Читать книгу How to Build New Hemi Performance on the Dyno - Richard Holdener - Страница 6
ОглавлениеWhether you have a stock, street, or strip Hemi application, the intake manifold is one of the three major players in terms of power production. Unlike the LS, the aftermarket has not stepped up with an abundance of aftermarket Hemi intakes, although we did manage to test just about everything available. Contrary to popular belief, intake designs do more than just allow airflow into the ports, they actually provide a tuning effect that aides in power production over a given RPM range. Not surprisingly, factory Hemi intake manifolds for the truck, Magnum, and SRT versions were not designed with peak power production in mind, but rather a combination of peak and average power combined with ease of production and even fuel mileage. Just as a poorly designed manifold can (literally) ruin an otherwise good Hemi, the right intake can help you produce impressive power, especially when used in conjunction with the right cam and ported cylinder heads. More than any other single component, the intake manifold’s runner length will determine where the engine makes effective power. Match the runner length to produce power in the same operating range as the cam profile, and you are a long way toward making a powerful Hemi.
All of the factory Mopar intakes perform very well, including the aluminum SRT8 manifold.
For 5.7L, 6.1L, and 6.4L Hemis and any other engine, intake manifold design may be broken down into three major elements, runner length cross section as well as taper ratio and plenum volume. These elements are listed in order of importance or, more specifically, in the order they most affect the performance of a given manifold. By this we mean that changing the runner length has somewhat more of an effect than altering the cross section or plenum volume. This is not to say that all of the elements are not important, it is just that proper care should be given to the elements in accordance with their eventual effect on performance. Intake designers, take note of this perspective, because fabricators often spend countless hours altering the plenum volume in an attempt to change the effective operating range when they should have simply increased (or decreased) the runner length. It should be stated here that manifold design is sometimes limited by production capability or rather ease of construction. Building a set of runners with a dedicated taper ratio and a compound curve is difficult if not impossible for the average fabricator. Despite the fact that this design produces the best power, it simply isn’t going to get produced unless a major intake manufacturer, such as FAST, Holley, or Edelbrock, steps up to the cost of such a complex combination.
The first element in intake design is the runner length. The overall intake runner length actually includes the head ports, but the discussion will be limited to those in the manifold. Unlike their carbureted counterparts, fuel-injected Hemi intake manifolds seem to be broken down into two distinct groups, long and short. It’s obviously not very scientific, but the terms long and short do not properly describe intake manifolds. The reason for the long and short designations is that, generally speaking, the longer the runner length, the lower the effective operating RPM. Obviously, the opposite is also true; shorter runner lengths improves top-end power. Production Hemi intake manifolds typically have a long-runner design to help promote torque production even though the SRT8 intake differs in length compared to the Ram truck or Magnum. It is often possible to design an intake that offers more low-speed or top-end power than a stock Hemi intake, but doing both can be proven difficult. The SRT8 intake was designed to produce power at a higher engine speed than the Magnum or Ram truck, but the trade-off is a loss lower in the rev range (also see Test 8 in this chapter where the author adjusted the runner length).
The next element in intake design is cross section or port volume. A related issue is taper ratio, but we will cover that shortly. The port volume or cross section of the runner refers to the physical size of the flow orifice. Suppose you have an intake manifold that has 17-inch-long runners and measures 2.00 inches in inside diameter. It is possible to improve the flow rate of the runners by increasing the cross-sectional area. Suppose we replace the 2.00-inch runners with equally long 2.25-inch runners. Keep in mind, the SRT8 head ports and intake runners are larger than 5.7L truck or Magnum runners. Accordingly, the larger 2.25-inch runners would flow a great deal more than the smaller 2.00-inch runners, thus improving the power potential of our engine. From a reflected wave standpoint, the increase in cross section will have no effect on the supercharging effect, but it will alter the Inertial Ram and Helmholtz resonance. Related to the cross section, taper ratio refers to the change in cross section over the length of the runner. Typically, intake manifolds feature decreasing cross sections, where the runner size decreases from the plenum to the cylinder head. The decrease in cross section helps to accelerate the airflow, thus improving cylinder filing, but the real difference is the effective change in cross section brought about by the taper.
Plenum volume is the final element of any Hemi intake manifold. Plenum volume refers to the size of the enclosure connecting the throttle body to the runners. The plenum volume is typically a function of the displacement of the engine. Most production intake manifold applications feature plenum volumes that measure smaller than the displacement of the engine (somewhere near 70 percent), but this depends on the intended application. A number of manufacturers have recognized the importance of the plenum volume and incorporated devices to alter the plenum volume to enhance the power curve, but the early Magnum, SRT8, and truck intake are all fixed volumes. The 2009-up truck intakes and 6.4L SRT manifolds offered dual runner lengths. Contrary to popular opinion, increasing the plenum volume does not increase the air reservoir allotted to the engine as much as it does affect the resonance wave. When excited, the area in the plenum resonates at a certain frequency. Changing the plenum volume changes the resonance frequency. The Helmholtz resonance wave aids airflow through the runner (acoustical supercharging). A number of things, but primarily the plenum volume, determine where this assistance takes place in the RPM band. The air intake length, inside diameter, and a portion of the cylinder (when the valve is open) are also used to calculate the Helmholtz resonance frequency and why air intake length and diameter have a tuning effect on the power curve.
The single-plane intake from Mopar Performance likes to rev, but it does drop torque production lower in the rev range.
Test 1: Truck vs Mopar Perfomance Single-Plane Intake for a 5.7L Stroker
When it comes to Hemi applications, both carbureted and EFI intakes are available. For carbureted applications, one of the common intake designs is the single plane. That particular induction system predates the modern Hemi engine family by multiple generations, but carbureted Hemi applications are becoming more commonplace. We all know that the Hemi was originally equipped with factory fuel injection, but MSD and others made the carb conversion ultra simple. In truth, the single-plane intake tested here may also be run in EFI configuration, but this test was run with a simple Holley carb. The real choice here has less to do with carburetion versus injection, and more to do with intake design. Choosing the proper intake design is critical for maximum performance, but just what defines the term maximum? In most cases, it doesn’t mean peak power, but rather maximized power through entire rev range, where acceleration actually takes place. Now factor in drivability, fuel mileage, and even torque converter compatibility and you start to understand the dilemma.
In this test between the factory 5.7L Hemi truck intake and a single-plane MP intake, we saw peak power numbers that differed by a scant 8 horsepower, but the power curves couldn’t be more different. We all like to brag about the peak power numbers, but the reality is that the vast majority of Hemi engines for street and strip spend most of their time well below the power peak. In fact, the vast majority of driving is spent well below the torque peak and even during hard acceleration, the engine spends most of its time accelerating between peak torque and peak power. Tested on our 370-ci stroker Hemi combination equipped with TEA-ported 5.7L heads, a Comp XFI 260H cam and internals from JE, Scat, and Speedmaster, the long-runner truck intake easily outperformed the short-runner single-plane intake up to 5,800 rpm. For 5,800 rpm on up, the single-plane intake was tuned to optimize power production and offered 8 more peak horsepower, but the difference grew to 25 hp at 6,300 rpm. Intake manifolds are optimized for power production in specific rpm ranges, a point illustrated perfectly by this, and every other test in this chapter.
Despite being designed for low-RPM truck applications, this factory intake offered surprising power production.
The single-plane design was designed for high-RPM power and was a tad out of place on this mild 5.7L stroker.
Truck vs MP Single Plane for a 5.7L Stroker
Stock Truck Intake: 436 hp @ 5,700 rpm
MP Single Plane: 444 hp @ 6,300 rpm
Largest Gain: 25 hp @ 6,300 rpm
The horsepower curves show a number of things including the fact that the single-plane intake did indeed make more peak power than the long-runner truck intake. Measured peak to peak, the single-plane, carbureted intake produced 444 hp to the 436 hp offered by the truck intake. The difference was a healthy 25 hp out at 6,300 rpm, where the power output of the stock intake fell off rapidly.
Truck vs MP Single Plane for a 5.7L Stroker
Stock Truck Intake: 440 ft-lbs @ 4,800 rpm
MP Single Plane: 410 ft-lbs @ 5,000 rpm
Largest Gain: 40 ft-lbs @ 3,500 rpm
In terms of torque production, it was really no contest. The peak torque production between the two intakes differed by 30 ft-lbs, but the difference was as great as 40 ft-lbs elsewhere in the curve. The additional torque offered by the long-runner factory intake in the low and mid-range is why it is usually chosen over the single plane for most street (and production) applications. The truck intake bettered the single plane up to 5,800 rpm, where the majority of driving takes place.
Test 2: Stock Truck vs Stock Magnum Intake for a 5.7L Stroker
In what will become a recurring theme, this test was run on two factory intakes that offered not just different designs for the different vehicles, which includes Magnum and truck, but represented significant changes in runner length. Since runner length determines where in the RPM range the intake was designed to promote power, choosing the right runner length for your Hemi application (and intended usage) is very important. Every bit as important as the runner length is the application; changes in things such as cam timing and displacement can determine the optimum intake design. For this test, we assembled a 5.7L stroker that displaced 370 ci. Thanks to a forged crank from Scat combined with rods from Speedmaster and forged pistons from JE, the stroker was ready for anything we could throw at it. For this test, the Hemi was equipped with a mild Comp XFI 260 H-13 cam that offered a .522/.525 lift split, a 208/212-degree duration split and 113-degree lobe separation angle (LSA). In addition, a set of ported 5.7L heads from Total Engine Airflow was part of the recipe. In addition to the porting, the Hemi heads received a valve spring upgrade from Comp Cams.
The stroker was used to compare a pair of stock intake manifolds. Both the factory truck and Magnum intakes offered long runners to promote torque production, but which one of them was best? Before testing, it was necessary to make a few minor changes to a couple of the factory components. First, we had to convert the drive-by-wire throttle body to manual operation. The next hurdle was to mill down the truck front cover to allow installation of the Magnum intake. The truck and Magnum 5.7L Hemis featured different FEAD mounting and therefore different front covers. The truck cover interfered with the position of the throttle body on the Magnum intake. A few minutes on the mill was all that was necessary to produce the desired clearance. Run with the truck intake, the Hemi stroker produced 436 hp and 440 ft-lbs of torque. After installation of the Magnum intake, the stroker produced 434 hp and 463 ft-lbs of torque. Despite a 2-hp deficit in peak power, the Magnum was the clear winner as the intake design offered a dramatic increase in torque production up to 5,400 rpm, with gains as great as 35 ft-lbs.
For this test we converted the electronic, drive-by-wire throttle body to mechanical operation. This allowed us to use a FAST XFI stand-alone management system.
Equipped with the Magnum intake and manual throttle body, the 5.7L stroker produced 434 hp and 463 ft-lbs of torque.
Stock Intake Shoot Out-Truck vs Magnum with a 5.7L Stroker
Truck: 436 hp @ 5,700 rpm
Magnum: 434 hp @ 5,800 rpm
Largest Gain: 22 hp @ 4,300 rpm
Tested on this mild 5.7L stroker application, there was little to choose from between the truck and Magnum intakes in terms of peak power. Although the peak numbers were within a couple of horsepower of each other, the curves differed dramatically before the power peak. The Magnum intake offered considerably more power through the majority of the rev range, up to 5,400 rpm.
Stock Intake Shoot Out-Truck vs Magnum with a 5.7L Stroker
Truck: 440 ft-lbs @ 4,800 rpm
Magnum: 463 ft-lbs @ 4,200 rpm
Largest Gain: 33 ft-lbs @ 3,500 rpm
The torque curve shows the real difference between the truck and Magnum intakes. The dramatic difference in torque production offered by the Magnum intakes makes us wonder why it wasn’t offered as the production manifold on the 5.7L Hemi trucks to begin with. Measured peak to peak, the magnum bettered the truck intake by more than 20 ft-lbs, but the differences were even larger elsewhere along the curve.
Test 3: Stock vs Kenne Bell Throttle Body Test on a KB Supercharged 6.1L
About the only thing better than a modified Hemi is one with a supercharger. Nothing adds zing to a Hemi like boost from a Kenne Bell, twin-screw supercharger. As efficient and powerful as twin-screw kits are, boost is only the beginning. The reality is that superchargers are only as good as their induction system. Nothing chokes off the power potential of a supercharger faster than a restrictive throttle body or associated inlet components. Knowing this, the question now is how much power is a throttle body upgrade really worth? As an airflow device, the power gains offered by the flow restriction inherent in the stock inlet system depend on the modified power output. What this means is that the more powerful the engine, the more restrictive the stock components become. This should not come as a big surprise because the factory inlet system and throttle body were never designed for the elevated power levels offered by a Kenne Bell supercharger. The inlet system that Dodge designed to support 425 hp has no business on a supercharged engine making 600, 700, or even 800 or more horsepower.
It is important to stress here that when it comes to the power gains in part produced by the throttle body, the gains are dependent wholly on the engine combination. As a simple airflow device, the higher the power output of the test engine, the larger the throttle body required. As an example, installation of a larger throttle body (with airflow) capable of supporting 1,000 hp will be of little use on a 425-hp 6.1L engine equipped with an already oversized throttle body capable of supporting 750 hp. The 750-hp throttle body is already oversized for the application, so there is no need to upgrade on the naturally aspirated engine. A larger throttle body may be necessary on (draw-through) supercharged applications in which elevated power levels are more commonplace. While naturally aspirated Challenger engines with 600+ hp are less common, supercharged 6.1Ls exceeding 600, 700, or even 800 hp are everywhere. This round of testing on a Kenne Bell supercharged 6.1L illustrated that a throttle-body upgrade on a 770-hp application (17.3 psi) was worth an impressive 54 hp. Performing the same test on a stock (naturally aspirated) Hemi might be worth nothing.
To maximize airflow into the supercharger, we compared the stock throttle body to this 168-mm throttle body offered by Kenne Bell.
Run on the dyno with the stock 80-mm Hemi throttle body, the supercharged 6.1L produced 770 hp at a peak boost pressure of 17.3 psi.
Stock Throttle Body vs Kenne Bell Throttle Body for a Supercharged 6.1L
Stock TB: 770 hp @ 6,000 rpm
KB Oval TB: 824 hp @ 6,300 rpm
Largest Gain: 55 hp @ 6,300 rpm
Obviously the stock throttle body was plenty restrictive at this elevated power level on the supercharged 6.1L. Replacing the stock throttle body and adapter on the Kenne Bell supercharger with the larger 168-mm oval throttle body resulted in a jump in boost of 3 psi. This improved the power output of the supercharged 6.1L Hemi by as much as 55 hp.
Stock Throttle Body vs Kenne Bell Throttle Body for a Supercharged 6.1L
Stock TB: 781 ft-lbs @ 4,200 rpm
KB Oval TB: 801 ft-lbs @4,300 rpm
Largest Gain: 34 ft-lbs @ 5,400 rpm
Typical of airflow improvements, the gains were most prevalent at higher engine speeds. The throttle body change increased the boost pressure by 1 psi down at 3,500 rpm, but the torque gains were as high as 34 ft-lbs higher in the rev range. More flow into the blower equates to more boost out of the blower. A throttle body swap such as this may be worth nothing on a stock engine, but airflow is critical on a high-horsepower, supercharged application.
Test 4: Stock SRT8 vs Mopar Performance Single Plane for a 5.7L Crate Hemi
Much like the comparison in Test 1, this test involved a substantial change in runner length. If you are interested in a graphic representation of the change in runner length, check out the results of Test 8, where the only change was runner length (good stuff). Although there was a distinct design difference between the Mopar Performance single-plane intake and the factory SRT8 intake, the major difference was the tuning effect offered by the difference in the intake runners. The single-plane design featured not only significantly shorter runners, but uneven lengths for the various cylinders (actually pairs of four). This means that four of the cylinders will be tuned for one RPM range and the other four will be tuned for another. Specifically, the longer (outer) pairs will be tuned for a lower engine speed than the four (shorter) inner runners. By contrast, the SRT8 intake featured consistent runner lengths that ensured even power production from all cylinders.
The test was run on a 5.7L Hemi crate engine from Mopar Performance. The MP crate engine was equipped with a Comp 273 cam, long-tube headers, and a Holley 750 HP carburetor with the MP single-plane intake. The STR8 was run with a FAST XFI management system and 60-pound injectors. The comparison was less about carbs versus computers. The carb version of the MP intake made slightly more power than the port-injected, EFI configuration. Equipped with the MP single-plane and Holley carburetor, the 5.7L Hemi produced 469 hp at 6,500 rpm and 411 ft-lbs of torque at 5,300 rpm. Torque was not the single plane’s strong suit, but it did produce a healthy peak power number. After installation of the SRT8 intake, the peak power dropped to 455 hp at 5,700 rpm, but torque jumped to 450 ft-lbs at 4,800 rpm. The MP offered 14 hp on the big end, but lagged behind the SRT8 below 5,800 rpm.
Swapping intakes on the Hemi was fast and easy. The SRT8 offered exceptional power up to 6,000 rpm. The short-runner single-plane design excelled at top-end power production but fell short below 6,000 rpm.
Stock SRT8 vs MP Single Plane for a 5.7L Crate Hemi
Stock SRT8 Intake: 455 hp @ 5,700 rpm
MP Single Plane Intake: 469 hp @ 6,500 rpm
Largest Gains: 20 hp @ 6,600 rpm
Looking at just the peak numbers, we see that the single-plane intake offered more power than the factory SRT8, but that isn’t the story here. Is a small gain in peak power better than having more power through the rest of the curve?
Stock SRT8 vs MP Single Plane for a 5.7L Crate Hemi
Stock SRT8 Intake: 450 ft-lbs @ 4,800 rpm
MP Single Plane Intake: 411 ft-lbs @ 5,300 rpm
Largest Gains: 41 ft-lbs @ 4,800 rpm
As much as we love an extra 14 hp, we love extra torque through the entire rev range even more. The torque curves illustrate the real difference between the SRT8 and the single plane; the factory long runners offered considerably more torque (both peak and average).
Test 5: Stock SRT8 vs Wilson Billet Intake for a Modified 6.1L
Some tests we run to illustrate what works, some we run to illustrate what doesn’t, and some we run just because the performance component under scrutiny is just so amazing. The test on this Keith Wilson Hemi intake falls into two of these categories. Once again, we applied the intake to a mild test engine similar to ported heads in Test 1 of chapter 2. The billet intake offered by the flow wizards at Wilson manifolds was significantly more intake than the mild 6.1L could utilize. We have seen these intakes on some of the most powerful Hemi combinations on the planet (usually with boost). The misapplication of the intake was by design because it shows there is a proper engine combination for every intake and our mild engine was not the ideal choice for the billet piece from Wilson. The second category this intake falls into is that it is a billet intake and looks like a piece of art when you pull it from the box. That alone is reason enough to want one, but knowing it comes from Wilson provides added confidence that the amazing form follows function.
We wanted to test the Wilson intake but didn’t have a 1,500-hp, twin turbo Hemi handy, so we ran what we had available. In this case, it meant we tested a modified 6.1L Hemi built for (eventual) boost supplied by a Kenne Bell. The 6.1L block was machined and treated to forged pistons from JE and a blower-proof ring package from Total Seal. Comp Cams supplied the XFI 273H-14 cam that offered a .547/.550 lift split, a 224/228-degree duration split and 114-degree LSA. A FAST XFI/XIM management system was used for all testing, while the guys over at Dr. J’s ported the 6.1L heads. All testing was run with American Racing long-tube headers with 18-inch collector extensions and no mufflers. Run with the factory SRT8 intake, the 6.1L produced 491 hp at 6,200 rpm and 453 ft-lbs of torque at 5,100 rpm. After installation of the Wilson intake, the peak numbers jumped to 510 hp at 6,800 rpm and 451 ft-lbs at 5,100 rpm. Once again, the shorter runners in the Wilson design offered more peak power at higher engine speeds (where race engines can take advantage of the extra power), but what the intake really needed was more cubic inches and some boost.
It was designed by the factory for this application, so it is not surprising that the SRT8 offered a good combination of average and peak power production on the 6.1L test engine.
If nothing but the best will do for your Hemi buildup, look no further than the Wilson billet manifold.
Stock SRT8 vs Wilson Billet for a Modified 6.1L
Stock SRT8 Intake: 489 hp @ 6,200 rpm
Wilson Billet Intake: 510 hp @ 6,800 rpm
Largest Gains: 19 hp @ 6,500 rpm
The Wilson billet intake easily bettered the factory SRT8, and (unlike other designs) did so without any significant losses in low-speed power. What the Wilson intake needed was actually more engine.
Stock SRT8 vs Wilson Billet Intake for a Modified 6.1L
Stock SRT8 Intake: 453 ft-lbs @ 5,100 rpm
Wilson Billet Intake: 451 ft-lbs @ 5,100 rpm
Largest Gains: 16 ft-lbs @ 6,500 rpm
In terms of peak torque production, the two intakes were within 2 ft-lbs of each other; the Wilson design came on strong after the torque peak.
Test 6: Single-Plane vs Edelbrock Dual-Quad Intake for a Modified 5.7L
This test was as much about the intake manifold design as it was about the method of fuel delivery. In this test, we compared an Edelbrock Dual-Quad intake with a pair of carbs to a single-plane intake with a 4-hole, EFI throttle body. The test engine was your basic 5.7L Hemi equipped with the usual modifications. The Mopar Performance crate engine was augmented with a new cam and ported heads. The original cam provided with the crate engine was replaced by an XFI 273H-14 grind from Comp Cams. The healthy XFI cam offered a .547/.550 lift split, a 224/228-degree duration split and 114-degree LSA. The cam required the use of a set of 26918 beehive springs and 762 retainers to provide the necessary coil-bind clearance and RPM capability. The last thing you want is to limit the performance of your cam with valve float. To ensure adequate flow for the new power potential of the modified engine, the stock 5.7L heads were given the once over by the guys at Total Engine Airflow.
Having run it on a number of other occasions, we knew what to expect from the MP single-plane intake provided with the crate engine. The design offered plenty of peak power (479 hp at 6,900 rpm in this case), but often lacked torque production lower in the rev range. The peak torque production of 404 ft-lbs at 5,300 rpm only hinted at the limited mid-range power production, but it became much more evident after we installed the dual-quad intake from Edelbrock. Not just any dual-quad intake, Edelbrock designed a dual-plane, RPM Air Gap, dual-quad manifold for the late-model Hemis. The new intake was designed to accept a pair of 500-cfm Edelbrock Thunder Series AVS carburetors. As we have come to expect from their namesakes, the Air-Gap design promised plenty of performance by isolating the charge air from contact with the heated engine. The real key to the power production was the dual-plane design. After installation of the Edelbrock dual-quad Hemi intake and carbs on our test Hemi, the peak horsepower output dropped slightly compared to the racy single plane, but was still adequate at 463 hp at 6,600 rpm. The peak torque output jumped, however, to an impressive 420 ft-lbs of torque at 4,900 rpm. The dual-quad intake outperformed the single plane up to nearly 6,000. Now toss in the cool factor and you have some very compelling reasons to top your Hemi with a trick induction system.
All of the powerful original Hemis had multiple carburetors, so it was only natural for Edelbrock to design a dual-quad intake for the modern Hemi.
The dual quad improved power production compared to the single plane up to nearly 6,000 rpm, but the single plane pulled ahead slightly on the big end. For most dual-quad Hemi owners, they will appreciate the look and extra grunt offered by the dual-quad setup much more than any missing horsepower past 6,000 rpm.
Single-Plane vs Edelbrock Dual-Quad for a Modified 5.7L
MP Single-Plane Intake: 479 hp @ 6,900 rpm
Edelbrock Dual-Quad Intake: 463 hp @ 6,600 rpm
Largest Gains: 20 hp @ 6,900 rpm
The dual quad offered improved performance and horsepower production up to 5,900 rpm, but the single-plane MP intake pulled away thereafter. The short-runner single plane was designed to improve power production higher in the rev range, but nothing looks as cool as a dual-quad Hemi!
Single-Plane vs Edelbrock Dual-Quad for a Modified 5.7L
MP Single-Plane Intake: 404 ft-lbs @ 5,300 rpm
Edelbrock Dual-Quad Intake: 420 ft-lbs @ 4,900 rpm
Largest Gains: 24 ft-lbs @ 3,700 rpm
The dual-quad intake from Edelbrock more than looked the part. The design improved torque production by as much as 24 ft-lbs, with substantial gains up to 5,800 rpm. The additional torque offered below 6,000 rpm will be much more useful than the loss in power above that point.
Test 7: Mopar Performance Single-Plane vs Speedmaster Downdraft IR Intake for a Mild 5.7L Hemi
Right off the bat, we should stress that the individual-runner intakes looked so amazing that most Hemi enthusiasts will chose it over any other intake based on looks alone. Nothing screams “look at me” like a downdraft EFI system with eight massive butterflies and full-radiused, polished air horns. These intakes are, however, not for the faint of heart, as tuning them for use as a daily driver can be difficult and time consuming. It’s not impossible. Just be prepared to spend some time getting the thing to idle, take throttle, and drive down the road with civility. The WOT stuff is easy by comparison; there is never a shortage of airflow from one of these systems. Many enthusiasts think that the massive airflow potential of these individual-runner intakes is the reason they make so much power, but the reality (for this test) is runner length produces the power. The single-pane MP intake offered more than enough flow to satisfy the power needs, but just check out the power differences lower in the rev range as airflow is less critical. Power gains that occur through the entire rev range are generally a function of runner length.
This particular test piece was run on a mild 5.7L Hemi equipped with a small Comp 260H-13 cam. The mild Comp cam offered a .522/.525 lift split, a 208/212-degree duration split, and 113-degree LSA. That the 5.7L pulled hard to 6,500 rpm is a testament to the high-RPM nature of the intakes tested. Run with the MP single-plane intake in EFI form with a 4-hole throttle body and FAST injectors, 5.7L produced 430 hp at 6,500 rpm and 391 ft-lbs of torque at 5,200 rpm. The mild cam and high-RPM intake were somewhat mismatched, but the combo produced impressive power given the minimal mods. After installation of the downdraft 8-stack induction system from Speedmaster, the peak power numbers jumped to 462 hp, which was at a slightly lower 6,400 rpm and 418 ft-lbs of torque at 5,300 rpm. Basically, the Speedmaster IR induction improved power everywhere, and would offer even greater gains on a wilder, more powerful combination.
The single-plane intake offered reasonable performance, but there are plenty of better intake systems available for your Hemi.
The individual-runner 8-stack system from Speedmaster looked amazing and offered plenty of performance.
MP Single-Plane vs Speedmaster Downdraft IR Intake for a Mild 5.7L Hemi
MP Single-Plane Intake: 430 hp @ 6,500 rpm
Speedmaster 8-Stack Intake: 462 hp @ 6,400 rpm
Largest Gains: 31 hp @ 6,400 rpm
The individual-runner intake offered impressive power gains through the entire rev range. Thanks to plenty of airflow and optimized runner length, the Speedmaster IR system improved the power output by as much as 31 hp.
MP Single-Plane vs Speedmaster Downdraft IR Intake for a Mild 5.7L Hemi
MP Single-Plane Intake: 391 ft-lbs @ 5,200 rpm
Speedmaster 8-Stack Intake: 418 ft-lbs @ 5,300 rpm
Largest Gains: 25 ft-lbs @ 5,200 rpm
The torque curve was even more telling because the long(er)-runner IR induction system improved torque output from top to bottom. Extra top-end power is always good, but more torque through the rev range will be even more useful.
Test 8: Effect of Runner Length of a Speedmaster Downdraft IR for a Mild 5.7L Hemi
Perhaps the best illustration of the effect of runner length in this chapter comes from this impromptu test on the Speedmaster downdraft system. With no adjustable intake system available from the aftermarket, the author whipped up something to work on the downdraft system offered by Speedmaster. Using a combination of slip-fit aluminum tubing, hose clamps, and duct tape, we were able to quickly adjust the runner lengths to optimize power production at different engine speeds on this 5.7L Hemi. The runner length acts as a tuning device to tailor the shape of the power curve. Longer runners optimize power production lower in the rev range than shorter runners. The downside to any given length is that there are trade-offs at the other end of the rev range. The additional low and mid-range torque offered by the longer runners is offset by a potential loss in high-RPM power. The opposite is true of short runners; they give up low and mid-range torque for optimization at high RPM. The idea is to tune the combination for the desired usage. Although the debate usually involves the generic terms long versus short runners, the best runner length is the one that provides the most average power production in the desired RPM range.
The downdraft IR intake from Speedmaster offered plenty of power over a standard single plane (see Test 7), but additional runner length can dramatically alter the power curve.
Having just run the test on the Speedmaster downdraft system (with the single plane), we decided to investigate the situation further by increasing the runners in the IR manifold. Run with the IR manifold as delivered by Speedmaster, the 5.7L produced 462 hp at 6,400 rpm and 418 ft-lbs of torque at 5,300 rpm. After extending the runner lengths by 4 inches, the peak numbers changed to 459 hp at 6,500 rpm and 432 ft-lbs of torque at 4,800 rpm. Note that the longer runners offered more torque but slightly less peak power than the shorter standard length. This trend continued after we increased the runner length another 2.75 inches (6.75 inches total). Run at this length, the peak numbers changed to 459 hp at 6,000 rpm and 443 ft-lbs of torque at 5,000 rpm. The longest runner length improved torque production by as much as 38 ft-lbs and dropped peak power by just 3 hp.
The author installed sliding runners inside the stock tubes to increase runner length. Additional runner length offered substantial gains in torque below 6,000 rpm.
Effect of Runner Length: Speedmaster Downdraft IR for a Mild 5.7l Hemi
Stock Runner Length: 462 hp @ 6,400 rpm
Plus 4 inches: 459 hp @ 6,500 rpm
Plus 6.75 inches: 459 hp @ 6,000 rpm
Largest Gains: 34 hp @ 4,900 rpm
Looking at the numbers, you might be tempted to pick the intake combination that offered the highest peak power. Unfortunately, man does not live by peak power alone. Each increase in runner length lowered the peak power number slightly but offered more power through most of the curve.
Effect of Runner Length: Speedmaster Downdraft IR for a Mild 5.7l Hemi
Stock Runner Length: 418 ft-lbs @ 5,000 rpm
Plus 4 inches: 432 ft-lbs @ 5,300 rpm
Plus 6.75 inches: 443 ft-lbs @ 5,000 rpm
Largest Gains: 38 ft-lbs @ 4,300 rpm
The results of this test on the Speedmaster downdraft system demonstrated the torque gains offered by changes in runner length. Increasing runner length on the downdraft intake dramatically enhanced torque production.
