Muscle Mustangs & Fast Fords
Ford Modular Motor Forced Induction Dyno Comparison - Boost Bash Part 1
Is all boost created equal? Obviously not or we wouldn't have a story.
Imagine installing nearly every available form of forced induction available for the four-valve 4.6 on the motor in a boost-bashing, charge-cooling, head-to-head test. Running all four of the different forms of forced induction on the same engine at the same boost level certainly demonstrated the advantages and disadvantages offered by each form.
In reality, the reason behind this particular adventure was exactly that, to illustrate inherent differences in the boost and power curves-not to crown an absolute winner. We will discuss shortly why it would be impossible to determine an absolute winner, but know this very basic fact: If one form of forced induction was vastly superior to all others on all levels, no other forms of forced induction would continue to exist. That we have so many forms to choose from only provides alternatives that might be best for our particular application. In the end, it will be the user and specific application that will determine the appropriate form of forced induction that best applies, but before you can choose, you need to see how they all stack up.
As luck would have it, we had the perfect test bed on which to run this boost bonanza. Our '03 Cobra crate engine (from the Ford Racing catalog) was not only factory-equipped with one of the methods being tested (Roots-style Eaton M112), but was also configured to easily accept the rigors of forced induction. The forged crank, rods, and pistons provided an ultra-sturdy reciprocating assembly on which to pump up the pressure, while the low-compression made such an exercise a safe and welcome proposition. The '03 Cobra mill had already proven itself plenty capable of supporting prodigious power levels, exceeding the 700hp mark at elevated boost levels (naturally with a blower upgrade) without ever so much as lifting a valve cover. Who knows exactly how much power these motors are capable of, but we may attempt to find out after running all four of our boost-meisters.
In addition to the reinforced reciprocating assembly, the '03 Cobra 4.6 also sported free-flowing four-valve heads. We were plenty successful in producing power from the two-valve GT motors in our previous "Mods for Two-Valve Mods" series, but even the best-flowing two-valve heads are no match for the stock four-valve Cobra heads. We will eventually take a look at ported versions of the Cobra heads, but for now, the stock heads seemed to offer plenty of performance.
Before getting to the test, we need to take a quick look at some of the problems inherent in such a comparison. The first obstacle to overcome was deciding on how to properly evaluate them. While enthusiasts might like the idea of running all four forms up to their absolute maximum power potential, this is both difficult and probably unrealistic. Sure, we could produce some impressive power results, but the reality is that not many (street) Cobras out there are running around with the blowers set on kill.
The vast majority are more likely dialed down to stun-meaning minor mods, additional boost pressure, and a few with blower or turbo upgrades. Short of the crazies that populate the Cobra classes at the dragstrip, how many have you ever run across sporting 25-30 psi? If we were running a comparison between two identical-sized superchargers, we would simply pulley them the same and see what happened, but since even the Kenne Bell twin-screw and Eaton M112 were sized differently (to say nothing of the Vortech and twin turbos), we decided to select boost pressure as our regulating device.
Since most Cobra owners are quick to crank up the boost well beyond the factory 8-9 psi level, we decided to start the comparison at 11 psi. To further illustrate the limitations of the factory blower versus the aftermarket upgrades, we also chose to run a high-boost 14-psi test. Running the two different pressures would (hopefully) serve to differentiate the boost and power potential offered by each form. When searching for a particular type of forced induction, more information is always better than less.
While a boost-to-boost shootout seems only natural, there are problems associated with choosing boost pressure to regulate the playing field. The main problem with using boost pressure is that since the four different forms produce decidedly different boost curves, where (in terms of rpm) do you take a boost reading? An example works well here. In the case of the Eaton supercharger, it provides a bell-shaped boost curve with boost being slightly lower at both extreme ends (high and low rpm) and highest in the middle. Thus the 11-psi max rule would have the Eaton blower spinning to produce a maximum boost pressure of 11 psi at roughly 4,500 rpm, where the pressure would have fallen to just 9 psi at the maximum rpm (for this test) of 6,500 rpm.
By contrast, the Kenne Bell twin-screw blower will make peak boost pressure at 6,500 rpm, not because the Roots blower produces better low-end response, but because the inefficiency of the Roots blower requires that it be spun so much faster (that the twin-screw) to produce the desired 11 psi. That the boost pressure offered by the Eaton falls off at a higher rpm is not a plus on the side of low-speed response but rather a negative on the side of keeping pace with the motor. Spun the same speed, the twin-screw design will offer more power and boost pressure (at any rpm), not to mention a lower charge temperature. To put this into perspective, the Eaton was spun over 2,000 rpm faster than the Kenne Bell blower to achieve 11 psi (at 4,500 rpm). That additional blower speed helped improve the low-speed power production where the airflow supplied by the Roots blower could keep up with the demand of the four-valve motor.
Obviously the centrifugal supercharger will also produce the desired peak boost level at the peak engine speed. The impressive power per pound of boost offered by the centrifugal supercharger (besting both the Roots and twin-screw designs) comes with a price. Though high-rpm (and peak) power is impressive with the centrifugal design, the low-speed boost response is significantly reduced compared to either of the positive-displacement blowers. Peak to peak, the Eaton is easily out-gunned by the Vortech, but Cobra owners do not live by peak power alone. Centrifugal superchargers (regardless of the nameplate attached to the supercharger), all behave in this manner. The boost curve increases with engine speed. In the case of our four-valve Cobra motor, the Vortech produced just 1.7 psi at 2,500 rpm before reaching a peak of 11.3 psi at 6,500 rpm. All the superior efficiency in the world won't overcome a boost pressure deficiency of nearly 8 psi at 3,000 rpm. It is at these lower engine speeds that the two positive-displacement blowers really shine. Note that neither of them matched the peak numbers offered by the Vortech at 6,500 rpm, but it is hard to argue with all that extra torque, especially on a street car.
The boost curve differential also applied to the twin-turbo kit from HP Performance (or any source for that matter). Unlike the superchargers, where boost pressure is a function of blower size and engine (and therefore blower) speed, the boost pressure supplied by the turbo is a function of turbo sizing, the power output (and attending exhaust energy) of the normally aspirated, and the load placed on the motor. For our purposes, the last variable is important, as the load placed on a stationary engine dyno is not the same as experienced on the street. While dyno testing, we loaded the motor at a predetermined engine speed (in this case, we started at 2,500 rpm) and allowed the motor to rev at a predetermined rate. In the real world, it might be difficult to duplicate this type of steady-state load at 2,500 rpm, unless you put it to the wood in Fifth gear going up a hill (something definitely recommended).
Despite the difference in the load experienced on the engine dyno, in chassis dyno testing and street driving, the HP kit has proven to be both responsive and powerful. Once loaded, the turbo motor can be essentially programmed (via a manual or electronic) wastegate controller to produce a set boost level. The wastegates open to reduce the buildup of exhaust pressure, thus limiting the boost pressure and power. We purposely chose a manual wastegate controller, which provided us a relatively consistent boost pressure of 11 (or 14) psi once there was sufficient exhaust energy to spool the 57mm turbos.
Ignition timing and air/fuel ratio were two other potential obstacles. Given the difference in boost curves generated by these different forms of forced induction, they will respond to different ignition timing curves. Since the centrifugal supercharger produces less boost pressure down low, it can be run with more ignition timing than the others. Is it fair in a direct comparison to skew the results by allowing one form to run a more aggressive timing curve than the others? In the end, we decided it was not and ran all methods with the same timing curves (23 degrees).
The same goes with air/fuel ratio, as the centrifugal supercharger can be run with a slightly leaner mixture down low. Obviously, octane comes into play here, but we purposely eliminated this variable by running 100-octane race fuel. The idea was not to see how much we could make from each on pump gas, but rather to illustrate the differences in the power curves.
A more detailed inspection of the various forms is in order, naturally starting with our baseline blower, the Eaton M112 supercharger (against which all others were eventually judged). This baseline status stems primarily from the fact that the Eaton Roots blower was chosen by Ford SVT as original equipment. As ecstatic as we are to have a force-fed Ford in the lineup (we can't say enough good things about this supercharged 4.6 Ford, so please continue with boosted Cobras in the future), the Roots blower simply cannot compete against the aftermarket blowers and turbos in terms of maximum power potential. If you've snuck ahead to check out the graphs, you'll realize that from a peak power standpoint, the Eaton was no match for any of the three.
What it did do was excel at producing low-speed boost (and torque) response. The centrifugal supercharger was easily outgunned at the lower speeds, but given free rein, the Vortech eventually caught and passed the Eaton. From an efficiency (hp per pound of boost) standpoint, the Roots blower will not measure up to any of the other three forms. What it does do is allow the motor to reach a predetermined power goal (and do so reliably) at the lowest possible price.
While the Eaton M112 may not be the prom queen, that's not going to stop this second-from-the-left cheerleader from spiking the punch bowl. True enough, the Kenne Bell twin-screw, Vortech centrifugal, and HP twin-turbo setup all promise (and usually deliver) hoards of performance, there are a great many Cobra owners out there who will never give up the factory blower. Whether on principle, economics, or factory loyalty, the supercharged Cobra came from Ford with an Eaton supercharger and by God, that's the way it is going to stay.
I seem to have gotten off track a bit there with all the cheerleader talk, but the Roots blower does have plenty to offer. In addition to the low cost (relative to the others), the Eaton provides immediate boost and therefore power response. Stick your foot in the throttle of an '03-04 Cobra and you are rewarded with something sorely missing on previous quad-cam Cobras, that something called torque. The immediate boost response provides impressive low-speed and mid-range torque. Too bad under-hood space constraints limited the runner length of the intake manifold or we would have even more of the glorious stuff present, but the current (ultra short-runner) configuration is somewhat masked by the rapid boost response offered by the Roots blower. Only the positive displacement twin-screw can match the immediate boost response of the Roots blower. This, combined with the improved efficiency (lower inlet charge temps and more hp per pound of boost) and additional power potential, is why Ford saw fit to select the twin-screw design for the wickedly fast and powerful GT supercar. The Eaton M112 Roots blower was simply not efficient or powerful enough to meet the needs of the 550hp Ford GT 5.4.
Next on the force-feeder list is the twin-screw design offered by Kenne Bell. Manufactured by Autorotor, the twin-screw design takes the positive displacement supercharger to the next level in terms of efficiency and performance potential. The Kenne Bell twin-screw combines the immediate boost response of the Roots with a reduced charge temperature and increased flow potential. To put this added potential into proper perspective, we ran a test on the Kenne Bell blower last month in "Mods for Four-Valve Mods, Part 2." Removing the Eaton M112 and installing the Kenne Bell 2.2L blower with the same pulley size upped the peak power output by an amazing 145 hp! Where the Eaton supercharger checked out near 540 hp, the Kenne Bell went on to exceed 680 hp.
For the '03-04 Cobra, the Kenne Bell twin-screw blower upgrade makes an attractive package since it requires neither exhaust system mods like the turbo setup nor punching a hole in the oil pan (both turbos and Vortech). While the Kenne Bell can't quite match the efficiency of the turbos or centrifugal superchargers for maximum power, we've seen these blowers pump out over 700 hp at the wheels on an internally stock '03 SVT. That power potential pretty much takes care of the 95 percent of the Cobra market. Combine the immediate boost response (and attending massive torque curve) with the ease of installation and prodigious power potential, and you have what makes for a pretty impressive blower upgrade.
The new kid on the four-valve block came from Vortech in the form of its Cobra upgrade kit. Naturally, the highlight of it is a centrifugal supercharger, in this case, a very potent V-2 T-trim. According to Vortech literature, the T-trim is plenty powerful and is capable of supporting 825 hp. Like the Kenne Bell blower upgrade, the T-Trim Vortech system can feed the needs of the vast majority of Cobra owners, even those looking to go into the 9s. Again like the Kenne Bell blower upgrade, the Vortech system retained the factory Cobra air-to-water intercooler and front-mounted heat exchanger. We've seen (actually data logged) this very efficient core knock 200 degrees out of the charge temperature at elevated boost levels-so feel free to crank up the boost.
Unlike the Kenne Bell blower, which directly replaced the like-positioned Eaton, installation of the Vortech required fabrication of an upper intake. The upper intake was designed to replace the missing Eaton supercharger and provide a mounting position for the stock (or aftermarket) throttle body and inlet assembly. The fabricated intake was designed to receive the factory air-to-water intercooler from the Eaton and positioned the throttle body in the stock location. Boosted air is fed from the front-mounted blower into a cast-aluminum discharge tube, through the throttle body and fabricated intake, and finally across the intercooler core. Tubing is also supplied with the kit to attach the factory mass air meter, but none of our blower kits utilized this since we programmed each using the F.A.S.T. engine management system.
The final contestant was supplied by the alien-friendly folks from Roswell, New Mexico. The gang from HP Performance has exceeded 730 wheel hp on a modified two-valve GT motor at 20 psi using its twin-turbo GT kit. Like the impressive two-valve GT kit, this four-valve Cobra setup featured a pair of powerful 57mm turbos. The standard kit comes with 46mm turbos, while huge 67mm units are available for those building an all-out race motor. The 57mm turbos tested on this Cobra motor were reported to be capable of exceeding 1,000 hp, so the kit has plenty of potential. Unlike the trio of blowers, the HP kit came equipped with a front-mounted air-to-air intercooler. The Eaton supercharger, intercooler, and even lower intake manifold were removed in favor of a factory '01 intake. The benefit of replacing the '03 Cobra assembly with the '01 intake was that the turbo combination was the beneficiary of the torque production offered by the additional runner length. Contrary to what some would have you believe, short-runner intakes do not make good blower or turbo manifolds. Runner length determines the effective operating speed of the motor, regardless of whether pressure is present or not.
Incidentally, we also ran the Vortech with the '01 intake (minus the intercooler), so check out the sidebar on the results of the effect of runner length in Part 2 of our "Ballistic Boost Bash."
There seems to be come confusion about comparing a twin-turbo system to each of the single blowers. That there were two turbos present compared to one of each of the superchargers was in no way an advantage to turbo kit. It is possible to produce the same power with a larger single turbo. The twin system was designed primarily for packaging, though commonly held theory suggests that two smaller turbos will spool up faster than one big one. In reality, the exhaust energy is divided in half, as each bank feeds one of the two turbos.
So is feeding one small turbo with half the exhaust energy that much better than feeding a single larger turbo with twice the exhaust energy? The wheel (compressor and turbine) sizes and weight have much less to do with spool-up than the exhaust energy supplied in relation to the flow orifice. Regardless of the theory, the twin system featured a pair of wastegates, an air-to-air intercooler, and custom tubular exhaust manifolds. The complete system featured Jet-Hot coating to keep the exhaust energy inside the tubing where it will do the most good. The boost pressure was controlled using a Turbo XS manual wastegate controller (essentially an adjustable bleed orifice). By bleeding the boost signal to the wastegate, they were tricked into staying closed (not bleeding off any exhaust energy), thus increasing the boost pressure. Of all the forced induction systems, adjusting the boost was easiest on the turbo kit.
I would like to go on record here and thank HP Performance, Kenne Bell, and Vortech Engineering for allowing us to put their systems to the test. It is a testament to the confidence in their products that such a banzai boost comparison was even possible.
Our '03 Cobra test mule was internally stock with the exception of a quartet of XE262AH Comp Cams cams recently installed. Externally, the four-valve motor was sporting a set of 1 5/8-inch Flowtech headers, a Meziere electric water pump (no accessories), and an Accufab throttle body and inlet system. The timing and fuel curves were controlled using a F.A.S.T. programmable engine management system. We kept the air/fuel ratio safe at 11.8:1 and the total timing stationary at 23 degrees under boost.
To produce the desired starting boost level of 11 psi, the stock Cobra (Eaton) supercharger pulley was combined with an 8.5-inch crank pulley. Thanks goes out to South Florida Pulley Headquarters for the Dial-Ur-Boost (DUB) hub and bolt-on pulley system that allowed quick pulley changes to dial in the boost pressure. Equipped with the stock blower and 8.5-inch crank pulley, the supercharged Cobra motor produced a peak boost pressure of 11.6 psi on its way to 572 hp and 533 lb-ft of torque. Please don't make the mistake of looking only at the peak power numbers. The supplied graphs tell a much better story that the peak numbers.
After a series of back-up runs, we removed the Eaton supercharger and swapped on the Kenne Bell Cobra upgrade kit. The upgrade kit included a 2.2L twin-screw supercharger designed to produce not only immediate boost response but also impressive ultimate power potential. Installing the Kenne Bell required removing the intercooler core from the bottom of Eaton supercharger. The 65-psi injectors were also swapped over and run in conjunction with an Aeromotive A1000 pump augmented with a Kenne Bell Boost-a-Pump to assure adequate fuel flow to our boosted combination. Kenne Bell also supplied a Boost-a-Spark to increase the supply voltage to our ignition. The Boost-a-Spark eliminated any misfires associated with insufficient ignition energy. The Kenne Bell was run with both a smaller crank (7.5 inches) and a larger (3.75 inch) blower pulley. The combination reduced the blower speed considerably (by roughly 2,000 rpm) compared to the Eaton, but that didn't stop the twin-screw from pumping out 629 hp and 525 lb-ft of torque at a maximum boost reading of 11.9 psi. This was as close as we could get to matching the peak boost pressures given the availability of pulley ratios.
After running the Kenne Bell, we installed the new Vortech blower upgrade. Like the twin-screw from Kenne Bell, the Vortech kit retained use of the factory air-to-water intercooler core used with the Eaton. It was necessary to fabricate an upper intake to facilitate use of both the factory intercooler and throttle body/inlet assembly. The fabricated Vortech intake positioned the throttle body in the stock location, as well as allowing use of the stock fuel rail. The downside to this fabricated upper intake is that it lacks any semblance of torque-enhancing runner length. Check out Part 2 for a test on the effectiveness of adding runner length, but for now know that the Vortech was being tested with the same intake used by the Eaton and Kenne Bell blowers. Equipped with the Vortech, the peak power numbers jumped to 667 hp and 532 lb-ft of torque. Judging by peak numbers alone, you'd think the Vortech was the way to go, but know that the added top-end power came with quite a cost. Given the linear boost curve supplied by the Vortech, the boost was off by as much as 8 psi and low-speed torque by 170 lb-ft compared to the Eaton. The beauty of the Vortech centrifugal supercharger is that you can easily crank up the boost pressure and power to levels not attainable by the Eaton, but at this relatively low (and equal) boost setting, all that extra torque produced by the Eaton looked tough to beat.
The final test involved the installation of the twin-turbo kit from HP Performance. Initially we were dreading the installation, thinking that the turbo kit must be much more difficult to install than either of the two superchargers. While this may be true to some extent in the car, on the engine dyno, hooking everything up was a breeze. We'd previously removed the pan to drill and weld a fitting for the oil drain-back used by the Vortech supercharger. Knowing we planned on testing the twin-turbo kit, we simply welded two fittings in place on the front of the oil pan. The Flowtech headers were replaced with the supplied (Jet-Hot-coated) tubular exhaust manifolds and hooked up to the pair of 57mm turbos, wastegates, and air-to-air intercooler. We relied on a fan and water mist to keep the core cool during our runs (the temps were monitored digitally). The inlet system on the HP kit from the intercooler to the stock throttle body was designed to be run with the mass air meter. Since we were running no MAF, we combined the Vortech inlet casting with the HP tubing to bypass the eliminated MAF. Using the Turbo XS manual boost controller, we upped the boost pressure until reaching our running pressure of 11 psi. Run at 11 psi, the turbo kit pumped out 750 hp and 679 lb-ft of torque, bettering all three of the superchargers.
Check out the supplied power and boost graphs for a complete rundown on how each system performed. As expected, the Eaton blower provided immediate boost response and despite the short-runner factory intake, produced impressive low-speed and mid-range torque numbers before falling off at higher engine speeds. The torque output of the supercharged four-valve motor exceeded 500 lb-ft from 3,100 rpm to 5,200 rpm. Spinning slower to keep the maximum boost pressure in check, the twin-screw lost out slightly at the lower engine speeds, but made up for the difference in blower speed with improved efficiency. Where the boost pressure fell off with the Eaton supercharger, the boost (and power) kept climbing with the Kenne Bell blower.
As expected, the Vortech out-powered the other superchargers at the top of the rev range, but lost out in terms of torque production big time compared to the positive-displacement blowers. Like the centrifugal supercharger, the turbos lost out at the lower engine speeds until the turbos came up on boost. Below 3,600 rpm, the Roots blower was the clear winner, but once the tach needle swung past 3,600 rpm, it was all turbo. How does an extra 178 hp and 154 lb-ft of torque sound? While the turbos were down by as much at 100 lb-ft at 2,500 rpm, they quickly made up for lost time by eclipsing the Eaton and producing the most impressive (post-4,000 rpm) power curves of the bunch. Check out the power and boost graphs and check back with us next issue when we crank up the test pressure on each system to 14 psi and bring you a test on the effect of runner length.