MK3/MK4 8v Deep Dive Info:
ABA:
Bore Diameter: 82.5 mm (3.25 in.)
Stroke: 92.8 mm (3.65 in.)
rod, ABA:159mm
Compression Ratio: 10:1
Horsepower: 115 @ 5,400 rpm
Torque: 122 ft-lb @ 3,200 rpm
OBD1 ABA's through 95 had forged cranks, under piston oil squirters and the dual valve spring heads, this allows for a cam with more lift without getting spring bind. Some had EGR, but that only made a small difference in the intake manny and management.
OD2's engines came out in 1996. OBD2 ABA's lost the good crank and squirters, went to a single valve spring design, and had a hole in the head for the smog pump.
The OBD1 ABA is an amazing engine to turbo. Rumor has it, VW originally set-out to design a turbo engine when they ran out of time / funding and decided to up the compression (with different pistons) and use it as a torquey N/A 4 cyl. Who knows if that story is true ... it certainly is possible as it is very unusual for a 115hp VW to have oil squirters ...!
AEG, AVH, AZG:
Bore diameter: 82.5 mm (3.25 in.)
Stroke: 92.8 mm (3.65 in.)
rod, AEG:141mm
Compression Ratio: 10:1
Horsepower: 115 @ 5,200 rpm
Torque: 122 ft-lb @ 2,600 rpm
the stroke length is maintained through different wrist pin heights and the shorter block (ie aba’s deck height change is equal to the change in rod length). Thus the shorter rod creates better low down torque but the ABA will rev better and make a wider torque curve (higher in the RPM band).
AEG heads do have swirl ports though… The intent is to swirl the mixture as a way to help it fill the cylinder at lower rpms. Its believed that this is one of the reasons the AEG makes better low-end torque than the ABA. However, the shorter deck and larger bore the the AEG is the primary reason for the greater torque though; this intake design is just a way to help fill the cylinders because of the shorter stroke.
Basically, the biggest difference comes in the power curves. The AEG+ engines give more torque and horsepower at the lower rpms. Mostly this is due to different camshaft profiles and the individual intake manifold runners compared to plenum in (MK3). The torque and hp actually comes LATER in the rpm range due to the different manifolds and the camshaft profiles! The MK4 intake manifold is longer and has individual runners. The longer the intake manifold runners, the earlier the torque comes. The shorter the runners the later the torque comes.
The AEG cylinder head is nothing more than a modified cast of the ABA placed on a 1.8T 06A block. Thats right,it is the block that has the 3 oil return drains in it NOT THE HEAD
Other Notes:
The AEG cross-flow is not a tall block, dropping back to 220mm like the earlier block (1.8). Suspect this was to enable it to fit the more aerodynamic hood lines that are becoming fashionable. Bore spacing, head-bolt pattern and all of that stuff is the same, but that's about where the similarities end.
The tall-block ABA cross-flow motor was transitional, and would bolt into a mk1 Rabbit. No such luck with the AEG-motor. There are no provisions for a front motor mount and no provisions for a side mount - in fact, there are no provisions for the MkI/MKII motor mounts at all. There is no intermediate shaft, and thus no distributor- and no place for one. The MkIV cars all have a coil-pack ignition, and since you can't add a distributor, you would have to swap the whole ignition system over to use this motor in an earlier car.
ABA’s have external water pump, intermediate shaft driven oil pump and distributor
AEG has a timing belt driven water pump and a chain driven oil pump thats built into the front of the engine, Audi style. Because of that, the AEG crank is not interchangeable with previous motors. It's a heavy cast crank, so serious, high-revving performance mods are probably not a good idea. The water pump is all in the block, and there is a different style of oil pan. The cylinder head bolts are 10mm diameter rather than 11mm.
Can you swap a AEG head on a ABA bottom end?
The AEG 8V head has the same 5 oil drain holes that the 20V heads have. You will hear a lot that, If you want to put it on an ABA block you will have to plug the three middle holes. However, this is not true! It is true only on the 20V head because the 20V head sticks out over the block, the 8V does not...
I believe its best to run dual valve springs on the OBD1 ABA head either way. This head seems to be the best flowing, least extra work head to use. Using the AEG head the manifolds should be fine, but since its on an aba anything you buy in the way of exhaust should be tailored to an ABA because of it's taller block.
In summary of AEG head on ABA… If it's an OBD2 ABA it wouldn't be any difference. If you have an OBD1 head, then I would stick with that over an AEG.
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