| Duratec 2.0L-2.3L - 2005-present
The Duratec is the universal engine designed around Ford's Zetec for use in both Mazda and Ford cars.
It is an all alloy block that produces 108kW in the 2.0L Ford Focus and Mazda 3. The Mazda MX-5 NC also has the 2.0L Duratec with VVT and produces 118kW. The 2.0L Duratec was also found in the Ford Fiesta XR4.
The 2.3L which is found in the Mazda 3 SP23 and Mazda 6, comes with VVT produces 120kW and more torque due to it being a stroked engine. It is however a taller engine due to the increased block and sump size and VVT. (changed 23/3/10)
The Australian demonstrator has the 2.0L Duratec found in the Fiesta XR4. GBS can provide engine mounts, exhaust manifold, alternator kit (to remove air conditioning pump) and fuel kit containing high and low pressure pumps and a swirl pot.
Zetec 2.0L Brand New The brand new Zetec from GBS is the predecessor to the Duratec. It produces around 91kW with throttle bodies from GBS and is likely to pass emissions. It is an engine which will be ideal for spritely performance and spirited driving. For people who want to race more I would suggest the Duratec or F20C as an alternative for more power and upgradablility.
Other Engines
Other common engines include the Toyota 4AGE 20V, Nissan SR20DET and Honda F20C. Of course potentially any car engine can be used provided it is known that modifications (to fit it in, torsional rigidity requirements increase with more cylinders etc) maybe required and that it can pass emissions (ADR37/01 in NSW) for registration.
The Toyota 4AGE 20V - 1991-1998
 The 4AGE 20V is Toyota's high revving 1.6L small car engine. It has and still is the most popular Clubbie engine in Australia due to its minute size and weight as well as its power delivery. It is also used because with the T50 gearbox it is easily converted to rear wheel drive.Pictured is the Silvertop, however the latest version, the Blacktop, is most recommended as it has more chance to pass emissions. Emissions and availabilty are the two biggest adversities regarding the 4AGE.
The Nissan SR20DET - 1999-2002
The Nissan 2.0L turboed engine is powerful, widely available and already comes in rear wheel drive format. It is commonly used in South Australian clubbies as regulations are more flexible.The latest version is the from the 200SX S15 which produces around 170-90kW. As previously stated it comes with a rear wheel drive gearbox. The gearbox is larger than most and may not fit in without modification. The SR20DET also has power delivery quirks being a turbo. It is also getting difficult to pass them through emissions. They are also a tall engine and may require modifications regarding this as well.
The Honda F20C - 1999-present
The F20C has been used more recently in higher end clubbies as it is a naturally aspirated 2.0L engine that produces around 183kW. It is a newer engine that is found in the Honda S2000. It is also rear wheel drive already and the 6 speed gearbox is apparently one of the best around.
I have done a bit more research on the F20C and found that it will more or less fit straight in the Zero's chassis. The standard Honda engine mounts are also apparently almost ideal. In order to fit it in without modifying the bonnet height the engine needs a dry sump.
(Update: 17/7/10) I have talked to GBS and have learnt that the F20C has been put in the Zero and the standard S2000 engine mounts work on the chassis mounts. They do have to do some modification of the chassis which means the torsional rigidity may be different and will require testing and possible modification.
The Toyota 1ZZGE and 2ZZGE 2000-2005 
The Toyota 4 cylinder replacements for the 4AGE are found in the Toyota Corolla and have been used in a few Clubbies now. The 1ZZGE 1.8L is the more tame engine producing a healthy 93kW and is found is Toyota Corollas. 
The 2ZZGE 1.8L is the more intense engine producing 141kW. It is more rare as it's only found in the Corolla Sportivo and Celica. With standard ancillaries these engines should pass emissions. You must be careful of the oil pump as I have heard it can explode under at high revs causing oil starvation. TRD has a stronger oil pump apparently.
To my knowledge they have no dedicated RWD gearbox so an adapter plate would need to be made to join the engine to a RWD gearbox. The Toyota W55 gearbox has apparently worked with the 2ZZGE.
Engine Bay Dimensions
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Engine Choices
