Supercharging
Part 1
Part one is just an over view and some teaser photos to get your attention more good stuff to come.
Introduction.
What exactly does a supercharger do? A supercharger forces air and fuel into the engine. This occurs when the engine is under full throttle or under load, not at normal cruise or most normal driving. A large displacement engine makes more power than a small displacement engine because it can convert larger amounts of fuel and air into energy. A supercharger allows a smaller engine to do the same thing but only when extra power is actually needed.
A supercharger is really only a mechanical device used to overcome some of the inefficiency of the internal combustion engine. In a nonsupercharged engine, mixture enters the cylinder by the fact air pressure at sea level is 14.7Lbs per square inch but due to inefficacy’s in the system The volume thus admitted is obviously not the full capacity of the cylinder at atmosphere pressure, and as such the efficiency of the engine suffers. Various traditional methods can be employed to improve this inefficiency, such as high lift cams and polished inlet and exhaust ports, but these can have drawbacks.
Oversize ports and high lift cams can lose the engines bottom end torque, very high compression can lead to ‘detonation’ and a restriction of the type of fuel used e.g. Avgas.
The advantage of supplying the mixture to the cylinder under boost instead of the piston sucking in the mixture is obvious. The power developed from a fully charge cylinder is as much as a third greater than normal. In addition to the greater power available from an engine when " fully charged " the power curve can continues to rise with the engine speed far beyond the point at which the normal engine begins to fall off.
Engine Wear.
The effect of supercharging on engine wear, As the supercharged engine develops more power, the first thought is that there will be proportionately more stress on the engine parts. In practice we find that supercharged engines wear equally as well or even better than before, if the original rev limit is used. Supercharging only increases the firing pressure, whereas it is the inertia stresses of a reciprocating engine increase dramatically with increased revs an important factors as regards mechanical failures.
Compression Ratio.
For normal roadwork where a maximum boost of not more than 5 to7 psi. Is used it is not usually necessary to reduce the compression ratio. However more than 9.0:1 is not recommend. An ideal ratio for an average engine with a maximum boost of 7.0 p.s.i. would be approximately 8.0:1
Timing.
Ignition Setting for most road installations will require less total advance, a trip to the dyno will establish what is required.
Camshafts.
Modified camshafts can be fitted to supercharged engines to improve performance, but generally speaking a full race camshaft with a lot of valve overlap should not be used as this will result in a loss of boost pressure and power, as well as a large increase in fuel consumption. This is due to too much of the unburnt or partially burnt gas going straight through the cylinder and out through the exhaust pipe, but if maximum power is required this is a necessary trade off
Exhaust
All supercharged engines will benefit to a large extent from the fitting of a bigger bore free flow exhaust manifold and system, allowing the engine which is handling a far greater quantity of gas when supercharged to get rid of this additional gas flow.
Cylinder heads
As previously mentioned, some engines will benefit from a reduction in compression ratio, which can be achieved by modifying the combustion chamber and or change the pistons for a low compression set, depending on what is available. Once again if maximum power is required the head will require a full porting job as a normally aspirated engine.