i have heard u can put a smaller head on to a block. is it possible to put a 1275 turbo head on a 1375 block. i have also heard decresesing the displacement is also good for trboing is all this true????????????????????????

There are only two different blocks for the a-series engine. The 998cc and the 1275cc. The 1375 being an overbored 1275.

The heads are interchangable between them, but there would be little benefit in going for the smaller capacity head.
When turboing an engine it is the compression ratio that is lowered not the capacity. This reduces the chances of melting the pistons.
If you have a 1375 lump then keep the head and fit lower comp pistons or thicker gasket.
For power or tuning bigger is always better!

The only thing that coul improve when going smaller is the response time especially when fitting a smaller turbo, however at the cost of top end power.

cheers mate

Hmm, i thought the valves used on Turbo cars where smaller than those on NA cars (Cosworth engines in the sierra being an example). No, I haven't a clue why.

spyderman, think you will find that a series engines came in 803 cc, 848 cc ,948cc, 997 cc and 1098cc versions as well. Rusty

thanks for the info

Hi,
Look for books by a guy called David Vizard. I have a book called somthing like "Tuning the A-Series Engine" by him and it used to be like a Bible to me when I was into mini's.
If you can, get a turbo engine from an old metro turbo as these have sodium filled exhaust valves which disperse heat more effectively. Other than that is is basically the same "12G940" (IIRC) head casting and if memory serves it used to run about a 9.4:1 comression ratio. Better to go a bit lower than that though if you want serious boost pressures.

As Syd said you can fit this head to the smaller engines if you "pocket" the block as the bigger head flowed air better and could make smaller engines go faster but this is only worth pursuing if you are running in a sub 1 litre racing class or something.

Also, try to use the later A+ Series blocks as these are stiffer and once again the Turbo engine had a few mods to make it better suited to higher power. I fitted a Metro Turbo engine to my Mini 1275GT and then up'd the boost to about 12Psi. It could absolutely fly and was actually quite scary at times. You felt like you were in one of those speeded up car chases!! Could top 130MPH!! Aledgedly!! and 0-60MPH was in the low 6's and maybe even high 5's on a good day...

Cheers,
Craig.

12G940 were Cooper heads weren't they? I think the old 1275S heads had an extra stud and were A series engines. The later 1275 (Metro GT)ones were A+ and didn't have the bypass pipe external but built into the head. I don't think they are interchangable. The Metro turbo used different exhaust valves because of the heat.

quote:

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Originally posted by Peteff
12G940 were Cooper heads weren't they? I think the old 1275S heads had an extra stud and were A series engines. The later 1275 (Metro GT)ones were A+ and didn't have the bypass pipe external but built into the head. I don't think they are interchangable. The Metro turbo used different exhaust valves because of the heat.

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Pete,
12G940 were used in a wide variety of engines form Sprite's and Midget's to MG 1300 and 1300GT's as well as the last production runs of Cooper S's. The earlier Cooper big bore engines (970,1071 and 1275) used a 12A185 and AFG163 according to the aforementioned Vizard book which I have now brought down out of the attic... The 12G940 continued into production for the A+ series engine in the Metro and the reputed best head is the 12G940 on the Metro Turbo although it has a thinner deck and thus can't be skimmed much. It does have improved cooling and the sodium filled valves which as you say are there to help to keep them cool and prevent detonation. The inlet valves on the turbo head (1.3125" are not as large as the ones on the Cooper head (1.4" but for forced induction this isn't a big problem.

The presence or absence of the extra stud is not a concern and you can use it or not without penalty. I think some heads had two extra stud holes didn't they?

The bypass hose was always a pain in the butt and most folk remove it and blank it off. IIRC you can just drill a hole in the thermostat to keep coolant circulating instead.

Anyway, getting back the original question, yes you can put any "big bore" head on any "big bore" block but you should ensure that you get an acceptable compression ratio for your application. The chamber volume of most "big bore" heads is 21.4cc but if they have been skimmed or gas flowed then they may differ from this. To calculate the (static) compression ratio you take this figure, add it to the following:
1. Volume of the piston crown
2. Volume between the piston and the bore-wall above the first ring
3. Volume held in the thickness of the head gasket
4. Volume between the piston crown and the block deck
Then divide this into the swept volume of the piston (engine cc / 4).

Hope this helps,
Craig.

I remember getting a 12G295 head from a MG1100 years ago and it fitted my 850 mini. I think it was skimmed by 90 thous to get the compression up and was a big improvement. A good 3 branch exhaust manifold and a longer inlet with bigger SU were always the first improvements to make on the old minis though. Usually followed by a crash or better brakes.

quote:

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Originally posted by Syd Bridge
The original 'Cooper' heads are the 12G295 casting.

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ps do u think this wold look cool on a black mini
[img]C:WINDOWSProfilespaulMy Documentskit car[/img]

u didnt load them on right. Put the pics in ur photo archieve first then create a link to them