There are other configurations, such as the 997cc Cooper and the 970cc and 1071cc Cooper S, that are not mentioned as these are too expensive and rare to be used to boost the performance of other Minis. Some engine configurations are lumped together such as the 1300GT and Cooper S, despite the engines not being exactly the same. Differences in camshaft, cylinder heads and carburettion account for the differing bhp figures for engines of the same capacity.
Improving your BHP - Standard Engine Swaps
It is often said that the cheapest way to add more power is to add more cc's. This is certainly true of the Mini - 1275cc engines are usually cheaper than extensive modifications to smaller engines, and are more tractable when giving the same amount of power. However, they are noiser and vibrate more than the smaller engines. If you have a small-engine Mini, consider replacing it with a larger engine as your first step towards more power, as a larger engine is a better base for future modifications.
Minis already equipped with 1275cc engines can be improved by swapping for one in a higher state of tune, most commonly the MG Metro engine. This method has the advantages that the engine is all-standard and there is no need to dismantle it for its camshaft and cylinder head.
Modifying your Engine - Basics
Modifying your Mini engine for increased power is a huge field. For more detail, read "Tuning British Leyland's A-Series Engine" by David Vizard - the definitive guide, if slightly dated. Specialists can advise you on the engine spec that will best match your needs.
Sometimes, it is not practical to swap an entire engine. This may be for reasons of insurance, effort required, or cost of a good quality replacement engine, or if you already have a good 1275cc engine installed. In this case, a Stage 1 kit can be fitted. This usually consists of a new exhaust, LCB exhaust manifold, intake manifold, performance air filter and replacement carburettor needle, and is available for all engine sizes. These elements are vital for future modifications and represent the cheapest methods of extracting more power from the engine.
Expect an power improvement of 20-25% and an improvement in fuel economy. These kits can be fitted by anyone with basic mechanical knowledge.
Another possible modification is to replace the carburettor with a larger one. This does not provide significant advantages for engines in standard tune but should be a prerequisite for fitting a stage 1 kit.
To obtain more power than provided by the Stage 1 kit requires modifications to the engine. The next modification is generally to replace the cylinder head with one that has modified chambers and ports and larger valves - the cost of a replacement head varies according to the aggressiveness of the modifications.
The next component to be changed is the camshaft. All camshafts represent a compromise between low-end and top-end power - a duration of 260 degrees is about as long as you can go without noticing a loss in low-end power. A duration of 280 degrees or more results in less power than a standard engine (or at best, the same amount), until 2000-3000rpm for "street" cams or 4000+ rpm for race cams, where the power curve rises rapidly, suddenly providing huge amounts of power.
Longer-duration cams cause the engine to idle unevenly and require frequent slipping of the clutch at low speeds, and also cause poor fuel economy at low rpm. An engine with one of these cams is likely to develop less power than an unmodified car below 3000 to 4000rpm. In all the car can be extremely unpleasant to drive around town. Camshaft manufacturers can recommend the appropriate cam for your intended uses.
Re-boring the engine is the other direction to go in to increase power. Given the expense, it is best to start with a 1275cc block. Standard overbores are 1293cc, 1310cc, and 1328cc, although the block is commonly bored to 1380cc while retaining reliability. Bores larger than this can take the engine to approximately 1400cc but bore strength is compromised.
To further increase cc's the engine can be stroked - the crankshaft journals are offset-ground and smaller con-rods are used to increase the stroke. This is expensive but yields 1430-1480cc engines. 1600cc engines have been produced by this method but can only be considered experimental.
What to Expect
The more power, the more wear on the gearbox and engine. The gearbox is mentioned first because it was originally designed for an 848cc engine delivering 34bhp and has not been significantly strengthened since.
About 90bhp with a 1380cc engine is the limit for good all-round tractability and reasonable longevity of the engine and gearbox, without serious modifications to strengthen either. Even at this level, expect to replace gearboxes more frequently, and engine life will be shortened if the car is driven hard.
Around 100bhp is common. Around this amount of power, relatively expensive strengthened engine and gearbox parts must be used and low-end driveability is compromised. It is not cheap to set up an engine to deliver this amount of power reliably, and peak rpm will be somewhere around 6000rpm or beyond - an engine in this state of tune must be revved high to extract peak power.
Any increase in power beyond that of a standard 1275cc engine can have some compromising effects on the rest of the car. It is usually necessary to fit an uprated radiator, and the brakes may no longer be up to stopping the car if it is driven more aggressively.
Engine Swap Details
There are some considerations when swapping a non-Mini engine or a Mini engine from a different year into your car: