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HP Twin Screw Rotors Image copyright (c) 2011.Supercharge your performance



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..

Tese 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 kompressor (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

Common causes include:
low octane fuel, suspect fuel quality, mixture too lean (check for vacuum leaks)
clogged jet(s) or injectors, insufficient fuel flow or pressure
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 ?

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 basically 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 MG A's and MG B'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 40 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 in traffic 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?????