and you can see that Hemi performance is alive and well. Back in the muscle car era, it took 426 ci to eclipse the 400-hp mark, and that was with the old gross power rating. Today, a modern 6.4L (392-ci) Hemi offers 485 (net) hp, and does so while offering a combination of mileage, reduced emissions, and drivability that were only a dream back in the 1960s. This book is all about how to take an already impressive Gen III Hemi and make it even better!
The Hemi engine family has naturally evolved since its introduction to keep Dodge owners one step ahead of the competition. The original 5.7L was rated at 345 hp, and was a solid step up in performance over the previous Magnum engines. The conventional V-8s just could not hold a candle to the power output of the new Hemi. To keep pace with the LS engine family offered by the power mongers at GM, the Hemi saw increases in displacement, first to 6.1 liters then to 6.4 liters. Each step up in displacement brought further power gains, but the Dodge engineers also threw in improvements in efficiency.
The 5.7L featured a 3.917-inch bore combined with a 3.578-inch stroke. The larger 6.1L offered in the SRT8 performance versions stepped up the bore size to 4.055 inches while retaining the same 3.578-inch stroke of the 5.7L. The latest 6.4L altered both the bore and stroke by combining a 4.09-inch bore with a 3.724-inch stroke. The hybrid 6.2L Hemi used in the Hellcat combined the 4.09-inch bore of the 6.4L with the 5.578-inch stroke of the 5.7L and 6.1L.
In addition to the increase in displacement, performance Hemis also received improved cylinder heads, revised camshafts, and even new intake manifolds to further improve performance. This becomes obvious when you measure the respective specific outputs of each factory Hemi. The specific output is a measurement of how much power the engine makes for each liter (or cubic inch) of its displacement. The 345-hp 5.7L produced right at 1 hp per cubic inch or 60.52 hp per liter. Stepping up to the 425-hp 6.1L resulted in a specific output of 1.148 hp per cubic inch or 69.67 hp per liter. The 6.4L iteration offered 485 hp, which equates to 1.237 hp per cubic inch or 75.78 hp per liter. The steady increase in the specific output means that Dodge did more than just add inches to its performance engines, it also improved the efficiency. This was accomplished with increased static compression ratio, improved cylinder head flow, and revised intake manifolds. On the SRT8 engines, Dodge also improved the exhaust flow with dedicated exhaust manifolds that easily out-flowed the log-style manifolds run on the 5.7L Ram applications (see chapter 4).
One of the things that made the new Hemi so successful was the impressive cylinder head flow. Even on the tuned-for-towing Ram applications, the 5.7L heads offered serious flow numbers. As-cast 5.7L heads flowed as much as 285 cfm, depending on the flow bench. To put these numbers into perspective, that is enough to support more than 570 hp on the right application. The flow numbers offered by the 6.1L SRT8 heads were even more impressive; the revised castings flowed 320 cfm, enough to support 640 hp on a serious buildup. These are flow numbers offered only by fully ported, aftermarket race versions of the LA or Magnum-style heads, and these came right from the factory. The most recent Eagle heads on the newer 5.7L Ram trucks flowed even more, with peak numbers eclipsing 330 cfm, good for an extra 20 hp (or more) more than the 6.1L heads. Of course, the Big-Kahuna Apache heads were offered on the latest 6.4L, and these flowed 340 cfm, enough to support almost 700 hp! Obviously, the Dodge engineers realized the importance of head flow when it comes to power production. They even left enough material on the as-cast heads to allow porting to further improve the flow.
The massive head flow offered by all of the stock Hemi heads is one of the major reasons why the Dodge engines respond so well to cam swaps. A performance cam is really the only thing missing in a performance Hemi combination, although it must be combined with the proper valve springs. If you check out chapter 2, you should realize that porting the stock heads is often of limited value. The reason is not the quality of the porting, but rather that the stock Hemi heads are already so good. The point here is that you shouldn’t expect huge power gains from a head swap, no matter what the flow bench says. If your modified 6.1L (or 6.4L) Hemi produces 580 hp with a set of (340 cfm) heads capable of supporting nearly 680 hp, don’t expect much of a change when you add heads with (380 cfm) flow numbers that support 760 hp. You don’t need better heads, you just need more engine!
In addition to camshafts and cylinder heads, this book contains separate chapters on nearly every aspect of Gen III Hemi performance, including intake manifolds, nitrous oxide, and even forced induction. With so much test data generated, I provided separate chapters on supercharging and turbocharging. Chapter 5 covers all of the various forms of supercharging, including Roots, twin-screw, and centrifugal superchargers. Chapter 6 covers both single and twin turbo testing. The different Hemi displacements all respond well to camshafts and they respond equally well to boost. Using boost from a super or turbocharger, it is possible to increase the power output of your Hemi by 50 to 100 percent or more. As illustrated by the test data in these two chapters, boost is simply a multiplier of the original output. Adding a turbo or supercharger to a stock engine will result in less of a power gain at any given boost level than adding the same boost to a modified engine. This book also covers the results of turbo and supercharged cam testing because the specs differ on cams designed specifically for forced induction.
One thing you will find out about the Gen III Hemis in this book is the relative lack of aftermarket support for things such as intake manifolds. Although the number of choices is limited compared to the offerings for the LS family, the stock Hemi intakes are already impressive performers. Testing has shown that it’s difficult to improve upon the factory Magnum and SRT8 intakes. It is often possible to increase power higher in the rev range (usually beyond 6,500 rpm) with a short-runner intake, but this usually comes with a trade-off in power lower in the rev range. The same can be said of producing power lower in the rev range, but this hurts power at high rpm. The tests on the adjustable intake manifolds (chapter 1) clearly illustrate this effect on the power curve. The comparison between the single-plane and long-runner electronic fuel injection (EFI) intakes show this as well; intake manifolds are designed to operate effectively at specific engine speeds. Short-runner (or single-plane carbureted) intakes should be combined with more aggressive cam timing designed to enhance power production higher in the rev range. Throttle bodies obviously work well with intake manifolds because they offer increased flow. The gains offered by throttle body upgrades increase with the power output of the engine. Tested on a stock engine, a throttle body upgrade might be worth nothing, but tested on an 800-hp supercharged combination, it can be worth as much as 50–60 hp (especially on a positive displacement supercharged application).
Chapters 7 and 8 include nitrous oxide, dedicated engine builds, and what I call overflow, meaning some of the tests that I ran out of room for. Nitrous oxide can be applied to any Hemi combination, ranging from a stock crate engine to a dedicated stroker (including turbo and supercharged combos). The amount of power supplied by nitrous oxide is a function of the jetting; larger jets allow more nitrous flow. Of course, this must be accompanied by the proper amount of fuel, but nitrous systems offer far and away the most bang for the buck. It is possible to add as much as 250 hp (or more) to your Hemi for about the cost of a cam swap. You will make more power with nitrous and a cam, but every Hemi owner should experience nitrous oxide once in his or her life.
I have broken down the chapters into individual components (for example, heads, cams, and intakes), but the reality is that the best way to produce optimum power from your Hemi combination is with the proper combination of components. The heads must work with the cam timing and intake design to optimize power production in the same RPM range. If you want to know how to make your Hemi more powerful with dyno-verified results, you’ll find it in these pages.
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