By Jim Roal


I was a service technician at a large Ford dealer in Southeastern Washington State for 15 years. I worked mostly with the driveability (formerly referred to as tune-up), electronics, and electrical systems. In 1993 a local Mustang collector came to work for us as a salesman so he could afford his Mustang habit. At that point he owned over 20 Mustangs between him and his son. When the Cobra came out in 1993, he decided he had to have one but he wanted it to have a Paxton supercharger dealer installed just as he had done when he bought one of his early Mustangs back in the 1960's. That Cobra install started us in the supercharger business.


We became a Paxton dealer and installed over 15 of them between 1994 and 1995 on brand new vehicles with only factory miles. Another technician (Brian Holsten) and myself did all these installs. Most of them were on F-150, 5.8 trucks but we did a couple 5.8 Bronco's, a 7.5 F-250, a 4.0L Ranger, a 1983 Mustang, and a Lightning. We were installing these kits on vehicles so new that Paxton's kits did not always fit right. Several times we had to modify the kits to fit. We would call them and explain the problems but they had never seen or heard of it since these were too new. For instance, when R134a air conditioning came out, the A/C system interfered with the mounting brackets for the supercharger. Then we ran into extreme vibration problems caused by the engine fan spacer from the Paxton kit. One customer made us take the Paxton kit back off because the vibration was so bad. More and more, Paxton was unwilling to help and they simply denied our concerns were valid. At one point, they told one of our customers that it was our fault!

A Few Problems

We began looking into other kits. I have always been a fan of positive displacement blowers since they provide instant boost even at low RPM. This results in more power under the curve which, in turn, generally results in lower ET's at the track for a given boost pressure as long as you can hook it up. I looked into Kenne Bell and we decided to drop Paxton and become a Kenne Bell dealer instead. My first install was on an F-150 that we had installed a Paxton kit on earlier and the customer was very unhappy. Things went well and the customer was much happier with the Kenne Bell. One nice thing about the Kenne Bell is that there were no additional crank pulleys or fan spacers so the vibration problems were history. The Kenne Bell blowers were also much more fun to drive. It felt like you just gained 40% more displacement.

Both Paxton and Kenne Bell include A/C modification kits in their F-series supercharger kits which I never used. These cheezy barbed fittings and worm clamps would not only look bad, but would also leak given time. Instead, I would bend and adjust the factory lines for a proper fit. This also kept the factory warranty in tact in most cases which was important since these were new vehicles. The 1993 Cobra was this biggest challenge here since the discharge line (just above blower input shaft) needed to be twisted 180 degrees to fit properly. To do this I had to cut the locating tab on the discharge line to compressor fitting and carefully bend the metal line around without kinking it. I was skeptical at first but it actually worked out quite well. The Paxton kits only required very minor bending on the suction line and I installed the compressor upside down from their recommendation (which was actually right side up anyway).

Kenne Bell

Between 1995 and 1996 we installed about 10 of the Kenne Bell kits, again mostly on F-150 trucks. I installed one on an F-250, 5.8 which required extensive modifications to the thermactor system. I also installed one on a 1993 Cobra. The Cobra really turned out nice. Now matter how easy you stepped into the throttle in first gear, by the time it was to the floor tires were smoking. The car was completely stock other than the blower and the customer told me later he ran a 13 flat with it at a local drag strip. The Kenne Bell kits installed nicely in the F-series trucks too but the performance gains were not near that of the Mustang. I think the problem was the way the air charge entered the lower intake manifold. On the F-series kits, the intake air exited the blower on the bottom right into the lower intake. The problem was that the blower exit is at the front of the blower and was only a few inches long. The rear cylinders would not receive near the flow that the front ones did. It was a direct shot into #1 and #5 cylinders but the air had to make 2 sharp 90 degree bends, and travel more distance for #4 and #8 cylinders. The Lightning uses a system very similar to the Mustang blower kit so it does not have the same airflow problems. We had a couple of Kenne Bell blowers installed on F-series trucks develop noise over time. Kenne Bell was good about replacing the noisy units. Another problem was the belt drive system. Some of the trucks would eat the belt in less than 15k miles. They would also squeal under heavy load unless the belt was very tight.

Allen Engine Development

Now that I changed careers, I had not installed a supercharger for about 3 years so I was itching to do another one. First a fellow Cat employee here in Peoria started building an 87 Mustang GT. He installed TFS heads and a powerdyne 9psi blower along with other mods. I helped him get it running right. Then I was contacted via email from a guy in Chicago who was interested in installing a supercharger on his '98 Mustang GT. I pointed him toward the Allen kit and did the install for him. It was the best quality kit overall and the most difficult and time consuming to install. If I had a 4.6L, I would choose the Allen kit. The car ran great. It was very responsive and provided instant boost. It also had very little ping even on regular fuel and no timing modifications. The blower was silent under normal driving conditions but you could hear it under boost. I thought the sound characteristics of the blower were just right. Then another guy called to have an Allen installed on his '98 GT too. The a 3rd, and so on. On the 3rd install, we also installed 30lb/h injectors, a C&L 80mm MAF, and an adjustable fuel pressure regulator (and did not install the FMU). He had already upgraded the exhaust as well. Even with the automatic trans, that car produced almost 300HP at the rear wheels and ran a 13.1 quarter mile at 105mph. Then I installed one on a 2001 Bullitt and got 335HP and 370ft*lbs at the wheels.

Fuel Systems

That same customer who had the Cobra with the Kenne Bell also had a 1993 Lightning which we had installed a Paxton on a few years earlier. Along with the Paxton, we installed an additional fuel pump kit from Paxton which was wired through a relay that was controlled by a boost pressure switch. The fuel pump was in series with the stock fuel pumps so when it was not running, the stock pumps would force fuel through it. The additional fuel pressure during boost really helped. In fact, once we put the fuel pump on, we were able to set the ignition timing back to stock. The pressure switch system was touchy however. Since the lightening had dual fuel tanks, fuel return was a real problem. When the Paxton fuel pump kicked on, the fuel would return to both tanks. This was due to the way the factory fuel pump modules worked. When a factory fuel pump module was turned on, a valve inside would open the return port for that tank by using fuel pressure to actuate the valve. When the Paxton pump kicked in, fuel pressure between the factory fuel pump module and the Paxton fuel pump would drop very low. This would close the return valve. The fuel pump modules had relief valves to allow return pressure back into the tank in if it was too high. The problem was both modules would see the high return pressure and open. Normally, you would not be in boost long enough for it to be a problem. If you set the boost pressure switch to come on under higher boost pressure, the engine would starve for fuel and surge due to the restriction of the Paxton pump which was not yet running. If you set the switch so the pump came on earlier, you would run the pump too much and transfer fuel between tanks causing gas to run out of the fuel filler. I finally learned how to solve this problem years later on my own truck. I removed the fuel pump module and dissassembled it to access the fuel pump inside. As I has suspected, this pump was nearly identical to a Mustang fuel pump. So, I bought a 255lph Mustang fuel pump (like a 1993 application) and installed it in my fuel pump module. I had to cut the plastic connector piece off the fuel pump and bend the electrical connections over a bit to make it fit but it works great. I think a 190lph would have been much easier to fit because it has more room near the electrical connection. You can buy 190lph Mustang fuel pumps for about $100. I used a Walbro.

Along with installing supercharger kits, we also serviced supercharged Fords. Some of these were not kits we installed but other brands like Vortec and Powerdyne. A friend of mine at the dealership bought a 1995 Saleen SR which was factory equipped with a Vortec supercharged 5.8. He bought the car with only 600 miles on it but when we first drove the car it ran real poor. Under moderate to heavy load, the engine would misfire and surge. We found the spark plugs all fuel fouled so we installed a new set of Motorcraft plugs and fixed the misfire. When we drove it again the surge was still very obvious and we noticed the fuel pressure would peg the 100psi fuel gage Saleen installs in the center of the dash. The fuel pressure would then fluctuate wildly while the car was surging. A call to Saleen proved unhelpful. They were more concerned about the car being out of their warranty (by time) than helping us fix the problem. We decided that the fuel system was cavitating under the high pressure so we monitored fuel pressure at several points along the system to find the problem. What we found was that the pressure between the stock in-tank pump and the Saleen installed T-Rex would go into a vacuum under load. Saleen had left the stock in-tank pump in place which was grossly inadequate for the 480hp engine even with the T-Rex. We also called Vortec and they advised us that the FMU was trying to boost the fuel pressure too high for 30lb/h injectors. We installed a 190lph in-tank fuel pump and recalibrated the FMU. The car ran better and stopped fouling plugs.

Most of the driveability concerns we experienced with supercharged vehicles centered around the fuel system. An FMU is really a poor way to get the additional fuel required into the engine. The problem is that the FMU increases the fuel rail pressure to between 8 and 14 times that of boost pressure. For instance, if you are using a 10:1 FMU, your fuel pressure will be 80psi at 8psi boost. A fuel pumps flow rate drops with higher pressure so you are actually decreasing the amount of fuel the system can flow by using the FMU. What you really need is more flow for a supercharged engine. The best way to accomplish this is with a package designed just for your application.

We installed a Powerdyne 9psi kit on a 1995 Cobra. The blower was very quiet and the car ran good but it did not feel right under full boost. We called Auburn Performance Equipment (APE, now out of business) and they had us install a 90mm MAF, 190lph in-tank fuel pump, 30lb/h injectors, and a special chip. Along with these goodies, we removed the FMU all together. APE claimed the improvements would be worth about 60-70hp and after driving the car I believe it. I think most the gains were from the larger MAF.


The factory late model ignition systems are very potent. Emissions controls and EGR demand a high performance ignition system to operate properly so that is what Ford builds. Factory TFI or DIS ignition systems are all you need until you get really crazy. Even the 480hp supercharged Saleen used the stock ignition system with no ignition problems. You will need to gap the plugs down to about .035" to .040" for a supercharged engine. At least one heat range colder is a good idea too. I would recommend Motorcraft plugs. If your car calls for AWSF-42 plugs install AWSF-32 (colder) instead. Be sure not to route plug wires too close together as this can cause induction crossfire. Ford had a Technical Service Bulletin for this problem. We began installing MSD boost retard systems with all out blower kits to help control detonation. With a boost retard, you could leave the base timing at the stock setting and retain fuel economy. Another option is a specially calibrated chip. Keep in mind the chip MUST be calibrated special for your particular application. Off-the-shelf chips will not work. In fact they can cause such severe detonation you could loose the engine. Overall I have not been impressed with chips at all. In fact, I avoid them if at all possible. In most cases, a MAF equipped engine can be tuned to support failry high power levels with properly sized injectors matched to an MAF designed to support them. I prefer C&L MAF sensors because they use the stock electronics in a modified housing with replaceable sampling tubes to match to injectors. Pro-M on the other hand uses some goofy electronics to modify the MAF signal. My experience with that approach has not been good. This works OK and it is simple to do. The best solution is to use the proper size MAF and injectors, and custom software flash. I prefer to use factory Ford MAF's such as the Lightning 90mm. Several companies now offer the ability to reflash the PCM instead of using a chip. This avoids the reliability problems associated with aftermarket electronics while still giving the ability to properly calibrate the powertrain for the changes made by supercharging.

Selecting a Supercharger

There are 2 basic types of superchargers used in kits today, centrifugal and positive displacement. Centrifugal superchargers build boost relative to engine RPM. The higher the RPM, the more boost they build. Most of the 6psi to 9psi centrifugal kits don't build any measurable boost until about 3,000 RPM. The higher pressure kits will start to build boost at lower engine speeds. Higher pressure, intercooled, centrifugal superchargers work OK in full race applications on an engine with a high RPM powerband and a drivetrain that keeps the rpms high.. The street kits can be quite disappointing. There are good reasons that very few professional race teams in only a few types of races select centrifugal blowers. I don't know of any major manufacturer using a centrifugal blower today. Back in the 1940's they were used much more. They are certainly better than no blower and they will give you the peak power gains. For a daily driver, they boost is never there whan you want it. You have to rev the heck out of the engine before you get to enjoy boost.

There are 2 types of positive displacement superchargers used in kits today. The older and more common type is the Roots. This uses 2 counter rotating lobed rotors to force air into the engine. The 2 rotors are either identical or mirror images of each other. The other type is a Lysholm, or screw type. They use 2 counter rotating rotors much like those in a Roots except the 2 rotors are different than each other and the lobes are twisted from one end to the other (like a screw). One is male and the other female. Both of these superchargers types (Roots and Lysholm) act much alike as far as boost characteristics. Both force air from the inlet to the outlet in a positive way, not just from centrifugal force. These superchargers build boost right off idle and keep that boost all the way through the RPM range. They will give you large torque gains and you will get these gains throughout the RPM range. These kits are great for daily drivers and tow vehicles, as well as race applications. In fact, top fuel dragsters use Roots or Lysholm blowers. Historically, more supercharged race applications use Roots blowers than any other type. Ford, GM, Mercedes, Aston Martin, and Jaguar all chose Roots or screw blowers for their OE applications as well. As you can probably tell by now, my personal preference is a positive displacement supercharger. Eaton makes the best Roots supercharger for late model applications and kits. See my Superchargers page for Eaton based kits. Eaton Roots blowers are about 60% adiabatic efficiency. Holley also makes Roots type blowers for street and race applications. Most other Roots type blowers out there are from Detroit Diesel, or a copy thereof. The old Detroit based blowers are usually less than 50% efficient. Autorotor makes Lysholm style superchargers used in Kenne Bell and older Whipple kits. Eaton began making Lysholm (screw) blowers and Whipple is now the distributor for those blowers to the aftermarket. The Lysholm blower is more efficient under higher boost than the Roots. This results in lower exit air temperatures, less back-work, more net engine power, and less ping. However, they will typically draw a little more power that a Roots when not in boost assuming both types incorporated a bypass valve in the system. A bypass valve should always be used with any positive displacement supercharger for street applications.

The chart below shows a real horsepower curve taken at the wheels from a car on the chassis dyno. The "power" line shows the horsepower measured by the dyno. I have added 2 more lines to the graph. "PD HP" is the theoretical power curve using a positive displacement supercharger making 9psi boost. "Cen HP" is the theoretical power using a centrifugal supercharger making 1.5psi boost at 3000rpm, and 9psi boost at 6000rpm (typical). This chart assumes equal supercharger efficiency.

Considering the fact that acceleration on a flat surface (neglecting rolling and aerodynamic resistance which is the same in all cases anyway) is affected by power by the following equation: acceleration = (power)/(mass*speed). It is clear that more power at any point in the rpm range will increase acceleration, assuming perfect traction. Therefore, the positive displacement blower shown above will give much better acceleration at say 3200rpm than the centrifugal. At 6000rpm they both provide the same acceleration. However, we do not have high power cars with continuously variable transmissions (CVT). If you did, then the centrifugal would be fine since you could always maintain 6000rpm under hard acceleration. Without a CVT, your engine rpm will vary between the shift point in the current gear, and the rpm after shift into the next gear. This is usually about 2000rpm to 3000rpm between gears. So the example above showing the 3000rpm to 6000rpm is a good one to demonstrate the value of positive displacement blowers over centrifugal blowers. In this example, the peak acceleration was the same but the average acceleration was 22% better for the positive displacement supercharged car. Most wastegated turbos will give near full boost by 3000rpm (on an engine with a 6000rpm redline) so the turbo will give similar results as the positive displacement blower in this example. Again, this all assumes equal supercharger efficiency. Turbo's will almost always be more efficient than any supercharger.

Don't forget Turbochargers

Turbochargers actually have the most power gain potential of all supercharging methods. Turbochargers take advantage of energy that is normally lost out the exhaust. The turbocharger uses exhaust pressure to drive a turbine connected to a common shaft with an intake turbine which compresses the intake charge. The back-work of a turbocharger can typically be around 1.5% whereas a typical centrifugal supercharger is more like 5%. Positive displacement superchargers can be even higher. Many of the highest power density engines are turbocharged. Turbochargers have been used widely in many forms of racing as well as OEM applications. Turbochargers work even better on diesel engines where they actually improve fuel economy and reduce emissions. On spark ignited engines they have a few drawbacks. First, the have a cut-in threshold which is usually 1/4 to 1/3 redline. Below that engine speed, no measurable boost will occur. Once you exceed the cut-in speed, they still have the lag problem. Turbochargers will give huge torque gains all the way from just after cut-in to redline. Turbochargers work very well with automatic transmissions. You can powerbrake the engine and get the turbo producing a small amount af boost. Releasing the brake and applying full throttle will result in nearly instant full boost. This method works well for drag racing. Camshaft selection for a turbocharger is important. Because turbocharger system can have as much as twice the exhaust backpressure as they make boost, high overlap cams don't work well with most turbo systems. It is important to know what the backpressure to boost ratio is to select the proper turbo cam. A well designed turbocharger system can actually give more boost than backpressure and in those cases, having overlap will still work well just as it would in a naturally aspirated engine.


An intercooler will significantly improve nearly any forced induction system. It is not air pressure you want but rather air density. As air is pressurized, it will heat up. That is just a law of physics we are stuck with. However, if an intercooler is employed, we can remove most of that added heat and really increase air density while also reducing ping tendency. 130F degree air at 7psi will yield more power than 250F degree air at 9psi. Adding an intercooler will reduce boost pressure for 2 reasons. The first is due to flow restriction. The second is due to cooling of the air. While restriction is not a good thing the temperature reduction certainly is. Intercoolers also reduce back-work by reducing pressure for a given air density. When you add an affective intercooler, you will need to change the supercharger drive ratio to get your full boost back. When you do though, you will have higher air density and more power than you did before the intercooler. On turbocharged cars, you can just locate the wastegate line to a point after the intercooler but before the throttle. If you can fit and afford an intercooler, get one! The engines' flow capabilities (called volumetric efficiency or VE) is still the limiting factor in filling the cylinders. Lower density means less air mass enters the cylinders in that volume. Higher density will have more air mass in the cylinders for that same volume.

There are 2 basic types: air-to-air and water-to-air.

The air-to-air (ATA) is simpler and it is a more efficient method for long term constant boost situations. The total resistance to heat flow is less in an ATA since it only has 2 boundary layers and the metal to flow though. ATA intercoolers are generally used on diesel engines since diesels will be in boost at normal highway speeds and the system is much simpler that a water-to-air intercooler.

Water-to-air intercoolers have their own advantages. On spark ignited engines, where periods of boost are short, the water can store heat (or cool if you want to think of it that way). When not in boost, the water will drop to near ambient temperature. Then when you hit boost, heat can be quickly stored in the water and removed from the air. The heat transfer from the air, through the metal, and to the water is better than an ATA intercooler. The total heat flow resistance when you consider both heat exchangers is worse however. This system is much more efficient at low speeds and works very well for drag racing and street applications in spark ignited engines. The heat can be temporarily stored in the water and dumped later as you drive. The water-to-air intercooler will allow a system design with a much shorter intake path which will also reduce restriction. For drag racing, very cold water and ice can be used (if you have it set up for ice) giving fantastic cooling for the 1/4 mile runs.

Compression Ratios and Supercharging

It is often said that supercharged engines require very low compression ratios. This is not totally true. Certainly for the street you do not want a compression ratio below about 8.5:1 or you will have horrible fuel economy, poor cold starting, and poor overall driveability. If you were building a top fuel dragster, then perhaps you would consider a low compression ratio so you could run 100psi boost. For the street however, it is best to have more moderate compression ratios. Take for instance the 2000 Jaguar 4.0 supercharged V8 with 9:1 compression and a supercharger, or the 2001 Porsche 911 twin turbo with 9.4:1 compression and around 12psi boost pressure. I am running 12psi boost in my BMW with 9.3:1 compression.

Combustion chamber design is important when you start running boost and higher compression. If you have really efficient combustion chambers, you can get away with more boost and compression, which will give more power and better mileage. Jaguar used the May combustion chamber design in their later V12 engines and were able to run 12.5:1 compression ratios on pump fuel. Rumor has it that they actually were testing some 14:1 engines on pump gas and they performed well but would not pass NOx emissions requirements. On the other hand, the old Ford 400M could hardy run with 8:1 compression on pump gas without rattling like crazy (as you can tell I don't like the Ford 351M/400M). The trick to good combustion chamber design is a short flame path, high quench/squish, and high swirl.

Other Engine Modifications

No matter how you are boosting the engine power, "O" ring head gaskets are always a good idea. The head gasket is probably one of the biggest trouble spots although most supercharged engines with less than 10psi boost should not have any trouble with the stock head gaskets.

The same things that make a naturally aspirated engine develop more power will work on a supercharged engine. A high flow exhaust system, better flowing intake manifolds, and better flowing heads will all give you gains just like they did on the naturally aspirated engine. Some people seem to think that if you supercharge you can ignore everything else. That is simply not true. The engine will only run as good as the weakest link. Supercharging will overcome many of these weak links to give you extra power but there are limits to what you can get out without other engine modifications.

Superchargers that draw through the throttle body or carburetor will benefit from larger throttle bodies and improved intake flow. The AFM power pipe is a good example. It is just a larger, better flowing intake tube for centrifugal blowers. It mounts between the MAF and the supercharger inlet. There have been reported gains of as much as 3psi boost from installing the AFM power pipe. Blow through systems, on the other hand, will see little if any benefit from larger throttle bodies unless you have added significantly more power..

Fuel Economy?

No form of supercharging will improve the fuel economy of a spark ignited engine, although the loss can be very low. The Eaton M90 for instance, draws less than 1/3hp at steady highway speeds in most applications. Turbochargers will increase backpressure although the loss in fuel economy under normal driving is still very low. In OEM applications, turbocharged and supercharged engines can take advantage of the increased torque and select overdrive transmission ratios accordingly. In that case, excellent fuel economy can be achieved. I have heard numerous Thunderbird Supercoupe (Eaton M90 blown 3.8L) owners claim as high as 36mpg at highway speeds. I owned a couple '88 Thunderbird Turbocoupes (2.3L turbo) that would get over 31mpg at 70mph. The owner of one of the 1998 Mustang GT's I installed the Allen supercharger on claimed he got around 26mpg on the highway with 2.73 gears. Expect a small loss in fuel economy for add-on supercharger or turbocharger kits although most of this will come from enjoying the extra power.


Supercharging, in my opinion, is the best way to make a very streetable, emissions legal car, fast. Installing a positive displacement blower like the Kenne Bell, Allen, Saleen, Whipple, or SVO Roots makes the engine feel like it gained 50% more displacement. This is not a power adder you can barely feel, its obvious. The fuel system setup is critical. Using an FMU is not the best way to get the fuel you need in my opinion. Large injectors matched to a large MAF, and larger fuel pump is. No matter what the kit instructions say, you can usually get away with bending the stock AC hoses so you don't need to butcher them with barbed fittings and worm clamps.

Last Updated: 6/19/2004


  • 15 years experience working as a mechanic at a Ford dealer specializing in driveability, electronic systems, and performance.
  • Professional installer of Allen, Paxton, and Kenne Bell superchargers. I have worked with many others including Vortec and Powerdyne as well.
  • Custom built my own turbo/intercooler system for a BMW 318i (see my cars page)
  • Ford SVT dealership team member.
  • Extensive Ford training and certifications including electronic engine controls (EEC/SBDS certified), ABS, and advanced engine performance.
  • Master Automotive and Master Heavy Truck certifications, as well as L1, from ASE.
  • Bachelors Degree in Mechanical Engineering from Washington State University.
  • Member SAE (Society of Automotive Engineers).
  • Currently working as a Mechanical Engineer for Caterpillar, Inc in Cat Electronics.