paul_mcq
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| posted on 11/4/04 at 02:18 PM |
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mini engines
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????????????????????????
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Spyderman
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| posted on 11/4/04 at 02:54 PM |
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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.
Spyderman
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paul_mcq
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| posted on 11/4/04 at 03:10 PM |
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cheers mate
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MikeR
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| posted on 11/4/04 at 04:07 PM |
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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.
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rusty nuts
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| posted on 11/4/04 at 04:26 PM |
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spyderman, think you will find that a series engines came in 803 cc, 848 cc ,948cc, 997 cc and 1098cc versions as well. Rusty
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paul_mcq
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| posted on 11/4/04 at 06:15 PM |
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thanks for the info
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craig1410
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| posted on 11/4/04 at 06:49 PM |
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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.
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Peteff
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| posted on 11/4/04 at 08:25 PM |
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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.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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andyps
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| posted on 11/4/04 at 08:46 PM |
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quote:
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.
I think it was the cooper S and the MG 1300 mark 2 which had the extra bolt in for the cylinder head.
You can fit an A+ head to an earlier engine, but need to use the later water pump as the early ones have a conection to the bypass hose which the head
doesn't have. If you don't the water losses are quite severe The head casting is still basically 12G940 - anyone want to buy one - I
have a spare!
Andy
An expert is someone who knows more and more about less and less
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craig1410
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| posted on 11/4/04 at 09:03 PM |
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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.
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Peteff
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| posted on 12/4/04 at 10:42 AM |
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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 .
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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craig1410
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| posted on 12/4/04 at 11:02 AM |
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quote:
Originally posted by Syd Bridge
The original 'Cooper' heads are the 12G295 casting.
Yes but the 12G295 was only used on the 998 Cooper (and MG1100, Mk2 Midget and Mk3 Sprite) which is obviously a "small bore" engine. You
certainly wouldn't want to put one of these on a 970, 1071 or 1275 engine. It also had a much larger capacity combustion chamber at 28.3cc so it
would reduce compression ratio if it was placed on a big bore engine.
You are correct that the 12G940's were not all equal but I am 99% certain that any 12G940 head can be bolted on to any big bore block as the
differences are in the subtleties of the castings and valve sizes. There are no "fundamental" differences which would prevent you bolting
it on and cranking up the engine although clearly you wouldn't want to run a Turbo engine which is already on the verge of detonation at 9.4:1
CR without the sodium filled valves.
Cheers,
Craig.
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paul_mcq
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| posted on 12/4/04 at 11:25 AM |
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cheers for all the info ppl
ps do u think this wold look cool on a black mini
[img]C:\WINDOWS\Profiles\paul\My Documents\kit car[/img]
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Crazy Jay
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| posted on 12/4/04 at 11:36 AM |
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u didnt load them on right. Put the pics in ur photo archieve first then create a link to them
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