Metro Turbo Engine Myths
Turbo Mini Forums

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.

Great info, Tom! Timely, too! I'm going to share this with another board, if you don't mind...

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1967 Cooper 's' Turbo build @ www.drjing.com

Just link it, we like the extra hits.
Good post, tom, useful info. Just FYI everyone, our engine started off as an SPI..

yep good stuff tom, good to see some more info coming into the beginner tech bit couple of other things worth mentioning that you could edit in if you feel it fits in with the theme of the post

- turbo head can also be spotted i think by having a screwed blanking plug where the bypass hose normally comes out, i think all other heads either have the bypass or this isn't even drilled ie. left as cast

- turbo cam is actually a standard 1275 metro cam, not a mg metro item

- Turbo inlet valves are the same as standard 1275 metro (33.20/33.32 head diameter according to leyland workshop manual) as opposed to the bigger MG Metro inlet valves (35.58/35.71 head diameter). Exhaust valve size does not vary across the engine range (29.25/29.38 head diameter) but some recent measurements by turbodave show some variations to exist taking the size to over 30.

- also spotted that the turbo with its double valve springs has a higher load at top of lift (44lbf +94lbf) as opposed to all others with (124lbf)

I was wondering about the valve size bit (I noted that on my turbo head). Is there a reason for smaller valves, as opposed to the larger, more VE-friendly MG Metro intake valves? I'm considering using my MG Metro head for conversion purposes...

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1967 Cooper 's' Turbo build @ www.drjing.com

Yes Dan good thinking I will add it in. A couple of the things you mention I knew about now you say them, a couple I didn't.

Doc, cannot say for definate but I think the reason the turbo's used the smaller (1.3") inlet valves as opposed to the MG spec (1.4") items was to try and avoid cracking problems between the valves with the amount of heat.

I'd also say that there's no need for concern on the ex valves. Mopst people here swpa these for regular 7,12mm ones. The standard A+ valves are of VERY good quality. The A ones aren't though :)

"- turbo head can also be spotted i think by having a screwed blanking plug where the bypass hose normally comes out, i think all other heads either have the bypass or this isn't even drilled ie. left as cast "

No thats not true.... the only definate is that the exh valves have visiblable larger stems as they are sodium cooled.

Alex

AlexF

From the Engines I've stripped, the superior CAM6581 crank appeared after 1986. Early engines have had the same CAM6232 as the stock 1275 engines...

Hmm thats interesting I thought the early ones were the better ones! Oh well you live and learn.

I don't claim to know everything there is to know about Metro engines, the plan was to have a bash at the write up above, wait a week or so for comments from you lot and then re-write and edit as required.

Keep the comments coming, cheers!

Edited by Tom Fenton on 10th May, 2005.

Agreed Tom - that's the best thing for these kinda posts - get the info up here, let peeps review it, then edit the original.

Another thing - re blocks - the early nat-asp blocks had solid upper/lower main shells, the turbo's always had the solid lower/grooved upper. They were standardised in '87 (ish) to the 'turbo' type.

pistons - might be worth mentioning the nat-asp pistons are 9.75:1 and not too bad if used with a modded head, but the MG pistons at 10.5:1 are too high a CR, unless a very large CC head is used (£££).

I know some who've modded engines to 'turbo' and the stock oil pump seems to be up to the job - ie no problems seen, but should really be swapped out as/when the engine is rebuilt

Yes the bit about the oil pumps is interesting, if the pump with the "standard" width lobes is up to the job of supplying the turbo feed as well, you have to ask where does the extra capacity of the turbo pump go? Over the relief valve would be my guess, only achievement being heating the oil up unneccesarily?

V true. And anything that goes out of the relief valve is wasted power.

you may want to add a bit about the camshaft! .. i.e. turbo cam is the exact same as any A+ 1275 cam.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

I'm sure I have heard that:

The metro turbo heads had their waterways closer to the chamber didnt they to help cooling even though they were the same cast number?

All the later heads have slightly better waterways. You can tell the difference as the early ones have the whole of the area around the thermostat machined, whereas the later ones have the area part machined and part left cast.
I don't know what the changeover year was, but all the _later_ heads be they MG, turbo, or just 1300 metro have use this casting.
All the common 1275cc heads are a 12G940 casting. The CooperS used the AEG163, these are rare and prone to cracking between valves as well.

Edited by Tom Fenton on 11th May, 2005.

Chris Barkas ere,
New to this website, very good, thanks Tom and you guys u have just cleared a few things up for me, Just a little about myself, I have been intrested in minis and a avid petrol head since i was about 8 and now that i am 17 and doing my driving test bring on a mini, and a possible turbo engine in the future, look forward to quising you lot for help and ideas. Stay safe and thanks.

Chris Barkas

Great Info Tom, helpful and intresting!
mark

1310 - Supercharged 16v Twin Cam - 190BHP - SC Built.
1293 - Turbo 16v Twin Cam - Build Commenced

Didn't the extra oil for the turbo pump go to the piston cooling oil jets?
I'm planning turbocharge of my standard ('sfar as I know, haven't taken it apart) Metro LE engine in the Mini. Since it has 75PSI of oil pressure now at anything above idle, I'm figuring it has the oil capacity to supply the turbo.

1275 with Back Door Turbo

No factory A-series ever had piston cooling jets...

The cooper S's also used the 12G940 head. It was only the very early S's that used the 163 head.

How thick actually is the piston crown on a turbo piston, versus a regular piston ???

For reference...

A CP forged piston that Ived used in Subarus, has a crown of only 5.5mm thick. I was quite amazed at this when we measured it.
Consider they can easily survive 250bhp/litre, I dont think crown thickness is the issue. Piston design, possible material, and fuel/igntioon control are a lot more critical than the piston crown thickness alone IMO.


As for the valves....Has anyone experienced the engine to be more detonation prone with regular exhaust valves ??

In order to keep me on the road, I once had to fit a Meastro head to my car. Ex valve was actually bigger then the metro turbo ones, and regular thin stem.
I dont think CR should have changed, but the car was far more prone to detonation with this head fitted. Although maybe it was fuelling related too...no such thing as widebands in those days tho. I had thought it may have been down to an extremely hot exhaust valve ???

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

AE Turbo piston section. Very thick as you can see; how anyone could disintegrate one of these I just don't know!!!

Steve - I'm guessing the subaru pistons are forged???

Edited by TurboDave16V on 11th Mar, 2007.

Is the ring pack not a little further down the piston in the turbo pistons too (where it's better shielded from detonation damge)? Maybe that's rubbish but I picked it up somewhere.