I tested that the UDP fits over the timing cover and installed the cover back. First I screwed each of the 5 bolts half way and tightened them in a second step. Installing the water pump pulley is a simple job, but it was annoying me that the pulley wouldn't stay put and kept wanting to spin.

With the timing cover in place the UDP was next. The OEM pulley is to be tightened to 103 Nm of torque. I think that since the new pulley is lighter it can be tightened a bit less, maybe 90 Nm or so.

Because the pulley has a smaller radius the belts have less contact with it. Therefore I think the belts have to be tensioned a bit tighter in order not to slip.

I got th Duralast 335K5 and 415K5 belts which are 1 size shorter than what I had with the original pulley (340K5 and 420K5).

I couldn't find a P/S and A/C belt in time and first ran the car without it. I forgot how much quieter the engine is without those two accessories running! Here's how the car feels now:

• Unlike other modifications the effects of the UDP can be clearly felt :)
• Higher idle. My idle was high as is with a 1.8 4g93 TB, with the UDP it's another 100rpm higher, at 1000rpm. To some extent this is good because it reduces the effect of accessory underdrive but it's using more gas at idle nonetheless.
• Faster rev-ups, especially through the first three gears.

I'm still thinking of a way to approximate the pulley's shape and volume to an equivalent disc so I can use some physics formula to calculate the freed-up power. It won't be much, but it's worth calculating because the physics involved are quite simple.

Just in case I ever need this info in the future, here are the dimensions of the accessory pulleys: