MG Metro Turbo engine
Turbo Mini Forums

On 9th May, 2005 Tom Fenton said:
For comment please all

Well this seems to be a common question, so I thought I would attempt a write up to cover the main points, the question being, "Do I need to use a Metro Turbo engine as a base to turbocharge my Mini??"

In short, the answer is no.

The Metro turbo engine is based on a 1275cc 4 cylinder A+ ("A Plus") engine. This same engine was fitted to other vehicles, the main examples are Minis, but also things such as Allegros and Maestros, not forgetting the Metro of course.

The Metro turbo was developed in around 1982, as it soon became apparent that the then newly introduced Austin Metro, was capable of dealing with more power than the current "Sporty" MG 1300 model produced. It would have been possible to tune the MG 1300 (later badged as the better known MG Metro) further to achieve the desired power increase, but it was felt that the raucous, rorty nature of such an engine would not suit the Metro. It was therefore decided to produce a turbocharged version of the Metro, to give quiet, smooth power. Thus the MG Metro Turbo was born.

British Leyland at any time in history were always tightwads, and the early 80's was no exception, so the engineers working on the project were tasked with creating the turbocharged powerplant using as many of the standard compnents as possible from the existing MG 1300.

Therefore, the main changes that concern us as potential Mini Turbo owners within the engine itself are thus-

1) The pistons are the main difference, and are a thicker crowned dished piston, to withstand the increased forces in the combustion cycle due to the boost pressure.
2) Some of the early cars were fitted with a toughened crankshaft. However, this was not continued throughout the life of the Metro Turbo, and so is considered unneccesary by many people.
3) The cylinder block itself is the exact same casting as all the other A+ 1275cc blocks. There is no difference in any of them, meaning a donor block to build an engine from is easily located.
4) The cylinder head casting itself is essentially the same as the other 1275cc A+ head castings. Where the turbo head stands apart is that it is fitted with exhaust valves with a larger stem diameter. This was done to increase the contact area between the valve stem and guide, to allow superior heat conduction away from the valve head. These exhaust valves are also sodium filled. This makes a "genuine" turbo head easy to identify, as the size difference between inlet and exhaust valves is visible with the naked eye.
5) The turbo engine uses a higher capacity oil pump, as it has to also supply the turbocharger itself with lubricating oil. These pumps are still available as reproduction parts, and are interchangable on all of the A+ blocks.
6) The nose support bearing on the end of the first motion shaft is different (larger) on the Turbo engines. The reason this was done is not clear. The bearing in the larger Turbo size is still available.

So the next question, can I turbocharge my standard Metro 1300cc engine?

Well this is more tricky, yes and no.
The main point of concern is the pistons. The standard normally-aspirated pistons are not up to the spec of the turbo pistons. Therefore if retaining them for use in a turbo engine, boost levels must strictly be kept down, or else holed pistons are likely.
The cylinder head can be reused, although it is prefered to fit the turbo exhaust valves and guides with the larger stems. A number of people also use stainless race exhaust valves to good effect.
The other problem is that of Compression Ratio (CR). The CR of the Metro Turbo engine is 9.4:1. Although this is not large in normal terms, for a forced induction engine this is very high. Therefore it is desirable to reduce the CR where possible. Many people run a CR as low as 8.0:1 or less. CR can be lowered effectively by either enlarging the cylinder head chamber volume, or increasing the piston dish size. Of the two options, increasing head volume is easier, and can be done on your existing head.