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~Boost is Boost?~

You have no idea how many times I’ve heard the phrase “boost is boost.” From a performance standpoint, all boost is not created equal. That is to say 9 pounds of boost from one supercharger is not the same as 9 pounds from another, or is it? What might happen if you ran a motor with identical boost pressure supplied by a positive displacement supercharger, a centrifugal supercharger, and a turbocharger? If boost is boost, then adding 9 psi from each should theoretically provide identical numbers.

Obviously, this is not the case, as three identical power curves would make for a pretty dull comparison. In fact, not only does the power output differ at identical boost pressure levels-due to differences in inlet temperature and the horsepower required to drive the supercharger used-but the shape of the boost curve provided by each form of forced induction varies greatly.

In order to demonstrate how these factors affect performance, we subjected our test engine to three different forms of forced induction. (It might have been nice to include a twin screw compressor in addition to the traditional Roots-style, positive displacement supercharger supplied by Holley, but one was not available to meet our dyno schedule)

The first obstacle was to overcome was deciding on a test procedure. Our first though was to create a no-holds-barred slugfest consisting of maxing out the power available to each form of forced induction. This would mean running the centrifugal at or beyond the maximum recommended impeller speed, ditto for the rotor speed in the Roots blower, and yanking the reference line to the wastegate on the turbo. Running insane levels of boost is now what you’d call scientific, nor was it safe for our precious little test motor. Instead, we elected to run each supercharger/turbocharger at a maximum boost pressure to 9.5 psi. This number reflects a measurement of pressure and not actual airflow (or power output), but it’s a commonly understood yardstick and seemed the best method of regulating the entrants. Even regulated for maximum pressure, the three force-feeders produced decidedly different peak power outputs as well as overall power curves. Results are show in the sidebars throughout this story.

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~Conclusions~

Is this the end-all comparison between the various forms of forced induction? Hardly. The test actually produced more questions than answers, many of which ar addressed in the accompanying sidebars.

First of all, we were not able to get the desired boost out of the Holley 174. In truth, a 600hp motor would probably benefit from a larger Roots blower, possibly a 250 Power Charger or even a 6-71. Given the artificial load offered by the engine dyno, a better comparison would have been a chassis dyno. This probably would have shown the true (street) boost curves and power production.

Should the turbo be punished in this test by not allowing an intercooler? Would it have made a difference? WOuld a larger centrifugal supercharger, say a Novi 2000, produce a better curve? What would the results be had we run the tests at just 5 psi, or how about 15 psi? How easy is each system to install?

The reality is that any of the three systems offer a combination of advantages and disadvantages. In the end, only the use will be able to choose the optimum combination of power production, cost, and ease of installation.

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~Test One : Naturally Aspirated~

Rather than the usual small-block Chevy, our boost testing was based on a stroker 5.0L Ford measuring 327 ci and spec’d as shown in the chart. To start our test, the motor was run in normally aspiration form equipped with a 750 cfm Speed Demon carb and ignition timing was set at 34 degrees. Sans boost, the low-compression 327 produced 392 hp at 6,000 RPM and 386 lb-ft of torque at 4,400 RPM.

1. The 327 stroker short-block was assembled using all the right components. The Ford Racing A4 block was stuffed with a billet Scat stroker crank (3.200 inch stroke), forged Manley 5.09 inch rods, and probe racing 8.4:1 forged pistons.

2. We chose to top off the stroker with a set of out-of-the-box Edelbrock Victor Jr. aluminum cylinder heads featuring 2.05/1.60 inch valves, 210/75cc ports, and 60 cc chambers. According to Edelbrock, these heads flow 291/195 cfm at .600 inch lift

3. Our cam choice for both supercharged and turbocharged applications was an Extreme Energy XE266HR hydraulic roller cam. The dual pattern cam offered a 216/224 duration split, a .544/.555 lift split and a 112 degree lobe separation angle.

4. The naturally aspirated combo used the same Edelbrock Performer RPM intake that was later pressurized with the Paxton and HP turbo systems.


Baseline engine:

Displacement: 327 ci
Bore/Stroke: 4.030/3.200
Compression Ratio: 8.4:1
Block: Ford racing A4
Crank: Scat billet
Rods: Manley forged, 5.09 inches
Camshaft: *see story*
Heads: Edelbrock Victor Jr.
Intake: Edelbrock Performer RPM
Carb: 750 Speed Demon
Jets: 83/88
Air Bleeds: NA
Exhaust: 1 5/8 tube Hooker headers, 3 inch Flowmaster mufflers
Peak hp: 392 hp at 6,000 RPM
Peak torque: 386 lb-ft at 4,400 RPM
Average street hp (2,900-6000 RPM): 310
Average street torque (2,900 RPM-6,000 RPM): 386 lb-ft

*The normally aspirated motor was only pulled down to 2,900 on the dyno. The blower motors were pulled to 2,500 RPM.

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~Test Two: Paxton Centrifugal blower~

Our first contestant was the Paxton Novi 1200 supercharger available in a newly rereleased small-block Ford blower kit complete with the custom carb enclosure. For this test, we swapped out the 750 Speed Demon for a 750 Mighty Demon due to adjustable air bleeds that help with fuel-curve tuning when the carb is pressurized.

The Mighty Demon had previously been run on a number of supercharged/carbureted applications, including an 811hp 383 Chevy and was set with 89/91 jets and .028 inch high-speed air bleeds. Once the pulleys were set to deliver our target boost (the boost gauge registered a peak reading of 9.4, right under the maximum of 9.5 psi), the Paxton produced 617 horsepower, again at 6,000 RPM, and 561 lb-ft of torque at slightly-higher-than-naturally-aspirated 5,200 RPM. The carbureted Paxton kit added over 200 hp and 200 lb-ft.

1. The Ford carbureted supercharger kit supplied by Paxton included all the components necessary to bring boost to your little windsor. In addition to the Novi 1200 supercharger, the kit supplied by Paxton included the carburetor enclosure, blower mounting bracket, and all the necessary hardware and pulleys to complete the kit. You can order PN 1001839 with the blower on the passenger side or PN 1001840 on the driver side; both go for about $3,000.
2. The key to success of the carbureted supercharger kit was the carburetor enclosure, allowing the carburetor to operate as it would in normally aspirated configuration. All that was necessary was to boost-reference the fuel pressure regulator so that fuel pressure increased at the same rate as boost.


Paxton Novi 1200
Blower inlet diameter: 3.25 inch id
Blower discharger diameter: 2.33 inch id
Maximum safe impeller speed: 48,000 RPM
Test impeller speed: 45,405 RPM
Internal step ratio: 3.6:1
Carb: 750 Mighty Demon
Jets: 89/91
High-speed air bleeds: .028 inch
Crank pulley: 7 inch
Blower pulley: 3.3 inch
Peak hp: 617 hp at 6,000 RPM
Peak torque: 561 lb-ft at 5,200 RPM
Minimum boost pressure: 1.7 psi at 2,500 RPM
Maximum boost pressure: 9.5 psi at 6,000 RPM
Average street hp (2,500-6,000 RPM): 412 hp
Average street torque (2,500-6000 RPM): 494 lb-ft
Average race hp (4,000-6,000 RPM): 518 hp
Average race torque (4,000-6,000 RPM): 542 lb-ft

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~Test Three: HP Performance Turbo~

Next up was the turbo kit, which we ran through the same carb enclosure as the Paxton centrifugal setup. The turbo kit supplied by HP Performance was designed as a bolt-on for ‘86-’92 fuel-injected 5.0L Mustangs and includes an air-to-air intercooler, but we elected to eliminate the cooler as neither the Paxton nor the Holley blowers were so equipped.

Testing showed that the turbo’s inlet charge temperature reached only 190 degrees at 9.5 psi of boost, so an intercooler was not mandatory at this boost level. With no changes to the carburetor, the turbocharged 327 posted peak numbers of 600 hp at 6,000 RPM and a massive 618 lb-ft at just 4,200 RPM. In fact, the turbo exceeded 600 lb-ft from 3,900 to 4,900 RPM. Neither of the superchargers was able to exceed the 600 lb-ft mark.

1. HP Performance supplied this single turbo kit originally designed to run on an early Fox fuelie 5.0L Mustang. We ditched the intercooler and connected the turbo to the Paxton carb enclosure.

2. Since the kit was designed to fit a fuelie 5.0L, a few mods were necessary to hook it to our carbureted stroker. We reclocked the compressor housing to position the discharged straight up, then fabricated a discharge tube to connect the outlet of the compressor housing to the 90 degree inlet into the carb enclosure.

3. Boost pressure supplied by the turbo was regulated by this manual wastegate controller. Basically a glorified bleed valve, the Turbo XS controller bled the pressure signal to the wastegate.

HP Performance Turbo
Turbo inlet diameter: 2.4 inch
Turbo discharge diameter: 2.2 inch
Turbine housing A/R: .92
Maximum safe impeller speed: 100,000 RPM+
Test impeller speed: NA
Internal step ratio: 1:1
Jets: 89/91
Air bleeds: .028 inch
Minimum boost pressure: 5.7 psi
Maximum boost pressure: 9.5 psi
Peak hp: 600 hp
Peak torque: 617 lb-ft
Average street hp (2,500-6,000 RPM): 460 hp
Average street torque (2,500-6,000 RPM): 564 lb-ft
Average race hp (4,000-6,000 RPM): 555 hp
Average race torque (4,000-6,000 RPM): 585 lb-ft

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~Test Four: Holley 174 ci Blower~

With backup runs indicating that the turbo numbers were for real, we pulled the HP Performance kit and installed the Holley 174 mini-blower (PN77175FSB-1 for the satin finish kit). WHile the turbo and Paxton had blown through an Edelbrock dual-plane intake manifold, installation of the Holley blower necessitated a dedicated lower intake manifold. Like virtually all Roots-blower setups, the blower manifold featured rather short intake runners that can ultimately hurt torque production despite the impressive boost response of the Roots blower.

Change number two involved carburetion: The 750 Mighty Demon was swapped in favor of a 950 HP Holley. We employed the larger 950 HP carb because, unlike the turbo and centrifugal blower, the Holley 174 blower was forced to draw through the carb and not pressurize it; pressurizing the carburetor increases its flow capacity. Limiting the Holley to a 750-cfm carb would skew the results in favor of the turbo and Paxton.

Equipped with the smallest supercharger pulley supplied by Holley, the blower could only muster a peak reading of 8 psi of boost. Like the turbo and Paxton, the Holley blower produced peak power at 6,000 RPM. At a slightly lower boost pressure of 8 psi, the Holley produced 535 hp and 513 lb-ft of torque. Had we been able to produce a full 9.5 psi, we’d expect to see another 15-20 hp and a like amount of torque.

1. The Holley supercharged kit was quite complete, including a 174 Pro Street blower, lower intake, and a tensioner system for the eight-rib belt. Note the short runner length and open plenum design of the lower intake manifold.

2. Both Paxton and Holley supplied a variety of different pulley sizes to allow us to adjust the boost pressure. Unfortunately, even the smallest blower pulley supplied by Holley resulted in a peak boost reading of just 8 psi.

Holley 174 BLower
Blower inlet: standard square-bore carb flange
Blower discharge: 2.55x7.5 inch rectangular
Maximum safe rotor speed: 14,000
Test rotor speed: 11,145
Internal step ratio: 1:1
Crank pulley: 6 inch
Blower pulley: 3.23
Peak hp: 535 hp at 6,000 RPM
Peak torque: 513 lb ft at 4,600 RPM
Average street hp (2,500-6000 RPM): 394 hp
Average street torque (2,500-6000 RPM): 483 lb-ft
Average race hp (4,000-6,000 RPM): 472 hp
Average race torque (4,000-6,000 RPM): 197 lb-ft

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~Turbo vs. Blower Cams~

There is a great deal of confusion about what constitutes an optimum cam profile, especially for forced induction. Why might a motor equipped with a supercharger respond to a given cam profile differently than one with a turbo? The answer lies in the drive method of each compressor wheel to create airflow and boost. In essence, the turbo relies on exhaust back pressure to produce boost pressure, and it is this presence of back pressure in a turbo application that can create specific cam needs.
More than other forms of forced induction, turbos are sensitive to overlap, the time when both the intake and exhaust valves are open. This occurs during the exhaust stroke, while the piston is forcing the exhaust out the open exhaust valve. As the exhaust valve is closing, the intake begins to open. The problem with this situation is that (in most turbo applications) the exhaust back pressure exceeds boost pressure. It is a common misconception that excess cam overlap will allow the boost to bleed from the intake tract into the exhaust. On turbo applications, the opposite is true: The excess exhaust back pressure can flow through the open exhaust valve and into the intake tract or simply limit the flow of boost pressure into the cylinder. Neither situation is good for power. Naturally, this reversion is not a problem ons upercharged applications.
While this reversion istuation is a possibility of excess overlap, there are a great many variables that can contribute to the success of a given turbo application. Things like cylinder head flow balance, desired operating range, and exhaust manifolds all play a part in choosing th optimum cam for a turbo motor. Perhaps the most important part of the equation is the turbo itself. Here is a very simple rule to remember regarding turbocharging: You will always make a great deal more power with the right turbo and wrong cam profile than the right cam and wrong turbo.
It is possible to choose a turbo that will produce backpressure readings that are equal to boost pressure readings. In this instance, the reversion is not as much of an issue, and the optimum cam timing will be quite different than with the presence of excessive backpressure. With so many variables, perhaps the best way to determine an effective cam for a given application is to test a few different profiles. Before running this comparison, the turbocharged 327 test motor was subjected to a series of cam tests. It is a widely held beliefe that turbo motors work best with mild stock cam profiles. THe power graph shows otherwise, as the XE266HR cam added 35-40 hp on the turbo 327 as compared to the stock 5.0L Mustang cams. Interestingly enough, similar power gains were posted with the superchargers.

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~Horsepower-Curve Comparo~
Going into the test, everyone expected the Roots blower to produce more low speed power, the turbo more midrange, and the centrifugal more top-end power. Unfortunately, life is not so black and white. These horsepower curves clearly demonstarte why peak power is not considerably less important than average power production.
If someone asks you to choose between a motor that makes 617 hp and one that makes 599 hp, there would seem to be little choice. After all, isn’t 617 hp always better than 599 hp? This bench-racing philosophy is exactly why I included the power curves. Note that the Paxton Novi 1200 produced the highest peak powr, 617 hp at 6,000 rpm. THe turbo, on the other hand, produced 599 peak horsepower.
Now forget the peak numbers anc check out the rest of the cruve. From 2,500 rpm to 5,700 rpm, the turbo posted significant power gains over the Paxton. Given equivalent vehicles, the turbo would easily motor away from the centrifugal in an acceleration contest. The minimal power gain above 5,700 would not overcome the huge power losses experienced throughout the rest of the rev range. In a street car, most of the driving is done from idle to 4,000 rpm or less. It is in this rev range that both the Roots and turbo offer more power than the centrifugal.

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~Torque-Curve Differences~
Given the relatively low rpm of our test motor, the torque curves are even more telling than the horsepower curves. All three power-adders added a good 150 lb-ft, but check out the respective shapes of the four torque curves. Note that the torque curves produced by the Holley blower nearly mimics the shape of the stock curve. This seems unlikely given the fact that the Holley 174 kit included a dedicated blower manifold.
The lower manifold offered a dramatic decrease in runner length compared to the Performer RPM used on the normally aspirated, turbo, and Paxton versions of the 327. Apparently the flow rate of the Roots blower combined with the change in intake effectively cancelled out each other, and we wound up with a torque curve that mirrored the baseline motor.
The Paxton supercharger literally bloted onto the normally aspirated motor, running the same intake and exhaust, yet the torque curves were quite dissimilar. Adding the Paxton increased the peak torque speed from 4,700 rpm to 5,200 rpm. Adding the turbo actually reduced the engine speed where peak torque occured, from 4,700 rpm down to 4,200 rpm. The turbo offered massive midrange torque production, the only system to exceed 600 lb-ft. Need more convincing?
At 4,000 rpm, the turbo was more than 100 lb-ft stronger than either the Roots or cnetrifugal. It should be noted that an engine dyno loads a motor differently than what would be experienced ont he street. The steady load at 2,500 RPM allowed the turbo to spool up more quickly than it would in the car.

84stang

Thanks Jeep and everybody else. But i think jeep broke it down pretty well. I think i am going to go ahead and try the turbo setup. Atleast it will be different than most people. Ill let you guys know what happens.