This article is written by Jim Roal and describes the basics of supercharging
Fords.
Details:
Background
I was a service technician at a large Ford dealer in Southeastern Washington
State for 15 years. I worked mostly with the drive ability (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 cheesy 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.
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.
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 re calibrated
the FMU. The car ran better and stopped fouling plugs.
Most of the drive ability 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. 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 nearly all cases, a MAF equipped
engine can be tuned to support very 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.
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,
inter cooled, centrifugal superchargers work OK in full race applications
on an engine with a high RPM power band. The street kits can be
disappointing. There are good reasons that very few professional
race teams in only a few types of races select centrifugal blowers.
They are certainly better than no blower and they will give you the peak
power gains.
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. Autorotor makes Lysholm superchargers used in Kenne
Bell and older Whipple kits. Eaton began making Lysholm (screw)
blowers and Whipple is not the distributor for those blowers to the aftermarket.
The Lysholm blower is more efficient under boost than the Roots.
This results in lower exit air temperatures and less ping. However,
they will typically draw 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.
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 turbo charged. 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 power brake the engine and
get the turbo producing a small amount of boost. Releasing the brake
and applying full throttle will result in nearly instant full boost.
This method works well for drag racing.
Inter cooling
An inter cooler will significantly improve 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 inter cooler is employed, we can remove
most of that added heat and really increase air density while also reducing
ping tendency. 130F degree air at 6psi will yield more power than
250F degree air at 9psi. Adding an inter cooler 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. Inter coolers
also reduce back-work by reducing pressure for a given air density.
If you can fit and afford an inter cooler, get one! 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 inter coolers 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 inter cooler.
Water-to-air inter coolers 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 inter cooler. 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 inter cooler 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.
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 drive ability. 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.
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
12psi 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.
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 back pressure although the loss in fuel economy
under normal driving is still very low. In OEM applications, turbo
charged 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
Super coupe (Eaton M90 blown 3.8L) owners claim as high as 36mph at highway
speeds. I owned a couple '88 Thunderbird Turbo coupes (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.
Currently working as a Mechanical Engineer for Caterpillar, Inc in Cat Electronics.
I live in Peoria, Illinois. I also have friends with similar background,
and still in the business, in Washington State. If you would like
to make your Ford more powerful, send me an email. I still enjoy modifying
Fords.
