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Mixing and matching.
Oliver Coles - 20/12/05 at 01:19 PM

Hi

Is it possible to put a MG metro turbo cylinder head and rocker cover onto a cylinder block of a midgit mk3 without it breaking when i turn the key?

Thank you.

andyps - 20/12/05 at 02:02 PM

Yes - they are both "large bore" engines so teh head is compatible. However, the turbo head is probably low compression so may not be ideal (I can't remember the figures).

Always worth turning it once by hand before using the starter motor just omake sure there are no bangs!

Oliver Coles - 20/12/05 at 05:04 PM

I have discovered that the compression ratio for the metro is 9.4:1 and that the compression ratio for a midget is 8.80:1.
Is there any way to modify the metro head so that it would work properly on the midget block?

cornishrob - 20/12/05 at 05:26 PM

a thicker head gasget would do the trick,

but i need to ask why you would want to lower the compression ratio? Also 9.4:1 for a turbo, espically and older one seems a little high are you sure its the turbo ratio you have?

Oliver Coles - 20/12/05 at 07:35 PM

This is the site where i got the information from.
http://www.metroturbo.com/tech.htm

MikeRJ - 20/12/05 at 09:02 PM

Is the Midget a 1098 or a 1275?

If it's a 1098, then it may or may not bolt on. The problem is that one of the valves (I think inlet, might be wrong) overhangs the cylinder, and if the lift is high enough it hits the block with predictably damaging consequences. This is a case of bolt it up with a used head gasket and check the clearances very carefully using plasticine etc. You can get the block "pocketed" to give more clearance, but to be honest it's all a bit of a bodge, and the resulting combustion chamber shape is most definately "sub-optimal". For a 1098 engine, you need to seek out a 12G295 head which offers signficantly better flow than the original. Fit large valves and have the ports tidied up and it flows pretty much the same as the large bore heads. 70 bhp from a 1098 should be fairly easy to acheive without compromising the reliability (they really don't like high revs though, it's a very long stroke engine).

If you really want to use the 12G940 on a small block engine, especialy the 1098, you will need to check the resulting compression ration very carefully, oddly the combustion chamber in the large bore heads is significantly smaller than thos in the small bore heads. You may have a bit of chamber grinding and/or piston machining/replacement to do.


If it's the 1275, the head will bolt straight on, though you will have to blank off the bypass hose that goes between the water pump and the older A series heads as the A+ doesn't use it. There are other issues you will need to consider:

All (unmolested) 12G940 head castings have the same chamber volume (21.3cc), the compression ratio is set by the piston dish and compression height on A series engines.

The Turbo head is not a great starting point for a normaly aspirated engine, it has standard 1.3 A+ valve sizes, and the exhaust valve stems are larger (sodium filled) with tulip shaped heads which means exhaust port flow is actualy worse. An MG Metro head would be a far better starting point as it has the late Cooper S sized valves. If you are swapping a bog standard 12G940 out for the turbo head with no other changes, you won't see any significant change in power.

As for compression ratio, you want to get it up to around 9.5:1 on a standard cam, or 10.5:1 on an MG Metro cam (and maybe up to 11:1 for a hotter cam, e.g. Kent 286) to make good power, the A Series thrives on high CR. Obviously igntion timing and fueling would need to be adjusted to suit. An LCB exhaust manifold coupled with a free flowing system and decent single SU inlet manifold is essential to make decent power.

MikeRJ - 20/12/05 at 09:04 PM

quote:
Originally posted by cornishrob
a thicker head gasget would do the trick,

but i need to ask why you would want to lower the compression ratio? Also 9.4:1 for a turbo, espically and older one seems a little high are you sure its the turbo ratio you have?


Yes, the Metro Turbo ran low boost and high CR to get good economy and improved off-boost response. The boost had to be fairly low from the factory anyway, as the gearbox "canna take no more cap'n". Even then, the gearboxes didn't last particulary long if the car was driven hard.

[Edited on 20/12/05 by MikeRJ]

andyps - 20/12/05 at 11:01 PM

I based my answer on the mark 3 midget being the 1275cc.

I had forgotten that the metro turbo ran such a high cr (for a turbo).

Oliver Coles - 21/12/05 at 11:57 AM

quote:

The Turbo head is not a great starting point for a normaly aspirated engine, it has standard 1.3 A+ valve sizes, and the exhaust valve stems are larger (sodium filled) with tulip shaped heads which means exhaust port flow is actualy worse. An MG Metro head would be a far better starting point as it has the late Cooper S sized valves. If you are swapping a bog standard 12G940 out for the turbo head with no other changes, you won't see any significant change in power.



What if the turbo is included in the engine? So theres the midget block, the metro tubo head and the turbo.

Thank you for the information

andyps - 21/12/05 at 01:49 PM

Better to use the turbo head if you are using the turbo. How does the manifold fit in a longitudinal engine bay? Can't remember how it was all laid out - long time since I had a Metro Turbo.

Oliver Coles - 21/12/05 at 02:06 PM

This is basically the turbo conected to the A series engine in the front wheel drive metro.


I have also discovered that the transmission in Midgets and MGAs in particular are good for 200 HP! So i souldn't need to modify the transmission
in any way.

[Edited on C:Wedocu12e12 by Oliver Coles]

[Edited on C:Wedocu12e12 by Oliver Coles]