Article on The Internet Mini Encyclopædia
For starters get a copy of "Tuning BL's A series Engine" and "How to modify your mini" both by David Vizard (aka God).
Ok, here is a quick run down on how to get more power:
All performance increasing methods can be classified into 2 distinct groups:
For a beginner look at the following areas:
Larger bore with less restrictive silencers lets the gas exit easily = more power to wheels. Beware that a too large a bore will reduce gas speed. Fast moving gas helps to suck the remnants out of the cylinders. 2" bore single box (silencer) is a good one.
Is the carb mouth diameter big enough? Large/twin carb lets the engine breath easier, but too large, again, reduces gas speed, necessary here to draw fuel from the jet. Use HIF38 1.5" on 998, HIF44 1.75" on 1275+.
Once the gas gets past the carb, can it flow smoothly to the head?
Avoid narrow/tight bends. Also keep the gas cool. Denser air holds more oxygen. Plumb in water heated manifolds in winter only.
The three areas above are known as 'Stage 1' tuning.
If you're feeling adventurous look at...
You can aid breathing by using larger valves and milling out the ports and chambers. You can raise the compression ratio (CR) by skimming the head face down to make the chamber volumes smaller. Stage 3 (big inlet, chamber/port work) heads are good.
Longer cam durations make the valves stay open longer and fill/empty the chamber better. Too long however will move the power band up the rev range and make the idle lumpy. MG Metro, Piper 285 or Kent 276/286 are good.
Crank/Clutch/Rods/etc will take less energy to accelerate if lighter.
A more powefull spark can be got from a sports coil. More precise ignition can be got from electronic systems. Use good plugs (eg: NGK). Don't bother with multi-electrode plugs.
Straight cut teeth zap less power than helical cut teeth.
Using a lower geared diff (eg: 3.44:1 as opposed to 2.95:1) will< give better acceleration but will mean higher revs when cruising.
The list is endless!
Join the Minilist share your joys and woes with 500+ mini owners worldwide via Minilist at Yahoogroups