The simple reality is that to controlling a car's engine doesn't take much processing power, so the car's ECU can be extremely basic.

If you're interested in learning about modifying your car's ECU there are tons of resources out there. I'm really into Nissans, and I've been getting more and more interested in tuning my 300ZX TT. It's a car from 1989, so the technology is pretty simple. The ECU is a basic 8-bit computer, less powerful than a Gameboy.

My setup basically consists of:
1. Software to read/write ROM files from the EPROM (the chip on the ECU that contains fuel maps, timing maps, rev limiters, idle rpm vs temperature, and a ton of other stuff). I like NIStune, but TunerPro is also very good (and free).
2. $0.89 28 DIP socket
3. $4 EEPROM
4. $85 Moates burn2 EPROM reader/writer.

First you dump the EPROM's data, which you can actually do with the ECU in the car, using a Consult cable (a USB or serial cable designed to work with Nissan's proprietary CONSULT interface) and some simple software. Then you use your software to modify the ROM. For example, you can change the injector multiplier (K-value) and latency, allowing you to upgrade to fuel injectors that flow more fuel, and increase the rev limiter, remove the speed limiter, etc.

Then, you burn the ROM to an EPROM. You desolder and remove the stock EPROM from the ECU and replace it with a socket, which allows you to just pop in a new EPROM whenever you need.

Pic related: It's my ECU with the EPROM removed.