The difference between a fuel injected setup and a carbureted setup is basically the difference between driving a 50 year old car compared to driving your modern car.
Carburetors work work using a principle called the venturi effect. This means that the air being sucked through the carburettor creates a vacuum in the fuel ports inside the carburettor which draws in the fuel needed. The big issue with this principle is that at low air-speeds (engine rpms), the venturi effect is not very strong and so the metering of the fuel is much less than optimal.
On a fuel injected engine the fuel is sprayed under pressure and almost perfectly atomised directly into the port. This means at very low rpms the fuel delivery is pretty much optimal. This helps give the engine much better response and a small increase in power at lower rpms. It also improves fuel efficiency as the correct amount of fuel is delivered.
At WOT (Wide Open Throttle) there is really no difference in power between the EFI and carbureted setups, as at maximum airflow both the carburetor and throttle body out-flow the heads and inlets.
Another big consideration and one often overlooked that makes a very large difference to the power delivery is the ignition management aspect of the EFI system, this is especially true on a boosted setup like our supercharger kits. The traditional distributor is essentially 100 year old technology and leaves a lot to be desired. To understand why this is it important you first need to understand the purpose of the distributor.
FACT: Fuel takes a pre-determined amount of time to burn. This means that from the time you ignite it using the spark plug, to the point that it has generated the maximum amount of force, is a fixed amount of time.
When this combustion takes place in an engine, the timing of this maximum force event is very important. It is essential that it occurs at exactly the correct time - essentially when the piston starts to travel downwards and with the crankshaft at the optimum angle to provide the maximum amount of leverage from the force operating on the piston pushing the conrod.
If this peak cylinder pressure happens to early or too late, the piston and crank location will either be less than optimal and the power will not be maximised, or in extreme instances this force can work against the piston and crankshaft and can cause engine damage.
The issue with automotive applications is that the engines do not operate at a fixed speed and so to ensure that the peak cylinder pressure event happens at the correct time, remembering that it takes a fixed amount of time to burn the fuel, when the engine spins faster you need to ignite the fuel earlier - essentially 'advance' the ignition timing event.
So in its simplest form the purpose of the distributor is to advance the ignition by the correct amount based on how fast the engine is moving. Mechanical distributors do this using weights and centrifugal force vacuum distributors do this using the vacuum in the manifold, which is at its maximum when the throttle is closed.
But this issue does not stop there as there are other factors involved in how long the fuel takes to burn. The other main factor that needs to be considered is pressure. At higher pressures the fuel mixture burns more efficiently and so take a shorter amount of time to burn. This means that if we do not adjust the ignition advance when under boost, the peak cylinder event will occur too early and can result in engine damage. So as well as advancing the ignition for rpms we also need to retard the ignition timing under boost, which sounds like we are now working against ourselves, which essentially we are.
This is where a regular distributor falls down as it can only advance the ignition timing. it has no provision to provide ignition retard under boost conditions. Normally the distributor is modified so that the maximum amount of advance does not exceed an unsafe amount, but this is essentially a massive compromise as it means that the advance is not optimal under non-boost conditions. There are some special vacuum advance distributors that can both advance and retard the ignition, it is even possible to retrofit these to the VW, but whilst much better, they are still a compromise.
Modern ignition and EFI systems do not use distributors, they control a coil pack directly. This has two distinct advantages over a traditional distributor. The first advantage is that the ignition timing can be fired at any time based on the manifold pressure, crankshaft location and rpms, which means that it is perfectly optimized regardless of whether it is under vacuum or boost. The other major advantage is the spark power, or spark voltage. This is approximately 10 times that of a traditional distributor and coil (about 50-60,000 volts compared to 6000v), which results in a much more consistent ignition event, especially in boosted applications where a stronger spark is required (higher cyclinder pressure requires nigher spark voltage to generate the spark). Even comparing a modern coil pack against traditional performance ignition systems such as magnetos (15,000v) and MSD systems (40-50,000v), the typical modern coil pack found on your average family hatchback it still heaps better. (MSD still uses a distributor which loses about half of the spark voltage at the air gap between the rotor and distributor cap.)
Both our ignition systems and EFI systems use a modern coil pack and manifold pressure sensors to deliver optimal ignition control.