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Background
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.
Paxton
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.
Ignition
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.
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.
Conclusions
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
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