The following versions of the 1.8T engine can be found fitted to the UK-spec Octavia with manual gearbox:

AGU 110KW/150bhp Octavia FWD 10/96 > 07/00

AUM 110kW/150bhp Octavia FWD 08/00 >

ARX 110kW/150bhp Octavia 4X4 08/00 >

AUQ 132kW/180bhp Octavia vRS 12/00 >

Dealing with the 3 current engines first, the AUM, ARX and AUQ share the same block, cylinder head, internal parts, intake system and throttle valve control unit, turbocharger, charge air cooler, exhaust manifold, injectors, spark plugs, lubrication system and fuel system (excluding fuel tank etc). Apart from the fact that the ARX (4X4) has a different return oil line for the turbocharger, and the AUQ (vRS) one different boot (rubber seal/pipe) on the charge air cooler, the 3 engines are identical. The ECUs for all 3 engines do however differ, as do the exhausts. The same clutch is used on all 3 engines (and by the AGU). The AUQ is fitted with a different crankshaft impulse sender to the AGU, AUM and ARX; and the AGU and AUQ use the same knock sensor and harness, while the AUM and ARX both use a different anti-knock sensor.

The AGU was fitted to the UK-spec Octavia 1.8T FWD between 10/96 and 07/00 and remains in production for other markets. Whilst the engine is substantially the same mechanically as the AUM, ARX and AUQ, the cylinder block and pistons are different, as are the cylinder head with valves and camshaft. Specifically, the cylinder head, camshafts, pistons, con rods and bearing shells are different, but the crankshaft, vibration damper, flywheel and clutch are the same. Other differences include the cable throttle, smaller injectors and turbocharger, and the absence of VVT. These differences mean that it is not as tuneable as the later versions. Moreover, unlike the later engines, the AGU was not designed for variable (extended) service intervals.

NB. The ARX initially shared the same turbocharger as the AGU, but from May 2001 is fitted with the same turbocharger as the AUM and AUQ.

Below is another brief overview of various 1.8T engines. This is not a comprehensive overview, but it will show the differences between the main engines used by Skoda and the other VAG brands.

AGU

Skoda Octavia 1.8T FWD (Elegance, L&K), 10/96-8/00

Also found in: Audi A3 1.8T and Volkswagen Golf GTI (earlier models)

Output: 150 hp

Drive: FWD

Emission: Non-VVT, EU3

Throttle: cable

Injectors: small

Turbo: K03

Gearbox: EVS, EGY

AUM

Skoda Octavia 1.8T FWD (Elegance, L&K), 8/00-now

Also found in: Volkswagen Bora 1.8T and Volkswagen Golf GTI (updated models)

Output: 150 hp

Drive: FWD

Emission: VVT, EU4

Throttle: drive-by-wire

Injectors: big

Turbo: K03sport

Gearbox: EBN, ENJ, ESV, FDC

AUQ

Skoda Octavia RS, 8/00-now

Also found in: Seat Leon 20VT Topsport

Output: 180 hp

Drive: FWD

Emission: VVT, EU4

Throttle: drive-by-wire

Injectors: big

Turbo: K03sport

Gearbox: EHA (Octavia RS, 5-speed, long gearing - Leon has 6-speed 'box!)

ARX

Skoda Octavia 1.8T 4x4 Elegance, 8/00-now

Output: 150 hp

Drive: Haldex 4WD

Emission: VVT, EU3

Throttle: drive-by-wire

Injectors: big

Turbo: K03sport

Gearbox: FEX (5-speed, short gearing)

Other 1.8Ts

APX: Audi TT, 225 hp, Haldex 4WD, K04, 6-speed

APY: Audi S3 (earlier model), Seat Leon Cupra R (earlier model), 210 hp, Haldex 4WD, K04, 6-speed

ARY: Seat Leon 20VT Cupra 4, 180 hp, Haldex 4WD, K03sport, 6-speed

ARZ: Volkswagen Golf GTI, 150 hp, FWD, K03sport, 5-speed

AWT: Audi A4 1.8T/1.8T quattro and Volkswagen Passat 1.8T, 150 hp, FWD or Torsen 4WD, K03, 5-speed

AWU: Volkswagen New Beetle 1.8T, 150 hp, FWD, K03, 5-speed

BAM: Audi S3/TT (updated models), Seat Leon Cupra R (updated model), 225 hp, Haldex 4WD, K04, 6-speed

BFB: Audi A4 1.8T/1.8T quattro (updated model), 163 hp, FWD or Torsen 4WD, K03sport, 5-speed

The 210 and 225 bhp engines as found in the Seat Leon Cupra R, Audi S3 and Audi TT have different internals and a bigger k04 series turbo, all which allow it to produce more power. A chipped version of these engines can reach 260-270 bhp.

Excellent info as i am currently doing a conversion. AUQ engine onto a 1.8t elegance with fex transmission. There is something worth noting about the transmission and my information is from a friend who works for a vag dealer.  The 5 speed on the auq fwd rs actually has the same gearing as the 5 speed fex 4x4 including the same final drive.