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SUPERCHARGER
FREQUENTLY
ASKED QUESTIONS
Some of the answers provided on this site
are candid replies to Emails sent to
Hans Pedersen and are included for your
ease of reference. If there are questions not answered in this FAQ please email
Hans and he will attempt to answer your question
as soon as possible.
What is a supercharger ?
A supercharger is essentially an air pump,
it allows a small engine to take in a similar volume of air (and fuel) as a
larger engine. The only reason a large engine makes more power is that it
converts a larger volume of air and fuel into energy.
A supercharged small engine achieves the same, but only when required, when
under load or full throttle, not during normal driving or cruising.
How does a supercharger improve power
output ?
The principal requirement to produce power
in the internal combustion engine is a precise mixture of clean air and fuel.
An engine's horsepower and torque output is directly proportional to the rate
at which this mixture enters the combustion chamber. In other words, the
density (or weight ) of the charge produces power, regardless of pressure, -an
engine responds to density.
Therefore the more mixture that can be crammed into the cylinders, the greater
the potential horsepower output will be.
What is an internal compression
supercharger ?
There are essentially two types of
supercharger, external compression and internal compression.
External compression superchargers (Roots type, Eaton etc.) are just an air
pump and actual compression of the charge takes place as it is pumped into the
cylinders and then backs up in the inlet manifold.
Internal compression superchargers (Autorotor - using the Lysholm principle)
actually compress the charge within the supercharger. This internal
compression ratio is the reason the Autorotor supercharger has the highest
thermal efficiency of any fixed displacement supercharger in production in the
world today.
These units give four primary advantages over all other style superchargers:
lowered discharge temperatures higher volumetric efficiency, reduced drive
power requirement and greater low rpm boost.
A cooler intake charge equates to a more dense charge = more power.
What difference does Internal Compression
make ?
Quite a lot. There are two important
parameters which determine how good a compressor (blower) is. One is
Volumetric Efficiency , the other is Adiabatic (or overall) Efficiency. The
twin screw supercharger is extremely efficient in both areas.
The high thermodynamic performance is due to internal compression of the
charge combined with very fine clearances rotor to rotor and casing to rotors.
The effect of this internal compression is to increase the density and
therefore the weight of charge per revolution and consequently reducing the
temperature rise usually encountered for a given weight of inlet charge.
Internal Compression - What
difference does it make in real terms ?
The cooler more dense charge can have the
same psi as a less dense charge. An internal compression supercharger
making 6 psi boost is going to make more power than an external compression
supercharger making seemingly the same 6 psi boost. Altho' the pressure
is the same the more dense charge produces more power. Some sources suggest
this equates to 1 - 2 psi more boost when compared to external
compression superchargers for the same fuel requirement.
The twin-screw supercharger has the inherent ability to produce impressive
boost pressures at low engine RPM's. No other supercharger can match the
Autorotor in this respect.
What is boost ?
A normally aspirated engine relies on
atmospheric pressure (at sea level = 14.7 psi. [pounds per square inch] or 1
bar) to push the inlet charge into the negative pressure (vacuum) area within
the cylinders. Boost is the amount of additional pressure created by the
supercharger. So if the supercharger makes 6 psi boost, the inlet charge
is atmospheric pressure plus 6 psi for a total of 20.7psi.
Bear in mind most engines struggle to achieve 70% volumetric efficiency.
With supercharging, even when the boost gauge shows 0 the engine is achieving
100% cylinder filling.
The difference in driveability must be experienced.
How
much boost ?
Every engine is different. Some engines can
take more than others.
It comes down to 4 basic elements;
Fuel octane rating to be used
Engine compression ratio
Engine susceptibility to detonation
Anticipated fuel and ignition control systems
Can boost be adjusted ?
Yes. Boost is adjusted by changing pulley
sizes. On the Hi-Flow kits a range of supercharger pulleys are available. The
smaller the pulley the faster the supercharger spins and more boost is
produced. Care must be exercised not to over-rev the supercharger.
Size range 1 Autorotor superchargers are rated at 18000 rpm continuous
(Mini and Spridget range)
Size range 2 Autorotor superchargers are rated at 15000 rpm continuous (MGB
and Triumph TR range)
Size range 3 Autorotor superchargers are rated at 13000 rpm continuous
Size range 4 Autorotor superchargers are rated at 13000 rpm continuous
How does Fuel octane rating affect the
compression / boost figures ?
Fuel octane rating determines the
"knock" sensitivity of the fuel. It is the effective compression
ratio (static compression ratio coupled to boost) which determines the fuel
requirement.
High compression limits the amount of boost that can be used.
Generally we
find that the 96 or 98 octane unleaded fuel (roughly equivalent to 91 - 93 octane US) is
the major restriction which limits power.
Each Hi-Flow kit is tailored to suit each individual installation.
How about "more fuel" ?
An engine must have the ideal air / fuel
ratio to produce maximum power.
Again every engine is different, but generally the best power is produced at
around 12 - 12.5 : 1. Additional fuel does not usually control detonation nor
produce more power. One of the best methods of controlling detonation is
octane.
How much compression ?
Again every engine is different. An engine
with lower compression can generally run more boost. Ideally, as much
compression and as much boost as the engine will take without detonation.
How about the dreaded detonation ?
By pushing the envelope in our quest for
more power, one very important factor must not be overlooked. Detonation. Again every engine is different.
In a well designed and maintained installation this should not be a problem.
If detonation (knocking) occurs - FIX THE
PROBLEM.
Common causes include:
FUELLING
low octane fuel, suspect fuel quality, mixture too lean (check for vacuum
leaks),
clogged jet(s) or injectors, insufficient fuel flow or pressure
TEMPERATURE
excessive coolant temperature, spark plug heat range too hot, charge inlet
temperature too high.
If I want more or less power, is that
possible, if so how ?
Yes.
We tailor each kit to suit the engine specification and chosen fuel. Boost can
be altered quickly and easily, say for daily driving using standard fuel, and
for a day at the track using a higher octane (such as AVGAS).
Many mod's which work with conventional tuning can also be successfully
employed on supercharged applications. A modified cylinder head, oversize
valves, modified camshaft (e.g. one of our "supercharger cams"),
electronic ignition and exhaust improvements all work well with a
supercharger.
Does my engine need to be rebuilt before
fitting the Hi-Flow Supercharger?
Our kit was designed to be fitted to a
stock engine.
Provided the engine is in good condition, and within manufacturers tolerances
there is generally no extraordinary mods. required in the milder stages of
tune.
Mini's, Sprites/Midgets and MGB's were supplied with a range of compression
ratios for various countries. Therefore we would need to ascertain the actual
ratio of each individual vehicle to establish just how much boost could be
safely used.
Bearing in mind that some of these vehicles are now over 30 years old, and may
have been rebuilt or modified with a higher than original comp. ratio.
Will installing the Hi-Flow Supercharger
increase wear on my Engine?
Actual power output has little bearing on
engine life.
Engine wear is determined more by how the engine is treated (i.e. maintenance
and driving style). Assuming the engine is properly tuned, properly
maintained, (with regular oil and filter changes) and driven in a similar
fashion, supercharging will generally not shorten engine life.
It is engine revs. which kill engines. With conventional tuning the aim with
MG engines is to run them up to 7000 or 8000 rpm. to make any real horsepower.
To achieve this, special cranks, rods, pistons, rocker arms, valves, valve
springs etc. etc. will usually be required. It is an absolute "pig"
to drive with very little "go" under 4500 - 5000 rpm.
You usually don't have to rev. a supercharged engine past 5500 rpm to make
max. horsepower.
An engine sees maximum component load the moment the piston changes direction
from going up to going down. There is a complicated (and commonly held) theory
that increasing the compression pressure, as the supercharger does, actually
reduces this maximum load and hence is easier on engine components.
A look at power loads versus inertial loads reveals doubling an engine's power
output does not double component loads at the same rpm. In fact far
less.
Driven hard continually, all cars will tend to wear out faster. The increased
torque and low down power provided by the supercharger kit generally equates
to using fewer engine rev's. hence lower wear rates.
Obviously, if you consistently drive your car hard, higher wear rates are to
be expected whether or not the engine is supercharged.
How much power?
A high peak power number, although very
impressive in "bragging sessions" to impress listeners, is not as
important as the area under the power curve which ultimately determines
driveability.
A high peak HP number with a really sharp rise followed by a rapid decline the
engine will presumably be extremely "peaky" and could be almost
undriveable in traffic.
Equally important is how smooth and linear the power curve is.
A well tuned combination will have a very smooth curve, without savage peaks
and troughs throughout the working rpm range.
For example the Hi-Flow supercharger conversions fitted with our
"supercharger" cam on the MG B Series engine display a
very rapid steady rise in power from the lower rpm range to a fairly flat peak
at around 5000 rpm. The torque curve rises rapidly to about 2800 rpm where it
is extremely flat to 4300 rpm followed by a gradual tailing off.
Also, never assume the power output quoted
in the Owners Manual is what your engine is producing. Particularly if
tested on a "rolling road".
"Rolling road" dyno's test output at the wheels whereas
manufacturers figures are usually taken at the engine flywheel. Particularly
during the horsepower race of the late 1960's and 1970's it wasn't uncommon
for a little exaggeration to creep into the quoted figures. Therefore don't be
surprised if your first visit to the dyno produces figures well below what may
have been expected.
Our first dyno test with an MG B was just such an occassion.
The Handbook quoted a figure of 98 bhp at the flywheel,
we tested a freshly rebuilt engine and we had 48 hp at the wheels,
Where did all that power go??????
With the Hi-Flow kit installed using a mild boost, tested under similar
weather conditions and on the same dyno with the same operator, we saw 85 hp,
more boost saw this climb to 100 hp. Can it be true that nearly half of the
engine's output is lost in travelling from the flywheel to the road
wheels?????
Specifications and availability subject to
change without notice.
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Supercharger Kits