I went with the earlier 2006 engine for the block because of this. We just did basic machining to the block, including tightening the mains to the lower side of tolerance, and some minor clearancing to the bottom of the cylinders for the stroker crank. The block required oiling system modifications because of the factory MDS system. We bored the block out .018-inch over, which happens to put it at the same size as a .060-inch overbore 283 Chevy small-block and there are a lot of good rings available at that size. I was able to get a 1mm, 1.2mm, 3mm ring package at that bore size which is a very low-drag set-up. I had Race Tec build a set of custom pistons for this combination with a 4.125-inch stroke.”

“I was going to build the 2010 engine because it was a very low mileage salvage yard engine, but the variable valve timing set up made it difficult…”—Daniel Boshears

Daniel continued, “The crank is from Molnar, but it started life as their 4.050-inch stroke crank, and I had it offset ground by Adney Brown at Performance Crankshaft to 4.125-inch stroke. We used a Quad Four GM journal size and Molnar offers a rod with that journal size and the Mopar 6.123-inch length. This is a widely used bearing size in endurance racing applications, so good performance bearings are readily available. The final combination ended up with a very light 1,663-gram bobweight. One area where we did run into trouble was clearance with reluctor wheel. With the 4.125-inch stroke, the piston will need to be modified to clear. We grooved the piston and also machined the inside edge of the wheel 0.050-inch to get it to clear.”

The oiling system in this engine was very straightforward, relying on stock replacement style parts. Boshears equipped the Hemi with a Melling Select standard-volume oil pump drawing oil from a stock truck pan though a modified stock pickup tube. With the stroker crank, a factory 6.4L windage tray offers improved clearance, requiring only minor modifications to fit. Daniel observed that the factory Gen III is well designed as far as controlling oil drainback, requiring no reworking or improvements to keep excessive oil from the rotating assembly. As an extra measure of friction reduction and insurance, Daniel had all of the moving parts of the engine treated to the proprietary RF85 antifriction process.

Top End, Cam, & Valvetrain

Traditional Mopar small-blocks need high-end race heads to make power in the range of this Hemi. In the case of the Gen III, there is no need to look any further than the production castings. As Daniel described his cylinder head combination, “The cylinder head is the factory production 2009-10 5.7 truck head. What we learned about this particular head is that it has the 6.1L sized intake port, a D-shaped exhaust port, and a 5.7-style combustion chamber with dual quench pads. The heads use a little bit longer valve than what is regularly available, so aftermarket valves choices are limited. We ended up using custom made valves, going up to 2.100- and 1.57-inch diameters to get a good valve job on the heads without having to change out the seats.”

Port work on the cylinder heads was a combination of CNC machining and custom hand porting by Randy Malik. Daniel explains, “The heads were CNC ported by Modern Cylinder Heads to the extent of the runners, and the rest was done by hand. Randy Malik finished the heads by hand doing the bowl work and the tight corner work such as the short turn. In the end we did not get final flow numbers on the heads, but looking at comparable port work that was very close to these in design, we are in about the 350-360 cfm range.” To put that flow level in perspective, only the very best of the old Mopar LA-series small-block race heads move a comparable amount of air.Pullquote:

To put that flow level in perspective, only the very best of the old Mopar LA-series small-block race heads move a comparable amount of air.

The camshaft is a custom hydraulic roller from Lunati. As Daniel tells us, “I worked with Lunati because they have been very good to me in the past on all of my business. They have some very good lobes that Harold Brookshire designed, and we looked through those and found the lobes that we really liked. The camshaft is actually a reverse pattern with the intake having slightly more duration than the exhaust. The Hemi tends to like this configuration. The lobe separation is 109 degrees, and that is just because the cam core would not allow anything any tighter than that without risking breaking through the hard case. We installed the cam at 103, and this too was limited by tolerance, since the piston-to-valve clearance wouldn’t allow us to go any tighter than that.”

“The camshaft is actually a reverse pattern with the intake having slightly more duration than the exhaust. The Hemi tends to like this configuration.”—Daniel Boshears

“The rocker arms are custom made from Harland Sharp, with a ratio of 1.7 on the intake and 1.65 on the exhaust. We didn’t feel the higher ratio would be beneficial on the exhaust, so we used the split ratio. The pushrods are from Trend and are heavy duty 5/16-inch diameter. The lifters started as tie bar lifters for an LS engine that were modified by Ron Shaver. The reason we used the LS lifter is that the factory lifter for the Hemi does not pump oil. We had to feed the valvetrain through the lifters, so we solved that problem with the GM lifter. The Hemi uses the .842-inch diameter lifters, which is the same as the Chevy, and the height of the lifter body was about right. There was a considerable amount of time put into a set of factory lifter guides to make it work.”

Feeding the stroked Hemi is a high-rise single-plane intake manifold and a carburetor. The vertical ports to the Hemi take maximum advantage of these tall manifolds, a design first seen with the Mopar Drag Pack induction. Daniel went with the similar Ritter Racing intake. “The intake is from Ritter Racing, which looks very much like the Mopar Drag Pack manifold. The Mopar intake is very hard to get and was really designed for a smaller fuel-injected application. With a carburetor, the Ritter intake seems to work better. I cut 1 3/8-inch off the top of the manifold, reducing the plenum volume, but otherwise the manifold was run as-cast. I didn’t want to increase the volume of the manifold at all, and the runners looked very good as delivered.”

“The Mopar intake is very hard to get and was really designed for a smaller fuel-injected application. With a carburetor, the Ritter intake seems to work better.”—Danial

Topping the engine is what appears to be a familiar 4150 Holley carb, but this unit is highly modified, as Daniel reveals. “The carburetor is an 850 body, and we modified the venturi diameter and put in a custom booster that is a downleg annular design. We put a third main circuit in the carburetor which helps output in the 4,000-5,000rpm range.” Beneath the carb is a unique spacer combination, featuring a short conventional four-hole spacer with an extended segmented spacer from Stealth below. Daniel details, “The spacer actually fits down into the plenum, dividing it into four parts—one for each barrel. We found improved distribution and torque with this arrangement.” Another unique aspect of the induction is the normally overlooked fuel log feeding the carb. Daniel and Randy Malik conspired over the design after finding information from the work of Smokey Yunick. The idea is to ensure no aerated fuel enters the bowls, feeding the carbs with nothing but a solid stream of liquid gasoline for improved mixture control.

One of the unique features of the Gen III Hemi is the factory’s dual spark plug arrangement, and word is this was done for emissions reduction reasons. The production dual plugs’ similarity to the famous Pro-Stock dual-plug race Gen II Hemi heads is unmistakable. Because the Gen III is a distributorless engine, firing the plugs is handled via individual coil packs and a stand-alone ignition controller. Daniel went with MSD components here citing ease of installation and use, as well as reliability for selecting MSD. Daniel remarked about the low timing requirement of the Hemi layout: “We programmed a curve into the ignition system, and in testing found the Hemi wanted very little timing for maximum power. We were in the range of 20 degrees total timing, which shows a very efficient burn. I have to believe there is an advantage in the Hemi’s twin-plug design.”

“We were in the range of 20 degrees total timing, which shows a very efficient burn. I have to believe there is an advantage in the Hemi’s twin-plug design.”—Daniel Boshe

Dyno Time

The engine was completed with a custom set of Schoenfeld headers, a Meziere electric water pump, and an ATI damper, and it was ready to run. Hooked to the DTS/Superflow dyno at Automotive Machine and Performance in Philpot, KY, we could see that the Hemi had evolved considerably from it production pickup truck beginning. The first peak number to cross the dyno display was torque, showing 570 lb-ft at 5,000-5,100 rpm. That, folks, is the twist you might expect from a strong big-block with an extra 40-plus cubic inches. Pulling up the rev range, the horsepower numbers rose steadily until reaching a peak of 613 hp at 6,100 rpm. This street style hydraulic roller Gen III handily crested the 600 horsepower mark, proving that Hemi performance is here to stay.

Fast Facts
402ci Gen III Hemi
Bore: 3.935-inch
Stroke: 4.125-inch
Displacement: 402 cubic inches
Compression ratio: 11.44:1
Camshaft: custom Lunati hydraulic roller
Valve lift: .637/.631-inch
Duration at .050: 249/247
Lobe separation: 109 degrees
Installed centerline: 103 degrees, intake
Rocker arm ratio: Harland Sharp 1.7/1.65:1
Piston rings: Total Seal 1-/1.2-/3mm
Piston: Race Tec forged custom
Block: factory 2006 5.7L Hemi
Crankshaft: Molnar
Bearings: MAHLE Clevite
Rods: Molnar 6.123-inch
Cylinder head: factory 5.7 Eagle casting
Intake valve diameter: 2.100-inch
Exhaust valve diameter: 1.570-inch
Intake manifold: Ritter single-plane
Carburetor: custom Holley 850
Ignition: MSD
Header: Schoenfeld stepped primary
Damper: ATI
Oil Pan: Moroso
Fuel: VP 100 unleaded
Oil: AMSOIL 5w20

On The Dyno
402ci Gen III Hemi
3,000 457 261
3,100 457 270
3,200 461 281
3,300 467 293
3,400 468 303
3,500 466 311
3,600 470 322
3,700 476 336
3,800 482 349
3,900 489 363
4,000 499 380
4,100 513 401
4,200 531 424
4,300 544 445
4,400 551 462
4,500 557 477
4,600 563 493
4,700 566 506
4,800 567 518
4,900 568 530
5,000 570 542
5,100 570 553
5,200 569 563
5,300 567 572
5,400 564 580
5,500 561 587
5,600 558 594
5,700 552 599
5,800 547 604
5,900 540 606
6,000 535 611
6,100 528 613
6,200 519 612
6,300 510 612
6,400 502 612
6,500 492 609

1. Daniel Boshears of Red Rocket Engines started with a common 5.7L Gen III Hemi truck block, and upped the cubes to 402ci via a slight overbore to 3.935 inches and a big move in stroke to 4.125 inch. Filling those bores are custom Race Tec pistons swinging from Molnar 6.123-inch rods.


2. The long stroke combination resulted in a tight fit with the factory-sized internal crank trigger wheel. Clearance machining to the piston pin boss as well as the wheel were required to make it all fit.


3. The Molnar crank started life with a stroke of 4.050 inches, and was offset ground to 4.125 inch by Performance Crankshaft. In the process the journal diameter was reduced to the 1.888-inch Olds Quad Four specification.


4. The Race Tec pistons feature a flat top configuration with the required valve clearance notches, producing 11.44:1 compression ratio with the shallow Gen III Hemi head chambers. Fasteners are ARP throughout.


5. A factory truck rear sump pan holds the AMSOIL 5w20 synthetic lube, while a slightly modified factory 6.4L Hemi windage tray controls the oil. The 6.4L tray features more clearance for longer stroke lengths than the production 5.7L piece.


6. Up front a Melling replacement oil pump circulates the vital lubricant, while a Cloyes duplex chain drive spins a custom 249-/247-degree duration-at-.050 Lunati hydraulic roller cam.


7. The production 5.7 “Eagle” head castings were CNC ported by Modern Cylinder Head, and hand finished by Randy Malik. The supporting valvetrain components include .110-inch wall Trend pushrods and Harland Sharp adjustable rockers. To accommodate the required pushrod oiling, GM LS lifters from Morel were reworked by Shaver Specialty Service and run in modified Gen III Hemi alignment guides.


8. Here we can see the Gen III Hemi combustion chamber. Note the filled-in quench pads found in the “Eagle” heads, and the twin plugs common to all Gen III Hemi engines. The chambers are filled with oversized 2.100-/1.57-inch valves.


9. The Harland Sharp rocker arrangement multiplies the cam action by 1.7:1 on the intake side and 1.65:1 on the exhaust. Note the clearance notch needed on the plug tube adjacent to the intake rocker. Lift at the valve works out to .637/.631 inch.


10. Starting with a modified Holley 850 body, the carb was custom built using parts from Stealth. Tricks include oversized venturii, and unique downleg annular boosters.


11. The custom oversized fuel log was inspired by the writings of Smokey Yunick, and built by Daniel Boshears. The idea is delivering a steady supply of liquid fuel without aeration.


12. The special carb spacer from Stealth uses a unique extended divider that drops into the plenum. Boshears cites improved distribution as a key benefit here.


13. The Ritter single-plane intake manifold resembles the virtually unavailable Mopar Performance Drag Pack piece. Note the idle vacuum evacuation system plumbed into the far side of the intake, and the evacuation fitting plumbed to the header on the near side.


14. Circulating the coolant is a Meziere electric water pump, while the ATI damper below helps the crankshaft survive. Boshears selected both of these components based upon their impeccable reputations.


15. With the lack of a distributor on the Gen III Hemi, these requirements are handled by an external electronic controller and the factory sensors. Boshears tells us the MSD 6 Hemi timing controller was easy to use and functioned flawlessly.


16. The MSD controller fires a battery of MSD coil modules—one unit per cylinder—to animate the Hemi’s dual spark plugs. The plugs are fired simultaneously, and Boshears notes that with total timing requirements in the low 20s, the Hemi is exceptionally efficient.


17. These beautiful headers were created by components from Schoenfeld. The primary tubes feature a 1 ¾ to 1 7/8-inch steps, leading to a Schoenfeld merge collector.


18. On the dyno the long-stroke Gen III delivered outstanding power, no doubt thanks to the deep breathing of the Hemi heads. On tap was a peak output of 612 horsepower, while a massive 570 lb-ft of torque was produced at 5,000-5,100 rpm. It’s hard to argue the power potential of the Gen III.


19. Builder Daniel Boshears (L), and his crew of co-conspirators Mike Phillips, Randy Malik and Cory Short can attest to the power-producing capabilities of Mopar’s modern Hemi.
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The Grumpy Grease Monkey mechanical engineer.
Staff member
Some good ol’ boys show us how to build a newfangled 640-horse pump-gas Hemi

There’s just something about a Southern man. There’s a modest nonchalance and reluctance to boast about his accomplishments that disarms you just before you realize that this good ol’ boy knows what he’s doing. Robert McDonald is that typical Southern man. Residing in the heartland of racing just outside Hickory, North Carolina, Bullet Bob and his friends at Atlantic Engines have been building high-performance gas and diesel engines for racers both local and nationwide. The kind of racers who would stop to help you fix your car for the race, put you in the wall to win the race, and then help you afterward. It’s just the way things work there.

Their engine started out life as a stock 5.7 belonging to a Yankee way north of the Mason-Dixon line. Salt and corrosion pitted away at the exterior cast iron while internal neglect put the engine in an overheating situation and lead to its ultimate demise. Bob bought the core and set about resurrecting the phoenix. With limited experience in building late-model Hemis, he had goals of learning the intricacies of the engine while building something that made tons of power, was reliable, fuel injected, and didn’t break the bank. Oh yeah, it was supposed to be going in an early Barracuda so you know it had to look good too.

When asked about his initial experience with the Hemi, he laughed and said: “I had no idea what I was getting into. I’d worked on a few but I had no idea. This won’t fit this because it’s a different year model and so on, but I said we’ll do it. The biggest problem I ran into was finding someone to machine the Hemi. It is a very unique engine.” In fact, even though the engines first came out in 2003, very few have passed through the doors of performance engine shops and even fewer still ended up as actual high-performance engines. Unlike the proliferation of GM LS and Ford Mod motors, it seems that most hot rodders have shied away from the late-model Hemi. Sure, most of the cars that the Hemi comes in are a little portly, but since designers at Mopar had the sense to make the bellhousing pattern identical to that of the early small-blocks, there is really no reason not to drop one in a lightweight muscle car–era A- or B-body for fun.

Prior to machining the block, Bob gave it a half-fill of Hard Block to be sure the semi-open deck cylinders didn’t move around too much under load. Once cured, the block was punched to an even 4-inch bore to give the heads room to breathe and add some cubes.

Decking the block is tricky, since it is super wide and can be a pain to set up in most surfacing machines. Fortunately, Bob hooked up with engine machinist Mike Ege, who performed the block work with as much care as he would on one of his top NASCAR engines.

For those looking to build a similar combo, the 6.1 block can be used as it comes from the factory with a 4.055-inch bore, but expect to pay extra for the core as it is a rarer entity that sports a factory forged crank, stronger rods, and piston oilers—perfect for forced-induction guys who don’t want to buy a whole rotating assembly. Another difference between the 5.7 and 6.1 is a revised water passage design. Bolting a set of 6.1 heads on a 5.7 block has been done, but Bob pointed out that due to the water passage differences between the blocks, care has to be taken to be sure there will not be a water leak from the mismatch.

Looking to stroke the engine to its final 407 ci, different crankshafts were considered. Mopar Performance and Scat both offer 3.795-inch stroke forged cranks, but ultimately the Atlantic Engines crew decided on using a Callies Compstar crank sporting a massive 4.050-inch stroke that was still able to use nice long 6.125-inch Callies rods. A side benefit of the Callies crank is that if it is installed in a standard-bore 5.7, it ends up at 392 cubes, and in a 6.1 block, it puts out a magical 426 ci. Serendipitous coincidences hailing back to the Hemi’s history!

Knowing that the Hemi comes with a 9.25-inch deck height, Bob subtracted the rod length and half of the stroke, leaving a 1.1-inch piston compression height that he related to Mahle, as they were contracted to manufacture the pistons for this build. Mahle coated the piston skirts with their Grafal coating and were able to reduce the piston-to-wall clearance, limiting piston rock and the wear, noise, and loss of compression associated with it.

All of the late-model Hemis came with hydraulic-roller camshafts, and Bob’s bullet was no different. Without the vast array of dyno data that would accompany something as mundane as a small-block Chevy, it took a bit of figuring to get a cam that would work the way he wanted it to. The requirements that Bob spec’d out were that the engine must make huge average torque and horsepower between 2,500 and 6,500 rpm, as the engine was to be used in the AMSOIL Engine Masters Challenge prior to its final destination in the ’Cuda. He spoke at length with Billy Godbold at COMP Cams who sent him a “small” and a “big” cam to try out. Like the older LA small-block engines, the Hemi uses a 57-degree lifter angle; however, they dropped the lifter diameter to .842 inch, like the GM camp. According to COMP, lifter angle and diameter, specifically the lifter roller wheel diameter, plays a huge role in determining the design of the cam lobes. Between those things and understanding how the Hemi heads flow compared to a typical Wedge head, the late Hemi cams are completely different than a “normal” cam. Additionally, the 2009-and-newer 5.7s employ Variable Camshaft Timing (VCT) that uses pressurized oil running to a unique camshaft sprocket, which can change the camshaft phase, advancing or retarding the cam as needed. Fortunately, Bob wasn’t saddled with that extra challenge when choosing a cam, since his was an earlier block. His final cam choice was cemented after speaking with cam guru Dema Elgin of Elgin Cams, who ground a COMP core to specs in between the “small” and “big” cams, which turned out to be the perfect choice.

When Bob was initially disassembling his engine and giving it the once-over, he did notice one more peculiar thing about the late Hemi: No oil galleys were feeding the lifters. Unlike most engines, the late Hemi feeds oil backward, from the top through the rockers, to the pushrods and finally to the lifters. Bob’s engine used the factory rocker arms for the engine, since at the time nothing in the aftermarket was available. “There’s nothing special about them, but there’s nobody making a set. Even at the PRI [Performance Racing Industry] Show you’re walking around and nobody’s making a set. You’re like ‘it’s been seven years guys and still nothing?’”

He had an “opportunity” for learning when freshening up the original rocker shafts. Since the heads were overheated originally, they were covered in scale, and being the fastidious engine builder that Bob is, he decided to pop out the freeze plugs in the ends of the rocker shafts to clean the inside. It turns out replacement plugs are not available, even from Chrysler. After much hemmin’ and hawin’, he just went down to the local dealer resigned to the fact that he’d have to buy new shafts to slide his nice, clean rockers on. Well, the shafts are only sold with rockers on them. (Head drops here.) On the plus side, a full set of brand-new rockers and shafts was only $140 a head. For both heads it was only $280! “Another lesson learned about the Hemi,” Bob says.

The perfect cam he now had was significantly dependant on the cylinder heads bolted on top of the block. To get the most out of the combination, Bob went straight to the horse’s mouth. He contacted Bryan Maloney of Maloney Competition Systems in Martinsville, Virginia, who was one of the original developers of the current 6.1 Hemi head and had a serious pedigree in the racing community. Maloney ended up porting the heads and intake to take full advantage of the big Ferrea 2.05/1.55 valves. On Bob’s flow bench, he says that while peak flow numbers aren’t everything, they do perform quite well, drawing in over 300 cfm by a scant .300-inch valve lift. That is one haulin’ port.

With a removable floor plate, a 6.1L intake manifold and plenum were ported by Maloney as well. Doing his best to make sure the port entries leading into each runner were even ensured that the air distribution among all eight cylinders was matched. Unlike a carburetor, which distributes fuel to each runner proportionally to the amount of air that the runner ingests, an EFI manifold will typically send the exact same amount of fuel to each cylinder regardless of the airflow in the various ports. That is just one of those little things that an experienced head and intake porter will attend to with the knowledge that if there is a cylinder that is typically lean or rich with a stock manifold, it can be fixed with the right porting.

To control the fuel, Atlantic Engines leaned on their friends from FAST and procured one of their FAST XFI 2.0 fuel injection systems. The new XFI 2.0 system is a logical progression of the original XFI unit. FAST took everything that racers and enthusiasts liked about the original XFI and added to it. Things like boost control features, wet and dry nitrous control, solenoid pulse frequency, and targeted air/fuel ratios for power adders make life mucho easier for making big power safely. They’ve also expanded their internal data logging controls to feature external arming. You know, for that guy who flips a switch at the back of your bumper before you launch. Intelligent Traction Control has also been upgraded to avoid erroneous starting line data. Naturally, it has a self-learn feature that Bob used to generate a working fuel map once the engine was up and running off the base map. One of the features that really stands out about XFI 2.0 is the fact that you don’t have to throw away your old XFI system to use the new features; they offer a firmware upgrade for a reasonable price that will bring your old system up to date. Also, their PN 301013 kit is a simple plug-and-play for the late-model Hemi crate or transplant engine using all of the factory sensors, which certainly made Bob’s life much easier.

Plain old pump gas was fed to the engine, and they didn’t do anything special for the squirters. “Those are just stock injectors out of a ’94 turbo Neon, and we got the fuel rails from Aeromotive.” Since the engine normally uses a drive-by-wire throttle blade, they chose to install a FAST 92mm throttle body that they adapted to the stock intake manifold via a custom spacer they built. With their friend Aaron at the laptop, tuning the XFI was as sweet as chess pie.

Bob’s learning curve was up and down throughout the process, and in one of the down parts, he found out that though the bellhousing was an easy match, finding a flywheel was not. “I had to get a steel flywheel for a late Hemi. Now you go to Hays or anybody else and they say ‘we don’t have that flywheel, sorry.’ So I go to Chrysler and say I need to order a high-performance flywheel. It comes in and I open the box and it says Hays.”

Exhaust for the Hemi-powered cars and trucks has come around as well. “The headers I had were just off-the-shelf Hedmans. I had flanges to build a new set, but didn’t really have the time. I wanted to do a set with equal length, one unequal length, a tri-Y, and do a lot of playing, but these turned out great.”

A Moroso road-race pan with the side wings cut back was chosen to hold six quarts of AMSOIL 15W-50 oil for the engine. Bob emphasized the importance of choosing the right viscosity oil for the engine. He’s found in testing with hydraulic lifter engines that using a traditional lightweight racing oil in a hydraulic lifter engine can cost an easy 5-10 hp by allowing the oil to bypass the lifter plunger, killing valve lift and duration. By using a Melling M-Select oil pump and the relatively thick oil, he was able to capture some of that lost power. A caveat to the thicker oil treatment is that if you are picking an oil for an engine that uses a displacement-on-demand or variable valve timing system, absolutely stick with the vehicle manufacturer’s recommendation.

Once the Hemi was complete, the boys stuck it on the dyno. True to form, the old feller running the dyno wore a set of coveralls, Carolina red clay-colored shoes, and had a drawl that belied the knowledge and experience that came from years of testing high-performance engines. Though skeptical at first, his eyes lit up a bit as the torque needle climbed and when the horsepower peaked at 644, he finally admitted: “You boys got yourselves a player.”

By the Numbers

Atlantic Engines 407ci Gen III Hemi

Bore: 4.000-inch
Stroke: 4.050-inch
Displacement: 407 actual cubic inches
Compression ratio: 11.2:1
Camshaft: Elgin hydraulic roller
Cam duration: 238/241 degrees at .050-inch tappet rise
Valve lift: .598/.598-inch
Rocker ratio: OEM 1.65 ratio
Top ring: .043 inch
Second ring: .043 inch
Oil ring: 3mm
Piston: Mahle dished
Block: OEM 5.7
Crankshaft: Callies Compstar
Rods: Callies Compstar H-beam
Cylinder head: OEM 5.7
Intake valve diameter: 2.05-inch
Exhaust valve diameter: 1.55-inch
Intake manifold: OEM 6.1
Fuel Injection: FAST XFI
Header: Hedman
Ignition: FAST XIM
Damper: ATI

Dyno Data

407ci Gen III Hemi

2,600 415 205
2,700 413 212
2,800 416 222
2,900 426 235
3,000 431 246
3,100 440 260
3,200 454 277
3,300 474 298
3,400 486 315
3,500 494 329
3,600 501 344
3,700 508 358
3,800 508 367
3,900 509 378
4,000 518 394
4,100 521 407
4,200 527 422
4,300 538 441
4,400 548 459
4,500 554 475
4,600 561 491
4,700 564 504
4,800 563 515
4,900 558 521
5,000 552 526
5,100 557 541
5,200 563 557
5,300 563 569
5,400 567 583
5,500 571 597
5,600 569 607
5,700 567 615
5,800 563 622
5,900 558 627
6,000 553 632
6,100 547 635
6,200 539 637
6,300 534 640
6,400 528 643
6,500 520 644


The Grumpy Grease Monkey mechanical engineer.
Staff member

Add Nearly 70 HP to a 5.7 Hemi With a Cam Change

John McGannAuthorRichard HoldenerWriter, Photographer
Apr 10, 2020

A cam and spring upgrade yield 67 additional horsepower on a Junkyard 5.7 Hemi.

In part 1, we introduced the Homeless Hemi, this 5.7L Hemi. The origin of this particular engine was shrouded in mystery. Long ago, it had been cast aside and forgotten, and only recently did it reappear, but it was in such disarray that we no longer knew if it ran. We decided to solve the mystery and get it running. In the process, we also added a little NX nitrous kit to up the power from 367 hp and 414 lb-ft to 495 hp and 530 lb-ft. With part one completed, we now turn our attention to increasing the naturally aspirated power output.

Hemi owners, even of the early version like our 2006, have one heck of a starting point. Much like LS engines, the Hemi responds very well to aggressive cam timing, because the engine is blessed with basically everything else needed to make big power. Having free-flowing heads certainly helps the situation, but the intake plays a big part, as well. Even the lowly truck intakes can support exceptional power, while maintaining impressive torque production thanks to their generous runner length.

With ample displacement and compression, the only thing stopping a Hemi from making power is the mild factory cam timing, especially on the Ram trucks. The mystery surrounding our Hemi was not so much would a cam swap increase the power output, but by how much.

Having run the baseline already with our nitrous test, we tore into the Hemi to replace the cam. The new Comp HRT profile we decided to run (0.596/0.582-inch lift, 220/230-degree duration, and 113-degree LSA) required a valve spring upgrade, so we installed a set of Comp 7228-16 springs.

Comp offers no less than three different spring packages for the various Hemi generations, allowing users to select spring kits based on their intended cam lift. Comp Cams also introduced a spring compressor tool for the Hemi at SEMA, but it was not available in time for our test.

After installation of the new cam and springs, we were excited about the power potential. You'll remember that our 5.7L produced peak numbers of 367 hp and 414 lb-ft of torque in stock trim with Hooker long-tube headers and the stock throttle body. The engine was plenty torquey, but we wanted to see this thing start to pull a little harder on the big end. The cam was just what the doctor ordered, as Homey the Homeless Hemi thumped out an additional 67 hp with the cam swap.

Equipped with the new HRT cam, the 5.7L produced 434 hp and 440 lb-ft of torque. We suspect that Homey was down slightly on power from where the old dog should be, both with the stock cam and the new Comp version, but we sure liked the gains offered by the HRT cam. It's hard to argue with an extra 67 hp, especially when the big gains on the top came with no penalty in power down low. Cam swaps can be tricky, as extra top-end power is often accompanied by a loss in low-speed torque, but it looks like the boys at Comp did their homework on the new HRT designs. What's next for the Homeless Hemi? It looks like his shopping cart is clattering ever closer with headers, intakes, and boost!

5.7L Hemi Dyno TestStock Vs Comp Cam
Much like GM's LS platform, the Hemi was blessed with ample displacement, head flow, and a solid induction system. The only thing missing from the performance equation on a Hemi is cam timing. This test is a perfect example of what happens when you add a cam to a motor that needs only cam timing. Run with the stock cam, the 5.7L Hemi produced 367 hp and 414 lb-ft of torque. After installation of the new HRT cam from Comp Cams, the power output jumped to 434 hp and 440 lb-ft of torque. The cam swap netted an impressive gain of 67 hp!