Remove the catalytic convertor - You can do this as the lambda probe is before the cat. You need to by an early front section (down pipes to the rear box near the rear axle), then weld an M18 fine thread nut onto the exhaust, just infront of the 1st exhaust box, to screw the lambda probe back into the exhaust.
The lambda probe must be put in, otherwise the ECU will put you onto an emergency get home scenario, which increase your fuel consumption & gives ****ty performance.
Doing these two has given better mid-range performance, a lovely induction roar & a better exhaust note,
Sniady: thanks for your reply. Can you give me more information about the supercharger? I have a 1991 Spider which I like very much, but I wonder if I can get maybe 20 30 more HP'S out of the engine. Thanks
rrudoy, you don't say where you live, but keep in mind that in most places in the US, eliminating or modifying any emissions equipment on a 1991 model will keep your car from passing state inspection and you'll be off the road.
Another thought from the "old fart" perspective (i.e. been there, done that) -- even assuming the emissions compliance issue is not a problem for you, adding 20 or 30 HP to a US-spec Bosch-injected 2 liter will not be an inexpensive process through any means. I rebuilt my 1974 2 liter last winter, just in parts I spent over $1,000 and I did all the work -- engine and trans removal and install, all the teardown, cleaning, measuring, and assembly. I spent another $300 or so on machine shop time. Most auto shops charge what, $60 an hour? I would easily have dropped another $2,000 on labor if I couldn't do my own work.
The twin-spark engine jamie mentions is not readily available here in the US, and the cheapest I've heard of someone getting one was about $800. Then they had to install it , not exactly a "drop in", it takes some work. And remember, it was never imported to the US, so you'll need to find a way to get it approved if you live in an emissions inspection area.
If you really want a more powerful car, buy one and save your spider for sunny day cruisin'. These cars have never been unusually powerful or fast, that wasn't their purpose.
If you're not in the US, just ignore my rambling. It still won't be cheap, but you probably won't face as many barriers to modifications.