306 1.6 - y2k+4
I've been offered a Peugeot 306 1.6 XS 98, it's a tidy example and done relatively low-mileage (52000). I know that it only has the 90bhp engine, but I wondered whether anyone knew what the revisions were to make it to a 100bhp and more economical (5mpg) in 2000? If it is just a new ECU, then I thought I could upgrade it...any thoughts?
306 1.6 - DP
If I recall correctly, the more powerful engine in the later 1600's is a 16 valve unit (still used today in the 206 XSi), whereas the pre 2000 1.6 is the old 8 valve TU engine which dates back to the Citroen AX / Peugeot 205 era.

I've not tried the 16 valve version, but I've driven a couple of 8 valve 306 1.6's and been very impressed. This engine works really well in the 306. Plenty lively enough for daily driving and it's one of those revvy engines which feels more powerful than it is. Good on fuel too - mine did about 38 mpg even driven hard. Also very DIY friendly if you do your own maintenance.

Purely as a drivers car, I still rate the 306 as the best mid sized hatchback ever built. Not quite as capable as the Focus, but so much more fun.
306 1.6 - P 2501
The 1.6 in 8 and 16 valve is certainly a great engine for the 306. Very lively and willing, and plenty of power for the car.

Have to agree with you DP though, the 306 as a drivers car is probably the best ever...
306 1.6 - Ross_D
The 100bhp (actually its 98bhp and 100PS) and the 90bhp engines are identical really, it was an ECu change that increased the power. The newer engine has 2 lambda probes, post and pre catalyst. The changes were prompted by emissions regs.